# Truck runs great with coolant sensor unplugged



## Moonlighter

I need some fresh ideas, my main salt truck is a 91 Chevy 3500 1 ton 2wd with a 5.7L TBI motor. If I have the coolant sensor plugged in it idles at 11-1300 rpm, if i unplug the coolant sensor (the one in the head next to the goose neck) it will run at perfect rpm and purr. This is not supposed to be so, as it runs best when the computer thinks it's at -30. However the other day I had it running great with everything plugged in for about 30 minutes, until I shut it off and restarted it, It was also before I replaced IAC.

Here is the list of what has been checked and or replaced.
There are no codes being thrown at all unless I unplug the CTS.
I checked all the vacuum lines 4 times, sprayed carb clean in and around anything that could possibly leak with no luck. New PVC valve, new IAC (due to me over tightening the original when I took it out to clean it). Warranted the CTS to make sure it was a new one. I looked down the throttle body to see if the base gasket was getting sucked in. I redid all the engine and harness grounds. I replaced vacuum lines as needed and ends that were cracked. Verified all sensor operations by a scanner computer. If I clear the code for the CTS while it's unplugged the truck will fall on it's face for a few seconds until the code pops again and it purrs again. There is no booster on this truck, has the hydro boost system in it.

Tomorrow I am going to disconnect the battery for a half an hour and check the plugs on the computer for issues and check for any grounds under the dash that may have not been redone since I rebuilt the body. Aside from that I am out of places to go with it, I ran it tonight to fill it up at the gas station with the CTS unplugged seamed fine but it's just not supposed to run that well with it unplugged.

I feel like I may be missing something, any ideas would be very helpful. P.S. I even threw in a picture of the truck shortly after I mounted the blade. Thanks in advance for any help it's driving me nuts!


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## Whiffyspark

Ok first you need to get a vacuum garage on it and verify you do not have a leak. Carb cleaner doesn't always work. You can use the line that goes into the master cylinder

Next, check your fuel pressure. That era is known for fuel pumps going out. 

Last, make sure your convertor isn't plugged. 

Your ect sensor should have a voltage range of .5-4.5. You want to verify that as well


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## dieselss

He's got hydro boost. No vacuum lines there 
Does the tk have a temp gauge on the dash?


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## Whiffyspark

Missed that. Theres a line on the back of the intake. I think theres one on throttle body too. Its been a few months since o worked on one. Doesn't really matter where you put the guage


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## Moonlighter

Whiffyspark;1674542 said:


> Ok first you need to get a vacuum garage on it and verify you do not have a leak. Carb cleaner doesn't always work. You can use the line that goes into the master cylinder
> 
> Next, check your fuel pressure. That era is known for fuel pumps going out.
> 
> Last, make sure your convertor isn't plugged.
> 
> Your ect sensor should have a voltage range of .5-4.5. You want to verify that as well


I will check it with the vacuum gauge in the morning. I will try and find a place in the line to test the fuel pressure. The coolant sensor is new and the values checked out via scanner - cold and warmed up. I didn't think about the converter because the exhaust flows freely, but I will check it as well.
I appreciate the suggestions I will report more findings tomorrow.



dieselss;1674557 said:


> He's got hydro boost. No vacuum lines there
> Does the tk have a temp gauge on the dash?


Yes it does but I plugged a mechanical one into the side of the block for a more accurate reading.


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## dieselss

What does the scanner read for the numbers? Is it close to accurate?


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## Moonlighter

Yes the scanner is accurate, all the sensor numbers are within spec. I can look at almost everything with it, o2 crosscounts, IAC fwd - rev and voltage, TPS voltage reads smooth from closed to wide open, CTS unplugged reads -36 plugged in reads right and climbs with engine temp rising, MAP sensor values read right, shows me 14.5 volts on fuel pump but no pressure reading, also shows injector pulse, batt volatge, ect. There is plenty of comunication between scanner and computer. I had a van last year that wouldn't read the computer & ran like crap, I cleaned all the grounds on the motor and body and cured it right up. I could write down all the numbers off it while truck is running good if that would help. I didn't get a chance to disconnect the battery again and check the plugs to the computer I am doing that first thing later today. Just really puzzled that I had it running good until I shut it down and restarted it, without doing anything of consequence.


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## Whiffyspark

Moonlighter;1675233 said:


> Yes the scanner is accurate, all the sensor numbers are within spec. I can look at almost everything with it, o2 crosscounts, IAC fwd - rev and voltage, TPS voltage reads smooth from closed to wide open, CTS unplugged reads -36 plugged in reads right and climbs with engine temp rising, MAP sensor values read right, shows me 14.5 volts on fuel pump but no pressure reading, also shows injector pulse, batt volatge, ect. There is plenty of comunication between scanner and computer. I had a van last year that wouldn't read the computer & ran like crap, I cleaned all the grounds on the motor and body and cured it right up. I could write down all the numbers off it while truck is running good if that would help. I didn't get a chance to disconnect the battery again and check the plugs to the computer I am doing that first thing later today. Just really puzzled that I had it running good until I shut it down and restarted it, without doing anything of consequence.


Need to check fuel pressure and vacuum. I doubt this is an electrical problem.


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## Moonlighter

I checked vacuum today and it reads 19 inch pounds, at the pcv port. Fuel pressure will be a little challenging but I will check it as soon as possible. Thanks Brian


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## Whiffyspark

Was it steady?


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## Moonlighter

Yes it was steady 19.


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## rjigto4oje

Does it use a knock sensor I had one on a motor home give me trouble once


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## Whiffyspark

Moonlighter;1675459 said:


> I checked vacuum today and it reads 19 inch pounds, at the pcv port. Fuel pressure will be a little challenging but I will check it as soon as possible. Thanks Brian


Most likely Fuel Pump. Do you have a tester? They have an adapter for your motor. I have one, but I'm no where near you


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## Moonlighter

rjigto4oje;1675820 said:


> Does it use a knock sensor I had one on a motor home give me trouble once


No knock sensor I think, scanner had no reading for it, I think sometime after 91 they started using them not sure.


Whiffyspark;1675827 said:


> Most likely Fuel Pump. Do you have a tester? They have an adapter for your motor. I have one, but I'm no where near you


That would kinda suck if it was, I replaced the tank when I rebuilt the truck this summer, would have been so easy then. I will have to buy an adapter or a whole kit new one for this truck so it will be a few days, doing a rebuild on the hydroboost tonight while it's warm.


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## dieselss

not understanding how the f.p. could be causing this???


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## maxwellp

There is something going on in closed loop. When you unplug the coolant sensor it runs in open loop off of a set of predetermined values. When it is warm and plugged in it goes in to closed loop and reads all the sensors. Keep looking there one or more that is making it do this. Did you check the knock sensor? Are there two temp sensors on this?


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## Moonlighter

maxwellp;1676015 said:


> There is something going on in closed loop. When you unplug the coolant sensor it runs in open loop off of a set of predetermined values. When it is warm and plugged in it goes in to closed loop and reads all the sensors. Keep looking there one or more that is making it do this. Did you check the knock sensor? Are there two temp sensors on this?


You are correct in your assumptions that's why it is puzzling me. Even when it's warm and plugged in it high idles, kinda leading to getting a computer out of a yard and see if it cures it. It won't go into closed loop I forgot to mention that, It hasn't since I started testing and trying to trace down the issue. However all sensors tested out properly both high idle and normal idle with the coolant sensor unplugged. Yes there are two sensors one in the head, is just the gauge sensor, I have a mechanical gauge in it, and the new one I put in the intake next to the goose neck is the computer sender.


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## rjigto4oje

DOES IT have a power steering sensor im the pressure line also if it does not go into closed loop maybe the engine is not getting hot enough


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## maxwellp

Moonlighter;1676579 said:


> You are correct in your assumptions that's why it is puzzling me. Even when it's warm and plugged in it high idles, kinda leading to getting a computer out of a yard and see if it cures it. It won't go into closed loop I forgot to mention that, It hasn't since I started testing and trying to trace down the issue. However all sensors tested out properly both high idle and normal idle with the coolant sensor unplugged. Yes there are two sensors one in the head, is just the gauge sensor, I have a mechanical gauge in it, and the new one I put in the intake next to the goose neck is the computer sender.


The computer was my first thought but I convinced myself that was just trying for an easy fix. But if you are thinking that also I would try one if you can come up with a bone yard one. I have spares for my older trucks I got off fleaBay for $50 because it also controls that alternator in a Dodge. 
One other thing could be that you have a wiring problem and the sensor signal is not getting back to the ECM.


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## Whiffyspark

maxwellp;1676691 said:


> The computer was my first thought but I convinced myself that was just trying for an easy fix. But if you are thinking that also I would try one if you can come up with a bone yard one. I have spares for my older trucks I got off fleaBay for $50 because it also controls that alternator in a Dodge.
> One other thing could be that you have a wiring problem and the sensor signal is not getting back to the ECM.


Take it to a shop and have them check fuel pressure. This is a VERY common 5.7 problem. No need to reinvent the wheel and piss money away


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## jb1390

Whiffyspark;1676699 said:


> Take it to a shop and have them check fuel pressure. This is a VERY common 5.7 problem. No need to reinvent the wheel and piss money away


Fuel pressure issues would not cause the symptoms indicated IMO. Where you would first see problems of fuel pressure is during high load periods, pulling a trailer or climbing a hill, when the truck is warm, and especially if the gas tank is low on fuel (fuel will be warmer, reducing the cooling capability).


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## jb1390

Moonlighter;1675459 said:


> I checked vacuum today and it reads 19 inch pounds, at the pcv port. Fuel pressure will be a little challenging but I will check it as soon as possible. Thanks Brian


Going to go out on a limb here and say that you still have a vacuum leak. I remember when checking the cats on my 98-you can give partial throttle, have it going down the road, and when you add throttle, the vacuum will decrease for a moment but jump right back to where it was, and hold steady. I do not think you can get a vacuum reading that is accurate enough to tell you that you do not have a minor leak.

Theory: Engine runs "normal" to you when coolant sensor is unplugged, because computer closes IAC valve to create a very rich mixture, in an attempt to warm the truck up.

Tests I would perform in this order:
EGR valve function: you need a vacuum pump for this. disconnect the vacuum line from the egr valve (do not need to plug it), and connect your vacuum pump to the egr valve directly. With the truck idling, pump the vacuum-the EGR valve should hold vacuum, and the truck should start having a poor idle. Release the vacuum, and the idle should smooth out. If this doesn't work as described, you need a new EGR valve before going any further.

All vacuum lines: Just replace them. There aren't that many, it's only a couple bucks, and it won't hurt

Throttle Body Gasket: Again pretty inexpensive, though more difficult to get to. You can try to find a leak here by using carburetor cleaner or propane. If you are checking for leaks, you need to have the temperature sensor enabled.

Idle Air Control Valve: Make sure it moves when you turn AC on or turn your wheel, or put the truck in gear (add load to the engine)

Intake Manifold Gaskets: Most difficult to change, and check. You can check the upper side of them using carb cleaner or propane, but you can't do anything to check for a leak internally, from the valley of the motor. A this point, you can decide how much the high idle bothers you.

I would not run it with the coolant temperature sensor unplugged, as your truck will run very rich I believe, and you are at risk of fouling valves, the cat, etc. If you cannot completely diagnose, I suggest unplugging the IAC valve, and setting it at a warm idle you are happy with. Might need to hit the gas during cold starts if you go this route.


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## Whiffyspark

jb1390;1676715 said:


> Fuel pressure issues would not cause the symptoms indicated IMO. Where you would first see problems of fuel pressure is during high load periods, pulling a trailer or climbing a hill, when the truck is warm, and especially if the gas tank is low on fuel (fuel will be warmer, reducing the cooling capability).


Wrong. These trucks have a known problem 
And FYI fuel pressure won't matter what Youre doing with truck. If it craps out its out.

Egr valve is cheap too.


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## Whiffyspark

Whiffyspark;1676731 said:


> Wrong. These trucks have a known problem
> And FYI fuel pressure won't matter what Youre doing with truck. If it craps out its out.
> 
> Egr valve is cheap too.


I was a master technician in a previous career. Its not a hard diagnosis you just need proper tools


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## jb1390

Whiffyspark;1676731 said:


> Wrong. These trucks have a known problem
> And FYI fuel pressure won't matter what Youre doing with truck. If it craps out its out.
> 
> Egr valve is cheap too.


EGR is around $50 for a cheap autozone one. Might as well perform the 45 second test first.

As for the fuel pump, they are more likely to crap out when the fuel pump is warm. You, as a master technician, must have seen at least one vehicle in your career that would crap out going down the road, but cools down a bit, and will start again due to a faulty fuel pump. I would hope so anyway, because I have been in a vehicle that has done just that.

I understand that it doesn't always work that way, and sometimes they just crap the bed. But, you must agree, if that had happened to the OP, we wouldn't be trying to diagnose a high idle, we'd be diagnosing a no start condition. Thumbs Up


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## maxwellp

I had a few 5.7 in this year, when there was a fuel pump problem you knew it because it would not run. As in the pump would work or not. 
I do not see how a fuel pump will create the problems that this truck is showing. Please expand on your theory - Whiffyspark. 
jb1390 has some strong points and is very possible.


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## Moonlighter

jb1390;1676727 said:


> Going to go out on a limb here and say that you still have a vacuum leak. I remember when checking the cats on my 98-you can give partial throttle, have it going down the road, and when you add throttle, the vacuum will decrease for a moment but jump right back to where it was, and hold steady. I do not think you can get a vacuum reading that is accurate enough to tell you that you do not have a minor leak.
> 
> Theory: Engine runs "normal" to you when coolant sensor is unplugged, because computer closes IAC valve to create a very rich mixture, in an attempt to warm the truck up.
> 
> Tests I would perform in this order:
> EGR valve function: you need a vacuum pump for this. disconnect the vacuum line from the egr valve (do not need to plug it), and connect your vacuum pump to the egr valve directly. With the truck idling, pump the vacuum-the EGR valve should hold vacuum, and the truck should start having a poor idle. Release the vacuum, and the idle should smooth out. If this doesn't work as described, you need a new EGR valve before going any further.
> I will test this out later today, the scanner shows it opening and closing properly but I did not do the vacuum test on it yet.
> 
> All vacuum lines: Just replace them. There aren't that many, it's only a couple bucks, and it won't hurt
> I found a few questionable lines yesterday and will replace them.
> 
> Throttle Body Gasket: Again pretty inexpensive, though more difficult to get to. You can try to find a leak here by using carburetor cleaner or propane. If you are checking for leaks, you need to have the temperature sensor enabled.
> I looked down the throttle body and didn't see any signs of it being sucked in, also sprayed all around it with no change. Does not mean it's not bad just couldn't find a leak so I didn't change it yet.
> 
> Idle Air Control Valve: Make sure it moves when you turn AC on or turn your wheel, or put the truck in gear (add load to the engine)
> 
> IAC is brand new and working properly.
> 
> Intake Manifold Gaskets: Most difficult to change, and check. You can check the upper side of them using carb cleaner or propane, but you can't do anything to check for a leak internally, from the valley of the motor. A this point, you can decide how much the high idle bothers you.
> 
> I would not run it with the coolant temperature sensor unplugged, as your truck will run very rich I believe, and you are at risk of fouling valves, the cat, etc. If you cannot completely diagnose, I suggest unplugging the IAC valve, and setting it at a warm idle you are happy with. Might need to hit the gas during cold starts if you go this route.


Thank you for your suggestions I will test them when I get a chance to, to me it has to be something simple that I can't pinpoint, as I stated in the beginning of the thread I had it running right for 30 minutes till I shut it off and restarted it. I checked all kinds of things that day, plugging and unplugging sensors along the way but it did run good for 30 minutes with everything plugged in. Looks like Monday morning may be our first push, hopefully I can get it right by then. Also an update on the hydro boost rebuild so far so good no leaks!
There is no sensor in the power steering lines.


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## Moonlighter

EGR tested out good. I will replace the vacuum line after I get new one later today and go from there. Like I stated IAC is new and seems fully operational. Thanks again for any help!


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## StuMpy77

My initial thought was intake gasket, our 89 Lesabre did the exact same thing and in my case the intake gasket indeed solved the high idle problem. I could not find the leak using propane or brake cleaner either. I bet yours is also an internal leak which is causing the high idle. I say that because the MAP sensor may not be picking it up(I assume you have, pretty sure my 88 k1500 5.7L did), so would have to guess its post MAP. Which by the way have you checked the map sensor to see if it holds vacuum? The extra fuel the ECM pumps in with the CTS unplugged compensates for the un-metered air that's sneaking into your combustion chamber, bringing the idle down closer to where the IAC can attempt to compensate.....that's the way it's working out in my head at this time anyway lol! Correct me if I'm wrong of course. Best of luck on your repair!


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## Whiffyspark

StuMpy77;1677502 said:


> My initial thought was intake gasket, our 89 Lesabre did the exact same thing and in my case the intake gasket indeed solved the high idle problem. I could not find the leak using propane or brake cleaner either. I bet yours is also an internal leak which is causing the high idle. I say that because the MAP sensor may not be picking it up(I assume you have, pretty sure my 88 k1500 5.7L did), so would have to guess its post MAP. Which by the way have you checked the map sensor to see if it holds vacuum? The extra fuel the ECM pumps in with the CTS unplugged compensates for the un-metered air that's sneaking into your combustion chamber, bringing the idle down closer to where the IAC can attempt to compensate.....that's the way it's working out in my head at this time anyway lol! Correct me if I'm wrong of course. Best of luck on your repair!


This also along the lines of low fuel pressure. When you unplug etc fuel pressure goes up to richen.

I would stop pissing money And have a shop diagnosis your problem. It is very very common


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## Moonlighter

Whiffyspark;1677511 said:


> This also along the lines of low fuel pressure. When you unplug etc fuel pressure goes up to richen.
> 
> I would stop pissing money And have a shop diagnosis your problem. It is very very common


Well for one thing I am not pissing my money away tracking this issue down myself. A shop is going to charge me to do all the diagnostics I can do myself, and have already done. Thanks for your input.


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## StuMpy77

I'm with ya there Moonlighter, I bet you'd be in over $3-400 easy at a shop said and done. I was wondering when you replaced your upstream O2 sensor last? The GM 1 wire unheated units had a very short recommended service interval, something like 20k and its pretty accurate. Truck will run fine they just get coated in crap over time and take longer to warm up to where it will provide useable info to the computer(around 600 degrees). Doesn't throw a code because technically it is working, just slow and the OBD1 systems don't always catch time delay issues. Longer in "1 size fits all cold start mode", gas mileage goes down. If you haven't done them in a long time I would pop on the under $20 O2 sensor first, if it's been a few years your looking at a win win situation anyhow. Don't think it'll fix your problem necessarily, but for about $20 and maybe 30 minutes, I bet you pick up an mpg or better which is always cool I don't care who ya are these days.

Test individual components you can test(EGR,MAP,CTS,Knock sensor). Check your timing. If everything is testing okay I would go upstream only oxygen sensor assuming you didn't do it last year. I wouldn't mess with the TPS, there is an adjustment procedure to reinstall if you take it out if I remember right. If it's not throwing a code or causing issues while driving I doubt its that. They usually dead spot where you operate the most while driving. From there I would have to really start questioning the intake gasket which is also dirt cheap from fel-pro btw. Very very common cause of high idle in many vehicles.


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## Moonlighter

StuMpy77;1678578 said:


> I'm with ya there Moonlighter, I bet you'd be in over $3-400 easy at a shop said and done. I was wondering when you replaced your upstream O2 sensor last? The GM 1 wire unheated units had a very short recommended service interval, something like 20k and its pretty accurate. Truck will run fine they just get coated in crap over time and take longer to warm up to where it will provide useable info to the computer(around 600 degrees). Doesn't throw a code because technically it is working, just slow and the OBD1 systems don't always catch time delay issues. Longer in "1 size fits all cold start mode", gas mileage goes down. If you haven't done them in a long time I would pop on the under $20 O2 sensor first, if it's been a few years your looking at a win win situation anyhow. Don't think it'll fix your problem necessarily, but for about $20 and maybe 30 minutes, I bet you pick up an mpg or better which is always cool I don't care who ya are these days.
> 
> Test individual components you can test(EGR,MAP,CTS,Knock sensor). Check your timing. If everything is testing okay I would go upstream only oxygen sensor assuming you didn't do it last year. I wouldn't mess with the TPS, there is an adjustment procedure to reinstall if you take it out if I remember right. If it's not throwing a code or causing issues while driving I doubt its that. They usually dead spot where you operate the most while driving. From there I would have to really start questioning the intake gasket which is also dirt cheap from fel-pro btw. Very very common cause of high idle in many vehicles.


That's interesting, the truck is relatively new to me and I have not replaced the o2 sensor, I did check it to make sure it was plugged in, it did have a layer of rust on the outside of it (normal from where it lives). Also when I checked it I noticed someone already removed the cat and straight pipped it. I will look into the o2 this week. The intake if it is leaking I would think it would have to be internal, I have checked all around it with spray with no change. Not a end all test on it I know that but it may have to wait a bit if I get down to it. Thank you for your suggestions.


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## jb1390

Moonlighter;1678754 said:


> That's interesting, the truck is relatively new to me and I have not replaced the o2 sensor, I did check it to make sure it was plugged in, it did have a layer of rust on the outside of it (normal from where it lives). Also when I checked it I noticed someone already removed the cat and straight pipped it. I will look into the o2 this week. The intake if it is leaking I would think it would have to be internal, I have checked all around it with spray with no change. Not a end all test on it I know that but it may have to wait a bit if I get down to it. Thank you for your suggestions.


Moonlighter, any updates on your idle issue?


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