# 1988 Chevy 5.7 tbi runs rough cold and....



## russ130

Ok here is the deal. This truck has a good motor but it lacks power all the time not like barely moves but it won't burn rubber. Now when its cold it runs really rough coughing and sputtering. If you have the throttle just barely open it will smooth out until it upshifts then it does it again until the rpms build. If I push in the throttle even 1/8th of the way in it just sputters and runs like crap. Once it gets to operating temp all these symptoms go away and it runs normal other than I think it should still have more power but there is no sputtering. I figured since it's a engine temp related problem that a new temp sensor might solve this problem. It didn't! The only code I'm getting is a lean mix. So without buying everything do I need an O2 sensor, ecm or eprom?


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## m2low

crazy question, but have you checked all the spark wires? replaed... they could be split when cold, but as the electricity runs through them they heat up and make connection... would also explain bad power and low RPM sputter... once the RPMS up the one cylinder does not make that much difference

also, those TBI injectors have a rubber 0 ring where they seat in the TBI - have you check when its idleing in the driveway take off the air cleaner and see if you are getting a good spary or is kind of dripping out of one of the jets?


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## russ130

Its got new wires and yes I have checked the spray pattern, no drips just a nice round spray. I also should add that the idle surges while in park and neutral but not while in drive or reverse.


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## Onebadbowtie86

if its got new plugs, replace the cap and rotor if you havnt done so


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## brad96z28

russ130;679017 said:


> Ok here is the deal. This truck has a good motor but it lacks power all the time not like barely moves but it won't burn rubber. Now when its cold it runs really rough coughing and sputtering. If you have the throttle just barely open it will smooth out until it upshifts then it does it again until the rpms build. If I push in the throttle even 1/8th of the way in it just sputters and runs like crap. Once it gets to operating temp all these symptoms go away and it runs normal other than I think it should still have more power but there is no sputtering. I figured since it's a engine temp related problem that a new temp sensor might solve this problem. It didn't! The only code I'm getting is a lean mix. So without buying everything do I need an O2 sensor, ecm or eprom?


 If its lean best to ck fuel pressure and volume.Then let us know. Im prety sure to do this u need a special adaptor that goes in place of the fuel filter on a tbi.It would also be essention to monitor what all ur sensors are doing on a scanner.


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## russ130

Cap and rotor are new and so are the plugs. I had originally thought about it being the fuel pump just based on the lack of power. I didn't know about it running bad when cold at first cause it was in the summer but now that it has cooled down it has shown itself. My thought is if it was the fuel pump wouldn't it not be such a big problem in the cold since the motor on the pump would be cooler?


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## Onebadbowtie86

you need more fuel the colder the weather, so look into it being the filter/pump


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## brad96z28

I could make guesses all day. U got to start checking things.


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## russ130

Onebadbowtie86;679242 said:


> you need more fuel the colder the weather, so look into it being the filter/pump


I'll bet you are right about it being the filter or pump. I overlooked the fact it would need more fuel when it is cold.


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## russ130

One more thing, anyone know off the top of their head what the pressure should be?


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## ed39

if you t in off the supply line in the back of the throttle body it should be 9-11psi, but if your truck is surging at idle i would check for a vacum leak, and check over the basics like timing, check the cap (make sure the contact points ware the rotor meets are good)


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## Spitz

To be honest you shouldnt have to hook a pressure gauge up to it. Any easy and accurate way it to pull the return line off the throttle body and start the truck, If there is enough fuel the fuel should come out of the return line port at a decent flow, if its slow or not much comes out chances are you have a flow or pressure issue. The regulator is spring loaded and if the pressure isnt enough the fuel doesnt even get to the return line.. Its a hell of a lot easier than checking it with a gauge.


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## bike5200

There is a temperature sensor that tells the computer how to mix fuel/air since the truck does not have a choke. I have had to replace this sensor on a few of my past trucks. It's located on the front top of the engine, next to where the thermostat is in that elbow.If you replace it antifreeze will leak out when it is removed. Mine had 2 wires to it and the plug went into the sensor. If you get a new one, the truck has 2 temperature sensor on it, one goes to the gauge in the dash and one tells the computer the temp of the engine so the computer can adjust fuel/air mix.I do not know if I ever got a code on mine.


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## russ130

Yeah I already tried a new temp sensor that was the first thing I tried


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## bike5200

russ130;680632 said:


> Yeah I already tried a new temp sensor that was the first thing I tried


Sorry that did not help


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## brad96z28

russ130;680632 said:


> Yeah I already tried a new temp sensor that was the first thing I tried


Yes but is the computer seeing the correct signal?


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## plowinzr2

i just had a similar problem with my 88 it had no power and a surging idle come to find out the wire broke off my O2 sensor i replaced it and it was like night and day


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## Beater_K20

pull the vac line off of the EGR and plug it. see if that makes any difference. my 94 was doing the same thing, unplugged the EGR, no more problems. though it does throw a CEL after extended periods at highways speeds. going to eventually turn it off in the tune once i get some more chips.


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## russ130

The EGR is already bypassed. I checked under the truck to see if the fuel filter looked new or not and it was kinda old but also noticed the line being rotted. So knowing that to replace the fuel filter the line will more than likely not make it, and to replace the line I have to drop the tank, and since the tank is already dropped I might as well go ahead and replace the fuel pump and do all this in one shot. So today I picked up a new pump and screen, fuel line and I already had the correct filter for some reason. Tomorrow I will drop the tank and make the repairs. I'll let you all know how things work out Sunday night.


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## Spitz

Like i said take the air cleaner off and remove the fuel line on the left side (passenger side) and start the engine, jump up on the truck and see how much fuel is coming out of the port on the tbi.. There should be a decent amount of fuel streaming out, if not, you have a pressure problem.. If no fuel is returning you have some sort of pressure or flow issue..

Ps- this is the return line, the pump should make well above what the truck needs, most can create 90psi or more if deadheaded, so if there is no fuel return then there isnt enough pressure and you need a new pump, or a filter that is extremely clogged.


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## 2COR517

It may be your Throttle Body gasket. I did some searching here
http://www.automotiveforums.com/vbulletin/index.php
and came up with this. My 95 Tahoe surges a little at idle. They said to spray carb cleaner around the base of the TBI and see if the engine speeds up. You could be in a situation where the gasket seals a little better when warm. I doubt you have a fuel delivery problem. My experience with pumps is they are fine or dead. And you seem to have plenty of power when warmed up. You might need slightly more fuel when cold, but nothing compared to high speeds under load.


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## russ130

*Spitz* - Two things wrong with that idea first is that its not going to tell me what its doing at cruising speed and second is that it also is not gonna tell me about the volume of fuel only that there is enough pressure and that there is enough volume at idle. I'm going to try the filter first and hopefully the fuel line will hold.


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## Spitz

You said it runs rough all the time and lacks power, its just a quick little test that i've used a number of times before i try to remove the fuel filter and install our inline pressure gauge. SImply put if the fuel pump cant make any fuel return its junk as even at idle if it cant produce the pressure to bypass the regulator its surely not going to make enough pressure when it really needs it. 

ps- This is why im telling you this, so you can try it before you wreck the fuel line completely. That and you will see fuel volume if you pull the line back, its quite a time saver, but what do i know its not like im employed as a technician..


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## brad96z28

Very easy to buy a 20 dollar gauge and filter adaptor and do it the right way it could save some real time.


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## russ130

Spitz;701750 said:


> You said it runs rough all the time and lacks power


I'm pretty sure I said it runs rough when cold and lacks power all the time. Of course the lacking power is really my opinion and it may very well be fine who knows. Look I'm not trying to insult you, not in the least and in fact I will try your method first. I'm just thinking what is this really gonna tell me? It runs fine at or around idle its when you are going down the road and it is not at operating temp that it spits and sputters if you try and give it the least amount of gas. I have all but let off the throttle and it smooths right out until it upshifts then it starts all over again. This symptom disappears as soon as it reaches full operating temp. Everyone here says its the fuel but to me it really feels more like a vacumn leak. I don't think it has a vacumn leak big enough to cause all these problems so that is why I started with replacing a sensor and now I'm thinking the eprom or the cpu. But at the same time it could be fuel. Truth is its cold and I'm in no hurry to lay on the cold ground to find out that the pump is ok. I can return the pump but not the filter since I had the filter for quite a while. As far as the line goes I saw it and I'm the type that has to fix it now. I'll let you know tomorrow during the day how things are going and what I find but I do appreciate all the ideas of things to check for whether right or wrong it does help.


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## Spitz

I know, im just suggesting. Anyone can take it for what its worth.. If its running rough when cold it may be because of open loop its running in, the computer will use presets for fuel delievery, any vacuum leak or fuel delivery issue, the computer will not compensate for untill it gets into closed loop, probably a few minutes of running on that old of a vehicle.


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## 2COR517

russ130;702172 said:


> Everyone here says its the fuel but to me it really feels more like a vacumn leak.


It won't take long to check that TBI gasket. Start her up and spray a little carb cleaner around the base of the throttle body. Needless to say, do this when it's cold.


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## bassfishn1

might help you, or might not, i have the same problem also, have not fixed it yet, but i believe it is my air cleaner assembly
the reason i say this is, there is an flap inside the snorkel, that is suppose to close at cold engine temps, so that the tbi gets warm air from your exhaust manifold, and as your engine warms up, the flap opens also
not sure if you have this on this truck, but worth checking


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## archeryhead4412

*give this a try*

just had the same prob on my truck found the intake sucking air...about 3 1/2 hrs later and a case of beer it was fine...i replaced the intake set.


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## Hall's Lawncare

I have the same problem with my 5.7 tbi. It doesn't lack that much power (that I know of) but when idling it rev's surge. It will stop once RPM's get over 1k. Ive also noticed if im sitting at a stop light and I put it in Neutral than it evens out alittle. I also haven't looked into it because the truck isn't that bad but first chance I get i'll be working on it. - By the way... new to the site and love it already. You guys have answered so many questions and I just browsed for acouple minutes.


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## russ130

Well I fixed mine the easy way. I sold it lol


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## lmiller

My 88 5.7 V8 wouldn't start when cold unless I pumped the throttle up and down for at least 4 minutes until the engine heated up, and the "service engine soon" light stayed on all the time. I changed the temperature sensor unit ($15) on the intake manifold and now it starts right up and the service engine light is off.


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## Sawboy

10 year old thread.....


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## lmiller

Sawboy said:


> 10 year old thread.....


31 year old truck, this 10 year old thread gave me the information I needed to keep it running.


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