# '99 7.4 Vortec Rough Idle



## MikeRi24

Truck recently had an exhaust manifold replaced. When its cold (as in first thing in the morning), it fires right up and high-idles for a few seconds and runs fine. After the first few seconds, its idles rough. The engine struggles to keep it above 7-800 RPM. When its in gear at idle (like a stop light), it struggles to stay above 500 RPM, and will occasionally stall. On any start after the initial cold start in the morning, it is hard starting. You have to feather the gas a little and it shudders but it will start then you have to give it a little gas or it'll stall. Driving, its fine as long as you are moving. It runs like a champ. Upon acceleration from a stop, there is a slight hesitation but once it's over that it runs flawlessly until it needs to idle at a stop again. I've been reading through the factory service manual, and theres more information in there on how to diagnose a squeaking serpentine belt than an issue like this. There is no CEL on and I scanned the computer for codes and nothing in the memory. I'm stumped. 

What I've tried/done so far:
-New Plugs/Wires/Cap/Rotor
-Remove and clean Idle Air Control, test resistance and it falls within factory spec
-Replace PVC valve and vacuum line for it
-Clean Mass Air sensor
-Replace Intake Air Temp Sensor
-Run a fresh tank of gas with fuel injector cleaner
-Remove and clean throttle body


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## the new boss 92

o2 sensor, sounds dumb my 92 did this and i replaced that and it worked. not saying its a cure cause your truck should have 3 and mine had 1. bust out your ohm meter and test them to see what happens. although mine tested good it was still shyt!


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## Milwaukee

Can you test oxygen sensors though scanner with real live data?


That sound like egr issues but that on my 94 explorer it would run poor no good acceleration after unplug vac line to EGR it run great. Mileage went up from 15 to 19-20 mpg.


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## MikeRi24

I will check that and look through the shop manual to see what the ohm spec is.


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## MikeRi24

Milwaukee;1320650 said:


> Can you test oxygen sensors though scanner with real live data?
> 
> That sound like egr issues but that on my 94 explorer it would run poor no good acceleration after unplug vac line to EGR it run great. Mileage went up from 15 to 19-20 mpg.


did you just plug the vac line up? and yes you can check the output of the o2 sensors with the scan tools, but it cant hurt to double check. Although if the scan tool was picking up a fault with them, then theoretically the CEL would be on, especially in an OBDII vehicle I'd think.


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## THEGOLDPRO

My 97 454 has a rough idle as well, I also tried a bunch of things that haven't fixed it either, Ny idle isnt as rough as yours tho it never stalls out or really stumbles.


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## MikeRi24

THEGOLDPRO;1320683 said:


> My 97 454 has a rough idle as well, I also tried a bunch of things that haven't fixed it either, Ny idle isnt as rough as yours tho it never stalls out or really stumbles.


I can understand it having a little rumble to it, from what I understand they put a good size cam in these things to get power, but nothing like what I have haha


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## NWanner

I'd look at the upstream O2 sensor as well, especially if the manifold was just replaced. The downstream sensor is just a nanny for the cat and doesn't effect fuel/air at all. The computer only throws a light when it starts throwing bad voltage, but just because the voltage is within specs doesn't mean it isn't fubar'd. EGR is only supposed to operate at highway speeds, but could be something to look at as well.


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## MikeRi24

NWanner;1320820 said:


> I'd look at the upstream O2 sensor as well, especially if the manifold was just replaced. The downstream sensor is just a nanny for the cat and doesn't effect fuel/air at all. The computer only throws a light when it starts throwing bad voltage, but just because the voltage is within specs doesn't mean it isn't fubar'd. EGR is only supposed to operate at highway speeds, but could be something to look at as well.


I looked at the EGR alright....unbolted it and when I pulled it off to inspect it the mounting flange cracked in my hands. $90 at the parts store later, it has a new EGR and it still idles rough so thats not the problem hahaha


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## the new boss 92

Milwaukee;1320650 said:


> Can you test oxygen sensors though scanner with real live data?
> 
> That sound like egr issues but that on my 94 explorer it would run poor no good acceleration after unplug vac line to EGR it run great. Mileage went up from 15 to 19-20 mpg.


you can only test an os if it throws a code, unless you have a really high tech scaner( witch i dont) then you have to ohm it and even then they test good sometime till they get hot from driving and they go ham and send weird shyt through the computer.


MikeRi24;1320658 said:


> did you just plug the vac line up? and yes you can check the output of the o2 sensors with the scan tools, but it cant hurt to double check. Although if the scan tool was picking up a fault with them, then theoretically the CEL would be on, especially in an OBDII vehicle I'd think.


in order to test the egr properly you have to pull it and see if it hold vaccume, cause most of the time when an egr goes the diaphram splits inside. it still kinda functions but will do what this guys trucks doing!


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## Holland

Fuel pressure regulator. And it sucks to get to. Time to pull the upper plenum off. Pretty common on the 7.4. They start to leak fuel into the intake.


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## B&B

Holland;1321090 said:


> Fuel pressure regulator. And it sucks to get to. Time to pull the upper plenum off. Pretty common on the 7.4. They start to leak fuel into the intake.


Ditto. Get a scanner on it and check the LTFT's. If they're pegged at +25 then the FP regulator is leaking. Very very common issue on the 7.4's. If you don't have scanner access connect a FP gauge, cycle the fuel pump a couple times and then watch the gauge for drop off. If it drops more than about 10 PSI in 20 minutes or so the reg is leaking into the intake.

They'll also lose injectors sometimes also. They'll stick open thus any time there's fuel pressure it's injecting unmetered fuel into the intake creating a rough idle/rich running condition. Those can easily be diagnosed with a temp gun on each exhaust port.


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## crete5245

I agree with the o2 sensor check, but there is a problem using a scan tool. It is not fast enough to see the voltage change above and below the thresh hold voltage of .450. 
You need a lab scope that will take reading as fast as several thousand a second. 
If you can get a shop to scope it, make sure it switches from rich to lean. And make sure it has a good sweep.


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## 70monte

I have a 98 K3500 with the 7.4L and it idles rough. I have replaced the distributor, cap, rotor, coil, plugs, wires, fuel filter, and both pre-cat O2 sensors. I would also guess the fuel pressure regulator or injectors because of your hard starting symptom. As others have stated, these are problem areas with this engine. 

A lot of people replace the factory multec injectors with Bosch 19 lbs injectors which from what I understand are a drop in replacement. They supposedly are a better design than the factory ones. Some people on another forum I go to that use the Bosch ones really like them.

Wayne


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## MikeRi24

Actually ended up being an EGR issue. After I replaced the EGR valve, it finally threw an EGR code, I cleared the code and it has been fine since!


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## Milwaukee

MikeRi24;1328473 said:


> Actually ended up being an EGR issue. After I replaced the EGR valve, it finally threw an EGR code, I cleared the code and it has been fine since!


Glad to know it was EGR valve issue Thumbs Up When it get bad then your mileage GO SUNK.


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## dieseld

Great reading here. Just bought a 1998 K2500 7.4L myself.


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## William B.

I know this thread is many years old but I'm having a very similar problem with my 454. We've(my mechanic and I) checked fuel pressure, replaced the IAC, EGR valve and upper plenum intake gaskets, cleaned throttle body as well as mass air flow sensor. My mechanic and I are lost as to what could be causing the problem. He had the truck last week trying all day to get it to act up and it never would. I hop in it to leave and it starts doing it. Everything is checked out in spec that we have checked.

Suggestions?


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## jonniesmooth

Anybody think it could be a crank position sensor?
I'm not a mechanic, but I know the 8.1's had issues with them,maybe the 7.4's do too?


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## Mountain Bob

jonniesmooth said:


> Anybody think it could be a crank position sensor?
> I'm not a mechanic, but I know the 8.1's had issues with them,maybe the 7.4's do too?


Yes,very possible. Also, the terminals in the crank sensor connector get loose.


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## William B.

This is what it is doing.


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## leolkfrm

watched an a/c compressor do that on a 6.0 yesterday, asked if ac was on, said no, but defrost was on so it was cycling the ac


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## Mountain Bob

Not prioritized--
crank sensor/connector
dirty fuel injectors
carbon buildup on intake valves
Worn out distributor drive gear
Dirty/corroded PCM ground connections
That's just off the top of my head


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## William B.

Mountain Bob said:


> Not prioritized--
> crank sensor/connector
> dirty fuel injectors
> carbon buildup on intake valves
> Worn out distributor drive gear
> Dirty/corroded PCM ground connections
> That's just off the top of my head


Where are the PCM ground connections at?

If it's fuel injectors, would it do it randomly compared to all the time?


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## WIPensFan

Sounds like it’s gasping for air!


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## William B.

It does it so random its hard to pin point what is causing it. I ran some errands for work and both times I backed out of parking spots it did it's thing and died. Fires right back up and drove fine. It seems to do it more in reverse vs forward but I don't know why that would matter.


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## Mountain Bob

William B. said:


> It does it so random its hard to pin point what is causing it. I ran some errands for work and both times I backed out of parking spots it did it's thing and died. Fires right back up and drove fine. It seems to do it more in reverse vs forward but I don't know why that would matter.


This sounds more like you have a rubbed spot on a wire on the engine wiring harness, shorting something out. Do a harness "wiggle"test, and visually inspect the harness.


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## Hydromaster

column shift?
Heavy key ring?


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## the Suburbanite

Mountain Bob said:


> This sounds more like you have a rubbed spot on a wire on the engine wiring harness, shorting something out. Do a harness "wiggle"test, and visually inspect the harness.


I had a couple plug wires on my 5.7 350 that were arcing causing similar issues. I couldnt figure it out until I looked under the hood at night.


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## William B.

Hydromaster said:


> column shift?
> Heavy key ring?


Just key and key fob. Has done it for the last 2 years that I've owned it and the previous owner had the same issue.



the Suburbanite said:


> I had a couple plug wires on my 5.7 350 that were arcing causing similar issues. I couldnt figure it out until I looked under the hood at night.


I've replaced the plugs, wires, and cap/rotor 2x now with the last time being 2 months ago with no change. The last time it was done was with all AC Delco parts.


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## WIPensFan

OP, you figure this out yet? I used to have one of these and I’m curious as to what the issue was/is.


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## William B.

WIPensFan said:


> OP, you figure this out yet? I used to have one of these and I'm curious as to what the issue was/is.


I have not. I'm lost on what to do.


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## WIPensFan

William B. said:


> I have not. I'm lost on what to do.


Take it to a dealership. They will have encountered it before if they're any good. Yeah, you'll have to sell a kidney, but the truck will be right. Priorities…


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## Mountain Bob

Like I said, check the engine wiring harness. 4.3's in Astro's and S10's had a well known problem,engine harness rubbed through,at right rear corner of intake,by one of the retainers. Caused all the problems you describe,especially "died when in reverse. Spend some time and check some chevy truck and silverado forums,you may find your answer quickly.


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## leolkfrm

is the a/c loading it down? remember it comes on with defrost


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## William B.

leolkfrm said:


> is the a/c loading it down? remember it comes on with defrost


Negative.


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