98 7.4 rough idle and stumble.

[FlyByWire]

Hey there guys. I’ve searched both the forum and some FB groups I’m in, and I’m probably more confused than when I started.

I have a 98 GMC 2500 w/ a 7.4. Not always, but sometimes it develops a rough idle (as if it’s got a lopey cam), and sometimes it sputters / stumbles from a dead stop. Not always… not even close to always… but sometimes. Whether it’s cold or hot, same thing. I put a new cap and rotor on the truck (it needed it anyway) and that didn’t fix the issue. The truck runs pretty damn good otherwise. Smells a little rich but that may be because the exhaust dumps under the cab.

I guess my main question is where do I START… as far as a diagnosis…? I’m not a mechanic, but I can watch YouTube and turn some wrenches and follow directions and so forth. What I’d prefer is to figure out WHAT the issue is, instead of just throwing parts at it. I don’t own a scanner but I’ve read that the innova 5610 is popular.

Thanks in advance guys!

 

[wirlybird]

I would look at the MAF and the EGR valve. Those have been issues on my '97's

 

[1998_K1500_Sub]

Any ECU codes? You mentioned none. Check them anyway, regardless of whether the "check engine" light is on.

When it acts up as you describe, that's your opportunity to identify the problem. While or shortly after acting up, have you...

- checked fuel pressure
- made note of ambient conditions (high / low temps, high humidity, rain, etc.)
- noted any unusual dashboard gauge readings
- used a diagnostic tool, e.g., like DIACOM et al., to see what the ECU has to tell you about...

- EGR commanded position
- EGR measured position
- TPS
- coolant temp
- knock retard
- MAF / MAP


Is the vehicle history a factor, e.g.,

- rust or corrosion due to sea spray / winter road salt, which can cause problems with wiring / connectors
- major body repairs (can lead to grounding problems)
- major power train repairs (trans swap, engine swap,...)

 

[Schurkey]

Smells a little rich but that may be because the exhaust dumps under the cab.

Extremely dangerous, totally illegal.

 

[RichLo]

^ yea, just a bad idea, at the very least get some extensions on that exhaust to get it under the bed.

But back to your question, I have a OBD Link MX+ and an iPad that I wasn't using for anything else mounted to the dash to live data log sensors that my be questionable. Its really handy to see what each sensor is doing when the problem is happening.

 

[FlyByWire]

Yeah I get it on the exhaust, I didn’t build the truck and I don’t like it this way, as I have my kids in it all the time. It’ll be fixed soon.

Thanks for the replies so far guys!! I never thought of the EGR valve, but the symptoms I’m reading about seem like it very much could be that.

 

[FlyByWire]

So I said I wasn’t going to throw parts at the truck, but I did it anyway. The EGR symptoms really sounded like my truck, so I threw a new one one, same issue. It threw a check engine light (it’s come and gone before, and I don’t have a scanner so I swung by Autozone and they pulled the code for me. It seems to idle better, but it still has the sporadic hesitation from a dead start.

P0507 is the only code. High idle.

Hmm??

Thanks again guys.

 

[MILLCMACK]

watching.. my 96 does the same. buddy of mine has a 7.4 burban and he said check fuel pressure first as the pumps get weak and dont hold pressure well. once i have some time here to get back to that truck ( working on the side unload bed for the HD now) thats been my only real issue with it as well. hate to add to your possibility chart but look at fuel pressure regulator symptoms too, those also sound good. maybe you go one way ill go another!? hahaha
my truck lived most of its life on a ranch in southern cali so we might not be looking at something too too different.
do you have extended crank times to start sometimes too?

 

[FlyByWire]

watching.. my 96 does the same. buddy of mine has a 7.4 burban and he said check fuel pressure first as the pumps get weak and dont hold pressure well. once i have some time here to get back to that truck ( working on the side unload bed for the HD now) thats been my only real issue with it as well. hate to add to your possibility chart but look at fuel pressure regulator symptoms too, those also sound good. maybe you go one way ill go another!? hahaha
my truck lived most of its life on a ranch in southern cali so we might not be looking at something too too different.
do you have extended crank times to start sometimes too?

No she starts up immediately every time.