Help me diagnose a rough idle condition that is driving me crazy

[DerekTheGreat]

Yep, mine are made in China. Didn't realize that until I looked at the box after I dropped them in.

 

[DerekTheGreat]

That was my thought too, but wouldn't my negative LTFT indicate the truck is running rich and the computer is trying to correct by pulling fuel?

Yep, that's correct. I glossed over that part in your first post. Opps.. I remember a video from South Main Auto where a rich condition was caused by a cracked injector and with what peeps on here say about the factory spider's poppet valve thingies (I'm not a vortec guy!) getting dirty that could be it. Could also be the plugs too. The chinesium A/C Delcos I've got seem to be ok so far. I've got about 5k on 'em now.

 

[jasons99hoe]

see how much play you have in distributor. i see alot of distributor gears severly worn. just something to check. seems to me like a timing issue

 

[Fozz]

Well I drove to work this AM with my code reader hooked up so I could watch live data. One surprise was how long it took to enter closed loop operation; about 5-8 minutes, about when the ECT hit 150 degrees. Once it entered CL, my bank 1 and 2 long term fuel trims looked pretty good, ranging between -3 to -5 at idle. Short term ranged from -1 to -3. The truck was running like a top, butter smooth idle! However progressively, as the engine continued to warm up to operating temperature, my LTFT for both banks continued to climb further into negative territory. Bank 2 LTFT maxed out around -8.6 and bank 1 LTFT maxed out at around -10.5. STFT for both banks hovered between -2.5 and -4. As my LTFT approached the -8.0 mark, idle quality started to diminish. MAP sensor reading was 6.2 with the truck fully warmed up in drive.

Any thoughts?

 

[Fozz]

I have now tried two sets of plugs. Pulled the original AC Delco platinum's that had 60K on them. Replaced with AC Delco Iridium which helped the rough idle for maybe a week, then it was back. Pulled those plugs and they were foul. Replaced with a pair of standard AC Delco copper plugs. Same thing, idle improved for maybe a week, then the rough idle was back.

 

[Fozz]

Another interesting development...noticed that my ECT was read 64 degrees this am on a cold engine. It was closer to 48 or 50 this morning. When I left work this afternoon, same thing ECT read 64 on a cold engine wheb it was probably closer to 53 out.

 

[GL1500]

Funny, I was just talking to my brother (cert. mechanic) about this issue and he said the first two things he would check are fuel pressure and engine coolant sensor. He said he's seen an ECT sensor (the one for the ecm) cause intermittent overfueling problems for the 5.0 and 5.7. Not sure if it works the same as the 7.4 but for less than $20 it's a pretty easy replacement. Since you're seeing questionable readings when cold, I'd throw in a new ECT sensor and see if that doesn't repair the issue. At least see if the cold readings change.

 

[Fozz]

Not a bad idea. I will do it this weekend if I have time. Drove to work this morning with the reader hooked up on live data again...This time my ECT started at 42 degrees which is about what the outside temp was this AM. Truck ran like crap this morning. One thing I noticed is that in live data, ECT reports fully warmed up at 188 degrees, but the gauge on the dash is just a tick or two over the 1/4 mark. It also seems like it takes a long time for it to reach full operating temp, about 10 minutes. But then again I am used to driving my 4 cylinder Colorado with an aluminum block that warms up right away.

 

[CoolDude1975]

Greetings all! New member here. Currently working on a 1998 GMC K1500 with a 5.0L. Truck was purchased by my dad in 1999 with 17,*** on the clock. I recent purchased it from him and it is currently sitting at 179,***.

Currently I am experiencing a rough idle that is driving me insane. Here are some facts on the situation.

-60K on a rebuilt 5.0L
-New plug wires
-New cap/rotor, but Dist. is suspect, due to a broken ear and oversized machine screw installed. Although it idled fine for many miles with this set up
-No check engine light
-Pulled the plugs with 60K on them when rough idle started. Cylinders 2,4 and 7 were severely fouled. #2 was black and crusty but not wet. #4 was crusty black and tan. #7 was crusty but tan only.
-Replaced with new plugs, idle was improved, but a couple weeks later rough idle was back.
-Cleaned MAF, some improvement, but still rough.
-Ran a compression test, all cylinders were 185-195 PSI
-I have tried running with the MAF unplugged, no change in idle quality noticed.

More information regarding symptoms:
-Truck runs good when the engine is cold.
-Long term fuel trims running -10 to -12 at idle warmed up.
-Truck runs good at all speeds/RPM except idle
-The symptoms are seem to be intermittent, although it never idles perfect. Some days are better than others.
-K&N air filter, which I plan on getting rid of soon.

Trust me when I say this.... Replace your entire distributor... I've had the same problem several times since owning my 99 Yukon. And each time it starts running rough or anything remotely close to that I always drive myself crazy trying to figure out what's wrong with it. And every single damn time it's ended up being something on the distributor.. Just get a whole new one so everything on it is correct, for some reason something is causing my cap to go bad.