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

[Fozz]

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.

 

[DerekTheGreat]

I'm not a vortec guy but my trolling makes me think intake gaskets or some other vacuum leak could be causing your issues..

 

[Fozz]

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?

 

[Fozz]

I should also add that when the truck is idling rough, its idling at like 550 RPM as measured on my scan tool, which seems low to me. Haven't got around to looking up the factory spec.

 

[Supercharged111]

Could be the spider, but my 1500 has done the same thing since 2015 and I have yet to figure it out. I have a marine intake and cam and changed plugs and all to no avail. New distributor gear and CMPRET set dead nuts made no difference either.

 

[Fozz]

I may just do the intake gaskets, upgrade to MFI, and a new distributer, since these are all known weak points of these motors. Big project and lots of $$$ though, and I hate to "throw parts at rigs".

 

[Supercharged111]

You can do 1 at a time. First just try intake gaskets as they're known t suck. Next you could look at the spider as it's super easy to go back in for it and won't cost any gaskets. Be very careful about your distributor selection, parts store electronics are GARBAGE.

 

[Fozz]

That's solid advice, although since I will have to pull the distributer to do the IM gaskets, and my current distributer has a cracked cap mounting tab, I may as well replace that too.

 

[Supercharged111]

Meh, doesn't Dorman make a thing for that anyway?

 

[Fozz]

Meh, doesn't Dorman make a thing for that anyway?

I have not heard of such a thing. Do you know the part number by chance?