Mystery Rough Idle Problem

[Smelly_Markers]

I have a 88 BII with the 2.9 V6 and the Auto Tranny. Okay so the engine starts up fine when cold but gets a little finicky when i have been driving it for awhile. At low RPMs (800-1000) it starts to get shaky and feel like its going to die but doesn't until it hits down to about 500-600 RPMs. It has hesitation upon acceleration and it blows carbon or black soot out the tail pipe at an idle when the engine is warm. There is no Check Engine light or OBD codes. I have noticed the oil level is getting low, but I know some oil leaks past the oil pan gasket. The engine runs strong (it puled through a locked caliper) and only has 114,000 miles. My 87 BII had 345,000 Miles and ran better than this one. I am very confused :[ so please help if you can!

Thanks, Tony

 

[TexCaliBII]

Odd,
I just put a rebuilt 2.9 from my 88 into my 87 and it's doing the exact same thing, minus the black smoke. I am getting a 3/4 shift solenoid error though when I scan it. I found that the truck runs fine even warmed up until I put it in gear then it started doing what you describe. The hesitation had me thinking the timing was off a bit so I double checked and I was about 12 degrees instead of 10. I reset it and now she runs much better but I’m starting to think the trans issue is pulling down the motor . I pulled the vac line form the tranny off while the truck was running and it died. So I can reasonably assume it is still pulling a vacuum and if the shift solenoid is bad it might be causing a vacuum leak in the tranny. That would cause the loping idle. That motor ran just fine when I had put it back into the 87, until it’s tranny shelled. Though the wore out 2.9 in the 87 also ran just fien burt leaked from every spot imaginable. I also put the computer form the 88 in the 87 due to the 87 having EGR and the 88 not. Anyway the truck runs much better now at idle but is still having issues once I'm driving. I’ll investigate more once I finish installing my lift kit. I plan to drop the tranny pan and replace the solenoid to see if this fixes it. Good luck on yours

 

[Smelly_Markers]

Thanks for the input Alaska, I actually stumbled upon a fuel related problem, hence the truck running VERY rich. The fuel pressure regulator diaphram is toast, it was spweing fuel from the vacuum line in the back. LOL who would have thought? A FPR... Thanks again though Alaska.

 

[TexCaliBII]

Cool,
Glad you found the issue. Still have to troubleshoot mine some more. I've seen/read about the FPR doing some wierd things though on other rigs. In my case I know that's not it but again glad you found the problem. Keep 'em rolling!

 

[big red 138]

you could check your fuel injectors too? that would cause a rough idle if they were running sporatic and really dirty, maybe not to that extent though, but might be work checking out. witchhunter.com does a cleaning and test, even send you the test results for $20 an injector

 

[TexCaliBII]

Yeah I found an article about cleaning injectors that looks like it might work. My reason for thinking its a trans issue is this engine ran fine in the original truck with a different tranny. I then pulled this engine and put it on a stand for a while but not long enough for the injectors to gum up, I do however have 2 other complete upper/lower intakes and since I have a topend gasket set I just might swap one of the others in to see, but I really like my black intake and valve covers.....