First the videos, then on to what I have done and checked...


YouTube - idle_engine.avi


YouTube - idle_exhaust.avi


I have replaced the throttle body as it was worn and leaking at the throttle shaft. The replacement was sourced from a 93 and is supposed to be one of the "sludge tolerant" design. It was spotless when I put it on less than 800 miles ago. The IAC is new with the TB and is still clean. The throttle position sensor is maybe 6mo old. Throttle closed voltage with KOEO is 0.97, KOER is 1.14. Max of sweep is @ 4.6-4.7 and the sweep seems smooth. The throttle is definitely closed and it will almost want to stick if the base stop adjusted down too far.


I have swapped the MAP sensor with a known good unit, no dice. I have checked all vacuum lines and intake gaskets and seals with carb cleaner and propane both. Brake booster is in good order as well. As far as I can tell there are no vacuum leaks. I have also run @ 1/2 of can of seafoam through it and watched the pretty smoke show. (I do that once a year to all my older vehicles) Every thing should be clean.


I have checked the reference voltage at several points and it is 5.0 and does not seem to drop, only increase slightly with KOER. I have run the diagnostic with a Snap-On MT2500; the only consistent codes are 25 and 44, 25 being no knock detected during testing. This could be due to the fact I run midgrade in the truck when I'm towing, more protection against spark knock. "44 is Thermactor Air System inop" I believe the passages in the heads may be partially clogged, but the air pump and diverter valve do function, I can hear it bleeding off when the ECU tells it to dump to atmosphere. EGR seems to be functioning normally; I can make the engine stumble by actuating it manually, and the EGR position sensor test within range. Now the funny thing is the EGR solenoid chatters allot, that is the random clicking you hear in the under hood video. I have yet to replace the O2, though it is only a few years old. I don't drive this truck all that often, it would be lucky to get 8k a year at most, mostly quick trips on weekends, and maybe a couple hours at most.

The engine also stumbles @ 1800-2000 RPM, right at cruising speed. Cap/rotor/wires/plugs etc seem good, only @ 9K miles on that stuff. Timing chain is new last year when I did the water pump, went with a 93^ double row roller unit.


I am waiting for a scanned copy of the EVTM so I can double check all the grounds.


This issue has been around since I bought the truck, it has just worsened this year, It can get much worse than what is in the video, to the point of violently surging in gear with foot on the brake, especially bad when there is 5k of trailer behind you when it does it. It does not do it all of the time, but is becoming a lot more prevalent and is worse at full temp. During warm-up it will run at fast idle, then when it goes to idle down, it will surge so low that it will want to cut off, sometimes it does, other times it hangs on by a thread. After it get's past that point, it will either run fine for a bit, or it will begin its dance. It has developed a new issue in the past few days, now it after it starts surging, sometimes it will kick the idle up @ 1200 and stay there. If I shut it off it will clear up for a little, but then it all comes back.


Sorry for the long windedness, but I wanted to be as descriptive as possible as to what I have done and what I am experiencing. Sad to say the neighbor's Chebby is running better right now...lol. I have 16 days to get this sorted out before Carlisle, towing the camper up there this year, and already pre-registered for everything, so I would really hate to miss it.



Thanks in advance for any help


Matt

 

Have you considered exorcizing the demon? Maybe you need a priest from the Church of The Holy Shelby.

 

The demon has been vanquished it seems. After checking the main grounds and finding a few variances, I added a few grounds, to the body and the motor. It still ran a bit funky, so I swapped in a spare 86 5.0 w/tow ecm we had sitting around and that seems to have done the trick. No more surge/hunt, the motor sounds and feels stronger. It will even break the tires loose now. No stumble, no breakdown. Now well find out in a couple weeks how much bettr she pulls the trailer.

 

See, I can perform exorcisms remotely.