TBI 350 Misfiring

[1Kurgan1]

My 91 2500 has had a misfiring issue for a bit. So I changed out the Plugs, Cap, Rotor, EGR Valve, and Fuel Filter. Hoping one of those would get it, also just doing a bit of maintenance as it has 284k on it. But none of those things solved it, so I checked the timing and it was retarded by about 4 degrees so I set it back to 0. After that the misfiring was gone, the truck still felt a bit more sluggish than it should have been. But at least the misfiring was gone, I took it on a 800 mile round trip on my vacation and no misfires it ran good. But now that I got back it's been 2 weeks and over the last 200 miles the misfiring has slowly returned, and now it's as bad as it was before I reset the timing to 0. Any ideas here? Why would the misfiring return, and beyond that, even after I fixed the timing it still felt a big doggy when on the power.

The only way I can usually fix misfiring is to drop down a gear and get it into the higher RPMs, then raise speed in that gear, then manually shift up to the next gear at a higher spot than it's use to.

 

[Southern Pride]

Your Check Engine Light isn't or doesn't come on does it? There's always the possibility of a sensor. Also is the timing still @ 0 or has it since crept back into the negatives?

 

[1Kurgan1]

The CE light use to be on a lot, after I reset the timing it was rare, but it's now back to being more common. It feels like the timing has gone back to being retarded, so thats possible. I have no cat or muffler on so wasn't sure if that would effect the CE at all, I'll have to check into learning how to read the codes and see what pops up. Any idea what sensor could cause this though? I know with my car if it acts a certain way it can usually be attributed to a single sensor, but I can't seem to find anything that would cause this with the truck, though I'm not completely sure if it has cam or crank sensors.

 

[Southern Pride]

Well heck man if your CEL is on then it will generalize a problem (or several) by giving you each code that's being thrown. This way you can replace the part that might be attributing to your misfire.

Here you go buddy read up. You can check your codes for free. All you will need is a paper clip. Literally.
http://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php

Come back with some codes for us and we can start diagnosing. Someone else should be able to tell you about not running a muffler or a cat. In highschool I did that but can't remember. I want to say it didn't throw a code and I think I read somewhere recently that it won't BUT don't believe me just to be safe. I can't believe noone else has responded in here. Anyway, read up check the codes and that will do a great deal for us in helping you.

 

[GMCTRUCKS]

Check your plug wires maybe one is burnt or change them all out. Open hood at night see if you see a spark jumping around.

 

[darren250r]

There is no O2 sensor behind the cat, so your truck will not throw a code for no cat or muffler.

 

[Southern Pride]

I dunno he said he did plugs wires cap and rotor...You know what, how sure are you that you got the firing order correct??? You def should re check all the wires and make sure your order and wires are correctly placed.

 

[1Kurgan1]

Ran the codes, got a 32, 42, and 44. Tried looking around to find a place to figure out what these are, but I can't find any list. Also, I only changed the plugs, cap, and rotor, not the wires as that was actually a decent price and figured I would save that for last if I had to do it. Wires are on right, it ran fine for over 800 miles after setting the timing, but now it's back to running poorly. But hopefully these codes can give me a hint where too look if anyone can provide me with a link to a place that lists the codes that would be awesome.

 

[Southern Pride]

http://www.gmtuners.com/OBD1_DTC.htm

There ya go buddy but you have...

32 EGR system failure
42 Ignition bypass circuit error
44 Oxygen sensor error (lean condition indicated)

Now. I also just had the 32 and 44. The EGR on mine was unplugged so I plugged it back up. Perhaps your EGR is bad or just not plugged up right and causing vacuum probs. Also for my code 44 with the o2 sensor it was not actually bad but I had a bad exhaust leak down around the sensor and so it would cause it to go off. Not sure about the ignition bypass circuit failure hopefully someone could chime in on that.

How old are your plugs? They very well could be the prob.

 

[1Kurgan1]

Interesting my EGR valve is new, I'll have to check the electrical plugs, but is weird to see an issue with that. I'll have to check around the o2 sensor, what kind of leak did you have? A hole in your header, or did you just replace your header gasket? And I wonder if this code 42 misfiring could cause code 44. But looking up code 42, that seems might be the cause of most of the issue here. A problem there would cause funny timing issues. Problem is, it looks like an absolute nightmare to diagnose. I hate following wiring....