LH0525

Hey everyone, my mind is running in circles and I am in need of some help. A few days ago my 98 T100 4x2 A/T with the 3.4 v6 (230k) developed a misfire read by the obd as misfire in cylinder #4.. only. No other codes. I have had this issue once before, ended up being the plug wires, changed the wires easy peasy done. This misfire is not so simple unfortunately. There are numerous misfire posts on this and other forums but none that I have read have the same symptoms, or the fix for many of their misfires did not work for me.

The first thing I did as many others is focus on the ignition/spark. I am 99% sure this is not where my problem lies, I have changed ALL the plugs NGK dual ground plugs. (old plug looked fine BTW, as did all others) I have moved the coils (twice) the misfire did not move. I have moved the plugs, (even though they are practically new) no change.
Next I decided to check compression on that cylinder and others. No issues, compression was not only good it was great 190-195.

So good compression, good spark, has to be injectors right? I buy tested re-manufactured denso injectors and replace not only cyl#4 but all the cylinders since its probably never been done and has 230k. The install went smooth no problems. I go to start the truck, Truck runs great no stumble no miss no hesitation... for the first 5 min. Almost as if once it got warm it would begin to buck. Same code misfire #4cyl.

The misfire is odd as well, first off it is intermittent. It misfires in park at idle (when warm) but when i rev the motor (in park or nuetral) the misfire disappears but promptly returns when returns to idle or placed in drive under load. Now to the intermittent part when i test drove it down the street it would be missing the whole time then would have periods where the misfire would resolve, I could actually feel the cylinder kick in and fire, that would happen for a few seconds then misfire again. this would happen randomly or sometimes not at all.

I am racking my brain trying to think of things i missed.

I've heard of the crank pulley coming loose causing the knock sensor to create random misfires, however this affects many cylinders and not just one. I also haven't ruled out vacuum lines but it is not high on my list of possibilities.
I would next like to check the electronic system of the injectors, as well as the ECU. Any one have any experience with this as I really don't know where to start other than testing resistance on the injector wire for cyl #4. I would live to swap out an ECU to see if that resolves my prob but they are expensive and I feel like that should be my last resort.

The only other thing I cant stop thinking about is the exhaust valve. could i have a bad exhaust valve even with great compression? I plan on taking the plenum of again to reach the injector wire, and taking the valve cover off while im down there to check the valve clearance.

So that's about it! any ideas?

tstockma

DOH - I just saw the date on your post, you probably moved on 5-6 weeks back...hey let us know what you found.

My reply today was:

I'm a newbie with the T100, so won't have experience to back any of this up...but here goes anyway. In the old days, I'd say cracked distributor cap right around that port, or plug wire, but the world has moved on, eh?

For me just like for you, the wire to that injector & also the ECU itself both come to mind as suspects. Possibly O'Reilly or Autozone or one of the chains, would free of charge, hook up an analyzer for ECU functionality & while you're at it, possibly they would confirm your thinking & throw on an ignition analyzer & rule out ignition spark issues. Sight but maybe all they can do is read codes which you've already done.

Fuel issues - check your fuel filters. Check your fuel pressure, a fuel pump that's putting out less pressure than needed can cause very flaky misfire symptoms. Why would this manifest only in cylinder #4? No clue, but I'd check anyway, an odd small variance in conditions might simply make it the weakest link. Look for any other cause of partial fuel blockage to just #4. I can't imagine a filter just on that line, instead the filter screen is embedded in the injector & moves when you swap them around, or so I think.

Good luck & let us know what you find!