Originally Posted By: armos
If you hold the throttle steady, does it have a persistent miss in the 1500-2000rpm range? Or does it only happen temporarily as the throttle is being moved?
Throttle position sensors have a reputation of developing a flat spot in the area of light throttle tip-in. I've never experienced that problem myself, but it's supposedly common.
Using a multimeter, you can probe the TPS and try to determine if it is behaving smoothly across it's operating range.
If the misfire is persistent at steady throttle, then I don't think it's the TPS. The throttle sensor is used to help with fuel calculation during transients. Once you're at steady state, the ECM will respond more specifically to the MAF or the MAP.
I don't really understand how the algorithm is programmed with cars that have both a MAP and a MAF. The cars I'm familiar with just use one or the other.
If you unplug the MAF, normally I'd expect it to run rich by default (likely staying in open loop). But if there's also a MAP available, then I'm not sure what it would do. I suspect unplugging the MAF is making it run richer though, which is why it cures the symptoms.
You mentioned that giving it heavier throttle clears up the misfire. Is it downshifting? If so, that's changing all the conditions. If it's clearing up without a downshift, then it could be a result of triggering power enrichment mode. When the various sensor inputs cross some preprogrammed thresholds, the ECM will run the engine rich instead of targeting stoich. This may clear up a lean misfire. It also argues against ignition - heavy throttle at same RPM should generally make an ignition problem worse, not better.
I seriously doubt that it's rich, I think it's lean. On a modern EFI vehicle you're far more likely to experience lean misfires than rich misfires. They try to run as lean as they can. The mixture could richen significantly and you wouldn't have a miss - in fact it richens intentionally during heavy throttle.
You might check the fuel pressure regulation - the fuel pressure should vary in relation to manifold pressure. The differential between fuel pressure and manifold pressure must remain constant or it messes with the injector flow rates, and that makes it impossible for the ECM to manage them correctly.
Did the truck have this problem when it was parked? How long has it been sitting? A vehicle that's been sitting is more likely to develop fuel system problems than anything else.
It is possible that you have bad injectors. They may be sputtering and giving improper volume/atomization in the lower duty cycle range of their operation. I don't know any practical way to confirm this though, and they are expensive to replace just on a guess.
A few years ago I had a frustrating stumble just off-idle on an 86 GM 2.8L. Somebody had put Multecs in it. Fuel pressure and regulation were tested and confirmed good. I replaced the injectors with a set of flow-matched refurbs (also Multecs) for about $90. I have not had the problem since then, and I believe the injectors are what fixed it. However, I did so much "fixing up" on that car around that period of time that it's kind of a blur. The injectors were replaced at the same time as all the intake gaskets were, so I can't rule that out as another possible reason for the fix. And this is just a single case - yours could certainly have a different issue.
The misfire/stumble is consistent at steady throttle input. If I hold it around 1k-1.5k rpm, it will stumble and chirp along.
It isn't downshifting with heavier throttle. I should have been more specific. If I input "parking lot" amounts of throttle, it has the stumble and hesitation. If I give it "merge onto the freeway" amounts, it feels normal.
I drive the truck daily, and the problem started while towing a small trailer during a trip. It had sat overnight and the next morning I started experiencing the symptoms.
I can definitely relate to the "blur" comment... I've read so much about the various sensors and system interconnectivity on this thing that I feel like I work for NASA.
Please see my comment above this where I provided additional symptom/symptom elaboration. Thanks for the thoughtful/quality post!
