1996 with a 3100. Put a new TPS, MAF, IAC, still having an idle above 1500 sometimes 2000. It will usually start low just below 1000 before jumping up. I don’t think it’s a vacuum leak since I’ve tried smoke test/starting fluid. Testing the TPS it sits around .63v with 5.05~ coming in. Any help would be appreciated.

 

Are you sure you did a thorough smoke test around the gaskets? Id be very suspect around the UIM / LIM.

If your new IAC is confirmed good and you're confident on your smoke test, how about the TB itself - does it look like its returning to closed - no buildup around the cable / gunk in the TB preventing the blade from closing all the way?

 

I cleaned the throttle body and egr passage. The plate appears to be seated. I bought two IACs with same results so I believe they are good. I think the gaskets are good, especially with how inconsistent it wants to act. I feel a gasket would either leak or wouldn’t, rather than jumping all around but I could be wrong. What’s the correct idle voltage coming out of the TPS?

 

.4-.6 is the usual range. Of course there's tolerance stackup between several components of both the TB and sensor so every car will be a little different.

One other thing - what are the fuel trims reading? That would be a good indication if its all coming through the MAF or not (ie if trims are running high).

 

I don’t believe I have a way to read those.

 

So I’ve bought a Bosch 1300 scan tool. Right now with the key on not running the throttle position jumps between 0.4, 1.6, and 2.4 with a new unit it. The MAP sensor was throwing 7.6”Hg so I’ll be looking into that. I unplugged it, just reads 3.0 now, and it ran at about 1500 instead of say 2200 before rising to ~1700. What else should I be getting from this? The short term fuel trim is all over the place between -10 and 10, I don’t know what is should be. I’ll definitely do some research.

 

Wow, if thats in volts (and not %), then that's problematic. Likely isn't the whole issue, but I'd start there as something is wrong. Itll at least be one problem fixed and off the list.

You unplugged the map? I'm not understanding what you're testing here or what was giving the 3.0 reading.

I've not tuned a 3100, but I know on 3800s, the primary fueling is done via MAF.

There will be some natural variation in fuel trims. Every car off the production line gets the same tune, but every engine and operating condition is a bit different. So if the O2 sensor tells the pcm its a little lean every time it hits a specific fueling cell then it makes the adjustment via short term fuel trim (which ultimately rolls into long term trim) so that the pcm is always giving the right fueling at that cell.

What I was curious about here was if there was air coming in past the MAF. If you've got a lot of air entering post MAF, the fuel trims should be strongly positive across the board as it adds a bunch of fuel to make up for what the MAF didn't measure. If you've got strong fluctuations both ways, then that doesn't seem likely to be the case.

 

Yes the TPS was the absolute position in %.

As for the MAP sensor, with the engine not running (no vacuum) I believe it should read around 29 for atmospheric pressure, correct? Since it was reading just ~7 that would lead the computer to think the manifold pressure was much lower than it actually is I would assume. I unplugged it to see how it effected the engine while running, and so far that has been the only variable change to have an effect.

The MAP reading, with it unplugged, from the computer is “3.0” with or with out the engine running. Probably not useful info but just felt like mentioning it.

 

So I’ve got a new MAP sensor in, now idles about 1000-1100 which is still a little high but better than before. It still likes to climb up in rpm after running for a while so I’m sorta at a lost for further ideas.

 

What are the units on those map readings? There's different ways readers can output data - raw or modified to various units.

Have you tried blocking off the intake inlet to see what happens? Its clearly getting more air from somewhere, that could help rule out if its picking it up via the MAF/ intake tube or of its getting sucked in elsewhere.