Transmission issue? Engine issue?

[kastein]

For a few months my '96 XJ (127k, 4.0L I6 automatic, NP231) has been giving me issues occasionally... it starts fine, it runs fine until I try to accelerate ever so slightly, as in pulling out of a residential street corner, then it sorta hesitates/chokes/runs rough for a few seconds, giving almost no power, and suddenly snaps back into normal operation. It does this only right after I start it, and only once in a while, but it's worrying me - I suspect PCM or transmission problems.

Some things I've already done:
* plugs, wires, distributor cap and rotor. Problem went away for a while, or it could have been luck, I'm not sure. Engine idles MUCH smoother now.
* intake/valve/whatever cleaner - seafoam. Runs better now, but the random issue still happens with about the same frequency.

I know that my alternator is fine (system at 14V as expected), and the transmission is not going into limp-in mode as it drives fine after the one issue right at the start of a run. It never turns on the check engine lamp, though I don't actually know if there is a bulb in there... I will check that at my first opportunity.

Any ideas? I've looked around at some other threads but haven't found anything that quite fits the symptoms.

 

[joe_peters]

Well, I would check/test/clean the idle air controller (IAC), the throttle position sensor (TPS), and the throttle body (TB).

The IAC is a stepper motor that handles both the idle and transition circuits in conjunction with the TPS inputs to the ECU (PCM).

 

[kastein]

Thanks, I'll definitely look at that. All of the parts you mention seem a good deal cheaper than the PCM/ECU, so I'll try them before it... probably will try cleaning, then junkyard parts, then dealership parts (from what I have read, I want nothing to do with aftermarket car electronics).

 

[kastein]

Another thing that's making me curious is why it only does it the first time right after I start it - I figured the TPS probably wouldn't do that, as it'd flake out randomly whenever it wanted to, which is making me think it's more likely to be the IAC because that could get stuck before it gets actuated a few times. I might just be nuts though...

 

[winterbeater]

IAC is more for idle control. I'd lean toward the TPS. Maybe it makes better contact after it's worked a few times after sitting.

 

[kastein]

Update - haven't had a chance to do anything about this, but I've noticed that the engine will surge randomly to 4k RPM while cruising with cruise control (for maybe a tenth of a second per surge) so I'm leaning more and more toward the TPS.

 

[jkelley330]

The iac will cause surge problems. Yes aftermarket is junk esp when it comes from auto zone. But the BWD brand is just as good as factory.

 

[kastein]

Thanks - looks like I'll be getting both at the junkyard this Saturday and hoping for the best. At least they haven't given out completely yet, it's just mildly annoying at this point.