After a good half hour of mostly highway driving I went to pull into my garage. I noticed that while in park or neutral the idle was all over the place, from 500 rpm up to around 900 and back, constantly. Once I put it back in gear it was rock solid at around 500 rpm and otherwise ran fine. I didn't even notice an issue until I went to open the garage door, and I haven't noticed any problems when it's cold or while stopped at lights. Any suggestions? Flakey IAC motor? Vacuum leak? Wouldn't either of those cause problems in drive as well? Wouldn't just about anything?

'90 GTA LB9/700R4.

 

Quick follow-up. I disconnected the IAC while running (and surging) in park. No change, still surges all over the place. Sprayed carb cleaner over the upper intake joints and the TB, no signs of a vacuum leak there. It's still rock stable in reverse or drive.

*scratching my head*

 

I have the exact same problem, it runs anywhere from 700-1300 rpms at idle in park and neutral and, runs a steady 500 in drive and reverse. I dont know why but i need help too...

 

No codes? Do you have a scanner? It could be rich/lean with a light load and trying to compensate. The ECM does compensate for being under load or not, so a bad park/neutral switch can also cause this. Any work done to the engine lately?

 

No codes. It had the same problem about two years ago before putting it away for the winter so before bringing it out the following spring I replaced all of the intake gaskets, installed full roller rockers, AFPR, valve stem seals, valve springs, and cleaned the EGR and TB. Later that year I replaced the alternator and it got a fresh Optima this year. It has gone maybe 2000 kms since so it's all 'recent'.

I'm confused, how would a bad park/neutral switch cause a problem? Or is that different from the neutral safety switch?

 

The P/N switch also has a set of contacts that tell the ECM whether you're in P/N or not. If not, then it expects more load and adjusts the injector pulse with accordingly.

 

i would say you need a tuneup. my 91 305v8 vert had the same problem got a tune up and now everything is peachy

 

Just to play with it, I increased the fuel pressure about a quarter turn before I went on a drive just to see what would happen. It got worse to the point where it was surging in drive as well. Cruising was fine but red lights had me worried about stalling. Once she cooled down enough that I could reach under the plenum I leaned it back out about half a turn and it ran much better. Still a bit of a surge in park but not nearly as bad as last night. Hmmm... I'll have to hook up my gauge and see where it's sitting right now, maybe that AFPR is acting up on me.

As for the park/neutral switch, doesn't that have just one set of contacts? Going by memory I thought it was just a simple microswitch. So if the switch is flakey and possibly causing my issue, technically I should be able to engage the starter while the car is in gear, right?

 

My '87 has multiple contacts and 5 or 6 wires. It's also listed in the FSM as one of the things to look at for a rough idle.

 

To update: I thought I was going to have to have it towed home this morning.

Since I last posted I lowered my fuel pressure to 38 psi (regulator disconnected) and it idled a lot better. It still fluctuated a little but only about a hundred revs versus the three or four hundred before. I still intend to replace my neutral switch but at least it was liveable.

Until this morning.

Once I hit the freeway it started severly bucking (felt like it was missing) and wouldn't rev. It stalled by the time I made it to the shoulder. It fired right back up and ran fine for a few minutes but died at least twice again on my way back home to grab my Jeep.

After work I put on my fuel pressure gauge. It took a few tries before it would idle, it would just catch on for a few seconds and sputter out. The whole time the fuel pressure would do exactly what I'd expect, sit around 30 psi with the regulator hooked up and 38 without and wouldn't idle any better with it disconnected.

Now, the only thing I did to it recently is fill the gas tank from half the evening before. Does this sound like a simple case of bad gas? It would have to be REALLY bad to polute a 'good' half tank though. I can't imagine that a tune-up would be needed so suddenly, athough I will check a couple of the easy plugs. Any advise?