Hi all,

When I first start the car (in park) she idles fine. When car is cold, I can change gears from P to D, back to P or N and there are no issues. When I'm driving there are no idle issues either, it stays steady around 800rpm. After the car has warmed up, if I choose Park or Neutral, I get the erratic 1000-1500 cyclic idle problem.

I have already:
1. Bled the cooling system
2. Cleaned EACV
3. Cleaned FITV (no adjustment necessary, it was already screwed tight)
4. Cleaned Air Boost Valve
5. Cleaned Throttle body


The symptom persists. Only when car is hot and only when car is in Park or Neutral will the idle surge. If I'm driving and stopped at a light, the idle is solid. If it was a vacuum leak or clogged EACV/FITV/ABV, wouldn't the problem present itself also in D? What could cause this to only occur in Park or Neutral?

Could it also be some kind of electical issue that occurs at closed-loop? Could the O2 sensor be sending back some weird values that the ECU is ignoring when not in P or N? Or could the TCU somehow be messing with the idle. Those both seem unlikely.

Any thoughts would be appreciated.

 

Is the check engine light on?

 

Nope, no codes at all in TCU or ECU. Other than the erratic idle it drives fine with no signs of any other issue.

 

Hmm...with everything you have listed, the only thought that comes to my mind is a vacuum leak somewhere.....

 

I think 92 auto-trans had the vacuum-operated rear engine mount. I don't remember whether the solenoid valve is normally open or closed, but it DOES switch over when idling in R or D.

So if that mount is broken, it may present a vacuum leak ONLY when idling in R & D.

 

Reset the ECU by pulling the 7.5 amp radio/backup fuse in the engine bay box for a minute. I have had strange problems if I don't do this when messing with the TB sensors/valves.

Even if you cleaned the valves, they may not be working properly.

You can start the car without the intake tube on the throttle body. There are two ports in the bore of throttle body before the throttle plate. Plug one at a time with your finger when the car is running. One should cause the car to stall (EVAC) port. The other port would be for the FITV.

There is also a vac hose #22 on the diagram. Pinch at #22 to block that hose.

If blocking the FITV or the Air boost valve stops the surging, then you found the problem.

 

An open PCV valve or damaged PCV hose could cause this symptom due to large vacuum leak.

I would check all vacuum lines for splits or missing connections.

good luck

 

Thanks for the advice guys, I'll let you know how it goes.

 

Blocking the FITV did not stop the surging, nor did blocking the Air Boost Valve. PCV and hose are about 6 months old - I doubled checked anyways and thats not the issue either. Also blocked the EGR hose, the hose leading the the solenoid for the engine mount, brake booster, and every other hose with no change. All the hoses look good.

Are the throttle body/intake manifold gaskets known to go bad? Any way to test them? I cant think of anything else. Any other thoughts?

Thanks a bunch!

 

Vacuum leaks can be anywhere, so it'll be good for you to check the TB gaskets, etc. Any other hoses you haven't checked? I never had a 92 so I don't know where all the hoses can be.

Around the injectors? There's an O-ring where the injector goes into the intake manifold. That can be a vacuum leak?

Spray TB cleaner around while the engine is idling; anywhere there's a vacuum leak it will make the idle rpm change. This can narrow down your search.