High Idle still...

[BenXJCA]

My idle was faltering below 500RPM's. I cleaned the throttle body and the IAC port, extremely well. Then the idle would stay at approximately 1000rpms. So I hooked up a scanner to the engine diagnostic ports. It concluded that the motor was not going into closed loop and that the O2 sensor was to blame. I replaced the O2 sensor and that would let the motor into closed loop, and it dropped the idle to about 900rpms give or take 50 rpms. It is still to high so I replaced the IAC stepper motor and it is still at 900rpms give or take 50.
What are your takes on this?
Let me know.
TIA

[BenXJCA]

I knew there werent going to be any takers to this post considering the engine performance teacher has a 90XJ and he cannot figure out what is going on with mine.
It was worth a try
TIA

[Moto]

I did search and didn't see anyone suggest this, or maybe I missed it, but if it hasn't been suggested before, have you checked for an intake leak?

[satan]

blocking off the IAT port in the TB to look for the engine's base idle can maybe help you locate something (base should be like 650 I believe - no likely that your bleed has been opened, but stranger things have happened - more likely a leak.)

On your '89 have you verified that the EGR system is closing well?
How did you eliminate the Throttle body itself as the source?
(you did check the ECU's TPS for smooth and correct output, after your cleaning, right?)

[BenXJCA]

I did check for vacuum leaks all over the throttle body and intake manifold as well as the map sensor and the vacuum that controls the EGR relay. I hooked it up to the snap on scan tool and everything looks good but the idle. Im thinking about sending the ECU out to get tested. That is what my teacher suggested. As far as the TPS goes, all it looks for is a voltage increas of about .07volts. It did not matter where it was adjusted to as long as it was under a volt. As soon as you move the throttle it would go into partial throttle. The teacher said it should go into partial throttle when the throttle is opened more.
Thanks for your interest.
TIA

[satan]

Have you watched the output for the IAC? (as in is the ECU trying to drive the IAC more and the IAC isn't responding - it's not to uncommon for them to stall at/near a common idle position)?

So you've verified that the EGR valve is ealing tightly with no leaks, and that the base-line bleed-idle is about 650 or less?

If you have not 100% confirmed by blocking off the IAC air passage that your idle falls below 700, then I'd highly recommend testing that - If your idle won't drop with the IAC supply sealed, then the ECU has no hope of controlling the idle in the first place (I set bored TBs to 450rpm on bleed air - works great and allows better TPS resolution).

The tool is nice, but since the ECU doesn't produce an idle-fault message, you're pretty much stuck with a mechanical evaluation - there's a nifty little AMC/Renix tool to block that port, but a thumb in a glove works quite well.

[BenXJCA]

Where is the base idle bleeder adjustment?

[PaulJ]

Its on the right side of the throttle body and is under a soft metal tamper-resistant cap. You can poke a hole in the cap and then remove it with a screw or easy out. The cap is really a disk that is in a recess. I don't think that misadjustment of this is the cause for your problem, assuming that it was previously running properly. Like Satan said, block off the IAC air passage with your thumb and see if the idle speed drops down to 300-500 RPM. If it doesn't, then you have a vacuum leak somewhere or the throttle plate isn't closing. If it does drop down when the passage is blocked, that means the ECU is unable to control the IAC motor. On the top of the throttle body, the round hole on the left is the base idle bleeder and the larger rectangular hole is the IAC passage. You know what the big round hole is. There shouldn't be much if any air getting by the throttle plate and you can block it off with your hand just to be sure. There is a stop screw that can be adjusted if the throttle plate isn't closing far enough. FYI, the ECU electronics seem to be quite reliable

[LynchMob]

I got really lucky with my high idle problem...it turned out to simply be a mechanical problem with my linkage and the FAT spring that joins it to the rear of the TB. It had lost a lot of tension and in combination with a gunked up linkage it would stick open on first start up, usually around 1200RPMs, and then sometimes higher as I came onto the pedal.

Might not be your problem...might help someone else...I don't know, but very easy to fix. I didn't buy a new TB, BTW (can't buy the spring separate), I just took the spring off gave it some more tension.

HTH

[satan]

Yep - the linkage stuff (failure to close to the stop) is very common on olderrides (stuff wears-out!)...

... and you'd be able to tell that with a finger in the IAT hole! (you'll hear/see the opening.

photo of TB top you'll find the bleed just below its intake...

[BenXJCA]

When I start it up it idles down to were it is supposed to be (below the 750 mark). Once I drive it and then I stop at a light for example the idle stays at close to 1000rpms. I know it is not the linkage. I was watching it while a friend was stepping on the gas all the way and letting off of it. It closes to where the linkage hits the stop screw.

[BenXJCA]

I blocked the IAC port and it does not effect the idle speed. Also the bleeder screw is turned in all the way. What now?

[Andy in Pa.]

I had the same problem, I wrestled with it and checked all vaccuum, got a new TPS (set the voltage correctly), IAC, checked it with a scan tool, you name it I tried it. You know what finally fixed my problem?? Turning the idle stop screw slightly out and bringing the idle back down to where it is supposed to be. Problem solved.... Sometimes after you clean the throttle body (and you have a fast idle and you have checked all of the known suspects), you have to adjust this screw slightly to get it back to normal again.

HTH's

Andy

[satan]

If blocking the IAC air passage doesn't affect idle speed, then you have air leaking into your intake somewhere else (NOT the computer's fault - mostlikely hardware related)

With the IAC intake covered - try looking around for other culprits (any loose vacuum line, the MAP sensor vacuum fitting at the TB, force your throttle linkage closed with moderate thumb pressure, try sealing the throttle bore with the heel of your hand...)

If you're idling smoothly with the IAC blocked-off I'd suspect that you have a major leak - either at the throttle plate itself (it does NOT touch the TB walls, but gets damned close) or at a gasket --

Look at the TB to manifold gasket for a tear or missing portion - you can "test" intake gaskets on a idling engine with a stream of carb-cleaner to see how the idle behaves. Check the manifold to head gasket as well.

Since you've proven that the Computer's only air source is not the issue - it's time to start looking for the mechanical solutions to your air leak --

Personally, I'd suspect a TB gasket or the linkage (since it was just cleaned, there's a risk that the gasket was improperly reinstalled or an old-one used -- or that the throttle shaft lubrication was washed-out leaving a certain degree of "bind" in that linkage)

[Greg Smith]

Higher idle after warmup is often caused by vacum leak at intake manifold due to loose manifold bolts. As the metal heats up the leak gets bigger causing higher idle. I have to tighten my intake bolts at least once a yr because of this. Tight the middle bolts first and work out to the ends. Not sure if this is your problem but hope it helps. Greg