Sudden High Idle

[verlane24]

The X gave me an unwelcome christmas present yesterday..... it started randomly kicking the idle high at stop lights and eventually got to where it pretty consistantly idles @ 1500 or so.

So I looked for any loose vaccuum hoses, etc with no success. I reset the computer, and the first time I started it, it ran good for about 1 minute, and then kicked the idle back up again.

Strangely enough, when I unplugged the IAC while it was running and the idle went down! I found this to be odd, because I had just changed the IAC about three weeks ago. The old one didn't seem to give me any trouble, I had just changed it because it looked old. Great, I thought had got a bad part. So I took the new one off and threw the old one on, reset the computer and ..... still idling high after about 10 minutes.

So I am a bit stumped. I have recently changed the ECT, and the IAT. I have not tried to pull codes yet, but I have not had a check engine light. I am fairly sure I have a leaky intake manifold, but other than highway pinging I have had no other problems stemming from that.

It came on suddenly, so I would think it would be a sensor but which one? Could the TPS cause this?

Thanks in advance!
Brian

 

[dogfriend]

I think its possible that the TPS could cause this, but you mention a leaky intake manifold which is more likely to be the cause of the problem. You should try to correct this problem first before starting to replace sensors, etc. The leaking intake manifold = vacuum leak = unmetered air which will screw up the air/fuel ratio and cause problems.

 

[verlane24]

After driving around this weekend, this is what I have found.

The idle becomes wierd after running the engine for 10-15 minutes. Cold it seems to behave like it should.

After 10-15 minutes it erratically bumps the idle up to 1.5 - 2K. Then sometimes it will normalize for a while, before acting up again.

I've ruled out the TPS, so is there anything else I should check before I dive into the intake manifold? I had been planning to do this anyway, I just wanted to wait until it was a little warmer outside.

I guess I'll try jiggling/tapping some of the sensors while the engine is running a make sure I don't have any loose wires.

Thanks
Brian

 

[dogfriend]

I think the significance of the 10min interval before the problem occurs is that the system is going into "closed loop" where the O2 sensors begin to control the air/fuel ratio. Before going into "closed loop", the system uses fixed, predetermined settings to control injector timing, etc. Then after the O2 sensors are warmed up, it switches into 'closed loop' and uses the O2 sensor data to determine injector pulse time.

Its possible that there is a problem with an O2 sensor or wiring, but I would still bet that the intake manifold leak is causing the problem by "tricking" the O2 sensors because of excess unmetered air.