I have been dealing with a minor/moderate rough idle when the engine is warm. It had been running a little rough at about 550 rpm, and occasionally bounces under load, enough to be a little embarrassing at stop lights. I disconnected the coolant temp sensor and that seemed to cure it, so I ordered a replacement sensor and installed it last night. This morning I noticed a small loss of power when driving at first, and then once the engine warmed up the bouncing got very bad, to the point I thought it was going to stall. problem gets alleviated when I disconnect the sensor, but when I measure the resistance, it seems fine (320 ohms warm) which leads me to wonder whether the new temp sensor is actually the culprit or whether something else is going on. Since it's measuring within spec I thought I'd ask whether anyone's run across a similar problem in the past.
When I check for codes the only ones I'm getting are for coolant (and a slew of unrelated codes: seat belt, door, brake circuit, boot, and washer)... Should I check anything else before I try to get another sensor shipped to me? Is the fact that it runs fine with the sensor disconnected a clue for another aspect of the system?
I appreciate any insights!
-A

 

One other question I meant to ask: Would a little air in the coolant line cause a bouncing idle like that? It occurred to me that a little air could have gotten in when I changed the sensor and I'm still waiting for the car to cool down to burp the system.

 

I don't think it's that (the coolant sensor)

You might have a small vacuum leak or possibly the IACV needs cleaning.

If you haven't cleaned out the air intake & throttle body, might be a good time to do it too. As far as the vacuum leaks go, the tee piece connections and vacuum line ends do tend to get a bit slack over the years, they should be fairly firm fits.

 

Codes aren't the illuminated lights on the dash before the engine starts!

Just to clarify, you have to press the VCM button with the ign on and look at the mileage display - that's the code display panel!

Larry

 

Oh thanks for the clarification! I was pressing the VCM button and looking at the computer screen, I'll check again later.
I did clean the IACV previously and that helped a little. I haven't gone into the throttle body yet mostly because the IACV seemed so much cleaner than I was expecting.
I'll do a once over on the vacuum lines to begin with and report back, I did notice a little slack in one of them earlier and I have suitable replacement silicone line on hand.

 

Well, Larry, you were close: It wasn't a small vacuum leak that was causing the problem but rather it was a very large one... The vacuum line into the intake manifold had separated from the elbow connector. I reattached and all is relatively well.
I still have a lot of vibration at idle, enough to cause the rearview mirror to vibrate, but it's down to the level of nuisance. I suspect cleaning the throttle body will help a lot. I did pull the MAF sensor a few hours ago and it appeared remarkably clean.
I tested the vacuum a few weeks ago and had a steady 18mmHg with no signs of major issue, but I think I'll plan to replumb all the vacuum lines when I get the throttle out just to be on the safe side.
Thanks again for the help!
-A

 

Great news!

Glad to hear you found a vacuum leak, that'll do it every time!

While you're at the MAF, try pinching the connectors together a bit, that maf plug can be a real PIA if it gets iffy.

In my experience a good intake clean and throttle butterfly de-coke can often get her purring again.

Good luck, nice to have another northwestern car up and about!

cheers

Larry