My car's been running well lately, except for one situation - when I idle too long in traffic.


If I continue driving at any speed, things are fine. If I sit at idle for too long, (if I'm backed up and waiting through a couple stop lights), what tends to be happening more and more is that I'll be idling nice and smoothly at about 650 rpm, and then all of a sudden the check engine light comes on (referencing an o2 sensor running rich) and the engine starts pulsating up and down/idle speed pulsating/etc..


I had a new O2 sensor in after the car was redone... and it seems to be fine as long as I don't sit at idle for too long...on a weekend I can drive around all weekend and never have a check engine light....or I can go to the car wash and get in line and I have the flashing exclamation mark coming back at me...


Connections are all cleaned/good to the o2 sensor...nothing looks out of place there...any tips are appreciated.


Thanks
Pete

 

Pete,

Have you put eyes on the troubleshooting section in the Haynes manual? I know it sounds too elementary, but sometimes reviewing it will jar your memory of work you have done to, or near one of the components affecting the irregular idle. I don't think it's your O2 sensor by the description your give.

I think there is another component affecting the behavior when the engine begins to get hot due to non-movement, but I can't put my finger on it this moment.

 

Hi Pete,

I agree with Rob that from your description, the O2 sensor is probably not the cause of your problem - something else is causing the O2 sensor to report a problem. Can you please tell us the exact Diagnostic Trouble Code you are getting?

One possibility is that the Coolant Temperature Sensor (CTS) is lazy and is continuing to tell the Engine Control Module (ECM) that the engine is cold, even after the engine has warmed up. This can cause the ECM to continue cold-start fuel enrichment that causes a rich-running condition once the engine warms up, and often this can happen without tripping a CTS DTC. You can test the CTS with an ohmmeter to see if its resistance changes as the engine warms up. The OBDI Diagnostic Manual gives the resistance specs at different temperatures. Also, the CTS electrical connector and wiring harness are prone to damage, so it would be worth inspecting them carefully.

Please keep us informed, and let us know the exact O2 sensor DTC you've been getting.

Cheers,

Don

 

Believe it or not, i'm finally getting back to this.


I checked the throttle position sensor, which read correctly and closed and full throttle. I checked the CTS and it's reading properly...although I wasn't really thrilled with the connection quality, so I'm going to swap that out (connectors seemed like they were rusting out. Anyhow, It seemed to match properly with the spec I read in the manual. I also tested the CTS and it had the proper voltage. I tried starting the vehicle with that disconnected and it wouldn't start at all. Putting it back on allowed the vehicle to start.


I walked through the idle set process for 3.6L engines, and noticed my idle was fairly high. Brought it back down a bit to the 580/700 (hot drive/neutral) setting. However, my concern is that I'm still getting the check engine alert and immediate rough idle kick in when it runs for a few minutes. I can turn the car off for a minute and turn it back on and get no check engine light and have the idle smooth again...then it'll just all of a sudden trip the message and run rough again.


I'm going to clean up this CTS area, but if anything else comes to mind besides potential vacuum leaks, I'd like to check it out some more.


Don, I have to double check the o2 sensor alert. I thought there was two, but indeed, even on the '88 and '89, there are two check engine alerts that may come up.


Thanks,
Pete

 

Hi Pete!

Please do let us know the exact trouble codes you read on the VCM and hopefully someone will have some helpful ideas.

Cheers,

Don

 

Once it triggers again I'll let you know. I will note that after adjusting the air mixture a bit, to get the idle down to 700, the next morning it was rough during the cold startup. It would run rough around 700, then bump way up to around 1200RPM, then start settling down slowly back to 700, then bump way up, etc... It was gradual, but I ended up just adjusting the mixture screw about 180degrees to the left and it smoothed out the idle (increased it a bit) and the jumping up to 1200RPM went away.

Right now when warm I'm seeing 675 in Drive, 840 in Neutral. I might try to back off my adjustment from the morning just slightly to see if I can get that down a bit. It seemed like I had it set properly earlier...hmm...

Pete

 

Could it be a vacuum leak in a hose that when the hose is hot gets more playable and presents the problem?

 

Don, fyi - the engine code is #4 - "Lambda Oxygen Sensor Feedback". I found this code on another site, but thought #4 was 'O2 sensor full rich'.

If I start up the car and let it just sit and idle...it'll go through the warmup period idling fairly smoothly... (nothing that seems incorrect), but then after a few minutes it throws the 'check engine' light and then starts hunting. I turned up the mixture about 180 degrees to increase the idle, which seems to avoid getting the check engine light unless i'm on the road and in heavy traffic, idling for a long time...

This is still happening after I swapped my temp sensor and connector harness. I'm going to try and borrow a smoke unit to see if I can identify any vacuum leaks next..

Pete

 

Pete,

On the '88 and '89 models code 4 does indicate a problem with the O2 sensor circuit and that the system indicates full rich. The definitions for the codes are in the Haynes manual, which you can download here:

https://www.mediafire.com/?hx8bttsy4kxxr#3267xri9vx4v6

For some reason the pdf is missing chapters 7 & 8, but fortunately the Engine Managment System / Emissions info is in chapter 6. See page 113 of the pdf file for the fault code definitions.

Cheers,

Don

 

Ok...yeah, I saw that I my Haynes manual as well...(But read the other description on the AJ6 engineering website), so I wasn't 100% sure.

What is that really indicating, though.... is it detecting 'full rich'...as in the sensor being out of spec, or is something wrong and making it 'full rich' and the engine code is just reflecting that? I pulled the connection to the O2 sensor, and thought that would cause it to be 'ignored' after the check engine was triggered.. but it didn't seem to effect the idle at all.

Pete

 

Hi Pete,

I don't have any engine management references for the 3.6L engines, but I assume the basics are the same as on the 4.0L cars.

The O2 sensor receives a 5V suppy from the ECM, and its resistance varies depending on the oxygen level in the exhaust gasses. The resulting voltage swings between just below 5V and just above 0V. You can back probe the O2S electrical connector and watch for the voltage to swing, but your meter needs to be able to react relatively quickly in order to see the swings. If the voltage seems to be stuck at, say, either 4.89V or 0.15V, suspect the O2S.

Other components the ECM references in determining the air-fuel ratio (AFR) are the MAFS, IATS and CTS.

It would be worth carefully cleaning your MAFS by flushing the internal hot wire and electrical connector terminals with zero-residue MAFS cleaner spray or electrical contact cleaner spray (don't touch the wire inside).

Did you ever check the CTS electrical connector and wiring and test the sensor resistance when the engine is cold and hot?

I don't have the '89 EMS manual but you can download the '93-'94 EMS Diagnostic Manual here:

http://www.jagrepair.com/images/Auto...stem%20S91.pdf


Don