Hunting at idle/stalling

KJSCULLY23

MEMBER
1987 JAGUAR XJ6
4.2L
6 CYL
2WD
AUTOMATIC
145,000 MILES

Idles okay when cold, when warm starts hunting at idle, will get bad enough to stall unless throttle is opened slightly.
Inspected vacuum hoses; replaced cracked hoses to the auxiliary air valve, also had removed the valved and tested per workshop manual: Voltage to valve okay. No change, proceeded with tune up; new spark plugs, cap, rotor, fuel filter and O2 sensor. No change. Replaced throttled body to elbow hose and elbow to air flow meter hose. No change. The emission check valve is working properly.

4 Replies

KASEKENNY

MECHANIC
18,907 POSTS

Do you have a scan tool that can read live data? Seems like you covered most of the basic items so just trying to think through what would cause a rough idle when warm and not cold, that points to a sensor issue or control issue.

The only thing that "knows" the engine temp is the coolant temp sensor which tells the PCM what the temp is. If this is not reading accurately it could cause the PCM to dump fuel erratically or insufficiently.

In fact, I have heard of Jags have an issue with this connector and sensor itself causing all sorts of issues when warm. I don't remember if it was the XJ or not that had this but it well worth checking.

Let me know if you have a scan tool that can monitor it and we can go from there. Thanks.

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Saturday, March 7th, 2020 AT 4:52 PM

KJSCULLY23

MEMBER
4 POSTS

Have an iCarsoft Vol II that I purchased for my 2007 Volvo XC70, can read live data, will try it on the Jag and let you know.

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Sunday, March 8th, 2020 AT 7:36 AM

KJSCULLY23

MEMBER
4 POSTS

Oops, the 1987 does not have on board diagnostics. Will replace temperature sensor and inspect wiring, the engine compartment wiring harness was hacked up by a previous mechanic or owner, had repaired the harness when I got the car in 2007. Did a valve job and had the car running great. Got an outside sales job with a company supplied car and parked the Jag and did not use it for about 3-4 years. Car was not happy with that plan!

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Sunday, March 8th, 2020 AT 7:43 AM

KASEKENNY

MECHANIC
18,907 POSTS

Haha. Yeah. Vehicles tend to hate sitting still. Based on that info, I bet you will get it with that or will find something else while looking at the wiring that caused it. Keep me posted. Thanks

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Sunday, March 8th, 2020 AT 4:39 PM