Rough idle when warm

[paulfrost88960]

My 1988 3.6 idles fine when cold, but as it warms up the idle becomes progressively worse until if stuck in a traffic jam the car ticks over very lumpily and almost stalls. What do you think is causing this problem?

[davidr]

Hi Paul - the first thing that come to mind is the coolant temperature sensor. If it is not working properly and not giving the ECU the right information as the engine warms then it will continue to fuel the car as if it was cold resulting in it running rich when hot. This could give the symptoms you describe.
Please bear in mind that you have 2 coolant sensors - one for the instrument pack and another for the ECU.
Hope that helps
David

[paulfrost88960]

Hi David. Would that affect the fuel consumption much as i'm still getting about 30mpg! How expensive/easy to fit are these sensors if they need replacing?

[davidr]

Hi Paul - yes I would expect consumption to increase.
Changing it is quick and easy. It is mounted on the thermostat housing and there is direct access from above. 5 minute job including opening and closing the bonnet! Cost is about £10 for an aftermarket one. Double that for the Jaguar item but they are currently showing as unavailable on the Jaguar Classic Parts site.
These sensors do have a finite life to the point that many of us consider them to be a service item.
Please bear in mind that we're only guessing here but if you want to take the gamble its only a tenner!

[paulfrost88960]

It seems worth a go doesn't it. I'll let you know what happens

[paulfrost88960]

David: i ordered a genuine Jaguar sensor from SNG Barratt, it was about £40. However the workshop manual says to use "appropriate sealant" on the threads, presumably to stop leaks? Which would you reccomend? or is it really needed?

Paul

[davidr]

Hi Paul - I've never used a sealant and not had an issue. The copper washer should make the seal. If you don't get a new one with the sensor then anneal the old one to soften it.
Good luck!

[paulfrost88960]

Thanks David, what i expected really..i'll let you know how i get on.

Paul

[paulfrost88960]

Hi again David. Brilliant diagnosis. As you say an easy job, no leaks and the tickover issue appears sorted. It's been irritating me for ages, but the fault finding section of the manual implied there were loads of things i needed to check and so i kept putting it off! Lesson learned
Many thanks

Paul

[davidr]

Great news Paul - I'm very pleased it worked for you!
Could you perhaps send me a few photos and we can put a piece in the magazine?

[paulfrost88960]

David, of course.

Paul

[paulfrost88960]

David, just had another idea. I only bought my car in feb, and despite lockdown have managed to drive 6000 miles since then. Why don't i write an article about my time with the car, including the idle problem?

Paul

[davidr]

Paul - that would be ideal. The views of a new owner are always interesting
Regards
David

[paulfrost88960]

David, i thought rather than putting it off i'd do it straight away, so i've taken a few photos and written an article for you this morning: i'll e-mail them to you.

Paul