I have an idle problem with my XJ8 3.6. No warning lights. When cold no problems. When warm the car can drop in idle to 5-600 rpm almost as if the engine dies then back to normal.

The pattern is that the idle drops for a second or two, back to normal again, then drops, back to normal and so on. This cycle last for typically 30 seconds and then the car settles to a normal idle. If I then rev the car it can go back to the unstable idle before back to normal.

Otherwise the car runs and drives well. When it is down on rpm it feels like the engine is choked rather than a misfire on some of the cylinders.

Any suggestions? I have checked for vacuum leaks, lose wiring etc.

br Martin

 

What is the fuel pressure during the erratic idle events?

 

not sure. Can I read it from the obd reader or do I need a gauge and read it from the fuel rail??

 

Use a smart phone with the Torque app to connect the vehicle's ECM/PCM with a Bluetooth interface through the OBD port. Read the fuel pressure directly.

 

oki Thanks I have icarsoft reader so I can read pressure. Around 380 I think

 

Hi Martin,

I am not sure which units you are using, but fuel pressure should be around 55 PSI.

 

KPA .........380 converts to 55 PSI

 

What are your fuel trims showing?

 

As mentioned the car idle has been erratic but no fault codes. Car has also been very sensible on the throttle pedal - very snappy sometimes like a ketchup effect.

When I got the tip trying to change the throttle position sensor I decided to give it a go also linking the idle problem.

Bingo - now the car runs smoothly again on idle and throttle is much easier to modulate.

So if you have a throttle which is very sensitive and an unstable idle the culprit might be the tps.

 

Thanks for posting this. an excellent tip. I think you meant 'sensitive' and not sensible though, maybe?

 

hi hi you ar right 😀

 

Thanks for that! Been in 'avoiidance mode' over nearly identical ......if still 'sporadic' issue.

 

mine was also sporadic, but getting worse and worse