My 1990 XJ40 4.0L has a very erratic idle while the engine is cold. It surges in RPM up and down. Once it comes up to operating temprature it smoothes down nicely.

 

I believe it should be from dirty throttle body , idle sensor .

 

Conrad:
Yes, you could be in for a good TB cleaning. Is the Check Engine Light on and, if so, what code does the VCM show when you turn off the engine, turn the key to position II (just before starting), and press the button?

Don't forget to introduce yourself to the forum in the correct place, and welcome!
Scott

 

MidwestJag agrees with you. I'll give it a try.
Thanks.
Conrad

 

Hello and thanks for the response. The check engine light has been on for the past couple of years but Jaguar diagnostics did not indicate anything wrong. The VCM says Fuse 1 FAIL. I'll check that out and see if I get another VCM reading. I'll also go with the TB cleaning. Any chance I can do it myself?

 

Conrad:
Hmmm... I don't think I would be satisfied with a perpetual Check Engine Light, but that's just me. Some CELs have multiple possible causes, and tracking down a fault may take more time than both owner or mechanic has patience for!

I am not all that familiar with fuse fails. I'm not sure I was even aware the VCM indicated those! I was hoping for it to indicate a "Fuel Fail" with a number which, probably, would have been much more helpful in solving your irregular idle. I would double check that and it wouldn't be a bad idea to get the more specific codes (probably from a mechanic, unless you have a code reader) from the CEL.

Although I have never personally cleaned my own throttle body, I know lots of guys have. Perhaps some of them can chime in to give you pointers. I do know that you should take great care to to disturb (don't loosen) the red Throttle Position Sensor on the bottom. It needs to be properly placed in order to work. It can, though, be disconnected and removed with the TB.

Cheers!
Scott

 

There are a few fuse locations , driver's foot side, passenger side, under armrest . I believe you need to remove the cover and have the fuses checked . Fuse 1 says there is a faulty fuse at that location (I don't remember which is which ,sorry . )
Still, I don't think erratic idling has anything to do with the fuse failure .I also get Check engine light time to time and when I press the VCM nothing happens lol

 

Fuse 1 is right side fusebox
Fuse 2 is left side fusebox
Fuse 3 is center console fusebox

bob gauff

 

I was playing with my fuse1 fail and seems okay now. Now I have VCM saying Fuel fail 13. rough as heck idle. What to do?

 

im sorry, Fuel fail 14

 

Apart from all the above, check the wire connection to the temp sender on top and at the front of the engine.

I had the same fail code, found the wire to be broken, taped it up 8 years ago and no more code/fail.

Reminds me, I must get around to making a permanent repair. LOL

 

You may have to replace that sending unit.

 

Did this ever get resolved?

 

The problem is still there. I will try to locate the sending unit this Sunday the 12th. The car is in a different location and I will be there tomorrow. Thanks, and I will post what happens.