My '98 XJ8 (127K miles) starts fine and idles smoothly until the engine reaches normal operating temperature. Then when I come to a stop, while in "drive," the RPMs drop from 650 to about 500, and the car starts idling rough and shakes. The RPMs will pick back up if I put the car in "park," but once back in "drive" with my foot on the brake, the same thing happens. The engine runs smoothly at speed. I've cleaned the throttle body, the MAF sensor, and the breather hoses. I also had the fuel filter, timing chain and tensioners replaced at 92K. Could someone suggest where I should go next? Thanks.

 

Clean the part load breather coming out of the left valve cover. A 3/32 drill bit will get rid of any buildup. Be very careful you don't break the hose, old plastic is fragile. Check for cracks/ breaks in that hose and the big flex between the maf and the throttle body.

 

Thanks, RJ. I cleaned both just recently at the same time I cleaned the MAF sensor and throttle body and got no improvement. I'm stumped as to why this only happens when the engine is fully warmed up and when it's patricularly hot outdoors. I get no fault codes when I run the test.

 

My guess: you have an air leak between the air filter box and the throttle body; maybe at the fittings for the part and full load breathers.

Clean the throttle bore and plate while you have the tube off to check for leaks. Might also try running a can of SEA FOAM through a tank of gas to clean the fuel injectors.

How old are the plugs?

 

Thanks, Jim. I just added a bottle of Sea Foam to a full tank of BP 93 octane gas. Will see if that helps. Will also check again for possible leaks this weekend, but I didn't detect any when I cleaned the throttle body and breather hoses a couple of months ago. The Jaguar dealer installed new plugs when the timing chain was replaced at 92 K miles in 2009. I may examine the ignition coils and plugs this weekend. I'm starting to wonder if I have an ECU problem. Neil.

 

I found another post on rough idle from forum member Blue_ XJ8 :

"My vote - BG44. I have tried Seafoam and other 'off the Advance Auto Parts shelves' products with no really noticeable result but since BG44 I have been quite impressed at things such as better idle, smoother running, etc.. This stuff is not sold at traditional retailers and is not cheap. I reached out to a couple of garages around my place and actually found two happy to sell me the stuff at ~$30 a can (ouch I know)."

https://www.jaguarforums.com/forum/x...ussion-114088/

Maybe worth it to try it.

Jim Lombardi

 

Check the vacuum pipe to the brake booster, they can crack and leak air when the cars get this old.

 

I tried a can of BG-44K a couple of months ago and got no improvement. I'll give the vaccum lines and hoses and good going over this weekend. Thanks for the suggestions!

 

There is a brake booster thread on the forum - vacuum hose runs from the lower throttle body to the brake booster (start with post 1):

https://www.jaguarforums.com/forum/x...l-38101/page2/

 

If you have an OBDII scanner you could check the fuel trims to see if it looks like you have a vacuum leak. You could also pull the plugs and see if oil is leaking into the spark plug wells. You could have pending OBD codes. They won't light up the CEL, but they could point to something. Does the engine seem to be running rich when this happens - black smoke from tailpipes, smell of unburnt fuel.

 

Things like a faulty crank sensor and temp sender can make the car behave as suggested, check wiring and test each.

 

Thanks to all for the additional suggestions. I can find no vacuum leaks, and there's no oil leakage into the spark plug wells. After climbing under the car and locating the crankshaft position sensor, then tracing the cable to the connector behind the left front wheel well, I think I will order a new one. I would rather replace the existing sensor than try and remove the old one for testing, even if the old one is still good. Regards. Neil.

 

Problem solved! I replaced the crankshaft position sensor and the rough idle problem persisted. I then removed all the ignition coil modules and pulled off the rubber boots from each one. On one of the left bank modules I found corrosion where the coil that makes contact with the spark plug slips over the post at the base of the module. After thoroughly removing the corrosion with a small wire brush and reattaching the module, all is well. No more rough idle. Thanks to all who weighed in.

 

So, who can say HONESTLY, that they had a noticeable drivability issue and put Seafoam or other such product in the tank and it got better? Not, " I put Seafoam in my tank every month and my car never had a problem!", but a real case of Seafoam fixing anything?????
I guess I am showing my biased opinion, huh?