Over run Valves

Hi all.

My 1988 XJS V12 (UK) idles far to fast, about 1400 revs. According to my CD manual the first thing to check is the overrun valves by blocking them both then unblocking one then the other and starting the engine each time. Doing this makes no difference to the idle speed. But if I block both of them the car starts much easier and quicker, on the turn of the key instead of a couple of seconds. unblocking one or the other or both then it's starts, but only after a few turns. To the best that I can tell (by putting my finger loosly over the hole in the air filter housing) there is no air going through the valves at idle (about 1400 revs). Does anyone know how these valves do their job, and maybe an explaination of why the car starts so much easier when the valves are blocked. And even better! Why this engine idles so fast. I have checked throttle linkage and butterfly valves and all seems as it should.

Thanks for any info

Ivar

Reply to
Ivar
Loading thread data ...

Hi Ivar, There are a couple of things to check. The clearance of the butterflys in the closed position. Does the idle slow when the engine is hot - been on a good run. Should be around 700 to 800 RPM. This may need adjusting at the Extra Air Valve. The overrun valves are to allow air into the manifolds when you have taken your foot off the accelerator to slow down. Chris

Ivar wrote:

Reply to
Chris Halpin

I think you have an ait leak somewhere.

Reply to
Blake Dodson

Hi people. Thanks for the feedback so far, does this shed any light on the problem? When the car first starts it idles at about 1400, after a few minutes at idle (I don't take it on the road) The engine revs will go down to 700 (ish) then back to 1400 then 700 then 1400. And do this every 2 seconds (the rev counter needle 'bounces'. At the back of the left hand manifold is what I think is the Auxiliary air valve. If I remove the lefthand air filter (Not the housing, just the filter) and Stick my finger in the pipe that goes from the air filter housing to the auxiliary air value ( There is a lot of air going through this pipe at idle) the engine revs decrease to about 800, remove my finger and the revs increase again. I would assume this valve is somehow temperature controlled, When should it be open and closed? Is there some way to test it?

Thanks

Ivar

Reply to
Ivar

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.