Get one problem sorted, and another pops up! Had an issue with an intermittent, slight misfire. Misfire sorted. I put in new bushings in the throttle linkage and corrected the length of the throttle arms. Removed the airbuses and adjusted the butterflies to .002 of an inch. Put everything back together. Now the idle is high when cold and after the car gets warm, its idle surges back and forth in park and neutral, as if the ECU thinks the car is still cold. I changed the coolant temp sensor not he left bank - didn't help. I took the sensor out of the block and started the car. It went to a fast idle again (no surging) so, it appears it is definitely related. Looked around. Didn't see any connections missed on reassembly. Any ideas would be appreciated!! Getting very frustrated ad this point!

 

Assuming an 80s-90s V12.

Assuming you removed the air cleaners entirely.....

Check the air cleaner-to-throttle body bolts. If not tight, you'll have an air leak.

Go back over your throttle and linkage adjustments. One throttle might be open further than the other. It happens.

Cheers
DD

 

You have enough of a massive vacuum leak (likely from the AAV being stuck or a puncture in the rubber between it and the manifold) that the ECU gets to the idle RPM fuel cut point (about 1200 to 1400 rpm). When the ECU sees the throttle as closed BUT the RPM crosses that threshold, fuel gets cut until the RPM falls to 700-800 at which point it gets reinstated.. and that causes your oscillation.

~Paul K.

 

AND

With the CTS out of the coolant, it will default to "rich side" of the scale. It is designed to "sense" liquid, not air.

Also, sometimes?, trimming the ECU idle fuel "pot" once he engine is at operating temp, WILL remove that surge, ONLY if all the other things are eliminated FIRST.

 

Definitely seems like a vacuum leak. I put my hand over the trumpet on the driver's side, and the idle decreases but, the engine doesn't die. I hear hissing from the front of the engine but, I can't find the source. There is virtually no suction from the passenger side trumpet (in fact, it almost feels as if it is blowing air out). It may be on that side.

 

Wild guess from Downunder.

Passenger and Drivers side means very little if we dont know RHD or LHD, hence A Bank, B Bank, or RH side, LH side, is better used, as viewed sitting in the car facing forward.

I know it could be seen as picky, but little things all help us help you.

I will assume LHD, just coz I can??

The B Bank (LH) can and does have a greater suction. This due to the AAV drawing its air supply from that side. As dows the PCV valve.

If you have a hissing at the front, have a GOOD look at the plug for the timing chain ratchet hole. On the A Bank (RH) side in the timing cover, and just below the cylinder head joint. Easily seen in my opinion. They generally leak copious amounts of oil, but with the PCV creating a small Vac inside the engine, maybe its sucking air there.
Another suspect would be Cruise bellows, OK, in the V more than the front, but they do leak vac, well documented in the archives.

 

Ugh! I am about to put this car up for sale. I have reached the end of my patience with it. I have been all over, I don't see any vacuum leak. The AAV and hose is new. I put in new gaskets between the airboxes and the throttle bodies. I have checked everywhere. Could it possibly be a bad smog pump?! I literally have no other ideas. Car is a 1989 XJS Collection Rouge US car.

 

Definitely believe it is a vacuum leak. AAV is new. I removed the hoses, replaced the top. No holes or leaks present.

 

Starting to replace hoses. Still haven't found the leak. Bought a vacuum tester/pump. Going to try that to see if I can track down the leak.

 

Right about now you need to do a smoke test.
Make a smoke generator (simple to do, instructions online), introduce smoke into the intake system under slight pressure (car NOT running), and watch where it comes out.
And There's your leak.
(';')

 

If you have the fuel vapour capture carbon cylinder/purge stuff on the car, then remove where it attached to the B bank manifold and block it off to test.
Have you capped off all the many little spigots all over each manifold including the large one under A bank throttle body that is the vac source for the cruise control. Capping them off is important as the hoses may be cracked.

 

You list two XJS in your forum profile and have three threads going on engine issues. This XJS Whack-A-Mole one and:

XJS V-12 Pesky Intermittent Engine Miss
https://www.jaguarforums.com/forum/x...e-miss-265366/

Vacuum Leak - 1989 XJS V-12
https://www.jaguarforums.com/forum/x...v-12-a-265472/


On a read through, I'm left confused about whether you have issues with one or both vehicles. This needs some clarity or the same set of knowledgeable members will be responding to each thread.

Graham

 

The idle issue seems to have started after you adjusted the throttle stops. Go back there, make sure the butterflies are tight on a 0.002" feeler gauge when closed, with the linkages all connected.