Jaguar Repair: 1985 XJ6, low pressure gauge, vacuum gauge

Question
Ihave just had the head rebuilt and installed. Everything went back together OK, but the engine will only run at higher RPMs and letting up on the gas peddle, it immediately stalls. I may have the vacuum hose on wrong?? Where can I get a Vac system diagram, or do you have any suggestions what else to look for. Thanks in anticipation.

Answer
Hi Gerry,
It is unlikely a vacuum leak that is causing the high RPM problem although it may be the low RPM problem.

Do this, remove one of the two caps on the exhaust manifold and put a low pressure gauge in the hole. I use a vacuum gauge that has a section for fuel pressure on it. Have someone do like a transmission stall test. (foot on the brake and add throttle) Work quick because the exhaust manifold will get hot enough to melt your hose.

Note the exhaust back pressure. Most cars I have tested can handle up to one and a half PSI. If you have a stopped up exhaust it will go extremely high. (5,6,7 + PSI) This is a stopped up exhaust system (most likely the first CAT or second or both.

I have a factory manual on the Series III XJ-6 and it does not have a good hose diagram but I have made up one of my own. I will try to locate it and put it on my web site so you can down load it and print it. It may take me a few days to do this but I will put it on my web site http://mg-tri-jag.net and I will add the page to that address when I locate it.
Howard