QuestionQUESTION: My 1987 XJ6 loses power on hills or above 40 mph. So far, I've changed the air flow meter, renewed the vacuum to distributor, changed fuel pump, cleaned both fuel tanks, blew out fuel supply lines to engine, changed change-over valve, changed fuel filter, checked pressure regulator, reset timing, checked coil.
ANSWER: Hi Carl,
Remove one of the hex plugs on the exhaust manifold and find a rubber plug to plug the hole and drill a hole in the rubber plug the size of a piece of pipe like a piece of brake line and put your vacuum gauge on the pipe (if your vacuum gauge has a fuel pressure section on it. If not get a low pressure gauge for it.) You don't need to heat the engine up (which would burn your rubber plug) Just start the engine and rev it up several times and tell me what pressure you get at high RPM. Or on a quick rev up.
The symptom you have could be a partially stopped up exhaust system. It should not go over 1.5 PSI or 2 PSI on a quick rev up.
Howard
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QUESTION: Hi Howard, Thanks for your quick response. I did as you suggested , using a vacuum gauge, and a quick rev of a cold engine produced an odd result: the gauge ran backwards, about 5 psi is my estimate.I checked the gauge on a vacuum line so I assume the gauge is telling me that I have aproximately 5 psi positive pressure? Does this means a partially blocked exhaust system, as you suggested?
Additional information: When the engine is first started, I don't have a problem, but as the engine warms, "flooring it" to get passing gear results in an almost total loss of power and quickly degrades to a stall, with no re-start until it cools down.
Your help is greatly appreciated...Thank you
Carl De Mott
ANSWER: Hi Carl,
It is a indication that you have an exhaust problem although a quick rev up does have a lot of air in the exhaust to move out of the way. You should rev it up and hold it up to see what the pressure will stabilize at. It should not stay above 1.5 PSI (positive pressure) Sometimes you will get the high pressure on a first quick rev up but then hold the RPM high for a couple of seconds to see if it drops to 1.5 or below. If it does not drop that far, it is an indication of a partial stopped exhaust. The final test is to loosen the first CAT from the manifold and put some sheet metal around the area so as not to burn anything. Some mechanics just drop the CAT down and wire it so as not to drop too far if the hanger is not holding it. You need about at least a 1/4 inch gap so as to allow enough exhaust to escape. Then drive the car under load to see if that is the problem 100%. One mechanic said he just removed both hex caps on the exhaust manifold and that allowed enough to escape and he just put a piece of sheet metal in to shield every thing from the exhaust. He claimed that worked too but I never tried it.
The vacuum gauge going to high pressure (3 to 7 PSI) is the clue and the open exhaust at the manifold is the 100% test.
The trouble is that you have two CATs in series and either or both could be stopped up. The front CAT can usually be looked at when it is off to see if the manifold end is burned and they usually look bad when they are bad. Looking in the manifold end you should see a clean honeycomb if it is good. And you can put a small light in the other end and see through the honeycomb. One XJ-6 I had looked good but the test of dropping the CAT down proved a stopped up exhaust. We found that he had stored the car in a barn for a year and mice or squirrels had filled the rear resonators with nuts and straw.
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QUESTION: Hi Howard, I just held the rev's high for the count of five; the positive psi still showed well above 1.5 psi (about 3 to 4 psi). You are a lifesaver!! We bought the Jag from a lady last July, and I have been trying to get it to run right ever since...by the way, she had the car stored for a long period, and we have plenty of squirrels! I am going to wait to verify 100% until the temperature warms up, it is 32 degrees and snowing now (we live in Oregon).I'll let you know the outcome.
Thank you again for sharing your knowledge with us.
sincerely, Carl %26 Audrey De Mott
AnswerThat sounds like what it is, but do the test that opens the exhaust up front to be sure. I ran the exhaust back pressure test on many British cars and some domestic cars and I never found a car that could handle more than 1.5 PSI. GM claimed that their cars can handle up to 3 PSI but I never tested a GM car. If you are able to show 3 + PSI just free running You will see higher than 5 to 7 under load. You can prove this by having someone hold the foot brake hard and put the car in gear and run the RPM up. This should not be done for more than a few seconds but you may be surprised at the pressure readings. They should not be higher than 1.5 PSI even under load.
Another indicator is to put your vacuum gauge on the intake manifold and run the hose out from under the hood and put the gauge under a wiper arm so you can view it from inside and read vacuum then take the car out on the road and slowly apply throttle and watch the gauge. The vacuum will drop but will stabilize but must not drop below 5 in hg even under full throttle and full load.
A stopped up exhaust system will have the vacuum go to zero. The trouble with using the vacuum on the intake for a diagnosis of a stopped up exhaust is that other things can cause loss of vacuum. But exhaust back pressure is a 100% good test.
Howard
