Vacuum pressure on '86 2.8L --- TOTALLY STUMPED

This usually indicates some type of restriction in the exhaust system. I would check the converter and also make sure the muffler hasn't come apart inside.

Chris

You're still thinking backwards. Vacuum is low because it runs like shit. It doesn't run like shit because vacuum is low. You need to find out why it's running so bad . Is it a 2SE or E2SE carb? Have you checked cam timing?

You're wast>I'M TOTALLY STUMPED with the vacuum output on 2.8L GM 6 in an XJ

Its a @ 2SE (non-Kalyfornia)

Cam timing is Correct

Yup, I know (now) that chang> You're still thinking backwards. Vacuum is low because it runs like shit. It

As someone else mentioned, a plugged up catalytic converter can cause your symptoms. It does happen, the guts can come loose blocking the passages.

The only way I know to test for it is to unhook the exhaust before the cat or at the manifold and see if the vacuum comes up.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Rich Hampel wrote:

Mike Romain did pass the time by typing:

You can put a pressure gauge in the O2 sensor hole and some of the old farts would say, real jeeps don't have catalytic converters. :)

Good, that eliminates a problem with electronic controls.

If you say so....

Two or three posts have mentioned restricted exhaust now, and you haven't responded to them. Have you checked this? A *symptom* of plugged exhaust would be low vacuum. This would cause a couple of problems.

1) If you can't get the all of the burned gas out, you can't get a full charge of fuel/air in for the next cycle. 2) That vehicle is equipped with a back pressure EGR valve. Too much back pressure could cause too much EGR action. Too much exhaust gas in the combustion chamber displaces too much of the air and fuel needed for decent combustion.

Once again, have you checked for restricted exhaust?

Can you put a number on the expected or acceptable back pressure (pressure ahead of the cat) for the 4.0 and/or the 2.5? I've seen lots of talk about excessive back pressure, but just what constitutes excessive??

Up here in Canada, an 86 does not need a cat to pass emissions. Well... I isn't 'looked' for on the visual test, but if it won't pass, then a cat might help it....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Now you're making me go out in the cold to look. I'm having difficulty absorbing the idea that I have a vacuum in both manifolds - that makes the engine one big sink sucking both ways .

I would suspect that the PCV purge could easily be accomplished > Any pressure. There is actually a vacuum in the header, and many Hot

I don't have a number for it. I don't know if there is one. Most that I see are pretty obvious just by the symptoms. The marginal ones can be picked up using a vacuum gauge.