Wierd engine/exhaust problem !!

Hey guys.. Before I tell you my problem I just wanted to say I have been on this Forum for many year and you guys always have helped my with the most amazing info.. THANKS !!!

Well Here it is.

She is a 1999 sport 4.0 w/ 92k miles. I did a tune up about one month ago.

I'm having two problems which might be related to eachother.

Problem #1 When I'm highway driving @ 60mph, and I press on the gas to either pass someone or just to excellerate, I get a loud muffled & thumping exhaust sound and a thumping feeling coming from under my feet. The more I press on the gas , the louder and more you can feel in the floor. It sort of sounds what a blocked exhaust might sound like. It drives fine at any speed under 50mph. Also when I hear this sound and get that thumping feeling as I press the gas pass 60mph I noticed a high power loss.

Problem #2 Just started a few days ago when I had the A/C on while stopped at a red light. She started to miss. It would idle smoothly then "PUTT PUTT" then smooth again. Once I turn the A/C off the problem is gone.

At home, parked, I put my hand in front of the exhaust pipe to feel any air restrictions. I noticed the exhaust was exceptionaly HOT! Hotter than normal I think. I couldnt keep my hand then more than 1 second.

Could all this be bad CAT and O2 sensors?
Neither have been changed before..


Thanks Guys !!

I would suspect clogged exhaust or CAT but you can do a vacuum test at the intake to be sure. You can also disconnect the cat, but that involves more work.

You might just take it to a diagnostic shop where they hook it to a computer for testing. Here in California, you could go to a "smog--test only" shop where they test but cannot sell repairs.

Alohabra , thanks for the info..

I have another question. This past weekend I looked at the exhaust to see if I can find any restrictions.. The one thing I did find was a big indent right where the header and the first exhaust pipe joint thogether. It was on the first exhausr pipe right before the flange that connects to the header.

Is this suppose to be there?

Alohabra , thanks for the info..

I have another question. This past weekend I looked at the exhaust to see if I can find any restrictions.. The one thing I did find was a big indent right where the header and the first exhaust pipe joint thogether. It was on the first exhausr pipe right before the flange that connects to the header.

Is this suppose to be there?

That dent is supposed to be there. It is for 4x4 clearance.

Awesome !!

I'm going to perform a vacumm test for restricted exhaust. I'm not too sure how to but this is what I found on the net..

"Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present."

Is this more or less correct or would you have a write up on a how -to.?

Other folks might want to chime in on this one.

But before you chase down the exhaust, you might read up on the crank position sensor and various problem associated with that.

Vacuum test is done with a vacuum gage at the intake manifold.

http://www.gregsengine.com/vacuum.htm
Yes, this is a good article.

The easy thing to do is to compare your reading with another car. It matters where you get vacuum to test from. Get it from an intake manifold source.

Per gregsengine.com article:

Normal Engine Operation
At idling speed, an engine at sea level should show a steady vacuum reading between 17" and 21" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 21" or more.