Loss of power

Recently my '94 4.0 Auto XJ has been having power problems. It started suddenly, and only seems to move well at low RPM with little pedal input. With lots of pedal input, especially at high RPM, it will actually loose speed. It has also been running hot. (Just below redline unless I run the heater) A few times the check engine light has come on and I've gotten code 51 (IIRC) that indicates a lean condition. It runs and idles smooth, and starts fine, it just acts like it's extremely bogged down.

First I tried cleaning the throttle body hoping that might solve the problem. While I was doing that I noticed a radial crack around the collector in the exhaust manifold. So next I figured that too much exhaust must be escaping before the O2 sensor and therefore screwing up the signal to the computer. Since I've had headers just sitting around waiting to be put in, I decided to replace the exhaust manifold next, hence my last post about the sheared stud.

Well, I got the headers in and everything seems fine, but I still have the power loss. Could this be caused by a bad TPS or MAP/MAT sensor? Clogged cat? The only thing that seems to make sense to me right now is bad gas. Any other suggestions out there? I'm planning on draining the tank tomorrow and trying some new gas unless someone has a better idea. TIA

Rob

You are on the right track with the TPS or MAP. Either can cause what you are experiencing.

I would be checking my exhaust flow. I have seen clogged catalytic converters that caused manifolds to glow red hot, overheating issues, loss of power, lean and ineficent 02 and cat. converter codes. Put your hand behind the tail pipe while someone revs up the engine and feel the exhaust flow. If you can not tell from this then disconnect the pipe before the converter and take it for a drive see if your condition still exists. You have all the symptoms.

Joe

I agree with that bad Cat converter. It sounds like a big restriction in the exhaust. Don't over look the muffler also.

I changed my cat 4 years ago and it was like taking the potato out of the exhaust pipe. I had more power but no more MPG. In the past year I had the muffler rot off and had the muffler & tail pipe replaced. I also noticed more power AND I grained 1 mpg in town. I also did better on the highway.

Since you have a new exhaust manifold/header It might be smart money to replace everything from the manifold back or at least the Cat converter on back.

I had a buddy who had his cat break up while on a trip, he burned out the electronic EGR on his 88 Oldsmobile with the 3800 and burned of some tube for the ERG. He had the new cat installed and fixed the problem. The car at the time had 140,000 miles on it.

I have observed and think it is a good practive to just change the cat out at 100,000 miles. It may still work but it no where near "stock" in performance. Also when you change the Cat install a new Oxygen sensor unless you had changed it just recently.

Once you change the Cat then you can look at the MAP and TPS.

PS: If the cat did frag out then you may have "junk" in the muffler. Might be good to change both.

I was just reminded that I never posted a follow-up to this thread.

I knew I needed a new muffler, but not because of constriction issues. I had broken the exhaust hanger off the t-case cross member and my borla muffler pulled apart at both ends because of the extra stress. Needless to say there wasn't much constriction there. So, since the most common suggestion I got was a plugged cat, I took my XJ to a local exhaust shop to have it checked out. They put it up on the rack and the guy lightly hit the cat with the palm of his hand. There was all sorts of crap sitting in there, so the cat had definitely "fragged out".

I had replaced the cat in '00, but about six months before this issue I'd had the O2 sensor go bad. The guy at the exhaust shop said that having too much unburnt fuel (running rich from a bad O2 sensor) running through the cat will break them up pretty fast. Anyway a new cat and muffler and the XJ's running great.

Rob