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Cherokee Lean-Surging Problem

Saturday, October 23rd, 2004
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Status: FIXED!

This was by far the most irritating problem I've had with this car. The car was very mushy; it had no power and generally ran like shit. You hold the accelerator steady (with clutch in) and the RPMs bounced around. It was sometimes difficult to clutch out (the RPMs would suddenly drop when loaded) and when driving at any speed if you step on the gas the just didn't go anywhere. It struggled to keep up 35 MPH on a flat road.

This is how the car sounded when idling (at the exhaust pipe): exhaust.wav (142 KB)

Original Diagnosis - Vacuum Leak

Right from the get-go this problem seemed like a vacuum leak. All sorts of things on this car depend on vacuum to operate properly: power brakes, 2/4wd selection, climate controls and a bunch of other stuff you'd never think of. What confirmed this theory was spraying something flammable near the vacuum hoses at the intake manifold. I shot the hoses with brakleen and all of a sudden the engine would speed up and sound much better for a few seconds. Obviously the leaking hoses would suck the flammable brakleen in and it would make the fuel/air mixture a little richer. Even though the car did (and still does) have a little vacuum leak, it wasn't the root of the current problem

Secondary Diagnosis - Ignition

It was about time for new spark plugs anyway. I bought a new set of plugs, wires and a new distributor cap and rotor. Wasn't the problem.

Tertiary Diagnosis - Exhaust Restriction

exhaust bypass

Cat basically disconnected from exhaust pipe.

If you can't exhale as fast as you inhale, you're not going to be able to run a mile right? At the time the catalytic converter on the car was falling apart and making noise, so perhaps it broke up and clocked the pipe. So I unbolted the cat from the exhaust pipe and bolted it back on to the opposite side. This way the exhaust gas would completely bypass the cat and muffler. I went for a really loud spin around the block, and found the problem was still there.

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Final Diagnosis - MAT Sensor

toasted

At room temperature this should be reading 3500 Ohms.

Over the couple months I was trying to fix this problem I came across a sensor that wasn't reading correctly. It was a temperature sensor mounted in the intake manifold that monitors the air temperature and adjusts the fuel mixture accordingly; the colder the air the richer the mix. Even though I found this problem pretty early on, I was unable to find a replacement. The problem was that everyone had their own name for this part, some (Jeep) called it a MAT sensor and some (Advance Autoparts) called it an air charge temperature sensor. I was finally able to find a replacement and the car was better instantly.

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