Isuzu 3.2l Severe Power Loss

Hi all. Just joined this community as it seems very active and I always liked listening to Car Talk on the radio. Looking for some advice on a 96 Isuzu Trooper with a 3.2l SOHC V6 and manual transmission.

Recently it has lost a dramatic amount of power, so the truck has gone from pretty slow to barely drivable. It cranks up instantly and idles perfectly. Everything feels normal taking off from a stop, except power does not increase as the engine revs, it might even decrease. Between idle and maybe 1500rpm it feels torquey and responsive, then just bogs down past that. Therefore it takes extended wide open throttle just to get to 45mph and I cannot maintain speed up inclines in any gear. Getting to 60 takes about 50 seconds on a flat surface. The Torque ODB app calculated 47 peak horsepower. There is also a rhythmic surging sensation under full throttle when it’s hot.

Other than having barely any power, I cannot find indication of a problem. No check engine light or codes. Dry compression is 150-165psi on all cylinders. Spark plugs look great. Fuel trims are normal not indicating a rich or lean condition. Just replaced the fuel filter yesterday. No obvious misfiring. Vacuum readings are normal and if I rev it to 2500 it stays there no problem, which I think rules out a clogged cat. I ran it with the MAF, MAP, Intake Air Temp sensors, and the EGR all disconnected individually with no improvement. It seems like poor acceleration would have to be an air or fuel supply issue, but I think my troubleshooting has ruled out both. Is there anything I’m missing here? This really has me stumped as the truck just isn’t showing any other signs of a problem. Thanks in advance for any advice!

What’s your fuel pressure at idle ? and at 2500 RPM ?

maybe your getting vapor lock. just a thought

I haven’t measured pressure yet, I’ll need to put a tee in. But ODB reading of fuel trims are around 0 all the time, I think I weak pump would cause positive readings but not sure. Vapor lock wouldn’t happen on a cold engine, right?

I’d say the fuel pump is OK based on that. I’d also say your catalytic convertor is clogged based on what you describe. 2 ways to confirm…

  1. Install a pressure gauge in the exhaust in front of the cat, tape it to your windshield, accelerate and watch the reading. Anything over about 1-2 psi is a plugged cat OR

  2. Unbolt the front of the cat and run the car “open pipes”… if the acceleration has returned, plugged cat.


To test for a clogged cat, you can also unscrew the pre cat oxygen sensor and tie it up out of the way. Might be easier than unbolting the exhaust from the manifold.

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I think I’ll order a exhaust pressure gauge to test with, Amazon has one next-day delivery. I tried driving with an o2 sensor removed, but I was only able to remove one from one bank and it was so absurdly loud I couldn’t do much “road testing” that way.

That is all you need if the Isuzu has a single exhaust and a single cat. The pressure gauge screws into that O2 hole… That’s what @Scrapyard-John was trying to point out. Makes a good pressure test point.

Offhand, sounds like an exhaust restriction. You state that you used a vacuum gauge but did not state what the gauge needle was doing.

At idle the gauge needle on a good system should show 19 to 21 inches with the needle being rock steady.
With the throttle opened and closed quickly the needle should drop to zero instantly.
After that, if the needle returns slowly to normal or if the needle drops back to a bit higher than normal before slowly returning to normal then that could be a sign of a plugged exhaust.


As for the loudness, pretend you’re one of those guys that likes that sort of thing for a few minutes :stuck_out_tongue_closed_eyes:. “Custom exhaust”.

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Got the back pressure gauge and hooked it up. Looks like I do indeed have an exhaust restriction. Here’s the gauge at idle and 2500rpm. I’m pretty sure it’s the cat, as I can hear a slight rattle if I hit it with my hand. If I can get the post-cat O2 sensor unseized I’ll take another measurement there to rule out the muffler, but that seems like a low probability. Looks like I’m about to spend the whole value of this truck on the repair! :upside_down_face:

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Just to wrap up this thread, I got the catalytic converter replaced and the problem is completely fixed. It’s back to its clattery and slow-but-perfectly-usable self lol. Also turned out to be much cheaper than I expected, only avoid $300 to weld in a new one. Thanks for the help!


Perfect! Thanks for the update. It’s always nice to hear what the resolution was.