krackle

Hello everyone, I'm brand new to this site and am hoping ANYONE can help me. Here's the situation:a few days back I bought a jeep cherokee (1996) and have started diggin in to getting it all up to par. Right now if I accelerate hard ( ie flooring it) the rpms bounce at 3500 like it's got a rev limiter. Sometimes it'll jump up to 4000 and do this, but it never increases in speed. It backfires when I do this. The guy I bought it from claims he put a new distributor, plugs, and wires in. So far I have replaced the coil, fuel filter, o2 sensor and tps, but nothing has helped. I've also checked the fuel pressure and it registers 48psi at idle. Additionally, I reset the tv cable (mine has the black push button style) and still no change in the issue. It has no downstream o2 sensor because the guy before me cut the cat out, this throws a o2 engine code. The jeep also keeps throwing the engine misfire codes.

Something else: this only starts to act up after the jeeps been driven about 10-15 minutes.

Can anyone help provide some insight for me.. I'm trying to not just keep throwing parts at it

Thank you for any help!

67 GMC

It looks like you've tried the most obvious things first which is good. My attention would be drawn to that O2 sensor. I had similar issues with a bad O2 sensor (but in a Renix system) and it would go in and out of closed loop when driving. The O2 was kinda working but not properly. This resulted in a lack of power, backfire when I "punched" it to get on the highway since it was basically messing up the fuel injection pulse timing, width etc as it tried to recalculated based on the crazy O2 readings. I changed the sensor, and everything went away.


Had a bad fuel pump a few years ago that was also the cause of some lack of power but if you have pressure and have changed the filter, probably not that. Did you check fuel pressure when revving the engine to see if it held or went down low?


Can't think of too much else at this point. Sorry.

XJmike0122

I had a timing belt in a 4 banger Wragler changed once and afterwords it had the same exact symptoms. Seemed like it had a rev limiter at 3500-4000 rpms threw all the gears. It was a standard. Took it to another shop turned out the timing was off. I doubt this would be your problem but just throwing it out there.

Fred/N0AZZ

Removal of the CAT can cause some of these issues and its illegal in any state to remove them, usually with a hefty fine if caught.


I would start with replacing the exhaust system from header back if it were mine so that the complete exhaust fits properly to the hangers and is up high so it doesn't drag on anything. Then replace the sensor where it goes and hook it up reset the ecm unit, start up see if that didn't take of it. If not at least your exhaust system is legal and it will run better than before.

cruiser54

I'd test the fuel pressure before proceeding any further.

Her's a vacuum test for clogged exhaust also:

CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION

Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

krackle

Hello all
First of all, thank you for the responses. I appreciate the time and dedication you all have
So here we go. Today I bought a vacuum tester and borrowed a fuel pressure tester. The vacuum tester came out bouncing between 12 and 15. It didn't sit smoothly, it bounced between the two numbers very jittery. Does anyone know why it'd do this? I checked all the vacuum lines and couldn't find any cracks or issues.

The fuel pressure gauge came out at 50 psi. When I shut the jeep off it immediately fell below 20 psi. I'm told this is bad. I tested it with a split and the rail holds pressure (so it's not fuel injectors I think).

All this leads me to believe it's the fuel pump. My 96 is the type that has the fuel pressure regulator right in top of the fuel pump, so they're getting replaced together.

cruiser54

http://www.classictruckshop.com/club...ts/vac/uum.htm

67 GMC

Great site. It has a printable PDF as well. I'm printing that out to go with my vacuum gauge. Thanks for posting.