I'm really beyond frustrated with my '96 4.0 HO Cherokee. It was in awful mechanical condition when I bought it six months ago. Since I've had it I did a full tune up including an ignition coil then a new coolant temp sensor, exhaust manifold, muffler, IAC, fuel pump and sending unit...and multiple other things in an effort to get it to a reliable daily driver status. A lot of effort and learning, (and help from all of you) and a few cans of seafoam finally left me with a pretty smooth, miss-free idle. It was good for about a month and now I seem to be back to square one with the rough idle and misfires. The parts store told me misfires on cylinders 1 and 4, so I cleaned everything off, and I replaced the plugs in those cylinders with champion plugs to eliminate that. I checked all the plugs, they didn't seem to be overly fouled, and they are getting spark. It ran without missing (as long as the AC is off) for a day, but this morning it sounds like garbage again. I'm going to get the battery and alternator tests this afternoon, but I also know it's running rich...the exhaust smells like gas and my mileage is terrible. SO, all that said, I'm going to check the O2 sensors, and move the 1 and 4 injectors to 2 and 3 and have it checked for misfire again to eliminate the injectors. Sorry for the long post, but I guess more info is better...can you guys think of anything else???? Thanks in advance!

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You ever try reseting the computer after you did all this? It may just need to be reset

 

Yeah, several times......

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Bump

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1. Verify fuel pressure. It isn't impossible to have a faulty regulator leaving you with too much pressure. For a 96, you should have 49 psi at idle, plus or minus 5 psi. This test MUST be done with a gauge. Anything less is guesswork. Many parts stores will rent you this tool.

2. Test your map sensor. It is involved in fuel/air ratio. Be darn sure that the vacuum line leading to the map sensor is pulling good vacuum.

3. Replace your oxygen sensors if you are unsure as to their age. These sensors have a finite lifespan of between 100k and 150k, depending on efficiency of the engine

4. A compression test of all cylinders is never a bad idea. You should have 120-150 psi, with no more than a 30 psi variation between cylinders.

5. Swapping injectors is a very good idea. If the misfire follows, you know what injector(s) are bad.

 

You're the man TJ! Thanks for the help. Now if you can tell me how to get my scorpion woman wife to stop busting my ***** about how much time I spend with my jeep...

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Sorry, not much help with the wife! I guess it goes with the territory when we own Jeeps??!!

Good luck and check back and let us know how you're doing. We'll figure this out.

 

Will do...I'm going to start with the new O2 sensors and go from there, they look OE and its at 154k...thanks again

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Does it matter if I don't change both O2 sensors at the same time? I ordered them today, but from 2 different places, and I want to get the upstream sensor in asap...

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dont matter, the upstream one is the only one that will control how it runs, the down stream just monitors the cat.

 

Just an update, I replaced the O2 sensors last night, not much change in how it ran but I'm hoping my fuel economy improves...the old ones were black on the ends, garbage. I pulled the TB and cleaned it, and pulled the fuel rail and moved my 2 suspect injectors. I cleaned the rail and injectors with Seafoam, then did the Seafoam treatment through the TB. The idle is 10x better now. Still hearing an occasional miss though. On to testing fuel pressure and doing a compression test.

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Some of the elbows on the vacuum lines are looking rough. Anyone know a place to get a set?

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Pull your rocker cover off and check all your valve springs are at the same height, this will let you know if any valve are blocked open which will cause misfires.

 

Yeah I'm planning on pulling it off anyway to change the valve cover gasket. I'm going to seafoam everything underneath while I'm at it. Thanks, I wouldn't have know to look for that

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Sooooo.... My Jeep's got 203k on it, and I don't think the O2 has ever been replaced... How much are they?