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A very interesting problem...

hornet98908

NAXJA Forum User
Location
Acworth, Ga
Alright i have a 1993 XJ with the 4.0L the 4 speed automatic transmission. at around 3k rpm it sounds like a cylinder is misfiring. this problem does not happen in first gear but it does in all subsequent gears. you can feel the loss of power too. i have new spark plugs (properly gaped), new spark plug wires, new distributor cap and rotating part inside cap, new fuel injectors from precisionautoinjcetors.com (very pleased with them, excellent customer service). all of these additions are in response to the misfiring i am experiencing (in other words the problem happened before i installed these new parts)

any suggestion would be greatly appreciated.
 
CPKS, fuel pressure, bad cat. One of those is your problem. May have needed the other stuff, but was a waste of money for this issue.

PEOPLE, Quit throwing money at problems. Diagnose THEN fix.
 
Any check engine light on or have you ever seen one? If so, pull stored codes for guidance.

I think I too would start with fuel pressure testing, and if that checks out, vacuum testing with a vacuum gauge can help diagnose a cat issue. Compression testing of all of the cylinders is never a bad idea and is a good "snapshot" of the internals.
 
Check for play in your distributor.
 
thanks guys for the input...
i have not seen a check engine light on and i have just today gone to autozone to have them pull the codes however their computers only do ford and Chevy so i struck out there...
i will check into the vacuum and fuel pressure ideas...
my next thought was also the fuel pressure. is there a place in the fuel system to check the pressure? and what is CPKS?
 
On the fuel injection rail you'll find a valve with a cap on it. You can check the pressure there. It should be 31 with engine running, and stay at that level throughout normal range. If you take the vacuum line off the regulator on the fuel rail, it should go up to about 39. The gauge needs to have the correct fitting. You can buy the kit for fairly reasonable money if you don't know anyone you can borrow it from. If you're really cheap and have stuff lying around including a gauge that goes to 60 pounds or so, you can cob up your own using the fitting from an old R-12 air conditioning fill hose.

I consider a vacuum gauge one of the first things you should use when you have unidentified engine problems. It will immediately identify mechanical issues. Although it's not a final arbiter of ignition-versus-fuel injection problems, I've found that usually an ignition problem will cause the gauge to fluctuate but a bad injector will not. Here's a bit of what else you can see with a vacuum gauge:

http://autospeed.com/cms/article.html?&A=2393

For a really stubborn and intermittent problem, put your gauge on a long hose, snake it out of the engine compartment and tape it to your side-view mirror, where you can monitor it as you drive.

Your 93 has the Mopar OBDI system which means you can get your own codes with the key. Ignition key on, off, on, off, then on for good: you will then get your codes flashed by the check engine light. Each code has two digits. "12" is a common one for recent battery disconnect, and sometimes appears spontaneously, and "55" is the end of sequence code that tells you the system is reading right. All others are significant. Naxja member 5-90 has a nice list of the codes HERE.
 
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