Topic: Diagnosed the cause of a misfire condition, need advise

[mack90]

I just purchased a 98 jeep se, and it ran beautifully when I test drove it.  Two days after I bought the jeep, it developed a misfire that was causing poor performance intermitantly.  I thought it was an ignition problem, so I changed the plugs, cap, rotor, and wires.  The problem still popped up here and there.  I was thinking a sticking valve that was masked by the exhaust leak.  Did a compression check, perfect.  Determined the misfire was the 3rd cylinder by pulling spark plug wires off while it was running.  Cylinder three was firing fine, compression tested fine.  I then went to the injectors.  While cylinder three was misfiring, pulled the connection from injector with no change.  Put a volt meter on the connection, and was reading .2 to .5 volts.  Tested the adjacent cylinder, 1.7 to 2ish volts.  Traced the wiring to the third cylinder injector, looks fine. 

Here is where it gets interesting.  I can make the #3 injector fire or not fire by putting pressure on the connections on the jeep computer.  I don't have a wiring diagram to determine which of the three connections would be related to this, but I can't put a little pressure on any of the three connections to lose or gain cylinder three.  Do I need to replace the computer?  Do you think there is an internal connection that is bad in the computer?  Where do I go from here?

Thanks in Advance,
Mack

[neale_rs]

I would suggest cleaning the connector before doing anything else.  Look for green corrosion on it and the pins.

[mack90]

That is good advise.  The jeep has absolutely no corrosion on the contacts.  It looks new. 

[neale_rs]

One good test would be to borrow a computer that works and see if it has the same symptoms.  If it does then it has to be wiring or connectors, if it does not then it is most likely the computer.  You can get used ones on ebay fairly cheap so that may be another option, it's good to have a spare anyway.

[mack90]

That is a good idea.  What year computers are compatible with a 98 5speed?

[neale_rs]

Sometimes you can get good information from the web pages of companies that sell remanufactured PCMs.  Here are a few:


http://www.autocomputerexchange.com/proddetail.asp?prod=91%2D5jpwrang

http://www.autoecms.com/?gclid=CPaXqOzv-I4CFQtYgAodqwvjwg

http://www.ecudirect.com/index.asp

Good luck!

[mack90]

I picked up a remanufactured car computer and it fixed the problem.  This 4 banger actually out accelerates my rubicon up to 70mph.  Thanks very much for the help!!!!

[neale_rs]

Wow, that was good and quick.  Glad it worked out well.