Running Rich
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Joined: Aug 2011
Posts: 43,971
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From: Prescott, Az
Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
Robert, the stock injectors were crap and prone to external leaking causing fires. there are later model injectors which are an upgrade and can be justified in the name of safety. Programbo here on CF has them.
CF Veteran
Joined: Aug 2011
Posts: 10,489
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From: Nor-Cal Coast
Year: 90,84
Model: Cherokee
Engine: 4.0,2.5
A friend was driving. I HAD had a slow leak there. It would seem it got low enough that the friction of the flexing rubber got the tire hot enough to simply give it up.
Last edited by DFlintstone; Aug 2, 2012 at 10:44 AM.
CF Veteran
Joined: Aug 2011
Posts: 10,489
Likes: 24
From: Nor-Cal Coast
Year: 90,84
Model: Cherokee
Engine: 4.0,2.5
If the 02 heater wire in that harness shorted to an injector wire it might hold it on.
Last edited by DFlintstone; Aug 2, 2012 at 10:45 AM.
Thread Starter
Junior Member
Joined: Jul 2012
Posts: 99
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From: Succasunna NJ
Year: 1988
Model: Cherokee
Engine: 4.0
Update: I rented a fuel pressure tester. Took the vacuum line off the fpr and it jumped up 10 PSI, so seems like that is ok.. It is running at 32 PSI at idle. One thing I did notice that did not make sense to me.. Honestly, I was expecting a fast bleed down due to a leaking injector(s) but it was actually the opposite. When I turned the engine off, the PSI actually climbed to 39 PSI. It's been 45 minutes and the pressure has not bled down at all, still at 39 PSI.. In your experience, is this normal?
Thread Starter
Junior Member
Joined: Jul 2012
Posts: 99
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From: Succasunna NJ
Year: 1988
Model: Cherokee
Engine: 4.0
Another update: After about an hour it started to bleed down to about 32 PSI. I brought it up to temp and it started to hesitate when I gave it throttle but immediately went back to normal. I turned it off and back on 3 times and it started normally. The 4th time it wouldn't fire until I held the throttle open to cut the fuel and then it ran normal. It had that "out of timing smell" to it. The fuel system seems to be ok. Do you think I should focus on timing now?
Thread Starter
Junior Member
Joined: Jul 2012
Posts: 99
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From: Succasunna NJ
Year: 1988
Model: Cherokee
Engine: 4.0
I circled back and tested the CPS. It is showing 248 ohms and 3.63 V when I crank it. The 248 ohms seems to be within spec however, the 3.63 seems low, if I am testing it correctly.. I've seem other postings indicate I should be testing AC on the volt meter, which doesn't make sense to me..
That would be about right. The CPS has 5Vdc applied to it, which it switches on and off as the flywheel rotates. That switching chops the applied 5Vdc into to a 2-4 volt AC reading on your meter, depending on how quickly the engine is rotating. The higher the RPM, the higher the voltage reading you'll get.
What is actually there is a square wave, alternating between 5V and 0V, but your meter will display the average as an AC voltage.
What is actually there is a square wave, alternating between 5V and 0V, but your meter will display the average as an AC voltage.
Thread Starter
Junior Member
Joined: Jul 2012
Posts: 99
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From: Succasunna NJ
Year: 1988
Model: Cherokee
Engine: 4.0
I did the AC test- it's reading a .1, should be .5 according to the specs. I have replaced the CPS already but I have a feeling it is too far away from the fly wheel. I'm going to see if I can push it up to the fly wheel and re test the reading. Thanks!
Senior Member
Joined: Aug 2011
Posts: 760
Likes: 6
From: Landers, CA
Year: Several
Model: Cherokee
Engine: 4.0
All this assuming it's the injectors. 3 pages of "expert opinions?"
If the ignition doesn't light the fire, it will get wet inside!
No mention except the first post about spark or coil. How did you test the coil? An OHMETER won't tell if it's working or not.
CHECK THE SPARK AT A PLUG = WHILE IT WON'T START! It must be a crispy BLUE spark, yellow, red etc won't light the fire needed.
If the ignition doesn't light the fire, it will get wet inside!
No mention except the first post about spark or coil. How did you test the coil? An OHMETER won't tell if it's working or not.
CHECK THE SPARK AT A PLUG = WHILE IT WON'T START! It must be a crispy BLUE spark, yellow, red etc won't light the fire needed.
Thread Starter
Junior Member
Joined: Jul 2012
Posts: 99
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From: Succasunna NJ
Year: 1988
Model: Cherokee
Engine: 4.0
All this assuming it's the injectors. 3 pages of "expert opinions?"
If the ignition doesn't light the fire, it will get wet inside!
No mention except the first post about spark or coil. How did you test the coil? An OHMETER won't tell if it's working or not.
CHECK THE SPARK AT A PLUG = WHILE IT WON'T START! It must be a crispy BLUE spark, yellow, red etc won't light the fire needed.
If the ignition doesn't light the fire, it will get wet inside!
No mention except the first post about spark or coil. How did you test the coil? An OHMETER won't tell if it's working or not.
CHECK THE SPARK AT A PLUG = WHILE IT WON'T START! It must be a crispy BLUE spark, yellow, red etc won't light the fire needed.
Yes 0.1 is too low. There are some posts around here about moving it closer to the flywheel to increase the voltage.
Thread Starter
Junior Member
Joined: Jul 2012
Posts: 99
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From: Succasunna NJ
Year: 1988
Model: Cherokee
Engine: 4.0
No, it doesn't start while the CPS is unplugged. I read somewhere that unplugging the CPS resets the ECU. Oh well, anyhow, I was able to recreate the no start condition and pulled one of the plugs. It had plenty of spark but definitely not blue. It was white and yellowish.


