Misfire.... No power

[da9r00d]

i have a 1988 XJ, it misfires when ive been driving for long distances. i drives great for shirt drives. at normal 210-220 temps after long drives it starts to misfire and the XJ has no power to accelerate. some times is dies out. just shuts off. i can start it back up but it starts and revs to about 2k then goes to 1k then off. if i wait 5min it starts fine but still misfires. ignition is all new CPS is new, Mustang injectors. just about every sensor on the jeep is new. i just had the Head bolts torqued and the timing chain replaced. it was misfiring before i had the work done. it actually ran better for a bit then started misfiring again.

im also looking for a cheap Renix scan tool to try and diagnose this. any one know if Jeep dealers would have the scanners?

[pvt.tadpolxj]

I have had similar issue's. What i found is one of the wire's inside my plug to the coolant temp. sensor was broken down inside the plug itself. I just stripped off a little insul. and pushed it down inside there real good, and pushed a wooden tooth-pic in behind it all the way and snipped it off at the plug. This made a realo big improvment. I hope this help's...:driving:

[JENSSEN]

Bypass your fuel pump resistor on the D/S front inner fender,.. small china looking thing, tie the two wires together , tape them up and go for a test drive

http://www.cherokeeforum.com/attachments/f2/5548d1251478336t-90-cherokke-cranks-but-wont-start-ballast-resistor.jpg

[4.3LXJ]

OK, something is worn out. So now to figure out what it is. The first thing to do is figure out if it is fuel or electrical related. What Jensen said is a good start. But I am thinking you have a sensor failure, particularly the O2 sensor. How long since that has been changed?

[da9r00d]

I just replaced the 02 sensor. also for the fuel bypass, im a little confused. just tape the 2 wires together or am i cutting anything?

[4.3LXJ]

I believe that is supposed to be a heated O2 sensor. You might want to check that. You need a good connection there. Any codes off the key switch thing?

[JENSSEN]

I just replaced the 02 sensor. also for the fuel bypass, im a little confused. just tape the 2 wires together or am i cutting anything?

You can cut the wires if you want but I would suggest for diagnosis purposes , just use a scrap wire or spade connector to bypass that ballast resistor. Pretty common failure on old XJ's

[JENSSEN]

Single wire = non-heated o2
multi-wire = Heated o2

but I believe the plugs are different so you cannot mix it up :)

[da9r00d]

the 02 sensor has 3 wires

[JENSSEN]

Did you try the fuel pump wires on the ballast resistor?

[da9r00d]

Jenssen I think your a Genius. I tried it and the XJ runs Great..... i drove it home from work as it misfired. i checked your post, wired it up and drove around for a while with no problems. im gonna drive for a while 2morrow and see what happens. i also put the wired back and it still ran great. ill keep you posted.

[JENSSEN]

Ok so what is happening is the ballast resistor has a hairline crack, when the engine temps in the bay come up the crack gets bigger. So in a perfect world your ballast resistor drops your voltage from 12V to say 8 volts ; when the hairline crack gets bigger due to the heat the voltage drops lower and lower which basically starves your fuel system,... your pressure will keep dropping and dropping. I would also want to bet when your misfire happens your engine temp creeps up pretty damn quick as its basically running really lean.

The ballast resistor is only there because in the past XJ owners complained about noisy pumps,.. lower voltage keeps the pump quiet.

[steph74]

what is the "science" behind this ? anybody knows ?
I mean the ballast resistor has to be here for a reason.... by passing it sure has consequences.... do you know what they are ? just curious as I have read this "fix" in different occasions...

For the O2 sensor, if you have an 88 you may want to check service recalls on that.... my 89 was recalled to change the O2 sensor that had some problems.... I am going to go see a dealership with the recall papers so they check if it is still the original one.... and if it is then BAAAM free O2 sensor ;)

[JENSSEN]

what is the "science" behind this ? anybody knows ?
I mean the ballast resistor has to be here for a reason.... by passing it sure has consequences.... do you know what they are ? just curious as I have read this "fix" in different occasions...

It's to quiet the Fuel pump,.. from owner complaints. My big jeep has been completely bi-passed for years now.

87 and earlier Renix 4.0 XJ's didn't have them at all, sometime in '88 they started voluntarily recalling them and put them in for "excessive fuel pump noise"

If I had more time today I would dig it up for you

[steph74]

I was just curious, and looking for someone to do my homework lol I'll look it up too.
So this should not interfere with gas mileage or stuff like that ?
I have a bit of a rough cold idling and also missfire and backfire especially when I let go the gas pedal.... maybe I should try that and see if it helps a bit....

[JENSSEN]

I was just curious, and looking for someone to do my homework lol I'll look it up too.
So this should not interfere with gas mileage or stuff like that ?
I have a bit of a rough cold idling and also missfire and backfire especially when I let go the gas pedal.... maybe I should try that and see if it helps a bit....

It will not effect mileage,.. regardless of the fuel the pump sends the injectors only deliver so much, the rest get returned to the tank on the return line.

[da9r00d]

i drove it for a couple of hours and still the same problem

[da9r00d]

at first i thought it worked, since i changed the wires and it ran good, today it ran good until it started misfiring after a while

[steph74]

is misfiring and backfiring the same thing ?

[da9r00d]

kinda, but i have a definitive misfire issue

[steph74]

Mine was shutting off sometimes and have absolutely no power (like in second gear, accelerate to 2/3K rpm, the jeep would just not take off and buck and spit...)
This would occur after driving a while and temp being just a little over 210/220
it stopped after I changed the radiator and never really worried about it... So I was thinking that maybe it was one of the injector heat soak thing even if my 89 was not known to be affected....
Could be your problem I don;t know, just throwing a bone in the discussion ;)

i still backfire a little though ')

[JENSSEN]

Well its defiantly not a fuel issue then,.. start looking at your IGN system,...

Take a video of the misfire when it happens and post it

[da9r00d]

ill do that 2morrow, i read that its an issue on later model XJ's with the 3rd Injector, they install a heat wrap

[da9r00d]

so i think its was my "Air Charge sensor" or MAT. i cleaned it and drove it all day with no problems... (as of now).

@ Jenssen: my XJ turns off suddenly while ive been driving a while, but it rarely happens. it shuts off but i can still start it back up. it revs and then dies out. will using the bypass help fix this. i already installed a new CPS a while back

[JENSSEN]

so i think its was my "Air Charge sensor" or MAT. i cleaned it and drove it all day with no problems... (as of now).

@ Jenssen: my XJ turns off suddenly while ive been driving a while, but it rarely happens. it shuts off but i can still start it back up. it revs and then dies out. will using the bypass help fix this. i already installed a new CPS a while back

Don't think so, I had a simlar issue but it was a bad egr that did it to me

[steph74]

Don't think so, I had a simlar issue but it was a bad egr that did it to me

yep that too... I took all the vaccuum lines from the EGR, and the EGR valve itself... cleaned them by running a wire through.... incredible how much stuff I removed... then I sprayed the EGR and made sure that the valve was moving freely (not sure it is a good idea but I watched a couple of video showing they did it)... ever since it seemed to solve my erratic idle ...
But I don;t think that the EGR would provoke the symptoms he sees (but what do I know eh, I am still a learning maggot)

MAP sensor is a good candidate too


Isn't it fun trying to diagnose a rennix engine ?

[JENSSEN]

Renix motors are a different beast,.. The no start Comanche was running and starting rather easily, now it's back to no starts and I am back to basics on the no start again,.. Ahh well atleast the others arn't giving me grief