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Hesitates/stumbles when hot

R

RyleyF

NAXJA Forum User
Location
Boone, NC
I just got a 91 Cherokee and on my last road trip to the mountains discovered that when it gets hot it likes to stumble and hesitate, sometimes pretty badly when I try to accelerate, especially up a hill and with some people in the back, in all gears.

Before I got it it had been sitting and had a bad fuel pump, so all that's new, along with the filter, plugs and wires, and distributor cap and so now it generally runs great, except after a few hours of driving, when it starts bucking if I try to push it past 2500 rpms.

I cleaned out the IAC which was a bit dirty and made it run better, but I don't know for sure. Could the fuel filter be clogged since it sat for so long? It seems like it's not getting enough fuel since I had an 89 that had a dead fuel pump and it stumbled like that, but at the same time, it was hard to start with a bad pump and this one starts up every single time.
 
CPS cuts spark, right? It feels like it's not getting enough fuel, not missing completely, as if it's just not making any power.
 
I had a issue with the factory wire splicing going to the fuel injectors acting up when it got worm. Yet this threw a engine code to me. Different years so your may not throw a code.
 
what youre describing sounds a lot like fuel starvation.

Really sounds like the fuel pressure regulator might be jammed with varnish, or have a vacuum leak.
 
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My jeep will do that sometimes when hot, its the plug for the CPS. When it starts up, I'll unplug the CPS and put dielectric grease on the plug - the problem will go away for 3-6 months or so. It won't happen when the engine is cold, though.
 
i thought the same thing you did, it was my CPS.
 
Onkover said:
I had a issue with the factory wire splicing going to the fuel injectors acting up when it got worm. Yet this threw a engine code to me. Different years so your may not throw a code.
Click to expand...
I had exactly the same problem with a 93. One bad splice, and I hope you didn't go through as many other sensors and tests and mis-diagnoses as I did before I figured it out! On that one, the voltage was enough to pass on an injector tester, but not enough to fire the ****ing injector! It also threw a code, since the problem was electrical. Non-electrical injector problems won't, though, at least in those years. Later models might get a misfire code (which is triggered by CPS/rpm calculations), but I don't think the 91 has that.
 
92DripCherokee said:
what youre describing sounds a lot like fuel starvation.

Really sounds like the fuel pressure regulator might be jammed with varnish, or have a vacuum leak.
Click to expand...

It idles just fine and will drive fine up until about 2200 rpms or earlier if the engine is under a lot of load which makes me think fuel starvation as well. This kinda makes me think it's just not getting enough fuel when it needs it.

Correct me if I'm wrong, but fuel starvation problems will show themselves under high load/higher rpms moreso than a spark issue, correct?
 
Matthew Currie said:
I had exactly the same problem with a 93. One bad splice, and I hope you didn't go through as many other sensors and tests and mis-diagnoses as I did before I figured it out! On that one, the voltage was enough to pass on an injector tester, but not enough to fire the ****ing injector! It also threw a code, since the problem was electrical. Non-electrical injector problems won't, though, at least in those years. Later models might get a misfire code (which is triggered by CPS/rpm calculations), but I don't think the 91 has that.
Click to expand...

Your the one that helped me figure it out. I would say that I did go through more wire, connections, sensors and everything else to piss off the POPE!! (Thanks for pointing me in the right direction, it works great now)

CPS cuts spark, right? It feels like it's not getting enough fuel, not missing completely, as if it's just not making any power.
Click to expand...

When my splicing was goofed up it was doing just as you say. The lack of power and spitting and sputtering. When my CPS was going out it would cut out quickly and fire itself right back up just within 2 secs. These 4.0s don't demonstrate a major bucking when one cylinder misfires like a v-6 does. I'm not saying to not check the CPS connection.

Could the fuel filter be clogged since it sat for so long?
Click to expand...

IF it sat for a long time and you just ran the old gas that was in the tank.. yes your new filter would plug up.
 
Well I just drove it today after I got home, and 4 miles down the road I try to accelerate up a hill and it felt like the RPMs dropped or I let off the gas, except it's a manual and I know for sure the clutch isn't slipping. I'll get another fuel filter tomorrow and check it out.
 
see if you can determine if it is indeed load, or 2200 rpm problem.
(level grade, 3rd gear maybe?)
I know we covered tach, and you said CPS plug is bypassed, but mine cut out at exactly 2800 rpms load independent, consistantly.
just chiming in again...
 
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cygnus58 said:
see if you can determine if it is indeed load, or 2200 rpm problem.
(level grade, 3rd gear maybe?)
I know we covered tach, and you said CPS plug is bypassed, but mine cut out at exactly 2800 rpms load independent, consistantly.
just chiming in again...
Click to expand...

The 2200 rpm figure is just basically where it starts losing power on flat ground, no matter what gear I'm in. It sucks because if I'm cruising in 5th gear and need to pass someone or go up a hill, the engine bogs down if I press the accelerator down without downshifting. I can't do anything about it either since it'll do the same thing if I downshift. New filter goes in tomorrow, I hope that'll do it.
 
i got a 91 that had been sittin for a few months, that was doin the same thing and after two fuel pumps, 3 fuel filters, and the whole tune up and always the same thing it would run fine for 20min then the boggin would start. finally i took the fuel tank out cleaned it (and wat was in the bottom of the tank was unbelievable)and replaced the filter again and this has been 6 months ago and no problems, it might help might not.
 
Put in a new filter today, drove great for about 15 mins and then started doing it again.

I checked the distributor cap, no cracks, in fact, it looks pretty damn good because its brand new, but is that little black carbon button that goes to the coil wire supposed to be spring loaded? I was checking it out and if it is, then it's definitely jammed up.
 
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