91 Cherokee Laredo question serious

stephapie

New Member
hey guys! new to the forum, i'll introduce myself, my name's Stephanie, i just recently bought a 1991 Jeep Cherokee Laredo, my first vehicle.

everything runs fine, EXCEPT this very strange thing that has been happening. okay, let me start at the beginning:

the guy i bought it from said in his ad "shudders going in and out of 3rd gear"... i figured okay maybe not going into overdrive something like that.

i drive 2 1/2 hours to go see this jeep, its decent looking, needs minor body work on some rust that i can handle, i drive it to feel what he's talking about, and it is definently not a "shudder", its a "thump" the best way to describe it is that it "bucks" when you accelerate and decelerate at certain points. NOT just 3rd, does it at all different speeds from 30mph-60mph, but runs fine after you pass that point. it's sporadic though, not a constant gear or speed.

i got the jeep for pretty cheap because my thought at that point was, "i'm going to have to put a tranny in this thing after this winter" so there was no way i was paying what he wanted.

we're trying now that it's home to figure out what it is, we honestly don't think its the tranny. we're leaning more towards engine. tonight, i was driving it and i made a point to look at the TAC while it bucked, and it dropped real quick suddenly a small amount when it happened, which tells me the engine is stopping, shutting off quickly for a moment????

we've checked the spark plugs, changed the thermostat (wasnt reading properly), cleaned out the rust in my engine coolant system and changed the antifreeze, AND (now this one was the most difficult) pulled the fuel tank down and checked the pump and filter and cleaned the filter thinking maybe the engine was "choking".

we've looked at a couple of other things, basically what i'm asking is, of all the jeep users on this site, have ANY of you experienced something like this???? any feedback is appreciated. Thanks!
 
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kowe69

wannabe
Wow! I wish more of my customers paid that much attention to detail! Well I'll admit I'm not that familiar with the quirks that are common to Jeeps, but since you say that the tach drops when it happens, it sounds like it might be maybe the crank sensor or maybe something in the PCM. Just guessing though since I can't look at it personally.
 

stephapie

New Member
thanks our next guess for tomorrow when the sun comes up was the PCM. but we can replace that from a junkyard MUCH cheaper than labor for a tranny, but anyway thanks for the feedback.;)

also, in 1-2 if i bring the RPM up to 3,000 the engine bucks and surges like it has a bad fuel pump or bad coil, but both are good.

we changed the fuel filter, checked the pressure at the pressure rail, its 30 lbs. idling, and 40 lbs. with the regulator disconnected. we checked the mount under the transmission and found both bolts loose, we tightened them (problem still persisted). also changed the coil for a known good one.

in addition to checking the computer we're also going to check the mass airflow sensor tomorrow. will update if anything changes.
 
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gijohn40

too poor to wheel... :(
Location
Layton, Utah
to the best of my knowledge and I could be wrong but there isn't a mass airflow sensor... there is a map sensor which stands for something my tired mind can't remember... but from what you have said it does sound like the crank sensor... a very common trouble with the 4.0 you might as well check that first and buy a spare to keep with you in the glove box...
 

mbryson

.......a few dollars more
Supporting Member
Go to the dealer (or source an OEM one from somewhere) and get a crank position sensor. You'll need one sooner or later anyway. It wouldn't surprise me if that's the problem you're having. I had a '91, and it acted the same way for a while when the crank sensor was going out.
 

Badger

I am the Brute squad
Location
South Salt Lake
as said get an OEM crank sensor from the dealer.it will be more but it will last a life time and not 2 months like the auto parts store ones will.as also stated you will need one sooner or later so just do it.the other thing it could be is the TPS (throttle position sensor) which is a bit tricky to setup when replacing.

since the trans and engine are both computer controlled and the 91 is OBDI .which is the first year both the engine and trans computers wanted to talk to one another.so the issue could be with transmission lock up.the trans torque converter could be locking up on you when its not supposed to or when its supposed to there is an issue causing the motor to lose power.

when it starts to get bogged down if you quickly slap it up to neutral does the motor come back to life?
 

stephapie

New Member
as said get an OEM crank sensor from the dealer.it will be more but it will last a life time and not 2 months like the auto parts store ones will.as also stated you will need one sooner or later so just do it.the other thing it could be is the TPS (throttle position sensor) which is a bit tricky to setup when replacing.

since the trans and engine are both computer controlled and the 91 is OBDI .which is the first year both the engine and trans computers wanted to talk to one another.so the issue could be with transmission lock up.the trans torque converter could be locking up on you when its not supposed to or when its supposed to there is an issue causing the motor to lose power.

when it starts to get bogged down if you quickly slap it up to neutral does the motor come back to life?
when it happens, i hear a loud thump bump and the tac just blips down like 100 rpm less and then comes right back i have a friend who has a 93 cherokee sport and when it goes into overdrive the tac noticeably drops like 900 rpm, this doesn't happen with mine.

it happens randomly at speeds over 35 mph, and usually when i get off the gas and then step back down again.

like i said, also, in 1-2 my friend can drive his all the way to 3,000 rpm and the motor runs clean, on mine when i get to about 2800 rpm the engine starts bucking (may or may not be related)?
 

Bart

Registered User
Location
Arm Utah
I'm guessing CPS also, but you might want to check the cat to see if it's plugged. It should run smooth at 3k rpm.
 

Jared

Formerly DeadEye J
Location
Ogden, UT
My '91 XJ once started running terrible at higher RPM's... the more you pushed the pedal the worse it would run. Turned out to be a cracked hose on the fuel pump inside the gas tank. It was letting fuel leak out of the pressure line, but not enough to drop the fuel pressure unless you were working the engine hard. Took me a LONG time to figure that one out!

Yours does sound a lot like a crankshaft position sensor.

Jared
 
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