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Stuttering/Misfiring under light load

R

Rushin

NAXJA Forum User
Location
Minneapolis, MN
Decided i will post a separate thread on this issue.

Vehicle detail: 1990 Cherokee, 150k, aw4.

Issue: Jeep stutters when pressing lightly on the gas while driving. It happens in every gear. Its apparent when accelerating lightly the truck feels like its pushing back and i can hear after fire in the exhaust. It seems like its around 2000 rpm. If i keep the truck in 1-2 gear and lightly accelerate i can make it misfire very violently from about 2k to 4k rpm. If i press on the gas harder the misfire is hardly noticeable it almost skips it.

What I have done so far: New TPS, adjusted and spot on. Replaced plugs, wires, rotor, cap, stator, o2 sensor. Got a coil and icm from a junk yard, still stutters the same.

Other modifications: New pacesetter header (old header had a crack), new magnaflow cat, used borla catback. Obviously new intake/exhaust gasket. All the CCV system lines are brand new. I checked every ground cleaned it and sprayed with anti-corrosion spray. Replaced braided ground with nice think insulated one. Cleaned pretty much every connector and filled with dialectic(sp?) grease.

The distributor rotor does not have much play in it, probably a degree or too, so i don't think the gear is warn.

I tried to check the timing with the light but i could not see anything.

What is the proper way to check timing with a timing light on Renix jeeps? Or is there even a way to do it?

This has been a huge pain for me to figure out. I am on a verge of bringing it to a shop but i know they will either charge me 500 dollars for diagnosis or will not figure out whats going on.

What scan tool does the shop have to have to be able to diagnose Renix?
 
Rushin said:
Decided i will post a separate thread on this issue.

Vehicle detail: 1990 Cherokee, 150k, aw4.

Issue: Jeep stutters when pressing lightly on the gas while driving. It happens in every gear. Its apparent when accelerating lightly the truck feels like its pushing back and i can hear after fire in the exhaust. It seems like its around 2000 rpm. If i keep the truck in 1-2 gear and lightly accelerate i can make it misfire very violently from about 2k to 4k rpm. If i press on the gas harder the misfire is hardly noticeable it almost skips it.

What I have done so far: New TPS, adjusted and spot on. Replaced plugs, wires, rotor, cap, stator, o2 sensor. Got a coil and icm from a junk yard, still stutters the same. Compression is consistent, on a low side but all cylinders are within 10% of each other.

Other modifications: New pacesetter header (old header had a crack), new magnaflow cat, used borla catback. Obviously new intake/exhaust gasket. All the CCV system lines are brand new. I checked every ground cleaned it and sprayed with anti-corrosion spray. Replaced braided ground with nice think insulated one. Cleaned pretty much every connector and filled with dialectic(sp?) grease.

The distributor rotor does not have much play in it, probably a degree or too, so i don't think the gear is warn.

I tried to check the timing with the light but i could not see anything.

What is the proper way to check timing with a timing light on Renix jeeps? Or is there even a way to do it?

This has been a huge pain for me to figure out. I am on a verge of bringing it to a shop but i know they will either charge me 500 dollars for diagnosis or will not figure out whats going on.

What scan tool does the shop have to have to be able to diagnose Renix?
Click to expand...
 
don't remember, i had a mechanic do it and he said it was normal. I remember for sure that it was not alarming at all. I think it was somewhere around 150ish...
 
line it up on top dead center and check where the distributor rotor is pointed. It should be pointed between #1 and #5. You could be a tooth off on on distributor indexing. My MJ did exactly the same thing until I got the distributor indexing straightened out...
 
I went to the local dealer and talk to the guy who has been a tech there for 35 years. He told me its the distributor. Apparently there is a 25 year old tsb that shows how to reindex the distributor. Apparently it has to be redrilled. He has a special tool that he made out of an old distribute cap that he uses to reindex the distributor. He gave me an oem original distributor cap and rotor, he todo me it will fix the issue for a short time but it will come back again soon. The issue is that the spark jumps from one cyl to another. At least thats what he told me is happening.
 
You need a DRB/DRBII/DRBIII scanner--good luck getting one of those. The DRBIII runs over $5,000.

A Snap-On MT2500 "brick" with the right cartridge and cable can read Renix--eBay, expect to pay in the neighborhood of $250 and UP.

You could have a worn timing chain giving you fits. Those suggetions on reindexing the distributor would be a good thing to try.

Is there ANY side to side play in the distributor? If yes, how much? Distributor wobble is going to screw you up.
 
joe_peters said:
You need a DRB/DRBII/DRBIII scanner--good luck getting one of those. The DRBIII runs over $5,000.

A Snap-On MT2500 "brick" with the right cartridge and cable can read Renix--eBay, expect to pay in the neighborhood of $250 and UP.

You could have a worn timing chain giving you fits. Those suggetions on reindexing the distributor would be a good thing to try.

Is there ANY side to side play in the distributor? If yes, how much? Distributor wobble is going to screw you up.
Click to expand...

Dealer has the original chrysler tool to read Renix.

There is some play when i move the rotor by hand, very little no more than like a degree or two. I will record a video of me moving it tomorrow.
 
What make and part # spark plugs are you using? I hear Bosch Platinum causes missfires.

Have you check each Sensor ground wire independently for less than one ohm to ground while wiggling the harness and wires?

CPS signal and its connections?

Have you checked the O2 sensor with a meter, 12-14 volts to the sensor heater, and 1-4 volts, sweep signal on the O2 sensor wires? Have you checked for shorted O2 sensor wires (on Exh manifold), Did you use the right "renix" O2 sensor, or did they give you an HO 91 model O2 sensor? They are entirely different.

Have you checked the fuel injectors and wires / connectors for a problem? Sticking fuel injector?
 
Ecomike said:
What make and part # spark plugs are you using? I hear Bosch Platinum causes missfires.

I use OEM Champion Plugs.

Have you check each Sensor ground wire independently for less than one ohm to ground while wiggling the harness and wires?

All grounds are cleaned, the ones that looked old and corroded I replaced.

CPS signal and its connections?

New CPS and CAP sensor, both reading to spec.

Have you checked the O2 sensor with a meter, 12-14 volts to the sensor heater, and 1-4 volts, sweep signal on the O2 sensor wires? Have you checked for shorted O2 sensor wires (on Exh manifold), Did you use the right "renix" O2 sensor, or did they give you an HO 91 model O2 sensor? They are entirely different.

Got a new o2 sensor for Renix.

Have you checked the fuel injectors and wires / connectors for a problem? Sticking fuel injector?
New sending unit with pump, new filter, injectors were soaked and cleaned.
Click to expand...



The issue is for sure ignition related. I btw, got the 1988 TSB for improperly indexed distributor. Apparently the issue affected other models models and years of Jeeps. The fix is pretty much a hack and only Chrysler could come up with something like that.

The fix involves cutting i window in an old Dist cap, so you can see the rotor inside the cap. then you have to remove the Distributor from the engine entirely, set the timing correctly by setting cyl one at TDC and oil pump gear and the damper so they all light up right. Then you have cut the dist hold down tab off, put the dist back into the engine, put the cap with window on the dist, and move the dist housing so the rotor is .22mm away from the cyl one post. Then you just tighten it all up and put the new cap on.

I guess the issue is that at light load at 1700rpm the spark jumps to cyl 2 when it only should go to cyl 1 causing a misfire. The tsb has a description of the issue and it is spot on to what i have.

Apparently new cap and rotor fixes the problem but only for a short time because carbon trail will cause the spark to jump again.
 
That "hack" TSB you describe fixed alot of Jeeps in it's day when I was service manager at the local dealership. Don't discount it. It really corrects the injector's sequential firing not the ignition firing. Just do it. Another thing to check before going to the trouble of indexing the distributor is to check the stator in your distributor. If it is bad, the Renix system wil default into batch firing the injectors , causing driveability problems.
 
What CPS spec voltage is it meeting?
 
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