hi guys. wondered if you guys might have some ideas to help me solve a strange problem with my '91 cherokee.
I have been going through it since I bought it in dec. and have done A TON to it but, I still get this erratic shudder randomly at idle that seems to smooth out but not completely go away with increased RPMs. the computer throws no codes for a misfire or anything presently. it was throwing a code 14 for the map sensor, I changed the sensor, but then discovered a vacuum leak and repaired it.no new codes since. the previous owner had the head freshly rebuilt right before I bought the Jeep. I discovered that the previous owner had recently installed new spark plugs and wires but hadn't gapped the plugs correctly. so, that was remedied with new champion double platinum plugs and correct gap, and new premium BWD wires were installed as an upgrade/precaution as the wires looked cheap and I didn't trust them.
that all did help some but I still have the erratic "misfire" but you can't see it on the tach! you can just feel it through the frame. I have also recently changed my injectors, and cleaned my IAC valve as well as the throttle body. the previous owner also just installed a new dist cap but I haven't looked to see what condition it's in on the contacts, or if he even replaced the rotor. I am planning on upgrading the coil soon, and it got me wondering if the stock coil is just weak or going bad as the plugs and wires and gap all seemed to make a difference but, not completely solve it. HELP! so frustrating! getting expensive too at this rate.
Jason
ALSO.... has anybody tried a throttle body spacer? if so what did you think? I have been looking at the air raid 310-511 on Summits site, I want more power/mileage. I am trying to get as much MPG as possible out of my jeep, so I can justify keeping it over my Grand Prix! thanks.

 

Well, I would fisrt visually check the cap/ rotor, then buy a $7 tool to stress test the coil.

 

Try different plugs. I've heard the double platinum plugs don't always agree with the 4.0 motor.

 

Agreed. I don't know much about the plugs in themselves but I had a similar problem to yours as mine felt a bit rough at idle, but when i drove it was fine. Replaced the plugs and wires and I was good to go. Good luck

 

Your 91 is an OBDI vehicle, so you won't get any misfire codes. Check for vacuum leaks and make sure the firing order is correct.

 

1. Verify distributor cap/rotor. Better yet, replace them. Chasing a driveability symptom without 100% confidence in tuneup hardware can be an exercise in "chasing your tail".

2. Champion copper plugs are actually the best for 84-99. Copper is actually a better conductor than platinum. Platinum is harder and can give you extended intervals before changing, but with the 4.0........run copper plugs, properly gapped, and change them every 30k.

 

ok, so I replaced the cap and rotor and it DID have a significant amount of carbon buildup. that said, it did help a bit but, it was still doing it, so i bit the bullet and bought a new IAC valve(piece of mind knowing it's a new part i can trust as I didn't fully trust the old one anyway). that helped as well but, the problem is still there. so, I guess it's time to warranty out my plugs for the copper ones?. thanks guys. Jason

 

yep, definitly switch to copper core, these motors dont like plat.

 

check the injectors

 

Personally, I have never liked any Champion plug.

I have had quality control and early failure problems with them.

I have NGK Platinum plugs in my '92 4.0 HO and it idles and runs as smooth as silk. So does my '87.

 

Cheromanche. I just replaced all the injectors with a precision rebuilt flow matched set from Precision Injectors about a week ago. they make a difference and my jeep has more power and starts easier(2nd crank most times) but, didn't solve the issue.
I was planning on upgrading my ignition coil anyway, and have it ordered a 42,000 volt Accel coil so, it'll be obvious when the new coil is in whether or not that was the issue. I suspect it strongly at this point. as i have done almost everything short of replacing the entire distributor, or computer...............

 

check or replace your tps and mat sensor and coolant temp sensor too.

 

have you checked your fuel pressure? is the check valve holding pressure like it should?

 

on the throttle body spacer(TBS)...

I found when I was a grease monkey in a lube shop that the net effect of a TBS depend entirely on whether you believe it will help before you put it on. Some folks swore it netted 1-2 mpg while others said it had no effect, but in most all cases the attitude of the car owner at the beginning seemed to make all the difference.

My recommendation if you believe or just want to believe do it. If not don't.

on the Grand Prix is it super charged?

 

Did somebody mention throttlebody spacers?