I thought I had cured the hesitation in my engine by replacing the fuel filter but it returned today. It doesnt occur everytime just every once in a while while in low RPM and hitting the gas hard. I have an auto tranny so when I step on the gas the engine shakes a little and sometime coughs like it want to backfire.

Any ideas?

probably wrong, but mine did that when my distributor rotor and cap was corroded to hell, fixed it for me, but noone else said they had that problem.

hope this helps.

Does the 91 and newer YJ use a typical distributor cap? I thought the EFI engines didnt use the typical set up with rotor and all that.

Also, the vehicle idles fine, runs fine, just hesitates some times when accelerating with from low RPM and going up hill.

Originally posted by Jeeptoy91
Does the 91 and newer YJ use a typical distributor cap? I thought the EFI engines didnt use the typical set up with rotor and all that.

Also, the vehicle idles fine, runs fine, just hesitates some times when accelerating with from low RPM and going up hill.
These are all basic questions that could be answered in 5 mins with a service manual. You do have one of those don't you? Yes, it's got a distributor cap, a rotor, and spark plugs. When the rotor and/or cap corrode or the plug gap is excessive you'll have these symptons.

I do not have a factory service manual. I know it has a distributor and plugs:rolleyes: What I didnt know was if the distributor operated the same way as the older ones did where you had to clean them out and replace the rotor when they became worn.

I have been using a Haynes manual to service the Jeep all these years and never had an engine problem. This is the first problem of it's type I have had with a 4.0. I have an XJ with over 220k miles and never had this issue.

Thanks for the help. I read in the Haynes manual that it actually could be the pressure regulator for the fuel. I am going to do some testing today.

You're saying you've never replaced a cap and rotor on the XJ in 220k miles :confused: All the 4.0 engines I've owned have definitely needed it more often than that.
My wife's Cherokee had run for 50k or so with no attention when it started exhibiting the same symptoms yours did. The gap on the plugs had opened to .060 and it stumbled badly on accelleration and at higher RPMs.

As sad as that may sound, no. I replaced the plugs, wires, and did all the maintenance and repairs myself and never changed the rotor and cap. It was done once during a tune up performed by others but I never changed them.

The xj never showed those types of symptoms.

Originally posted by Jeeptoy91
As sad as that may sound, no. I replaced the plugs, wires, and did all the maintenance and repairs myself and never changed the rotor and cap. It was done once during a tune up performed by others but I never changed them.



It's been my experience that changing the cap and rotor make a bigger difference then changing the plugs

Ahhhhhhhh. I have replaced the MAP sensor, distributor, rotor, and fuel filter and still the cursed hesitation. I dont think it's the throttle position sensor since it doesnt display the other symptoms of this going bad (poor low idle and stalls).

I dont know how to check for vacuum leaks. Any suggestions other than taking to the stealership or shop?

Fuel pump? Mine just went south on my 92. It was OK at idle and cruise, but under accelleration or hills it seemed to have no power.

This is literally a brand new pump. I installed the new Bosch pump in January. It is the exact same pump that came with the Jeep.

I dont want to change the stinking pump again unless I know for sure that is the problem. Is there a way to test the output (pressure) of the pump under load?

Couple more things for you to check.

When you installed the fuel filter did ya make sure you installed it with the flow arrow in the right direction? It might be starving under hard acceleration.

The plugs, did you check the gap before installing the new ones?

Did you check and double check the wires to be certain they arent out of firing order?

How's the air filter?

Vacuum leaks you could hear usually. Check for broken plastic and cracked rubber lines.

I havent changed the plugs yet. I am going to do that today. The fuel filter is in correctly. I am sure. I didnt change the cables and I disconnected the cables from the old distributor cap one at a time just to make sure.

Today another Jeeper and I are going to do a fuel pressure test, fuel pressure regulator test, throttle position indicator test and see if we cant find the little pest.

;)

Airfilter is a slightly dirty K&N. Still tons of flow since I can see daylight through the thing :p

Mine's done this twice. Once the catalytic converter had broken up and a chunk of it would blow back to the back of the converter and plug up the exhaust when I stepped on the gas. Another time one of my vacuum lines dropped down and hit the exhaust, which melted it and blocked it off. Good luck.

OK here is the latest:

Replaced fuel filter, dist cap & rotor, MAP sensor, TPI sensor and checked the plugs and fuel pressure. o2 sensor is new.

The problem seems to get worse the longer the vehicle operates. It doesnt see to be affected by the operating temperature of the engine, its something else heating up.

Another Jeeper diagnised the crank position sensor (I ordered a new one) and if not that the fuel pump.

The question I have is can the CPI be tested? Also is it possible for the fuel pump to test perfectly on pressure only to fail when the vehicle is under load and low RPM?