01 Cherokee check engine light

My check engine light has been on for quite some time now. I get codes of evap system minor and evap system major. changed the gas cap already. New plugs, run fuel injector cleaner every tank of gas. I get misfire in cyl. 3.

any fixes to these problems? Is it the charcoal canister or something?

The 4.0 is notorious for exhaust valve problems and cracked heads. It will surely throw the misfire code or a random misfire if you have this problem. Make sure you do a compression check before you invest too much into anything else.

dzjeep said:

The 4.0 is notorious for exhaust valve problems and cracked heads. It will surely throw the misfire code or a random misfire if you have this problem. Make sure you do a compression check before you invest too much into anything else.

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I am not the best mechanic in the world, but i feel i need a professional to look at this. I have only had to replace tires and serp. belt and front brakes and radiator. It has just under 80,000 miles on it. I am only second owner, i love the thing to death, i want to keep it and its paid for. I just worry i will wait too long to get thigs fixed and it will be all downhill after that.

Only the 0331 head is has notoriety! Not ALL 4.0 engines. They have been around a very long time and have a great reputation!
To find out if you have a 0331 head, look at the head on the drivers side somewhere around cylinder 4 on the top surface next to the valve cover. If you see this number, then it is time to have the oil checked for water and the water checked for exhaust gasses and oil.

Check the Evap Leak Detection Pump and the charcoal cannister, inspect the vacuum lines that come and go from them. Look for loose or damaged rubber fitting and split hoses.

Address the misfire with a complete tune up. Test the coil rail. Test the #3 injector.

I agree with Tim's advice. An evap leak can often be as simple as a deteriorated vacuum hose. If you're having trouble finding it, a shop can perform a "smoke test" where smoke is introduced into the evap system followed by a close inspection for escaping smoke which is the fault area.

As for the cylinder #3 misfire, always change the plugs first to see if that resolves. Simple stuff first. Always.

There is also a technical service bulletin for a "heat soak induced" misfire that affects cylinder #3. This may or may not be related to your misfire.

TSB
1803103 - 4.0L ROUGH ENGINE IDLE AFTER RESTART FOLLOWING A HOT SOAK
Date: 09/05/03 (supercedes 1802702 dated 09/06/02)
Model Year(s): 1999-2004

Description: NOTE: This bulletin applies to vehicles equipped with a 4.0L engine. This bulletin involves the installation of a fuel injector insulator sleeve.

Details: Customers may describe a 20 to 30 second rough idle following the restart of a heat soaked engine. This condition may be most noticeable when the engine is restarted following a prior 10 to 20 minute heat soak in hot ambient conditions of approximately 32 C (90ºF) or higher. This condition may be consistent with short city stop-and-go driving trips and can be aggravated by the use of fuel with a high ethanol content. Depending upon various conditions a MIL may occur due to DTC P0303 - Cylinder #3 Misfire.

This condition may be caused by heat from the exhaust manifold that following engine shut down migrates to the area around injector #3 and causes fuel vapor to form within the injector. This in turn may cause a momentary misfire of cylinder #3 until the fuel vapor is cleared of injector #3. The insulator sleeve lowers the injector #3 temperature to a point below which the fuel will not normally vaporize.

If vehicle exhibits a rough idle, and if a misfire of cylinder #3 is observed following a 10 to 20 minute hot soak, perform the Repair Procedure.

Parts required:
56028371AA Ignition Wire Shield

Repair procedure:
1. Cut insulator sleeve, p/n 56028371AA, to make two (2) insulator sleeves about 25-30 mm (1 in.) in length. See diagram.
2. Install one sleeve around injector #3, with the slit on the upward facing side of the injector. Install the other sleeve with the slit on the downward facing side of the injector.
3. Confirm sleeve is flush to intake manifold surface around injector.
4. Check injector #3 wire and ensure that the injector is rotated to a 2 o'clock position.


TSB for the cold start rough idle and P0303 code: TSB 18-010-01

Excessive use of fuel injector cleaner can also cause heat soak misfires due to lowering the boiling point of the fuel. Check for a cracked charcoal cannister. I had that on my Crown Vic. That caused a lean mixture.