So, I rescued a 1999 Jeep Cherokee 2wd from a field in south texas. It's in amazing shape, for the most part.

I dropped a new battery in it on site, and fired it up. It drove home on the old gas, without a tune-up, and purred like a kitten.

When I got it home I filled it with fresh gas (Sadly, 10% ethanol) and dumped a can of seafoam in on top, just in case. I also changed the plugs, rotor, cap and wires. Yes, I checked the manual to insure the wiring was correct, and it matched the old wiring.

I used dielectric grease on the fittings, and changed the oil, oil filter, and air filter.

not ten miles after all this TLC, it started misfiring, and the engine light came on.

I double checked the gap. I confirmed that the plugs I was using were factory standard.

The misfire only occurs when the engine is -not- under load. When accelerating, or driving over 70mph, it doesn't misfire.

take your foot off the gas, and it starts misfiring.

It also doesn't misfire when it's accelerating.

I'm inclined to think it's fuel related, because it ran fine at first, and -then- started misbehaving after I drove it for a while.

I have since learned that it was a police package vehicle. I don't know if that's relevant, but I figured I'd include it.

 

The strategy for when you see the check engine light is ALWAYS the same. Retrieve the codes.

For a 99, you'll need either a scan tool or a code reader. Or bring your XJ to a parts store like Autozone where they read codes for free.

You must use the clues that codes provide. Anything less is pure guesswork. Post the exact code numbers here (not a description) for comment. With OBD, the devil is definitely in the details.

 

I did take it to a parts store, and the code result was 'engine misfire cylinder 4'. I did not see the actual code. I will, however see if I cannot get a more detailed answer.

 

A dielectric material is a substance that is a poor conductor of electricity.

 

I'm originally from coastal Maine. Coastal Maine is a very wet, salty place, year round. Corrosion is a never sleeping monster that eats anything mechanical and electrical. When working on boats, I learned to use DG on any and all electrical contacts, because the salt would eat anything you didn't protect within about six months.

I've never had problems with using DG on plug wires in the past, and more to the point of this problem, the plug wires that were in the jeep when I got it, -before- the misfire problem occured, had DG on them. So, I'm fairly certain the grease isn't the problem.

Product in question-



"Improves electrical performance by reducing arcing, voltage drop and other conditions"

"waterproofs electrical contacts"

 

Noalox

This is better and is conductive.

 

Have you tried swapping that injector?

 

I have not. do you mean replacing it with a new one? or simply moving it to a different cylinder?

 

For plug wires? Most plug wire sets include a small packet of dielectric grease right in the box.

 

For a single cylinder misfire on a '99, this is how I would proceed. Best to proceed from simplest to more complex. Always.

1. Plug. Swap it or replace it, even if it's new. I've seen brand new plugs with tiny cracks in the insulator (probably from being dropped) that create misfires.

2. Plug wire. Same strategy. New doesn't mean it can't fail.

3. Distributor cap. See above.

4. Fuel injector. Swap injector from cylinder 4 with a different cylinder and see if the misfire follows

5. Compression test. Never a bad idea on a "new-to-you" vehicle. The compression spec on the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders

 

Yeah, but that's only because it's the correct thing to use.

 

The dielectric grease is supposed to be used on the plug insulator (boot), to make pulling the wires easier, while electrically insulating the connection between boot and plug base, so as not to have a problem with arcing. Dielectric grease is to be used by coating the wire connections AFTER the attachment is made when used to deter corrosion. It IS a poor conductor by design.

 

curious too to see an actual code...
i throw an intermittent PO206, all new shyt as well

quite possibly a wiring to 'puter problem, once in a while just not grounding when required

 

The code for a cyl #4 misfire will be P0304. A P0206 code is for a fault with cyl #6 injector or injector circuit.