mntn-biker

Going to replace the plugs on my 2000 XJ 4.0L (160,000 miles)this weekend and am debating whether to replace the coil rail while I have it off. It occasionally throws a P0300, 0303, & 0304 code (not all at the same time) but yet it runs fine and does not overheat nor does it look like a milkshake under the oil cap or on the dipstick. I was told it might be the cracked head syndrome but I am not having any other tell tale signs of this so I thought I would start with new plugs for certain. So...my question is, when the coil goes bad, does it just not work or could it start going bad and not create as strong of a spark throwing a code? I plan to start with the plugs and if they look real bad, I will probably hold off on the coil before dropping a Benjamin on 1. If the plugs don't look that bad, then this is where I will need everyone's advice on when a coil goes bad, what the symptoms might be.

Thanks - Mike

Turbo X_J

Why not do a compression test first? Then pump up the cooling system to 16 psi & see if it dumps coolant where it is not supposed to be? Are you planning on reinstalling the spark plugs if they don't "look real bad"?, if so please don't, new plugs are cheap.
In the end, no sense in wasting money on anything if it really needs a new cylinder head and/or motor.

mikesignal

I would not replace the coil rail until you know it is the problem. The 2000 and 2001's with the california emmisions are known to get misfires on a hot start, or even sometimes just sitting at a red light in the hot weather. Change the plugs, clear the codes, and take notice when/if the check engine light comes on, and if it stays on or goes off.

mntn-biker

My plan this weekend is to change the plugs (with new Champion OEM copper cores), inspect the boots (& hope for no cracks), apply some di-electric grease in the boots, put it together, and keep my fingers crossed. The guy at the parts store said that if the plugs haven't been changed in a while and the boots are dry (or cracked) that this could also cause a P0300 (random/multiple cylinder misfire). We will start with the plugs.

My mechanic also said that the fuel injectors could probably use a fuel injector service cleaning (I ran some SeaFoam thru last tank). Might try that since my guess is it's never been done.

salad

There is a TSB from Chrysler on heat soak in the fuel injectors, I believe cyls 3 and 4 are affected. If you don't see any heat wrap on them you can take it get to get it done.

Spark plugs should be Champion RC12ECC or NGK ZFR5N gapped to 0.035"

mntn-biker

Those Champions are the ones I just picked up.

I think I will do an online search for that TSB prior to contacting a Jeep dealer as #3 and #4 injectors do not have the heat wrap. Thanks for that tip!

salad

Sure. While you're at it a compression test is always a good thing. Especially on a 0331 with misfires. Never hurts to be sure

mntn-biker

Good point. I will have to see if I can get my hands on a loaner to check the compression and I will do a search on the forum to see exactly how to do the check. Seems like the injectors need to be disconnected or something so fuel doesn't spray into the combustion chambers???

mikesignal

I bought some "heat sheath protective sleeve" from advance auto parts, cut it to fit the injectors and the fuel rail, and secured it with wire ties. I will post a pic if i can find it.

salad

Yep. Here's what Chrysler put in the FSM:

CYLINDER COMPRESSION PRESSURE TEST
The results of a cylinder compression pressure test can be utilized to diagnose several engine malfunctions.

Ensure the battery is completely charged and the engine starter motor is in good operating condition. Otherwise the indicated compression pressures may not be valid for diagnosis purposes.

(1) Clean the spark plug recesses with compressed air.
(2) Remove the spark plugs.
(3) Secure the throttle in the wide-open position.
(4) Disable the fuel system. (Refer to Group 14, Fuel System for the correct procedure)
(5) Disconnect the ignition coil.
(6) Insert a compression pressure gauge and rotate the engine with the engine starter motor for three revolutions.
(7) Record the compression pressure on the 3rd revolution. Continue the test for the remaining cylinders.

CYLINDER COMPRESSION PRESSURE
Pressure Range: 827 to 1,034 kPa (120 to 150 psi)
Max. Variation Between Cylinders: 206 kPa (30 psi)

DISABLING THE FUEL SYSTEM
Remove the ASD and Fuel Pump relays from the PDC. There's a diagram under the cover as to which is which.