P0403 misfire, need some input.

Follow along, i'll try to make this short.

Few weeks ago i got a call at Midnight from one of my friends, i'm his mechanic. It's an 00 4.0L XJ and it would start and die. I had my hands full that weekend so he had it towed to a shop. They found sand in the fuel tank, dropped, cleaned the tank and replaced the fuel pump and plugs.

Two days later he calls me, he has a CEL that comes up P0403, cyl misfire. My best guess was sand made it to an injector, so we bought new injectors from Five-O-Motorsports. I wiped the ECU during replacement. Jeep idled in driveway for 45 minutes and a few rides around the block, no codes. It popped a CEL on the way home, again P0403.

Now, while it was here i noticed his coolant was low again, i always check and top off fluids whenever i service a rig. It was a gallon low, just as it was the last time it was here about a month prior. I suspect a head gasket or cracked 0331 head, walked around to the tailpipe, white smoke and water drip every few seconds.. smells sweet like coolant. No evidence of water in oil.

I'm wondering if water in cylinder would throw a CEL basically steam cleaning the plug? Since this is an 0331 head whats the chance it's just a haedgasket vs a cracked head? Where do i get a cylinder head if i need one? Is there an upgrade head that less prone to cracking?

Am i missing anything?

Inspect the plugs. Test the coolant for combustion gasses. Do a leak down test.

As far as head gasket versus cracked head, you won't know until it is torn down and checked. 0331 head, expect the worst. If the head is Ok, and you replace the head gasket, then you have probably just put a time bomb back on top of the engine.

Alabama Cylinder Head is a source for new, claim that the heads are an "improved" 0331 casting.

Have the original 0331 head tested for cracks before you reinstall it or get an 0331 from a 2002+ 4.0L. By 2002 production run the 0331 problem is supposed to fixed.

Alabama's heads are stronger castings. By far and away the better choice.

just an FYI,the revised head is supposed to have "TUPY" cast between cyls 3&4 under the valve cover.

And u meant P0303 right?

mrtosh said:

And u meant P0303 right?

Click to expand...

I did.. :laugh:

You have all the classic symptoms of the cracked head. Could be the gasket but my money is on the head.

The most common spot for cracking on an 0331 head is between cylinders 3-4. There is a casting flaw there. Many have been able to visually see the crack(s) by removing the valve cover. Worth a try.

Clearwater Cylinder Head and Alabama Cylinder Head are good sources for replacements with a beefed casting. Or go with something 2003 or newer (Wrangler/Grand) which is when Chrysler started sourcing better heads. As mentioned, these are stamped with the word TUPY inside the head somewhere.

Test to be sure, as Joe mentioned.....But even if it was the gasket, I would replace the head while going through the work of removing it if you are planning on keeping the vehicle.

Thanks again guys.

I dont have a leakdown tester so i'll pull the valve cover and see if it's a visable crack, otherwise i'll pull the head.

I'm betting on a crack at the #3 exhaust valve. I'll know tomorrow if we have to order a head.

i totally missed the 403/303 mixup...lol!