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Really bad misfire

C

Chuck B

NAXJA Forum User
Location
Fredericksburg, VA
2000 4.0L AW4

I started it up about 15 minutes ago and it immediately started having a really bad misfire, MIL came on and started flashing and it wouldn't rev over around 2750 RPM.
It ran fine the last time I drove it.
I pulled the codes and unsurprisingly got P300, P301, P302, P303, P304.
My question is, what could be the problem and where should I start looking to find it?

I've already searched and couldn't find anything helpful.
 
Have you done any work on it recently or is this something that just popped up? Sorry I can't be of more help, hopefully someone more knowledgeable will be along soon...
Good luck
 
Has it started running normally again? If it doesn't, it could be a lot of things... fuel pump going south, fuel pressure regulator, fuel filter, injectors clogged, bad gas, some sort of electrical problem, timing chain jumped a few teeth... not sure really.

Did you just fill up? Anything else stand out? Check the tailpipe for obstruction?

Doesn't sound like a check valve issue to me unless it stops misfiring after a little while. Also doesn't sound like heat soak.
 
It had been sitting about two days before I started it so I am fairly certain it is not heat soak and I haven't done any work to it in quite a while but I did fill it up and drive it about 25 miles right before I last parked it a couple days ago with no problems.
 
One other thing I just thought of is that it has been making some valvetrain noise that I have assumed was a stuck lifter. I don't know if that might have anything to do with it.
 
I'd pull the spark plugs first and take a look. You might need new ones or one of them may have crapped out. The color and condition will give you a lot of information.

While they are out, you may as well check the compression. You want to verify the condition of the insides. Kind of like an early colonoscopy for your engine mechanical condition.

If all is good, synch the cam position sensor using the "toothpick method".
 
disconnect the fuel line and let the pump cycle on&off a couple times. Catch the fuel in a glass container and let it settle for a couple hours. You'll be able to see a layer of H2O separate if it's water in the gas-- definitely possible based on the symptoms.
 
Any coolant in your oil?

You can rent a tester for the coolant at most parts stores, you have to pay for the chemicals, then check your coolant for combustion byproducts.
 
A flashing check engine light indicates a serious mifire, which if not corrected, can damage the catalytic converter. You need to resolve this ASAP.

So assuming that it continues to run rough with the check engine light and that it hasn't cleared itself?

A few things that come to mind include a vacuum leak, a flaky crankshaft position sensor and a fuel delivery problem. Are you using any coolant/reservoir level okay?

If it were mine, first thing I would do would be to install a fuel pressure gauge on the schrader valve on the fuel rail. For yours, you are looking for 49.2 psi at idle, plus or minus 5. Fuel delivery should be ruled in or out very early in this troubleshooting process.

Here is a good article that deals with the "random" misfire (which P0300 is).

http://www.random-misfire.com/
 
I figured out what's wrong.
That's the exhaust valve pushrod on the #2 cylinder.

IMG_0088.jpg


This is the intake pushrod from the #2 cylinder.

IMG_0090.jpg
 
Anvil, ball peen hammer, good as new!
 
wow... that just might do it! :shocked:

Guess it's time to start looking for a head, or a long block if you don't feel up to replacing the head yourself.
 
The dislodged exhaust pushrod is perfectly straight and is just sitting in there like that. it would seem that the intake pushrod bent because the exhaust valve didn't open and the intake vale tried to open against the pressure in the cylinder after combustion.

I have done compression and leak-down tests and both returned normal results.

I removed all the rocker arms and pushrods and cleaned all the gunk out of there and I can't find any visible damage to the head. Should I still replace the head?

Also, there is no coolant in the oil, oil in the coolant, bubbles in the radiator, or any other sign of a bad head gasket so I'm fairly sure it's good.
 
What does the lubed area of the rockers and fulcrums look like? It looks to me like a lubrication failure due to all the sludge. That engine is probably on its last legs.
 
The oil pressure has been a little low but nothing alarming 30ish on the interstate turning 2200RPM and just above 10 at idle; and the rockers and fulcrums look fine aside from tons of sludge.
 
To tell you the truth I would source a couple pushrods and put it back together.

Also, I would recommend you IMMEDIATELY start using a diesel motor oil, and change it at 3 months or 3000 miles RELIGIOUSLY.

Do a lot of cold start-short trips in that XJ? What thermostat are you running?

While I have seen a lot worse, your engine reminds me of the 50s running cheap 30w NON-detergent oils.
 
I do tend to make a large number of short trips and the thermostat is a stock 190°.

I use Castrol GTX 10W-30 oil and change it as close to 3000 miles as possible which used to come much more often than three months; though in the past six months I've barely put 1500 miles on it and its at about 2500 miles since the last oil change.

This is the first time since I got it in 2007 that I have had the valve cover off and I did not expect to see that much sludge in there.
 
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