I have a 2002 Jaguar X type. I am getting a P0300 Random Cylinder Misfire Code. This misfire and an accompanying Cruise Not Available is occurring once every 5 miles or so. It happens more frequently when not under load (either idling or when I remove my foot from the gas to decelerate).

I have done the following:

1) replaced all 6 spark plugs with Autolite Dbl Platinum ( I noticed some improvement in driveability when I did this though the problem still occurs).
2) checked all hoses and connections for air intake leaks (found none).
3) ordered replacement ring gaskets for the upper and lower intake tuning valves off of the exhaust manifold as while I did not see or hear a leak they are the original and all the forums say good to replace them.

What is the most likely prognosis?

1) Air leak that I have not found?
2) Faulty Mass Air Flow Sensor?
3) Faulty Ignition coil?
4) Faulty ECU?

What would be the next thing to check ?

Thank you!

John

 

John,

Random misfires are not typical symptoms of a vacuum leak or MAF issue. Bad plug or coil would be my first thought, but the plugs are new and, if it was a coil, I would think (1) you would be getting a misfire code for a particular cylinder, but not random misfires, or (2) you would see even more misfires while under heavy load.

The ECU doesn't sound too likely either because it runs OK under load.

I would check fuel pressure. JTIS claims P300 may be thrown for fuel pressure that is too low or too high.

 

Thank you Mark...you reminded me that I did have this vehicle in the shop quite some time ago and they told me that it looked like it was injecting too much gas into the engine. They said that it sprayed twice per stroke which they did not think that it should be doing.

Is there a way to adjust the fuel pressure?

Thanks.

 

I don't think there is a wat to adjust fuel pressure, that is weird that they would say that an injector is firing twice. I'd look in the JTIS to see what actually tells the injector to fire, seems that unit is pooched.

 

being a ford engine i would put my money on a bad fuel pressure regulator, or check the hose from the brake booster to the intake manifold, that particular hose has a check valve which is know to deteriorate on the bottom side, its really easy to miss