Got a 1996 Ford Ranger with the 4.0L in it that has a misfire on the #3 cylinder and giving check engine code: P303. Have tried new plugs, wires, distributor, switching out the coil pack with a known good, checked injectors for pulse, checked injectors, moved injectors and still the misfire remains on #3. Removed the valve covers and all appears to be well. Any ideas???

Thanks!

 

Any other DTC's?

Check compression?

Might be PCM defective?

 

No other DTC's and compression is at or above 150 all the way around. The PCM is all I can think it could be at this point... Hope to change it out today.

 

Still no luck with this truck and have had no luck finding a replacement PCM. Put the truck on the machine to make sure the injectors and the fuel rail weren't clogged and that did nothing.

 

Located a PCM today off a 96' Ford Explorer. Installed the PCM and there is still a dead miss on the #3 cylinder. I, and my mechanic, are at a complete loss on this one.

Doesn't make any sense, has fire and is getting gas, but the miss is still there. Compression is good all the way around with the lowest cylinder reading 150. Changed plugs, wires, distributor, coil pack, moving injectors, ran the truck on the motorvac and now replaced the PCM and still no luck.

Any suggestions would be greatly appreciated.

 

might try swapping that injector with a different one or atleast different cylinder and make sure it doesn't follow it.

 

lifter fail? That's out of the box thinking. That would still give you a compression but a no fire due to no gas charge to fire.

 

Swapped the injector and the misfire stayed on cylinder #3.

At one point we took off the intake and all that to check the valves/spring to make sure there wasn't a broke spring. All was perfect and we put it all back together and fired it up and it ran good for about 5-10 minutes... turned it off then started it back and the misfire was there again.

Not sure about the lifter, that's certainly something to check.

I'm at the point of just pulling the motor, but hate to because of a misfire.

 

Did you check the compression at operating temp? Might be a issue with heat expansion causing a head gasket leak?

 

Yep, checked it several times at operating temp, leak down test. I give up. I am pulling and putting in a motor from a 99 Ford Explorer tomorrow and saying forget it.

 

Well, the 99' motor didn't work out after all. After checking it out further it had nearly 160K on it so I decided to look for a lower mileage motor.

In the meantime I am going to continue to look at this motor. It seems that it would be a burnt valve or something of that nature, but it has good compression and the leak down test was good so I am at a loss.

Anyone?

 

Did you leak test the intake? Allow me to explain. If you have a leak in the intake runners, usually due to a bad gasket at either the upper or lower intake, it will cause some of the cylinders to be much leaner than the others. This lean condition can cause a bad misfire. This problem would not be detected by swapping injectors and plugs, and would not cause bad compression.

As an alternative, a faulty crank position sender could cause an erratic signal that the computer may interpret as a misfire.

 

That's a new idea and makes complete sense. I have considered just buying a complete head gasket kit and replacing the head gaskets, intake gaskets and all that just to see what happens. I just hate to give up on this motor, it is a good clean motor and has been well cared for.

 

EDIT: Welcome to FTE.

Good diagnostic trouble shooting you've done.

Have you considered CCDF = Combustion Chamber Deposit Flaking???? It's known to cause phantom misfire problems.

If #3 cyl has intake valve, or combustion chamber deposits, from say, worn, leaking valve stem guides or seals, or maybe some blow-by from a stuck oil control ring, causing deposits that are flaking off, they can cause intermittent mischief by lodging on a valve seat & causing compression loss, or spark plug misfire, before moving on.

SO, maybe consider doing a professional decarbon treatment & see if you get positive results.

More thoughts for consideration, let us know how it goes.

 

^I also think this makes good sense and is something to consider. A failed lifter is also a suggestion worth keeping in mind.

If you are going to do the a whole gasket job, THIS might be of help. I would definitely pull the heads and have them checked while you're at it.