Calling all car guru's!!!!

I NEED HELP figuring out what is wrong with this car.

Here are specs
2001 Ford Taurus Wagon with the 3.0 DOHC vin S
57,000 Miles

The Problem:

Car has a P0304 misfire code (CYL 4), I can tell you that the amount of load on the engine has no effect on the misfire. It is noticed at idle, will do it consistently for 10-20 seconds then go away.

So far here is what I know has been replaced on it
-PCM
-Ignition Coil
-Fuel injector
-Spark Plugs
-Wires

The misfire is random, I can sometimes duplicate it by reving up the engine for a few seconds, and letting it idle down, then in about 30 seconds, it will misfire.

I noticed that when the car is misfiring, the #4 injector stops working, and After using a scope on the injector signal wire, the injector signal is dropped during the misfire event. To my knowledge it is just limited to cyl 4. This is what warrented the PCM replacement. I figured the transistor in the PCM was bad/overheating and stopped working. After replacing the PCM, it still does it, same situation.

So today I scoped the cam sensor for a good pattern, and it is fine, even during the misfire.

I am at a loss,

My plans tomorrow are to check the injector ground signal by backprobing it off of the PCM, to rule out a bad wire.

Next step is to make sure the power wire to the injector does not have any issues.

But other than that I have no idea where to go.

To the best of my knowledge the cam sensor is the only thing that controls the firing of the fuel injectors. If that is ok, and the PCM is ok, it must be a wire that connects them. Thats all I can think of!

So many thanks to anyone who has any ideas for me, I even welcome someone willing just to throw something out there, even if it doesnt make sense.

The tech working on the car before me pretty much just gave up and is telling my boss that it is a bad valve causing the misfire, and it needs a new motor. I know given the loss of injector pulse, there is no way it can be true.

So anything to help would be great!

 

I think you're on the right track with checking the wires. Wire could be rubbed through anywhere in the harness though, might take a while to find it.

 

If I told your that the wrong starter could cause the injectors not to fire, you prolly won't believe me, but that is EXACTLY what happened on some 1991 Explorers.

 

I had a 00 that did the same thing after I replaced the battery. I cleaned every ground that I could find under the hood, and the misfire code never came back.

 

You have done the basics so far. I would not think it is a valve if you lose injector signal. But I would do a compression and leak down test to rule it out.
If I had to say what I would look for, I would look at the wiring from the ECM/PCM to the injectors and coil. I have heard tales of broken wires at the ECM/PCM (but I have not found one).
I would disconnect the battery and ECM/PCM and check the ohms on the injector wires while someone shacks them. I bet you find a broken wire.

Rod

 

Check the engine connector on the harness, and the wires at the harness anchor on the fenderwell, those are the most common failure points.

 

Thanks guys, we'll see how it goes tommorow.

 

The injector signal goes away after the misfire because the PCM has recognized there is a misfire on that cylinder and turned off the injector for that cylinder. This keeps raw fuel from going to the catalyst, which can melt if it gets enough raw fuel. Losing the injector signal isn't the problem, it is a result of the problem.

I suspect you have a bad wire on the power side of the injector or a bad injector. If there was a bad wire on the ground side you would have a code for an electrical failure on #4 cylinder.

 

Find the main connector for the injector harness and try running a jumper from the #4 wire to the #4 injector.

 

AHH!

That is a handy piece of information!

(I wish I would have found this earlier)


A question I have regarding that. Is there a set time limit the computer will stop firing the injector for? Obviously if it stops firing the injector the engine will continue to misfire, and the computer wouldnt know when to start it again.

Thanks for that info!

 

I think it will keep the injector off until the next engine start.

 

When the car is running the injector goes out and comes back on. Usually it will say work for 5 min, go out for 30 seconds (or so), comes back on for 5, goes out for 30 sec, etc...

 

Really? Why is that?

 

I didn't even think of the meltdown protection-
In that case, it could even be rust on the crank tone wheel, or something on the cam one, affecting the CPS signal-

 

If there were any problems caused by a CKP or CMP wouldnt that throw a code?

Also, its not that I dont believe you about the TWC protection, I just want more info on it, and have been searching for the past hour. What sources were you using for your information?