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Help/Suggestion for DTC PO304

For the past few days I’ve been trying to track down the problem with my '02 Ford Escort. First thing I did was change the spark plugs which smoothed out the engine some, but I still have the PO304 (misfire #4 cylinder). Next I swapped the plug wires between #3 and #4 and reset the ECU hoping the code would come back PO303 (misfire #3 cylinder), yet it still set a code for cylinder #4 meaning the plug wires were good. I then tested the coil and camshaft position sensor which both tested good. After this I checked the compression on #4 and it was good at 175 PSI. When I removed the #4 plug to test the compression it was gas soaked so the injector is firing, but apparently not the plug. My Chilton’s manual says the center pin (ignition/start/run) on the wiring harness for the coil should have 10 volts with the ignition switch in the off position. I checked this and only had 0.1 volts. Does anyone know where this wire receives it’s voltage from and able to tell me more about what to look for or test? I never expected to have so much problem finding a cylinder misfire. Any help is greatly appreciated.

Is this a lost spark ignition? That is one coil for a pair of plugs with a plug wire on each end of the coil?

It is lost spark - I forget which two fire together - I think its 1/4 and 2/3.

Are you sure you read the manual right? I can’t imagine why the coil should get 10V with the key off. I’d also say that if your info is correct and you have .1V rather than 10V at the coil then you’d have a bigger problem than a single cylinder misfiring. How did you check the coil? Just basic resistance? This can really only verify that the coil is bad - you can easily get fine resistance resistance readings and still have a coil issue. (Ask me how I know…)

What was the compression on the other 3 cylinders?

What kinds of symptoms does the car display?

Even given what you said I’d still swap the #4 injector with another one. Not firing and thus having a lack of fuel is one thing. But its also possible for it to be leaky and flood the cylinder.

I agree with cig.

Yes this is a wasted spark system. Cigroller, your correct one side of the coil fires 1/4 and the other side fires 2/3. Resistance reading is all I was able to test on the coil. I didn’t check the compression on the other cylinders since the misfire was on #4. Hopefully this weekend I’ll get the opportunity to look into some of the other suggestions. I’ll have to check, but I think the coil on my '97 Escort is identical to the one on the '02 so I may pull the one off the '97 and see how it does. The car starts fine, symtoms are just poor performance while at idle or under load and performance seems to degrade significantly right after the torque converter locks up.

Just swap the two coils on this car and see if the problem moves to either p0302 or p0303.

I just noticed it was you Fordman - which means I don’t need to mention what I’m always thinking when I see the words “Escort” and “cylinder 4 misfire” all in the same place. 175psi is fine - but I was wondering about the comparisons. Those valve seats can seal up one minute and drop the next though, so I don’t even know if the other numbers would say anything. But the misfire issues tend to be very different from what you describe. So I wouldn’t be suspecting that.

The coils on those cars are known to be problematic, so if they do swap between the years I’d also do that next - before messing with the fuel rail. The one I have on my '97 right now cost me all of $50 - not too bad.

Have you tried to listen to the injector with a stethoscope while the engine is running? Maybe the injector is sticking open or being help open due to an electronic glitch and this is leading to the injector spraying constantly.

You can use a long handled screwdriver as a stethoscope and you should hear the injector clicking.

I would add that a dead plug for whatever reason can also appear gas soaked.

ok4450, The misfire began while it had an old set of plugs in it. I changed the plugs and still have #4 misfire. Not likely the old plug and a new one both would be bad and happen to both end up in #4, but I guess anything is possible. I haven’t listened to the injector, but I have a mechanics stethoscope so that’s no problem.

Cigroller, Hopefully tomorrow the weather will be nice and I can try to do some investigating and come up with something.

My guess is the coil has an internal short between the #4 output and ground.
Swapping #1 and #4 plug wires at the coil should make the misfire move to #1.

"Swapping #1 and #4 plug wires at the coil should make the misfire move to #1. "

Good point - much faster than swapping coils.

Since changing the coils between the two cars was so easy I just took the one off the '97 and put it on the '02, that was the problem. I really appreciate the help. New coil is on order.

Thanks for the update FordMan1959. Good to hear you found the trouble.

Anytime I ask for assistance with a problem I try to update it when the problem is found in case someone else has the same problem at a later date they’ll have an idea what to look for. The coil was one of my first thoughts as to what the problem might be, but I checked the resistances and they were all good so I thought it was OK, but some of the smarter mechanics than I said it was still possible that was the problem and when I exchanged it with one off my '97 Escort everything was fine. Since this is the car my son and wife drive to work I just took the one off my '94 which isn’t driven much and put on it until the new one arrives. I was able to order one online for about $25. and the local auto parts wanted $70. so I though it would be worth waiting a few days to save $35. It may actually be a good thing it went out otherwise I wouldn’t have changed the plugs and they were getting in worse condition than I expected.

Great work, guys.