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Having a problem that is showing up as code P0304 – a misfire on cylinder #4. Here are some general bits of info and thoughts. I’d appreciate any further ideas or thoughts anyone might have.

1998 Escort 2.0 SOHC – automatic trans

Head redone approx. 10k miles ago – compression good on all cylinders. Engine runs great most of the time.

Misfires generally caused by:

Faulty spark plug (all plugs look great – new when head was done), faulty plug wires (new when head was done) – switching #4 plug and wire to #2 cylinder made no change when fault code comes up so believe plug and wire is NOT the problem (also eliminated the coil pack since all plugs are firing as evidenced by plug condition when removed)

Faulty crankshaft or camshaft position sensor – As the problem is intermittent BUT always the same cylinder this does not seem likely as I would expect a faulty sensor to cause a misfire on any of the four cylinders not the same each time (this because the sensors only send an electronic signal based on magnetic pulses caused by teeth on the crank and/or cam).

Faulty fuel injector – as a misfire code will trigger if any single cylinder develops slightly different power than other cylinders this seems as though it could be the cause providing the injector is marginal (which would account for the engine running properly most of the time?)

Faulty ECM – since it references crankshaft/camshaft sensor input to perform engine management and send signals for timing, injector firing, etc it seems as though this could be the problem if the flaw is a repeatable glitch.

Faulty wiring or connector – assuming the connection/wiring is marginal this certainly could be an intermittent problem

Anyone have any experience with a similar problem and if so what did you do to correct? I am leaning towards a faulty fuel injector or electrical problem but would appreciate any additional insight anyone might have.

Thanks in advance for any ideas.
 

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After the head was "redone" was the check engine light off for a period of time?
How many miles on the vehicle?

Misfire is generally a lean or too lean mixture. Unless the ignition system has been neglected. You have kind of verified this by swapping plugs and ignition parts. Other maintenance would be air and fuel filters if those were not changed 10,000 miles ago with the head work.

I would generally rule out ECM issues because the failure rate of the ECM is astronomically low. And if it failed a whole lot more would be coming up as an issue. Especially if this is intermittent.

Getting back to running lean. Possible areas to look at.
Verify there is no vacuum leak. Post intake manifold vacuum readings at hot idle.
Not sure why the head work was done, and running a compression check would eliminate a head gasket, valve or some other issue.
Possible low fuel line pressure. Check fuel pressure with hot engine running and post the pressure.
EGR partially open at idle or stuck in any position. Normal EGR function does not occur until the engine is hot and engine RPM is off idle. By forcing the EGR open at idle, the engine should stall. If not there is an issue. Or if the EGR can not be opened the mixture will run lean at vehicle speeds between 25 and 50.
It may be possible the ECM is running lean because the feedback to the ECM is showing something different. IE the oxygen sensor is not reporting fast enough because it is old. If the vehicle has over 100,000 miles replace the O2 sensor.

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After the head was "redone" was the check engine light off for a period of time?
How many miles on the vehicle?

Misfire is generally a lean or too lean mixture. Unless the ignition system has been neglected. You have kind of verified this by swapping plugs and ignition parts. Other maintenance would be air and fuel filters if those were not changed 10,000 miles ago with the head work.

I would generally rule out ECM issues because the failure rate of the ECM is astronomically low. And if it failed a whole lot more would be coming up as an issue. Especially if this is intermittent.

Getting back to running lean. Possible areas to look at.
Verify there is no vacuum leak. Post intake manifold vacuum readings at hot idle.
Not sure why the head work was done, and running a compression check would eliminate a head gasket, valve or some other issue.
Possible low fuel line pressure. Check fuel pressure with hot engine running and post the pressure.
EGR partially open at idle or stuck in any position. Normal EGR function does not occur until the engine is hot and engine RPM is off idle. By forcing the EGR open at idle, the engine should stall. If not there is an issue. Or if the EGR can not be opened the mixture will run lean at vehicle speeds between 25 and 50.
It may be possible the ECM is running lean because the feedback to the ECM is showing something different. IE the oxygen sensor is not reporting fast enough because it is old. If the vehicle has over 100,000 miles replace the O2 sensor.

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Odometer showing 142k but when I pulled the head I found telltale yellow painted date indicating that the original engine was replaced with a used one so do not know actual mileage. Bought the car needing work as the head gasket was blown (engine still ran quite well) so had the head completely rebuilt at a local shop that specializes in heads only. Have not replaced O2 sensor but car did pass emissions test for registration renewal recently.

Air filter replaced two months ago - have not changed fuel filter but if this was causing the problem wouldn't it be apt to occur on more than a single cylinder (unless in conjunction with a faulty injector on #4)?

Thanks for tip on EGR - I will have to check that out. Compression is good. Engine usually runs fine and when I delete the error code it usually does not come back for a week or more (and if I don't get around to deleting it normally will clear itself after the car is driven enough without an additional episode - normally do not wait this long as I am curious if there is another code - has not happened yet).
 

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The O2 sensor will pass everything. (unless it is no longer functioning) The emissions test which will be a look see at the codes in the computer. The computer will pass the o2 sensor because the O2 sensor will return a signal. However in the world of engine management time is critical. Data is looked at on an individual engine revolution basis. (Some times a 1/4 revolution) So if the O2 sensor is returning data it is still good. the problem the data it is returning is old. As an O2 sensor wears the data about oxygen or complete fuel burn is slowed down. As the data takes longer to get there the ECU reacts as that data comes in. If the data is old (say several RPMs later) the ECU makes an adjustment for a condition that no longer exists. (You have accelerated or decelerated slightly or greatly) The ECU is making fuel adjustments to the injectors based on old data.

This can impact all or some or one cylinder depending on individual cylinder efficiency. The end cylinders usually do not fire like center cylinders. All it takes is one cylinder that is marginal and the ECU fuel mixture adjustment which is for all cylinders (injectors) and that one cylinder misfires because the fuel mixture is too lean. This assumes no other issues. And it is important to check all other issues too.

The issue you have is not a total failure of something. It is something that is on the edge of total failure. That something will fail and the engine will run much worse. The question becomes do you want to wait for that or address it now?

A partial fuel restriction because of a fuel filter plug can create the same result. Heck all of maintenance items are the same way. You can leave spark plugs in until they fail or an air filter until it does not pass enough air. But why???

For maintenance it is not about replacing when the items have failed but replacing before that point. Replace the fuel filter and the O2 sensors! Both are cheap. The O2 sensors for 3 and 4 wire sensors are less than $20. When they totally fail the engine will go to limp mode and will barely run. then you are not replacing when it is convienent but you have to drop everything and do it!

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Update for anyone. Turned out to be a faulty spark plug wire.:( Everything checked turned out to be okay (EGR, injectors, etc...) so I checked each wire for continuity - all good but if I twisted and pulled on the wire the continuity would break up even though the all was okay most of the time (hence the intermittent problem). A new set of wires and the problem has gone for a couple of weeks now. So simple a fix but hard to pin down. Thanks to all with suggestions.
 

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Thanks for posting the result.

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