Ghost in the Machine Misfire


#1

In need to diagnose an intermittent misfire problem, and am looking for an intelligent list of things to check.



This is a '97 Ford Escort, 2.0L SOHC, 4cyl, 8 valve engine. The car has about 211K on it - the engine more like 170-180K. (Dropped in used about 60K ago).



Mostly it runs really well - idles smooth, and even though you don’t discuss a car like this in terms of “power” - it has all of the power it’s ever had. The mpg is fine and hasn’t changed. It burns no oil.



Once in a while, for no apparent reason, it starts to misfire really badly - like it wants to stall. It will do this for anywhere from 5-30 seconds and then return to smooth and just keep on going like nothing ever happened. I’ve gotten it twice on cold start, once on hot start, once while accelerating lightly, and once while coasting. I’ve gotten in in all kinds of weather. It might go 3 weeks and not do it. It might do it once a day. So it is completely unpredictable.



It does set a code for cylinder 4 misfire (P0304), and the light blinks at me whenever it has one of its “fits.”



The plugs and ignition coil are new. The wires have only about 20K on them, are Motorcraft, and I tested them for resistance when I put in the plugs. I do all other regular maintenance items (e.g. filters/oil changes). The intake and gas tank were recently “Sea Foamed” and it gets the occasional bottle of injector cleaner/fuel dryer.



I have just had it into the shop as it was due for inspection, and had compression tested - all cylinders fine. There is also no nasty valve noise when it misfires, so I’m not looking inside the engine (unless someone says there’s still a good reason to look).



I was told that since it only misfires on occasion, locating the cause would be a hunting expedition (i.e. lots of labor cost). So I am going to hunt myself.



Given that it is isolated to one cylinder and occurs randomly - what is the best place to start, and what is the most likely list?


#2

The car has a built-in diagnostics computer. It’s, also called the engine computer. The diagnostic code has been delivered. It is P0304. Since the misfire can be caused by fuel or spark, try fuel first. SWAP #4 fuel injector with another. Erase the code. Drive a few “trips”. When the DTC comes back, and it has followed the fuel injector, then, that swapped fuel injector is the problem. If the misfire code didn’t move to another cylinder, then, you can assume that the misfire is from the ignition -----probably, the ignition coil. You could cross-wire the control wires going into the ignition coil to swap which part of the coil fires which spark plug. That’s a bit tricky. You could try a new ignition coil. From that result, you take you next step in troubleshooting.


#3

The injector swap I will do - though there are separate codes that get set for injector problems. Does anyone know if that goes by electrical readings? I.e. if I have the misfire code but not the injector code can I assume that the power feed to the injector hasn’t had a problem? Or is it actual fuel flow that is monitored in which case I assume it isn’t fuel flow?

The ignition coil is new, and just to be sure I checked it (measured resistances as directed for testing) before installing. Its first misfire “fit” was also before I put that coil on, so I don’t think I need to go there.


#4

if you you checked the plug wire resistance,what were the results? and what were the specs.
please post back, need to know the length of the wires tested ,and the actual reading on the meter.

thanks


#5

I actually don’t remember b/c I looked at all 4 and just wanted to see that they were w/in specs - specs are less than 7,000 Ohms/ft. The #4 wire is about 14in and - I think - it was around 5,000. My certainty is only that it was comfortably < 7,000.

If knowing the actual value will help I will pull the wire and get it.


#6

The #4 spark plug wire could be the problem, even if it passed the resistance test, and even if it is almost new. Swap the #4 spark plug wire with another. If the misfire moves to that cylinder, that wire is faulty. Same thing with the #4 spark plug. Swap it.


#7

Thank you for the suggestions - I plan to swap and injector for a while and then a wire for a while, and whatever else people come up with. I will also double check the coil and such.

But here is some more news - this morning I pulled in to park at work and heard an intermittent valve-like noise - the sharp thwack thwack thwack. It was not persistent - it would make noise a couple of times, then quiet down, then a few more whacks, etc. The car was not misfiring at the time. This is the first time I’ve heard it - and it doesn’t always do it b/c I have now spent a lot of time listening and poking and prodding under the hood (with the car both hot and cold). I had just finished about 70 Interstate miles during which time the car behaved perfectly - no hint of a misfire. But now I am a little worried about valves - I have been assuming that, given the precision with which valves have to work they would either fail or not. But is there some reason that the valves could periodocially get messed up for 5-30 seconds, then right themselves and go on their merry way? It sounds implausible to me - but I am the amateur.


#8

Valves don’t suddenly fail, unless one actually broke. Then, it would stay broke. Valve lifters can get a bit of slack if they fail to pump up for some reason (oil pressure, blockage, wear). Once the engine is warm, and there is oil pressure, valve click and clatter won’t worsen. There are always, “…buts, and ifs…”.
See if you can make the thwack thwack noise happen again. Open the hood with the engine running. Listen, and work the throttle to raise rpm high and low. If you can get the noise to repeat, at will, take the car to a mechanic. I can’t hear it (at least, with fidelity) through cyberspace. The virtual mechanic, Ms. Diag Nostic is still in development. It’ll be a long time before it’s ready for dispatch through cyberspace to someone with a problem.


#9

O.k. - I am getting valve noise. (Sometimes at idle, sometimes not - once up to full temp it seems to be reproducible by revving). I am going to run the car into my shop tomorrow because I can probably reproduce it for the mechanic. (Though I may be able to get and post a recording - who says it can’t be done via cyberspace?) I still cannot reproduce the misfire, however.

But what I’m wondering about is - given that 99% of the time the car runs just fine - whether the valve noise can be related to the misfire. What I was sort of asking in my earlier post was whether a valve problem that pretty much shuts down a cylinder could just do it once in a while and then go away on its own leaving the car running fine. Deterioration over time from wear and build up I get. But sudden complete failure followed by proper functioning is what would have to be happening if the misfire was coming from a valve problem. I’m just wondering if there is some plausible explanation for that - or if I should treat the valve noise and misfire as 2 separate things.

Thanks.