My god, this is annoying. Misfire problem. | Ford Explorer Forums - Serious Explorations

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My god, this is annoying. Misfire problem.


August 7, 2010
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Year, Model & Trim Level
97 XLT
Been trying to fix this issue for a while. Heres what I have:

Misfiring on cylinders 1 and 6. When I start the engine, the car runs perfectly, no misfires, nothing at all. I drive for about 5 minutes at ~70km/h, and the car starts shaking. Read the codes, P0301, P0306.

The car use to misfire on cylinders 5 and 6, but 5 has since 'moved' to cylinder 1.

Also, when going ~80km/h, if I open the throttle, say, half way, the car shakes violently, but any more or any less than that, the car just shakes slightly.

Recently changed: Plugs, plug wires, coil pack, EGR valve, cleaned IAC and intake. Cleaned MAF sensor.

Took plug and wire from cylinder 4, switched them to cylinder 6, same thing happened.

Shutting the car down, and starting it 2 minutes later fixes it, until I drive for around 5 minutes again.

Any ideas at all?

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what did the plugs you took out look like?
carbon deposits?
electrodes ?
oily ?
are you losing any coolant ?

plugs had a bit of oil on the threads, nothing to worry about though.
electrodes had nothing on them.
not losing coolant, but it is a dark color, needing a flush real soon imo.

are you sure that you gapped the plugs to specs? try putting the old coil pack back in and see what happens.

tried putting the old coil pack back in, same thing. i cahnged the pack to try and fix this problem, as well as the old one having 250,000k on it. the plugs that were in before were rusted right to the block (friend who sold me the car didnt change them for 200,000k.) so i took it to the dealer, who of course, used oem parts, so the gaps are correct.

Chadrax, did you do a compression check? If so, what were the readings?

I feel your pain because I had a stubborn misfire that I could not fix by doing anything on the outside of the engine. I had #3 misfiring. Changed plugs, wires, coil, and a bunch of other parts that I thought were questionable and replaced anyway. I spent about six months chasing this problem. The miss even moved to #5 occasionally. Compression wasn't real bad but #3 was 20lbs lower than 1 and 2.

Now, I don't want to mislead you but I bought a head gasket set thinking that at best, it's only head gaskets. At worst, the heads are shot. I pulled the right bank head and found the exhust valve on #2 was worn into the head 7/32nds". Looking at the top of the head, the stem was that much higher than the rest of the valves. You can't see it without removing the rocker shaft. On closer inspection, there was a small crack in the head between the valves in #2 combustion chamber. I had a similar situation on the left head as well. I'm a retired mechanic and called friends to get the best price. I bought 2 new castings @ $300 each. I did all the work myself and the engine runs smooth and has more power than when I bought the car.

The car, by the way, is a '96 EB with 4.0 OHV engine. I cannot explain why cylinder 3 was throwing a misfire code when it appeared that most of the obvious problems were in #2. All I know and care about is that the engine runs lots better and the mileage is much better. This engine has over 200k on it.

I don't know if your engine has the same problems. I just got tired of messing with it and felt the next step was to pull the heads. I was lucky that it was the heads and didn't have to keep digging. The compression readings had a lot to do with my decision to pull the heads.

I wish you luck in your search. I also hope I might have helped you some with my story.


@lugnuts55: I haven't done a compression check yet, its dark out, so I will do one tomorrow and report back my findings, really hoping its not the heads.

2 things come to mind: Vacuum leak, or fuel injectors.

I looked at the freeze frame data from my scanner, coolant temp ~78 degrees celsius. Engine load 74%. Short term fuel trim ~2%-5%, Long term was %21%-25%.

Should I look for a vacuum leak first, or check the injectors?

lugnuts55's post made me curious enough to do a forum search on compression tests. This thread popped up which has some interesting tips particularly in post #7 by Maniak: Link.

lugnuts55 said:
On closer inspection, there was a small crack in the head between the valves in #2 combustion chamber. I had a similar situation on the left head as well.

Seems odd that both of the heads would be poorly cast. What do you think would have caused this? Excessively high combustion temperatures? I think that would cause the head gaskets to go first though.

Is the long term fuel trim a positive number? Those percent signs are confusing me. If it is a positive 21-25%, the computer is adding fuel which would indicate a lean condition. Lean condition = vacuum leak.

This is most often the case. It's easier to check for a vacuum leak than a bad injector unless you have a noid light. There are several ways to check for vacuum leaks. I use carb cleaner and spray it along the intake gaskets and the plenum.

You haven't said which engine you have and I can only speak about the 4.0 OHV since that is what I have in mine. On this engine, there is an upper EGR tube that goes into the plenum which is sealed by an O-ring. That is one common area for leaks. Make sure to check any and all rubber vacuum hoses including the one on the PCV valve.

If your search finds no vacuum leaks, you could check fuel pressure. When was the last time you changed the fuel filter?

Try to do the easiest things first and work your way into the more complex processes. I don't know what your level of experience or knowledge is. Don't do more than you know because you don't want to cause a problem in your attempt to find what's causing your problem.

I'll be interested to see your compression readings.

lugnuts55, thanks for the quick reply.

Yes they are positive, I am running lean, as once in a while i will get a p0174 - bank 2 lean code, which I am just assuming is caused by the misfires, as it came up a few days after the misfires started. Come to think of it, they started after I changed the air filter, cleaned out the intake hoses and throttle body. Any common mistakes I could have made to create the leak?

Fuel filter was replaced about 3 months ago. Any places i should check for vacuum leaks in particular? PCV, brake booster, air intake hose, EGR hoses, anywhere else thats common?

1997 Explorer XLT. SOHC, 4.0L btw.

OneLever, I don't know that the heads I replaced were bad castings. I was told by the automotive machinist that he has seen this a lot and he believes the wall between the valve seats is too thin. I don't know any more than that about this issue except that replacement heads fixed the problem I had.

I don't mean to imply this is the issue every time there is a misfire that can't seem to be fixed. There are many factors that need to be checked before pulling heads. I just felt that I was not having any luck trying to figure out what the problem was and that I had done everything I could do without head removal. I was doing all the work myself and was ready to take that step. I was lucky that at least the problem showed itself.

I did go into it with the thinking that I would be lucky if it was only head gaskets. That's what I was hoping it would be. The heads cost me $600 for the pair. The head set cost around $100. I have no idea what the job would have cost me if I took the job to a shop.

Thanks for your response, though. It is a good question.

Chadrax, all the places you mention are good places to look. I'm sorry I'm not familiar with your engine. But since you think the problem started after changing the air filter and cleaning out the intake hoses, I would definitely start by backtracking. Make sure the air intake hoses are installed correctly, especially at the throttle body. Check anything you took off and/or replaced. Connections have to be correctly installed and if there is a clamp, it needs to be snug. I think you will probably find your problem just by checking your work.

I was told by the automotive machinist that he has seen this a lot and he believes the wall between the valve seats is too thin.

That is interesting to know even though it applies to a 4.0L OHV.

I don't mean to imply this is the issue every time there is a misfire that can't seem to be fixed. There are many factors that need to be checked before pulling heads.

Understood, I like picking up details on things which happen that are out of the ordinary. The kind of problems that stump you for days until you trace down their solution. Thank you for sharing your knowledge.

compression check showed that they all are within 5 psi of eachother.

re-assembled air intake, putting duct tape around any seals or hoses. Car runs better, fuel trim is down to +6.5%, so something was wrong there. Still getting the misfires on cylinder 1 and 6, cant feel them as much, but can still feel them shaking.

The compression is good and you were able to reseal things as good as you can, I assume. Did you happen to notice anything that might have been a vacuum leak as you were backtracking? Do you think the duct tape is helping or is it more to make you think it's helping?

Do you have a vacuum gauge that you could read engine vacuum? If the vacuum is steady and about 16-20" HG, it should be good. If the needle bounces around, there could be a valve problem.

Also, see if you can check the fuel pressure. I should have asked sooner, but do you have a repair manual of some sort, Haynes, Chilton? It would be a good idea if you plan to do some of these tests yourself.

How many miles are on this engine? I just have a feeling that it is something simple. Is there a tube that goes into the plenum from the EGR valve? On the 4.o OHV engine, the O-ring on the end of the tube that goes into the plenum sometimes doesn't seat right or just wears out by hardening over the years. I wish I knew more about the SOHC but I don't.

My final suggestion is to go over the vacuum hoses again and recheck everything you did before. Let me know if there is something like I was trying to describe regarding the EGR tube. If a hose is soft, replace it. Don't forget about the PCV hose. I think it's in a place that is not too easy to get at. I wish you luck, Chadrax. I'll be watching this thread, but I think it's time for someone with more knowledge of that engine to see if they can help. Good luck, Mike

didnt notice anything that would be a leak, but i did take all the hoses off again and reseal them, so it may have been a leak. the duct tape may be helping, or it may just be in my head.

ill head on over to my buddys house when hes home and test the engine vacuum, and yes i have a haynes manual.

260,000 km, engine was rebuilt at about 200,000km. checked the egr valve, everything is snug, im pretty sure.

i just cant seem to find a leak anywhere.

I noticed the problem occurs as soon as the engine gets to operating temeprature, (the temperature guage on the dash is at 1/4) I did take a freeze frame of the data, and the coolant temperature is @72 degrees celsius. Could it be a bad thermostat, as the guage only rises to 1/4, and is that an ideal temperature for the coolant. I have noticed the coolant in the resevoir is brown, time for a flush?

The thermostat could be contributing to the problem. The computer reads the engine temperature and the load and adjusts the fuel delivery according to the information it reads. If it thinks the engine is cooler than it is, the fuel will not be the correct amount.

Besides all that, if the temp only goes up to 1/4, it is probably opening too soon. The heater might only deliver warm air when you need it to be hot to clear the windshield.

A cooling system flush might be a good idea, too. I don't think the misfire will go away with a flush but I would replace the thermostat.