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4.0 misses in 94 Explorer

Sscolax

Member
Joined
June 5, 2006
Messages
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City, State
Colfax, CA
Year, Model & Trim Level
94 Eddie Bauer
A couple of weeks ago I noticed our 94 Explorer started developing a miss. It was very subtle at first, most noticeable at idle. It's been getting worse and the power and torque just aren't there anymore. There's also a definite gasoline smell in the exhaust. According to the computer nothing's wrong (no codes). The plugs were new about 3000 miles ago, as was the O2 sensor on the passenger side. I just tried replacing the wires with a new set, which seemed to help a little, but it's still missing. I also inspected the plugs and found that cylinders 2 and 3 were clean with a gasonline smell while the rest had the healthy grey ash appearance. Does this sound like something obvious to someone with more experience one this engine?
 



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I double checked today and realized that it was the driver side O2 sensor that got replaced a few months ago, not the passenger side. Also of note is the horrible gas mileage I'm now getting. I just filled it up today and only got 9 MPG on the last tank! This afternoon I tried swapping the coil pack with a spare I have, but no change.
 






From the clean plugs, it sounds like cylinders 2 and 3 aren't firing, which might explain the power/torque loss and horrible mileage. Either they aren't getting fuel (injector/fuel system issue, less likely due to gasoline smell), or theres no spark (ignition/electronics issue).

Has the issue been present since before the new spark plugs? If not, I suggest you purchase (or go to the parts store and demand replacements for) two matching new ones to replace those in cylinders 2 and 3 and see if that helps. What kind of plugs did you use? I always suggest Motorcraft, as they really do seem to work best in these engines.
 






Thank you for the reply! The plugs were replaced several months ago (Autolite), but the missing just started within the last couple of weeks.

Some history in case this helps: I bought the vechicle cheap last summer knowing that it had a blown head gasket and/or cracked head and had been sitting this way for a couple of years. It turned out to have both heads cracked (worse on the driver side) and a blown driver side head gasket. I replaced the heads with new aftermarket ones, along with new plugs and all new gaskets, of course. Once I got it running, I had to replace the MAF and driver side O2 sensor to get it to pass a smog check. I also changed the oil several times during the first couple of weeks to get any accumulated crud out from when it sat. It's been running like a dream until this miss started.

I'll go ahead and try putting a new pair of plugs in cylinders 2 & 3 today and see what happens.
 






Might try a compression test. Two dead cylinders right next to each other like that might indicate the head gasket has new issues.
 






Well, I just went out and performed the "other" version of a compression test, putting compressed air into the cylinder with it at TDC on the compression stroke. I removed the plugs from cyls. 1, 2, and 3, then put the air into cyl #2. When I did that I felt air coming out the #3 plug hole, but not #1. So it looks like I get to tear it all apart and replace the head gasket again. Grrrrrrrrr.........

I don't understand why it failed so soon. I carefully followed the torque procedures and used a new set of head bolts. The only thing I can think of is that my wife said it got hot one day in July when she was driving it (hot day, stop-and-go traffic, A/C running), but it never started running any worse until recently. Hopefully the new head didn't crack or warp. I'll have to check once I get it all apart.
 






While you have it apart anyway, you might want to change the thermostat, and all of the antifreeze/coolant too, as a preventative measure.
 






The coolant and thermostat were both new in June, but it would be cheap to replace them again, so it couldn't hurt.
 






So I got it all torn down, only to find that the head gasket (and everything else in there) looked just fine. No signs of any damage. The combustion chamber on cylinder #1 had a slight grey ash-looking coating. Cylinders 2 & 3 had less deposits on the head, especially the exhaust valves. Since I didn't see anything that appeared to need repair I put it all back together. Once it was complete I started it up and it's still got the miss, though it's not as bad as it had been lately. This is starting to get quite frustrating!
 






I still haven't gotten this engine to run right. The other day after replacing the head gasket I charged the #2 cylinder with compressed air again. It started coming out the #3 plug hole, but that's when I realized it was also coming out the intake. Suddenly it made sense: leaking #2 intake valve. Cylinder #3 is on its intake stroke when cylinder #2's plug is firing, which explains how the air was getting into Cylinder #3. So I pulled the head again, removed the #2 intake valve, and saw it was not seating properly. Replaced the valve, lapped it, and verified no valve leaks by bolting the head to a block of wood covered with cork gasket material and applied compressed air. Put it all back together again, started it up, and there's still a miss. It ran better than before, but now its main issue is that it's cutting out and backfiring when the throttle is opened more than 2/3 or so. I'm just about ready to take it into a shop and pay the $$$$$$ to have them figure it out for me. I'm out of ideas.
 






Update: It seems to be working now. One at a time I pulled each plug, grounded it, connected the plug wire, and started the engine to check for spark at the electrodes. All the plugs fired except for #2 and #3. I tried some old wires I had, but that didn't help. Finally I tried some spare plugs I had and then I got spark. It turns out that those 2 plugs failed for some reason. So that means that you, Anime4x4, got that part right. I don't understand why they failed along with the bad valve, however. Weird coincidence, or could the problems be related? What's also strange to me is that the 2 bad plugs had the same resistance (6K) as the good ones.
 






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