Explorer misfire issue - stumped

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  #1  
Old 07-03-15, 08:05 AM
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Explorer misfire issue - stumped

All,.

Seems I am having an issue with 02 explorer 4wd 4.0 ltr. engine, 78000 miles. The issue comes up when the vehicle has sat for a while and it has rained quite a bit. When we go to drive it, that is when it acts up. It runs fine with no issues when it is dry out or rains a little bit. After a rain storm, if we drive it, it runs fine for maybe 2 or 3 miles then acts up. IT is never right away after starting. it is random and the last time it did it it was driven a couple miles with no issues and then parked then on the way back it started to misfire. The check engine light flashes telling me it is misfiring and the truck lacks power as it won't go very fast when you step on it. It wants to stall when stopped. IF you put it in nuetral or park it stops and idles and rev's fine so it seems like it load related. This will happen for a few miles and then will go away and drive fine and not reappear until it rains again. I have had it to the garage but they couldn't find issue as it was not doing it and no codes were present. I have installed new plugs and wires and also a new ignition coil. I have went out there at dark and completely hosed down the engine compartment and windsheild with a garden hose to soak everything. No evidence of arcing and it didn't cause the misfire issue. I also hosed down the underside good and that didn't cause it. The last hard rain it did it again after hoping the new ignition coil would work. IT didn't and it misfired and the check engine light lit. Below are the codes set when I scanned.

Po152 - High voltage bank 2 sensor 1
PO175 - bank 2 system to rich
PO300 - Multiple misfire detected
PO306 - Cylinder 6 misfire

NOt sure what to check next. I am assuming because this is water related it is probably an electrical issue somewhere, just not sure where to look next beyond plugs wires and coil. the strange thing is it goes away shortly after it happens so troubleshooting time is minimal. Any thoughts would be greatly appreciated. If I can't find issue, I Will need to wait until next rain storm and drop of at the garage. THx a bunch
 
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  #2  
Old 07-03-15, 09:06 AM
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Location: USA
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I had similar issues with my ranger but not just when it rained. The first thing I would do is clean the mass air flow sensor (google it) and see if that cures the problem. I'm thinking that driving it in the rain is introducing more humidity into the air going in and the sensor may be dirty enough to cause an issue? Just a shot in the dark on that. Regarding the misfire code I would pull that plug and inspect it for moisture on the tip and rust around the threads, then I would put a new plug in that cylinder and drive it a while to see if you get another PO306 code. I had this happen on my 4.0 on the #4 cylinder and found in the end that the head had a crack between the valves letting water into the engine and fouling the plug. I pulled the head thinking it was only a bad head gasket but it wasn't. It isn't all that uncommon for heads to crack on newer engines according to my research.
 
  #3  
Old 07-03-15, 10:09 AM
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Join Date: Feb 2014
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Had one years ago at shop: Ford Ranger V6 with intermittent misfires and bad running symptoms. Eventually tracked it to wiring harness from ECM that ran across an AC hose. Vibrations had worn through the outer plastic loom and then rubbed insulation from several of the wires inside. This allowed a voltage changing problem with the ECM wiring. I separated all the wires and individually taped them and reinstalled in loom. Rerouted the loom away from AC hose and used plastic wire ties to keep it out of harms way.
Problem solved!
You might check for similar issue.

Check all wiring plugs for insulation shrinkage exposing bare wire.
Had this happen on '85 Crown Vic with 302 ci engine. The wiring to fuel injectors was shorting especially on left turns. Turn right and it would run like the proverbial 'spotted a$$ ape', turn left and it would sputter and sometimes die. A few pieces of tape and problem cured.

RR
 
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