4.0 SOHC suddenly misfiring on cylinders 1 and 6

[Justin_]

Hey guys, I have a 98 SOHC Sport 2 door that I am having problems with suddenly.

Bit of background to start, I have had this vehicle for about 11,000 miles and 18 months and I rolled it about 3000 miles ago. Over the time I have had the vehicle I have had some minor issues (fuel pump failure, exhaust try to fall off, etc) but no real engine issues. The entire time I have had the vehicle I have had bank 1 too rich and bank 2 too rich, but I have never been able to find out why. Also I still get 12MPG with a 4.10 axle, so I just kinda left it alone.

When I got the vehicle it had a vacuum leak issue, but I believe I have resolved that at this point by replacing a number of seals and hoses. When its working it idles very well and stable at right around 600 RMP. A few days ago I went up a mountain and came back down and all was good. The path was a bit icy, but nothing that demanding on the engine. Took the highway about 30 miles to my folks house and had dinner. A few hours later I left to go home and I started to get Cylinder 1 and 6 misfire issues. I started looking into it the next day. First off I replaced the plugs, wires, and coils as I had never done that and they were kinda old anyway, figured I would get them out of the way. When I pulled out all the old plugs they looked worn, but not overly clean or dirty and they were all relatively even. I also tried disconnecting the battery to reset the ECU however there was not much of a difference.

Over the whole day of messing around I still ended up with 1+6 misfire codes, I also once got a cylinder 3 code. Sometimes I will get misfire codes, but the vehicle will run mostly fine, sometimes I will get misfire codes and the vehicle will be shaky. This could be because of the engine switching from individual fuel injection to banked fuel injection (read from somewhere online, could be wrong). When I am getting misfire codes and the vehicle is running well I will only have the CEL light flash under high acceleration, when it is shakey it will almost constantly have the CEL flash.

Here is the fun part. Yesterday when I was about to give up the engine decided to run as good as it had ever run. The acceleration was great and I swear the throttle response was better than ever. However today I went for a ride down the highway. For the first 10 miles it was great, for the next 10 it was a bit sluggish, then it started to misfire and I got off the highway. Form that point on I have had misfires again.

While I was poking around yesterday I checked the fuel pressure and it was ~50-55. Pretty shaky between them, i'm guessing that is from the fuel pump being pulsed. I also checked compression in cylinder 6 for the hell of it and it was about 150, so that seems fine.

I don't know if it does or does not have anything to do with it, but it has only just started to get above 40f on a regular basis here, got up to 70 over the weekend.

Some other testing I did involved pressing the gas when starting the engine to check for injector leak and did not hear a single firing. I also tried coasting down a hill in low gear and hitting the gas at the bottom, and there was no smoke out the tailpipe.

When I rolled the vehicle I somehow managed to put a hole in the passenger valve cover and that did leak some oil onto one of the cats before I replaced the cover. To clean that up I took a plumbers torch to the cat and burned off all the oil.

Some other information, when I got the vehicle, the vacuum lines that control the EGR and EVAP systems were removed and plugged. When I did a body swap after the rollover I got all the vacuum lines back to factory.

I have already replaced the upper and lower intake seals

I have always done regular oil changes myself and have never seen anything in the oil while doing it. I have ran synthetic in it the whole time, but still replaced about every 3000 miles. I also did an oil change right after it rolled.

Things replaced ~5000-11000 miles ago:
Intake gaskets
PCV valve
Upper O2 sensors
Fuel Pump

Things replaced >3000 miles ago:
Spark Plugs
Wires
Coil Pack
All small vacuum lines
ABS module (ABS never worked and I was trying to figure out why while I had another vehicle I was pulling parts from)
Passenger valve cover+seal (cracked in rollover)
Thermostat
Muffler, resonator, and tailpipe
All steering and suspension components
Brake pads and rotors
Front wheel bearings
Passenger side front diff seal and had diff serviced

Probably more, but I think that is everything relevant and more.

About 4000 miles ago had a transmission coolant line break and I had to replace it as well as all the transmission fluid.
Im running 31" tires, but with 4.10 gearing I don't think that is even a factor.





So, does anybody have any ideas on where I could go next with troubleshooting this? As it can go in and out I am wondering if there is a loose vacuum line or something, but I don't know where to look. It could also be related to the banks 1+2 too rich codes, but I have not been able to source that issue either.

Thanks

 

[Justin_]

Im kinda thinking that 1 of 2 things happened.

1) Somebody replaced the injectors with slightly bigger injectors at some point and the computer cannot compensate for that.

2) The computer is failing somewhere, like on the lines that control the firing of the spark plugs.

Any other guesses? I currently have another 98 SOHC that I can pull parts from to test with, so the next thing might be to try swapping the computer, then spark plugs.

 

[boominXplorer]

Usually rich codes point to a bad mass air flow sensor.

 

[Justin_]

When I was first looking into the rich codes I thought about the MAF. back then I ordered a new one but the new one seemed to be given completely wrong numbers and I ended up returning it. I ended up looking at the MAF on a code reader and the numbers seemed reasonable, however I can definitely double check it and swap it between my other good 98 SOHC and see if it makes a difference.

I'll give an update tomorrow.

 

[boominXplorer]

To be honest I've had better luck with a used oe maf than any "rebuilt" or new one.

 

[XploderPhil]

1) Somebody replaced the injectors with slightly bigger injectors at some point

That is a very real possibility. Last Nov I purchased a Bronco II, (between two owners) they replaced a lot of parts including the injectors. I rechecked everything they had done plus I replaced a bad MAP sensor. Once I got it running it was very rich. The replaced injectors were 24lb units from Autozone. In researching Ford used 14lb injectors in most 4-6cylinder application. Every parts wholesaler and retailer listed 14 and 24lb injectors. Somewhere I believe in typo error the 24lb injector became available for the 2.9L. Anyway I installed new Motorcraft 14lb injectors and she runs great and I have a set of 24lb injectors for sale.

 

[Justin_]

I took the entire intake hose off my other Explorer, dropped it on this one, and reset the ECU and still not really seeing anything different. Haven't officially posted misfire codes yet but I'm sure it's about to.

Guess I'm going to dig further into the ECU and/or injector potentialitys today

 

[Justin_]

I think I have a lead on what might have happened. I was poking around and realized that when I swapped out the engine bay wiring harness during the body swap I did not fully tighten down the connector between the engine bay wiring harness and the engine wiring harness. I had only hand tightened it. Because of that it was not fully seated.

I'm wondering if what happened was that when I was running around in the mountains where it was very wet some water got in that connector and shorted something out. I already tried tightening the connector and that didn't help and then I removed and reinserted it a few times to see if there was any gunk that needed to be rubbed off but that also did not help.

for all the guys that are more familiar with the electrical system out there, is there anything that would have reason to fail with a short that wouldn't recover after the short has been removed?

I was going to try swapping the ECU, but then I remembered that because of all the pats components I probably won't be able to do that without swapping a lot of stuff. Do you guys think it's worth trying to swap the entire pats system over?


I wish I had just tightened that damn bolt the first time. But you live and you learn.

 

[Justin_]

Well, I spent the time to swap out the ECU and all of the pats system modules and unfortunately that didn't change anything. New ECU and still having the same issues.

I'm also learning that the GEM module from a four-door 98 XLT does not properly function in a two-door 98 sport. I get the flashing 4lo 4hi lights and cannot switch into 4x4.

 

[donalds]

Did you check the capacitors in the pcm

 

[Justin_]

Did you check the capacitors in the pcm

I just tried swapping out the PCM, GEM, PATS, and key cylinder modules and it did not change. All of the modules worked in the other car, so I think that bypasses the need to check that.

 

[Justin_]

Interesting little update here. I have not yet switched back the ECU and all the associated components including the GEM module however the 4x4 system is working now. I'm not getting the flashing 4hi 4lo lights and it can switch into 4x4 just fine now. Not sure what's up there. It doesn't really matter since I'm going to switch the whole ECU and PATS system back over shortly anyway.

 

[Justin_]

Well in not excited on just randomly swapping parts, but I got the injectors out of my other explorer (had to tear apart the intake anyway to replace the timing chain tensioner).

I'll try swapping them into my 2dr hopefully some time this weekend.

I'm not sure what else there is to try. Compression is good, plugs, wires, and coils are new, it's not the ECU, fuel pressure is fine, there /could/ possibly be a timing issue but I hope not. The chains, guides, and tensioners are still behaving. If this does not work I might just take it to a shop and see what they have to say.

 

[Justin_]

I have not swapped out the fuel injectors yet, but looking around it looks like the crankshaft position sensor may have recently made contact with the harmonic balancer. Is that potentially something that could have caused the issue? I hate just throwing random things out there but I'm not sure what I'm really even doing at this point.

 

[Justin_]

Okay so going back to random troubleshooting where I don't really know what I'm doing. I tried running the vehicle and pulling the spark plug wires off the coil pack one at a time to see what happens. For cylinders one through five I will pull the wire off and the engine will have a bit of a jump and take a second or two to get back to rhythm, but when I pull the wire on cylinder six basically nothing happens.

I already tried checking compression on cylinder 6 and it's fine, when I pull the wire for cylinder 6 the spark is obviously jumping up to 3-in between the coils and the wire so I really doubt spark is a problem. Fuel pressure up to the fuel rail is fine, so I'm wondering if it's just an issue with injector number six and it's throwing off cylinder one which directly follows it and the computer sometimes interprets it as an issue with cylinder 3 which immediately precedes it.

This would explain why most of the vehicle's power is there but not all of it. It doesn't feel like I'm missing two or three cylinders, but there is a noticeable reduction in power. It could potentially also explain why it popped codes for cylinder six and one and occasionally three, and it would be a single point of failure which makes more sense than multiple simultaneous failures. So I'm hoping that's what the problem is.


I already pulled off the upper intake and have the lower intake almost removed but then it started hailing out of nowhere so I went inside for a little bit.


I'm going to start by trying to replace only fuel injector 6 because they're kind of a pain to get out and I think I might have torn some of the rubber seals on my other Explorer when I pulled them. Side note, are those pretty easy to order either just as a single or as a set?

 

[Justin_]

That was it. Fuel injector #6 had failed and was not injecting fuel. Replaced it and we are running at 100% again.

Probably just going to order a full new set so I don't have more trouble with them in the future.

 

[XploderPhil]

Glad to hear you got it fixed

 

[EB4X]

Good job!