Cylinder #8 Misfire?

[mercurygm88]

Alas I have encountered my first problem with the '01 Town Car. When I bought the car the check engine light was on for a cylinder #8 misfire. I had it reset to see if it came back, it stayed out for about 2 weeks or 2k miles, yeah I drive it a lot. Then it came back, the car had 108k miles on it and I figured hey the previous owner probably never changed plugs lets try that first. There was no coolant or oil in the spark plug wells=good sign and all 8 plugs looked good and fairly new. Changed plugs and had CEL reset. Check engine light came back about 3 weeks and 1500-2k miles later so I bought a Motorcraft coil and put it on #8 all goes well for about another 2 weeks and now the light is back on. I can't hardly believe it's the fuel injector because I'm getting 25-27mpg on the highway, and I've gotten damn near 30mpg on a trip averaging 55-60mph. I've never felt the slightest miss or hesitation either. Yesterday morning I had my buddy at Oreilly's reset the CEL with his fancy Bosch scanner and it came on about 50 miles later. Again cylinder #8 misfire, again I never felt a miss or even the slightest hesitation and I paid attention for it. I had the same problem on my '95 GMQ and it was a bad plug wire on the #8 cylinder and you could definitely feel it miss especially going slow uphill. Is there even the slightest possibility that the ECM is detecting a problem thats not there? This isn't what I'd call a big problem because as far as I can tell and as far as my fuel mileage goes there isn't a problem, but as far as the ECM and the annoying orange CEL are concerned there is a problem. Any help would be greatly appreciated.

[1990LTD]

It's odd that you're getting a misfire code so quickly and it's not running poorly. I've seen Fords with a dead cylinder take a while to throw a misfire code...they take forever sometimes.


For shits and giggles, move the injector to a different cylinder. See what happens.

[IPreferDIY]

This might be a longshot, but since you don't have any overt indications of a misfire, it just might be relevant. The computer has some kind of fancy misfire monitoring stuff, and there's a process for it to learn a "profile correction". Here's a link that should take you to a PDF dealing with OBD II stuff for 2001:

https://www.motorcraftservice.com/vd...f/obdsm109.pdf

Alternatively, you can Google "[year] my obd system operation", and it should come up.

The misfire stuff for 2001 starts at page 8, and there's stuff about the profile correction at the bottom of page 9. Essentially, you're supposed to decelerate from 60mph to 40mph a few times without touching the brake or gas pedals. They say this represents deceleration at exit ramps, but I've gone months without ever having done this during normal driving. My code reader/scanner can read some mode $06 data, and the profile correction is reflected in the results for "cylinder events tested". I have to go out of my way to do the decelerations when no one's around before I get proper results.

Like I said, it's a longshot, but it's one of those cheap "can't hurt" things.

[mercurygm88]

It's odd that you're getting a misfire code so quickly and it's not running poorly. I've seen Fords with a dead cylinder take a while to throw a misfire code...they take forever sometimes.


For shits and giggles, move the injector to a different cylinder. See what happens.

I thought it was odd too I don't know if it's true but I read somewhere that the ford OBD II misfire threshold is something like 24,000 misfires? I reset it this afternoon and since have only driven 25 miles, I'll report back when it comes back on and if I'm feeling real ambitious I'll move the injector. I should probably try moving the coil as well only separately, it's possible that a brand new coil could be screwy, unlikely but possible.

[mercurygm88]

This might be a longshot, but since you don't have any overt indications of a misfire, it just might be relevant. The computer has some kind of fancy misfire monitoring stuff, and there's a process for it to learn a "profile correction". Here's a link that should take you to a PDF dealing with OBD II stuff for 2001:

https://www.motorcraftservice.com/vd...f/obdsm109.pdf

Alternatively, you can Google "[year] my obd system operation", and it should come up.

The misfire stuff for 2001 starts at page 8, and there's stuff about the profile correction at the bottom of page 9. Essentially, you're supposed to decelerate from 60mph to 40mph a few times without touching the brake or gas pedals. They say this represents deceleration at exit ramps, but I've gone months without ever having done this during normal driving. My code reader/scanner can read some mode $06 data, and the profile correction is reflected in the results for "cylinder events tested". I have to go out of my way to do the decelerations when no one's around before I get proper results.

Like I said, it's a longshot, but it's one of those cheap "can't hurt" things.

I will look into that, it can't hurt to try. That'll give me something to do tonight I know where theres 25 miles of wide open road that no one hardly uses.

[mercurygm88]

Back on today at lunch, it's been 367 miles since it was last reset. As usual no hesitation and no discernible miss. Averaged 24.7 mpg with about 80% rural 20% city driving. At this point I think I'm just gonna keep my scanner in the car so that I can reset it whenever it comes on. If it actually gets to where it's noticeable in any way then I'll look further into it. I really don't feel like going through the trouble of swapping injectors or changing the intake manifold gasket if theres no discernible problem. I am going to swap two coils around and see if it moves to a different cylinder though, it's possible that I could have bought a bad coil.

[sly]

for giggles... run a tank of fuel system cleaner through. If an injector is starting to clog up, it could cause the issue and that stuff will clean them out. And it's just $8 for a bottle at the most expensive place I have access to. I usually use the STP version myself.

[gadget73]

The injectors are easy to move on these at least. Should be just 2 nuts and the fuel rail lifts up. No need to remove a bunch of nonsense to get at them. The 4.6 was often the injector donor of choice for HO swaps just because it was so much easier to snatch them vs off a 302.

[mercurygm88]

for giggles... run a tank of fuel system cleaner through. If an injector is starting to clog up, it could cause the issue and that stuff will clean them out. And it's just $8 for a bottle at the most expensive place I have access to. I usually use the STP version myself.

I will do that, the Lucas stuff is almost always on some kind of sale around here. Not sure about the STP but I used to run a can of seafoam in the tank about once a month in my '88 it seemed to keep it going just fine. Not that theres any indication that it wouldn't have been fine without it.

The injectors are easy to move on these at least. Should be just 2 nuts and the fuel rail lifts up. No need to remove a bunch of nonsense to get at them. The 4.6 was often the injector donor of choice for HO swaps just because it was so much easier to snatch them vs off a 302.

I figured it was easier than on a 5.0L but wasn't sure just how easy, I had a leaky #8 injector in my '88 and I remember what a bitch it was mainly because of the upper intake. At least theres no Plenum to deal with on the 4.6.