Missing

[JoesSHO]

New this week- a miss coming either as a "popping backfire" (only able to detect it in the cabin) when cruising, at startup as a stuttering RPM needle as it revs on start, and as a stuttering needle as it comes down off an acceleration. All of them are rhythmic, and nothing is there when it is under acceleration, neither hard not soft acceleration. Before I replaced the rear bank, I had a few incidents where I would start the car and it would act like it was way out of balance in a miss, but in a rhythmic miss, it was nearly perfectly timed.

I've replaced all of the plugs, and the rear bank of coils are new 2 weeks ago.

Does it sound like an injector, or a front coil? I do not have a stutter at idle.

Does the SHO use the common Ford 19lb. injectors? I don't remember what was in there, but I thought they were yellow.

Thanx!

BTW- got 25 MPG on the last tank of gas, lots of highway, a couple of good runs though.

 

[kit352]

could be one of the front bank coils.

 

[Exitwound]

Could be one of the front bank coils, and it could also be a TPS sensor. I had the same problem last week and I replaced the back 4 coils and the TPS at the same time and it went away. I was able to get my TPS voltage to .998 key on and 4.8v WOT and it was still the same 2 days later. Check yours and see where it's at.

 

[JoesSHO]

I'll have to do that. Same as on my Mustang? ...probe the green and ground the other multi lead for the reading?

Thanks for the help...

 

[Exitwound]

I just grounded my voltmeter on the battery and probed the middle wire with a paperclip.

 

[SHOZ123]

Front coils get just as hot as the back when you use the appearance cover.

 

[JoesSHO]

Does it sound like a coil Paul? It's so specific how it happens, like a cylinder is misfiring. I picked up a TPS tonight just because, but I didn't put it on yet. Any other thoughts on the symptoms? And I am indeed using the appearance cover...

 

[SuperHO]

Joe's gotta have a pretty engine....


Kinda hard to check coils while it's idling if the problem isn't present at idle. considering your fuel mileage, however, I'm inclined to think that it's not an ignition related problem and would lean more towards checking the TPS as was stated earlier. 25mpg on a V8 SHO is pretty damn good, all things considered.

 

[JoesSHO]

That does make sense. I just don't know exactly how the coils cycle under load, or if their voltages change at all, or if it is merely the occurance of signal. I'll plop the TPS on tomorrow, if that fixes it, great. If not, eh, it's a SHO and I'm used to this by now.

And get this- all the grease and crap from when I had the engine torn open for valve cover gaskets/plugs/coils all over the intake, etc, from 3 weeks ago- it's still there. I've washed it once in the last 3 weeks. I'm seriously slacking on her... No wonder she's running like neglected ass. Not always, she was a runner when I left work today ****** off... Ha ha!

 

[SHOZ123]

I would guess coils. They are random it seems in when the misfire.

The coils are hot all the time and the PCM grounds them to fire them. Just like any other coil.

I seriously doubt if it is the TPS.

 

[Exitwound]

I wouldn't think the TPS would cause a misfire, maybe a rough or hiigh idle but not a misfire. When I changed my TPS I also changes 4 of the oldest coils and i'm more inclined to think it was one of the coils.

 

[JoesSHO]

That's the odd thing- it's random only in the sense that it doesn't occur when under acceleration- OR- I can't tell. It is rhythmic though. It's constant on startup, it's constant on RPM rundown after letting off the gas, or when running down when shifting gears. The only think that made me sort of think TPS was the rhythmic super-miss it had a couple of times- it literally shook the car- like one or more cylinders were dead. It only did it twice, and both times from a startup. Weird.

I've got 4 coils from the rear bank, good news is, they're easy now. I'll toss the TPS in anyway, 220k on the original is good for retirement. I'll be checking this out tonight...

 

[NebraskaSHO]

My coils that took a crap always were fine under acceleration but acted up under off throttle, idle, etc. situations. Until the coil completely failed, they smoothed out under a heavy foot.

 

[JoesSHO]

That was also my experience with my old '97. I think one of them is acting up, (maybe more) I'll try swapping one in tonight. It would be nice if it was something $2.00 to fix. Ha ha! I love my car...

 

[Exitwound]

Maybe under idle the voltage drop in the system from a slower running alternator plus the aged coils are enough to make one of the coils not able to produce enough voltage sometimes to fire correctly. When you get on the accelerator it ups the voltage just enough to feed the coil that little bit more motivation it needs to fire correctly.

 

[JoesSHO]

Nice thought- never thought of that- maybe not a bad coil, just weak... Heading out to the garage in 20 minutes... Also, the alternator was new last year- New Ford part.

 

[JoesSHO]

Well, it isn't the coils. Tried the unplug, all affected idle. Swapped in a known good one to each front bank, rotating the replaced good one to the next. All ran exactly the same after the swap.

When I unplugged the TPS, obviously it didn't return to idle as normally as it would, but it did not stutter when I goosed it, as it did when the TPS was plugged in. But, it still stuttered when starting it with the TPS unplugged.

Of course, I would have liked to have put the new TPS on to try it out- but as usual, the heads of the screws used to hold it on the TB are all stripped out so I couldn't get a screwdriver to grab them. I'll have to Dremel out a flathead to get them off.

Search continues, but I'm worrying about the motor and tranny mounts first, then rear brakes, then the miss.