What da prolem is? VIII

[decipha]

alright this is an easy one

said vehicle is a 96 crown vic 4.6L w/ 4r70w trans, completely bone stock

vehicle runs fine but has a random misfire at idle and very low loads, its a random misfire but occurs on both banks of the engine and at random times while idling and cruising. Has no rhythm to it and sometimes you can go days without a single hiccup.

The owner of the vehicle says it's not really an issue just a nuisance and the vehicles runs fine otherwise. WOT is nice and strong and the engine feels very responsive. The engine is completely bone stock with 140k miles on the clock but a complete full tune up was done less than 15k miles ago. The problem has persisted since 90k miles and nothing has seemed to helped it. When it first occurred he brought the vehicle to the dealership (car has always been a dealer only vehicle for maintenance) and they replaced the following. Note that since the issue arouse 50k miles ago the following has been changed to no avail

ECU is throwing no codes

timing chain and tensioners
plugs
wires
coil packs
cam and crank sensors
pcv valve
fuel filter
fuel pump (went bad at 130k)
air filter
maf sensor (mechanic changed originally thinking it was the problem)
EGR valve and flow sensor

The vehicle has been to 3 different ford dealerships and 6 different independent highly regarded repair shops that specialize in fords. The issue was never resolved. I was contacted by a friend of mine that owns a local repair shop to come check it out, he had the car for 2 weeks and was completely baffled but was determined to get it fixed.


I come over and first thing I do is plug in to see whats going on in the ecu, everything looks perfectly nomal but when the engine misfires i can see that hego go lean and the lambse richen up to compensate then come back to normal. I see its occurring on both banks of the engines and there's no repeatability to it, it's a completely random misfire and when you rev the engine up it doesn't do it.

So I get to snooping under the hood and voila!!! The problem is staring me right in the face.

anyone want to take a stab at it?

[YaBoyHalle]

this actually happened to me for a loooong time. coil was too weak, couldnt gap the plugs wide enough. loose alternator mounting, wiring. loose engine/bat ground.

[decipha]

nope all that was checked and/or replaced no issues there

[ranga83]

O2 wiring melted/broken but still making contact most of the time?

[decipha]

nope all's well there

[decipha]

as you can see, i changed the name of the game to be more accurate

cause of carnage indicates damage ensued

[red5.0fogger]

On a side note: if there is a missfire detected at cylinder x, How does the ecu figure out which cyl it was?

[decipha]

by the rpm drop when that cylinder reaches tdc

[red5.0fogger]

Is it a tiny vacuum leak? I heard of explorers sucking in the intake gaskets slightly causing weird shit that seams random.

[YaBoyHalle]

ignition switch, if they were still used.

[decipha]

nah no vac leaks

yea all ford up until recent have ignition switches lol, nothing wrong with it

[red5.0fogger]

Sounds so much like egr stuck open just a tad but I see they changed it. How about a loose crank sensor?

[decipha]

nope

[ranga83]

im at a disadvantage here, I don't know what the engine bay looks like on all these cars haha

[decipha]

lol no different than any other

[decipha]

I guess this one was a little more difficult than I thought it would be

The capacitor next to the coil was bad, it had residue of it guts visible on it

replaced it with a spare I had laying around and all was well! Not an issue since.

[red5.0fogger]

This segment gives US a taste of what it's like to try and help with a car from long distance. Without sight ,sound ,smell. Not easy, decipha does a good job.

[YaBoyHalle]

you're exactly right in that. i had no idea they had caps for this car.

[decipha]

yeah they have little resonator capacitors that bolt down next to the coil packs, most people think its there to prevent radio interference but its actually there to help regulate the coil voltage, its a very small cap but that's exactly whats needed to prevent random misfires

[slimmons2009]

Mike,
I know this is an old subject but would this still be an issue with after-market coils/ignition systems? Or better yet, should we keep the capacitor when using after-market coils/ ignition systems?

[decipha]

yes the capacitors MUST be there

[slimmons2009]

That could be an issue with mine. I notice a little miss at idle and was going to investigate next time home. Guess I will be adding a capacitor to the coil to see if this clears the issue. And you are saying to use a capacitor even with a MSD ignition system?

[decipha]

with any, all coil packs need a resistor to buffer voltage drops/spikes