Misfire


#1

I used a fluid pump to suck out 1 gal of trans fluid thru the fill tube. I refilled. Started car, have engine miss. Flashing cel, comes and goes. I literally touched no other parts under hood. Yes, the fill tube is within 1 foot of maf. Perhaps I touched maf? What the heck? Life is not fair. Can’t find my code reader. Lent it to kid.


#2

Transverse v6. I pulled front plugs. All 3 have same gap, about .100. Ground lug looks good. Center tips are all worn down equally. Installed new plugs. Miss is gone. Runs the same as last week. No better, but it seems to be ok now.


#3

Well, a lot of help we were. Glad it’s straightened out even if the cause isn’t clear.


#4

When are you going to replace the back 3 plugs?


#5

What is the gap spec’d at for your car?


#6

Spec is .045. I pulled 1 of the front plugs since it is real easy. Than I saw .100 gap. So I pulled all 3 fronts. Same. Than I got new plugs and all are in now at 045 gap. Autolite 104 plugs. I started with checking plugs since its easy to do. Car ran fine last week. Miss showed up. New plugs. Runs fine now. I have only had car for 3 weeks. My question was, what caused miss? I thought I bumped a wire or harness doing the fluid. Turns out plug gap was bad but may or may not of been issue with miss


#7

It very likely WAS the cause of the miss. That’s a huge gap difference.
My suggestion now is to go through the recommended maintenance in the owner’s manual (or in a repair manual maintenance schedule if you lack the owner’s manual) and get everything else up to date. Clearly the prior owner wasn’t religious about maintenance.


#8

poor picture. no flash


#9

Looks like what I’d expect with a .100 gap. That little nub in the center should be a post, an electrode. The material has all been vaporized over time by the heat of the spark, the vapors having been drawn away from the metal electrode by the flow of the arc. It was actually sent out the tailpipe molecule-by-molecule.

The main thing that makes iridium so good is that it doesn’t do this anywhere near as rapidly as the other metals available. The gap stays much better much longer.


#10

A .100 thousandths plug gap is obscenely wide and even a new plug gapped at that much can misfire. That kind of gap is also rough on plugs wires, coils, etc.


#11

I just looked at the picture

That’s just a worn out, old, probably relatively high mileage plug

It just proves that simple maintenance actually fixes some problems


#12

My Corolla developed a definite ping when the spark plug gap increased due to normal wear 50% wider than the spec. Replacing the spark plugs (& with the correct gap) immediately cured this ping. So its not surprising that you were noticing some problems. I expect you already know that it’s important to replace the other three plugs too.

Edit: What caused the miss? The spark occurs because when the electric field in the gap increases beyond a certain number of volts/inch, the electrons in one atom of the fuel/air mixture start to jump to the next atom creating a current, the current is the spark. If you’ve got 1000 volts across 0.050 inches, that’s 20K volts/inch. When the gap increases to 0.100, that’s 10K volts/inch. So the electric field strength decreases when the gap increases. Not enough electric field to pull the electrons from one atom to the next, so no spark. Hence, a misfire.


#13

question #2. car has cold miss. starts fine. but it stumbles if i try and back out of garage during initial 30 sec of starting. if i sit for 1 min or so, the miss is almost gone. car seems to drive ok initially while cold. it had this cold miss 3 weeks ago. i thought new plugs might fix cold miss too. have not cleaned or checked throttlebore for carbon buildup. i assume nothing has ever been touched on this car.


#14

So far all you said is that you changed the 3 plugs in the front. Until you deal with the other three plugs (did you?) I wouldn’t bother to look at or speculate about anything else. A new set of wires would also be a fine idea.


#15

I changed all 6 plugs last week


#16

I would move on to checking fuel pressure. It has to be done with a gauge. One simple thing you could try is to make sure that the pump has fully pressurized the lines before starting. Do a whole bunch of on-off-on-off-on-off etc. key cycles to energize the fuel pump at least a half dozen times before cranking.

You should also still worry about the spark plug wires. And you need to worry about the coil packs now because having those old plugs in there beat them up. I would think, however, that they would first show problems under heat rather than on a cold start.


#17

I’m gonna go with the fuel line pressurization possibility too. Except I think it’d only need one or two cycles of the key to repressurize the system. I doubt of the shutoff delay on the pump circuit is less that 4 seconds, and the pump can easily pressurize the line in 4 seconds.


#18

Mine takes 3. I’ve checked it. Of course, that itself may indicate a weak pump, but I have no driveability problems. I always say a half-dozen or so just to be sure for “testing” purposes. The return line will send it back if you keep doing it after it’s up to pressure. Of course, a fuel pressure gauge at a time like this is priceless.


#19

Agree. I too have lived with a fuel system that depressurizes and simply turned the key to ON once or twice. Never did put a fuel pressure gage on, but I suppose it would have been a nice touch to see the line pressurize.