wierd bizzarre occasional misfire

ok so compression checks out even on all cylinders at 195 cold engine. coils are fine does not follow the coils, injectors same thing does not follow. leakdown is good too. suspected vaccum leak so replaced intake manifold orings and egr oring and throttlebody oring and same thing. swapped entire intake manifold assembly from my spare engine and still have it. all wires ohm out perfectly. maf has been replaced same with all 4 02 sensors. im perplexed. occasionnaly tho under 2350 rpms it will miss like crazy and then hit 2351 and it stops completely according to the pids. it will do this regardless of load for 10 or 15 minutes of driving even doing it at idle then it will go away and come back weeks later. its really getting on my nerves. vehicle is 2003 mazda 6s v6 with headers p&p heads and lower intake cold air intake and maf custimizer fluiddamped crank pully, weapon x coils and ngk iridium ix plugs gapped at .059(been running that gap on plugs since car had 30k on it now have 138k. yes they have been changed every 20 k) this problem has been ongoing since vehicle had 85k on it.

A piece of rust flaked off the inside of the gas tank, rested itself hear the fuel line inlet where it lays flat. At RPM higher than 2300 the need for fuel is greater so the flounder-like rust flap now becomes an obstruction.

OK, that was a guess, but if you put a pressure gage in the fuel rail you'll know for sure.

Blow the fuel rail and lines out and buy a good quality fuel filter.

Dan

its under 2300 not above.... fuel sock has been changed out every 20k also. and i even swapped the fuel rail with my spare. but you tried.

What engine event happens at that RPM?

Variable valve timing switch the programming?

Bad ground/short on system that only operates intermittently... like AC compressor or air pump...

Stuck or partially failed relay

Damp mouse nest in the ECU box traps moisture and wreaks havoc 2 days after a good rain?

Silly stuff like that.

vvt switch isnt untill about 4400 ac has no effect.... nothing specific. ive messed with this thing. had friends keep the car for weeks at a time looking at it. none of us can come up with a reason

KATYB wrote: vvt switch isnt untill about 4400 ac has no effect.... nothing specific. ive messed with this thing. had friends keep the car for weeks at a time looking at it. none of us can come up with a reason

Leaky injector?

ya and rain has nothing to do with it or humidity its completely random. fuel pressure stays consistent during the miss. and miss does not show up on scope as far as injectors or coil. only knock sensor picks it up.

doesnt follow injects only cylinder 1 no matter where it goes

im getting to the point of just swapping out the harness and engine and seeing what happens.

Faulty harness or cam sensor? A failing cam position sensor could explain how it is random and no specific cylinder. Harness would be similiar too. Harness could be checked with a running wiggle test.

KATYB wrote: doesnt follow injects only cylinder 1 no matter where it goes

You're saying it is always (and only) cylinder #1 no matter what you do with injectors / plugs / etc?

if yes, then i'd be looking for mechanical issues affecting cyl #1. valve hanging open so it doesn't build compression, lifter/HLA collapsing so it doesn't get a full inhale or exhale?

I am with P.

Carbon on intake valve, bad valve spring, HLA oil flow failure or possibly debris in intake.

ok well gonna rip that head off and go through it. see if i find anything.

WAITAMINUTE! Aren't you a graduate of a mechanic school and looking for a job? Re: some post called Whining.

And you're asking us?

I tried.

In reply to KATYB:

mild threadjack: How bad is it to change the fuel sock? Remove back seat and pop off an access cover and it's all right there? I've got an '03 6s sport that sucks up air bubbles (or gets the supply blocked?) in right hand turns when the fuel level gets down to 1/4-tank. I'm going to tackle that job as soon as I'm done rebuilding the front suspension in wifey's '03 Odyssey.

Always #1? Before you snatch the head off, see if the pressure line from the pump enters the fuel rail at #1. We had a couple of Mercedes V6's (112 engine) with corrosion in the #1 injector but nowhere else. Our theory: water and corrosion are heavier than gas so they get sucked into the #1 injector. Or maybe it's linked to sunspots.

Waaayy back in the 1980's, Ford TFI (Thick Film Ignition) modules could be driven bugE36 M3 by high power transmission lines.

no inlet of fuel is by cyl 3. and yep pop off cover pul assembly and done.

914Driver wrote: WAITAMINUTE! Aren't you a graduate of a mechanic school and looking for a job? Re: some post called Whining. And you're asking us? I tried.

lol well ya because sometimes you want another opinion. and grad of mech school no. i am a master tech tho yes. i didnt finish school because i was hired before the end of school :)

Compression is ok, and you can't find this on the ignition or fuel injector traces. So how do you know it's always the #1 cylinder?

knock sensor corresponds with cyl 1.

Beware the red herring.

Are you seeing a trace on the knock sensor, or seeing a trace on the #1 cylinder, and are simply blaming the knock sensor?

Certain era Toyotas are notorious for elusive problems due to partial failure of the shielding for the knock sensor. Some have gotten band-aid improvements by disconnecting the knock sensor, others had to remove it from the ecu connector.

the pids are showing a miss on cyl 1. picked up by the knock. nothing shows up ignitition or fuel wise. before i rip the head off im gonna close the intake falves and give them an atf bath. then ill do the same for the exhaust. and see if it comes back

i have nothing else to add at this point except that i'm haunted by the on/off nature of the problem at a particular RPM. that almost screams out "processed signal" and makes me want to find out what the ECU does differently above / below that speed, and if there are six separate outputs from the ECU for whatever function it is that gets controlled differently above / below that speed. it is the combination of "on / off at 2350 rpm" plus "cyl #1 only" that troubles me.

you haven't found anything useful on the duratec or mazda6 forums?

edit: additional thought, possibly not even feasible: is there any kind of electronic control on the oil pressure, ie regulated below 2350 rpm and full-flow above, such that a weak HLA or restricted oil flow to an HLA would be "fixed" by the increased oil pressure available above that rpm?

nothing i can find. and ya the mazda6 forums we wont discuss those. i am not a member of any of those. and the duratec forums only seem to know the 2.5 and 3.0 ford and lincoln versions the jaguar and mazda aj versions no one knows and cylinder heads are completely diff cause of the vvt.

Have you tried switching the fuel injectors around? I'm wondering if it's partially clogged or faulty, but the problem only occurs at a certain duty cycle.