The car is a 95 maro w/ 3.4
The Problem is: A missfire that occures more @ WOT, when you back off the throttel it goes away SOMETIMES, and sometimes its constant when the accelerator is deppressed, no SES codes moreso occures when warm, and days will go by with none @ all


Plugs and wires
02 sensors
map sensor
tps sensor
had ICM and coils tested all ok
IAT
fuel filer
fuel injectors test ok

runs 45 fuel psi
180psi and good first stroke on compression



what am i missing? Cam sensor or crank? PCM? all i can think that is left


Don

 

I have had that exact same type of misfire happen twice. It was not very noticeable at idle but quite noticeable under acceleration.

Once when I bought cheap off-brand wires, after about 9 months they started sparking through the jacket, into the lift bracket IIRC. Found it by idling the car with the hood up at night.

Second when I replaced them with Taylor SpiroPro, after 2 years one of the boot crimps failed. The jacket and molded boot slid right off the connector when I went to check plugs.

I've had the MSD SuperConductors on for 3 years now and not one problem.

 

thanks for the reply, i am currently running accell preformance wires, replaced them 2 rimes now with no luck

don

 

I had a similar issue and it was my catalytic converter

 

That's what I was going to suggest. When the cat clogs, it will prevent exhaust flow, so it will cause misfires. Next time it starts doing it, pull over and check up under the car. If the cat's glowing, it's dead. Either gut it, remove it, or replace it.

 

check fuel pressure at wot it could be fallin off but the accel wires suck so i would change them fast anyways

 

thanks all, put msd wires on it today, still doing it, and the cat had been removed, just had a borla exhaust installed also,i havent checked fuel psi @ wot, my guage isnt long enough to road test it, there has to be a lod on the enginwe to cause it


Don

would tha be the pump or regulator in most cases?

 

If it was the pump I would think you'd see a loss in pressure even at idle, and usually it will get noisy before they go out as well. The regulator is a simple vacuum-controlled device, however a loss of vacuum results in higher fuel pressure, so you would think it would be running rough at idle and fine at high rpm's if that was the case. The FPR also has a rubber diaphragm that can crack with age and leak fuel past but again you'd be running rich under high-vacuum conditions there as well.

Cam sensor controls the SFI but loss of signal should throw an SES and the system will revert to batch fire, basically turning it into an MPFI system. I've known guys who put the Crane or GMPP cams in their 3.4 which don't have the lobe for the cam sensor, and the car runs great. The 2.8/3.1 used MPFI as well.

IIRC the front crank sensor is required to start the car and controls timing during idle, loss of signal there would result in a no-start condition. The passenger-side crank sensor I believe takes over in the upper rpm's, and might have something to do with your miss, but again should throw an SES.

Have you tried a good, thorough cleaning of the air filter, intake, TB and plenum?

 

well ive had the same problem since febuary and i havent been able to figure it out im thinking its a vacuum problem but i haven found it.

 

do you have boush spark plugs in it? i had them in there and had the same problem put ac delcos back in went away

 

i had rapid fire pluggs and replaced them with autolite pluggs and bosch wires

 

Yes i have had the TB off and cleanded and ran an upper intake cleaner in it, it doesnt seem to have a vacuum leak, idels perfect, and most vacuum lines are new, the only place that i think i may have a vacuum is in the evap purge, but still non ses codes

also im running AC plugs i it, i hat bosch

Don

 

IIRC fuel pressure should be 41-47 at key-on (not running) and drop 7-10 at idle.

Mine stock was 41 at key-on and 35 at idle, so a bit on the low side. Pulling off the vacuum line brought pressure up to 45.

At what point are you seeing the 45psi?

 

KOEO

and 38 ish running, havent been able to road test guage isnt long enough, would disconecting the vacuum line and roadtesting tell me something

 

That is excellent pressure, I would not suspect the pump at all. Disconnecting the vacuum and roadtesting would probably not tell you anything other than how it drives while rich. You can snake the line over the cowl and tuck the gauge under the wiper, face to the glass, I've done that before to see what the pressure is doing on the road.

I am running out of ideas here. Have you tried a different brand/grade of gas?