Persistent Misfire, please help!

[aston@ultrasw.c]

85 carrera, 3.2, stock motor (except for K&N filter). 200K miles.

Engine misfires during acceleration, small throttle opening and WOT. The engine stumbles and hesitates.

It will reach redline and power seems strong.
Idle is OK, overrun is OK.

Checked/changed so far

plugs
dist cap
rotor
wires
AFM

Vacuum leaks (propane torch method)

fuel pressure
fuel injectors
fuel filter

A clue:

I live out in the desert so it never really gets very cold, but last night the air temp was around 50F and it ran very much better, not perfect, but a distinct improvement.

So what next?!?

Chris L.

[cdrik915]

What about your Bosch airflow meter system ?

Verify his plug and values , it's a common issue on our cars .

Just an idea.

[aston@ultrasw.c]

I have already cleaned out the airflow meter and checked it's function as a voltage divider.

I also checked the temp sensor and the throttle position sensors.

Maybe I will make up a test cable so I can monitor AFM function while actually driving.

[al lkosmal]

I have had a coil intermittently fail during acceleration and at higher rpms (4500 and up). couldn't find the problem until it failed completely. New coil...problem went away.

[aston@ultrasw.c]

Originally Posted by al lkosmal

I have had a coil intermittently fail during acceleration and at higher rpms (4500 and up). couldn't find the problem until it failed completely. New coil...problem went away.

It's worth a try, Wayne send me a new coil!

[aston@ultrasw.c]

OK so I tried a new coil.....and it's fixed.....err.......no it isn't........

It went from persistent to intermittent. Sometimes the engine will accelerate cleanly other times it will misfire.

Balls!

Nice idea though. Could it be the grounding of the coil...does the coil need to be grounded?

Anything else worth a try?

[stlrj]

It might be a bad 02 sensor. Try unplugging it and see if it helps.

[aston@ultrasw.c]

It's not the O2 senser.

I made up my test cable for the AFM so I could monitor its output while driving. All seems OK there - no erratic readings.

Bastard.

So now what?

[BReyes]

Have you checked gap/function of crank sensor(s)? Cleaning them and then ...

Regards,

[Steve W]

Sounds almost like a fueling issue. You need to determine if it's fuel or ignition spark and narrow it down from there. Monitor your O2 sensor voltage and if it's too low (not including when it's misfiring) you may be running too lean. Too wide of a spark plug gap will make it also hard to fire under load. Also check the reference sensors for (a) gap and (b) wire deterioration. Check your head temp sensor too.

[aston@ultrasw.c]

Originally Posted by BReyes

Have you checked gap/function of crank sensor(s)? Cleaning them and then ...

Regards,

No I haven't. I read in the manual about checking resistance, easy enough.

If they pass a resistance check they might still be putting out erratic signals, how can I check function while on the road?

Lastly, does checking the gap/clean/replace require a motor drop??!?

Cheers

[aston@ultrasw.c]

Originally Posted by Steve W

Sounds almost like a fueling issue. You need to determine if it's fuel or ignition spark and narrow it down from there. Monitor your O2 sensor voltage and if it's too low (not including when it's misfiring) you may be running too lean. Too wide of a spark plug gap will make it also hard to fire under load. Also check the reference sensors for (a) gap and (b) wire deterioration. Check your head temp sensor too.

I thought so too. I have installed a fuel pressure gauge and the pump/reg/check valve are OK. I also had the injectors cleaned and checked the vacuum circuit. I'm not sure what else there is!?!

I'll make up some test leads to monitor the o2 sensor. I have a wide band somewhere and a data logger.

Plugs are OK. Temp sensor ohmed out correctly.

I'll check the CPS and ref sensor.

Cheers

[aston@ultrasw.c]

I pulled the harness off the ECM and performed electrical tests as described by Bentley

Crankshaft speed sensor = 1020 ohms
Ref Sensor = 1010 ohms

Both in range.

The only unusual thing was a single brown wire pulled out of the harness with a connector on each end. The connectors were disconnected.

I traced one wire to terminal 10 on the ECM which is assigned, bridge to ground California cars. The wiring diagram has it connected to the altitude correction box.

So should it be connected or not?

[rcaradimos]

Richen the fuel mixture 1/16 to 1/8 turn clockwise on the air box. With the cold/cooler temp you said it ran better, the CHT sensor must of registered the need for additional fuel. Does it run worse on humid days vs cool dry days? Could be a touch lean, Steve W I think nailed it.

[Scott R]

Take it for what its worth, but I just fixed an odd misfire on a 3.2 for a buddy. The air boot that goes between the afm and the throttle body had some micro-fine cracks in it. Was just enough to let false air in and cause a misfire.

[aston@ultrasw.c]

Originally Posted by Scott R

Take it for what its worth, but I just fixed an odd misfire on a 3.2 for a buddy. The air boot that goes between the afm and the throttle body had some micro-fine cracks in it. Was just enough to let false air in and cause a misfire.

Well, Scott R - you called it!!!!

I sniffed around the boot with a piece of tubing stuck in my ear, and sure enough all kinds of air flow could be heard.

I loosened the clamps and gave the boot a medium tug, it tore in half

I have a DE tomorrow so a friend was kind enough to lend me a spare, misfire fixed yay.

But what a PITA.

Thanks everyone for your contributions.

[rcorujo]

Originally Posted by aston@ultrasw.c

Well, Scott R - you called it!!!!

I sniffed around the boot with a piece of tubing stuck in my ear, and sure enough all kinds of air flow could be heard.

I loosened the clamps and gave the boot a medium tug, it tore in half

I have a DE tomorrow so a friend was kind enough to lend me a spare, misfire fixed yay.

But what a PITA.

Thanks everyone for your contributions.

Can you please show a picture, diagram or photo of the part you replace (The boot). Im not to familiar with all the hoses, boots from the air intake system. I think my car started a similar problem these days. I started replacing the spark plugs but miss-firing continues.

Thanks in advance!