Some mis-firing input please....

coupe · 07-11-2005, 05:16 PM

Car is a '93 GT. It has heads, cam, intake, etc...5-speed car.

It starts, runs, idles fine and revs fine at a stop. Nothing wrong.

Start to take off and there's a bad bobble/misfiring under part-throttle like spark issues. Give it more gas (ie: roll into it hard) and it is not noticable...

Or, let it accelerate through the bobble to about 2400 RPM and it clears up. Otherwise no problems.

I've set timing, replaced TPS, tried another MAF, sensor and complete meter (19# injectors), and checked cap/rotor/wires, etc...). I also disconnected the MSD and ran stock ignition...no change. Still a bobble/sputter up to 2400...unless you're about 1/2 throttle or better.

My next step will be try another computer and/or distributor.

Any other suggestions?

Italian LX · 07-11-2005, 05:27 PM

What's the results of a compression/leak-down test?

Disney Lincoln · 07-11-2005, 05:28 PM

Pull codes.

coupe · 07-11-2005, 05:42 PM

No compression tester.
But no crankcase pressure problems. Fairly fresh motor.
Could a O2 sensor or both be causing this??? It's ONLY at very slight throttle up to 2400 RPM. Any hard throttle or past 2400 and it's fine.

QWKSNKE · 07-11-2005, 08:53 PM

does it do it when cold or all the time?

QWKSNKE · 07-11-2005, 08:54 PM

any mods on the engine?

Wicked · 07-11-2005, 09:09 PM

Quote:

Originally Posted by coupe

No compression tester.
But no crankcase pressure problems. Fairly fresh motor.
Could a O2 sensor or both be causing this??? It's ONLY at very slight throttle up to 2400 RPM. Any hard throttle or past 2400 and it's fine.

Vacuum leak possibly.

I have seen similar with O2 sensors. Even had a friend whose sensor coupler got burned on the exhaust such that he could plug it in wrong. He chased an intake backfire for weeks.

Wicked · 07-11-2005, 09:10 PM

Quote:

Originally Posted by QWKSNKE

any mods on the engine?

Quote:

Originally Posted by Coupe

Car is a '93 GT. It has heads, cam, intake, etc...5-speed car.

coupe · 07-12-2005, 07:45 AM

Quote:

Originally Posted by Wicked

Vacuum leak possibly.

now why didn't I think of that?!?

I bet that's it. Will let you know.

It does it cold AND warm.

(Oh I also removed the spout connector and ran straight 25Âº timing... it still did it so it's NOT the computer control of timing. Either vacuum or dist.)

Disney Lincoln · 07-12-2005, 11:51 AM

Also try cleaning the MAF.

07-11-2005, 05:16 PM	#1
coupe Super Moderator Join Date: Dec 2004 Posts: 6,313	Some mis-firing input please.... Car is a '93 GT. It has heads, cam, intake, etc...5-speed car. It starts, runs, idles fine and revs fine at a stop. Nothing wrong. Start to take off and there's a bad bobble/misfiring under part-throttle like spark issues. Give it more gas (ie: roll into it hard) and it is not noticable... Or, let it accelerate through the bobble to about 2400 RPM and it clears up. Otherwise no problems. I've set timing, replaced TPS, tried another MAF, sensor and complete meter (19# injectors), and checked cap/rotor/wires, etc...). I also disconnected the MSD and ran stock ignition...no change. Still a bobble/sputter up to 2400...unless you're about 1/2 throttle or better. My next step will be try another computer and/or distributor. Any other suggestions? __________________ " If you can't explain it simply, you don't understand it well enough. " - Albert Einstein

07-11-2005, 05:27 PM	#2
Italian LX Daddy x4 ...we're done Join Date: Dec 2004 Location: Oxford, AL Posts: 11,551	Re: Some mis-firing input please.... What's the results of a compression/leak-down test? __________________ -Brian '86 Mustang GT Convertible '88 Mustang LX coupe '11 Odyssey TE '21 Insight Touring

07-11-2005, 05:28 PM	#3
Disney Lincoln Senior Member Join Date: May 2005 Posts: 290	Re: Some mis-firing input please.... Pull codes.

07-11-2005, 05:42 PM	#4
coupe Super Moderator Join Date: Dec 2004 Posts: 6,313	Re: Some mis-firing input please.... No compression tester. But no crankcase pressure problems. Fairly fresh motor. Could a O2 sensor or both be causing this??? It's ONLY at very slight throttle up to 2400 RPM. Any hard throttle or past 2400 and it's fine. __________________ " If you can't explain it simply, you don't understand it well enough. " - Albert Einstein

07-11-2005, 08:53 PM	#5
QWKSNKE 3v's are slow Join Date: Dec 2004 Location: Here and There Posts: 17,152	Re: Some mis-firing input please.... does it do it when cold or all the time? __________________ 2009 Porsche Carrera S 2008 ///AMG C63.. Eurocharge Tune. RIP 2019 AMG GLC43..wife's 03 F250 SD.. Edge Evo, AFE Stage 1 CAI 08 GT. JBA axle back, FRPP springs, FRPP swaybars.. daughter's 01 Cobra vert... wife's I need a new toy

07-11-2005, 08:54 PM	#6
QWKSNKE 3v's are slow Join Date: Dec 2004 Location: Here and There Posts: 17,152	Re: Some mis-firing input please.... any mods on the engine? __________________ 2009 Porsche Carrera S 2008 ///AMG C63.. Eurocharge Tune. RIP 2019 AMG GLC43..wife's 03 F250 SD.. Edge Evo, AFE Stage 1 CAI 08 GT. JBA axle back, FRPP springs, FRPP swaybars.. daughter's 01 Cobra vert... wife's I need a new toy

07-12-2005, 11:51 AM	#10
Disney Lincoln Senior Member Join Date: May 2005 Posts: 290	Re: Some mis-firing input please.... Also try cleaning the MAF.