Some mis-firing input please....

[coupe]

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? :shrug:

[Italian LX]

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

[Disney Lincoln]

Pull codes.

[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. :think:

[QWKSNKE]

does it do it when cold or all the time?

[QWKSNKE]

any mods on the engine?

[Wicked]

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. :think:

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]

any mods on the engine?

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

:jester:

[coupe]

Vacuum leak possibly.

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

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 25º timing... it still did it so it's NOT the computer control of timing. Either vacuum or dist.)

[Disney Lincoln]

Also try cleaning the MAF.

[93Cobra#2771]

The MAF you swapped with - was it a known working MAF off a working car? If not, it does indeed sound like classic MAF issues.

Any detonation occuring at the same time?

[coupe]

Not really detonating. Just sputtering EXACTLY like timing or a vacuum leak. The MAF was a stocker from a working car. Checking into the vacuum & distributor today.

[93Cobra#2771]

In that case, I'm leaning towards a vacuum leak as well...

[Disney Lincoln]

A vacuum leak will usually show up at idle as well, and cause a rough/missing idle.

Try something else. Get it to misfire/bobble like normal, then put it into open loop and see if it still exists. Easiest way to do that is just go WOT when it's happening.

[Wicked]

[Quote=Coupe]
Give it more gas (ie: roll into it hard) and it is not noticable...

Still a bobble/sputter up to 2400...unless you're about 1/2 throttle or better.[/Coupe]

:)
Sounds like he has already done that which is why I suspected vacuum leak...there is no vacuum at WOT.

[Italian LX]

...there is no vacuum at WOT.
That doesn't matter when it comes to vaccum leaks. What makes a leak so problematic is that it allows unmetered air into the motor. Even though there is no vacuum at WOT, it still is allowing air into the intake that hasn't gone theough the MAF which can cause issues.

However, I'm sure it is unlikely to be noticable since a vacuum leak is usally very small compared to the 58+mm opening into the intake. :D

[Disney Lincoln]

If it goes away at open loop then I wold suspect something in the 02 sensors or harness.

[Wicked]

That doesn't matter when it comes to vaccum leaks. What makes a leak so problematic is that it allows unmetered air into the motor. Even though there is no vacuum at WOT, it still is allowing air into the intake that hasn't gone theough the MAF which can cause issues.


Yep, thanks, I know how it works. :jester:

My point was also that the small leak compared to the TB will be negligible at WOT.

[Italian LX]

Yep, thanks, I know how it works.
Sorry, I couldn't tell by your post.

My point was also that the small leak compared to the TB will be negligible at WOT.
Well, maybe next time you should word it that way. :shrug:

[coupe]

Found a bad ground for the MSD-type (other brand) ignition...STILL didn't fix it! I replaced distributor with a known working piece...Still didn't fix it. Cannot find vacuum leak; it's a cobra intake with only a small vacuum tree under the plenum. Seems to be fine. The 2400 RPM thing has made me think the ignition box...but the stock ignition does not fix anything. Overall; changing these things has slightly improved the problem...but not repaired it...I'm thinking O2 now... :shrug:
Or maybe plug gap??? Checking it next.

[Italian LX]

I think you should do a cylinder leak-down test.

[93Cobra#2771]

Other places for vacuum leaks to occur other than just the vacuum tree - loose intake ducting, PCV malfunctioning, etc.

Also, have you checked your TPS to make sure it isn't going bad? Should have a linear voltage reading throughout it's movement. Sometimes "flat spots" will show up and cause havok with things.

If the O2's are going bad/not switching correctly, you will have EEC error codes. Have you tried pulling codes yet?

[coupe]

New TPS. Just gonna put an LS1 in it :jester:
It's going to be something simple like a loose
connection or something somewhere.

[93Cobra#2771]

I can't remember if you said or not - did you try pulling codes?

[coupe]

no. I have the cheapo scanner box. Will see what it gives me. Keep in mind: the car idles great, runs great (at more than about 1/8-1/4 throttle and past 2400RPM).

WHat could be causing the 2400 RPM thing? Some pickup in the dist. right? Already changed it. Guess the CPU... :shrug: