302 light backfire

1365 Views 13 Replies 5 Participants Last post by jon.young40, 2 mo ago

LFugate

Discussion Starter · 4 mo ago

I installed a 95 5.0 from a 96 Explorer into my 69 with a T5 trans. I've had it back on the road about 10 months and have put about 1500 miles on it. It has a Trick Flow Stage 1 Cam, upgraded rockers and springs and has ported/polished heads and intake. In the last couple months I have been getting a light backfire in the intake. I can drive through my neighborhood with no issues, but once I get on the main road and start accelerating to about 35mph it was back firing once or twice. Now it happens 5 or 6 times and is getting worse. After that initial backfire it never backfires again, unless it completely cools off.

I have no O2 Sensors or EGR installed.

I'm thinking I should change the plugs and Ohm the spark plug wires. They were all new when I installed the engine though. What else would cause this?

Thanks for the help.

Hood Motor vehicle Vehicle Automotive design Automotive air manifold

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EvoRaat

· #2 · 4 mo ago

My guess, Timing and or a vac. leak...

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LFugate

Discussion Starter · #3 · 4 mo ago

EvoRaat said:
My guess, Timing and or a vac. leak...

Thanks. I'll invesitgate those.

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evintho

· #4 · 4 mo ago

What EvoRaat said^^^.
BTW....that's a sweet '69!!

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Krem12

· #5 · 4 mo ago

So you're running a factory tune on a modified car, with no adaptive fueling?!?!??!? Why?

The EEC is running in failure mode at all times....

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EvoRaat

· #6 · 4 mo ago

Krem12, for my benefit, being new to FoMoCo ...can/how do you reset the EEC? Tnx.

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Krem12

· #7 · 4 mo ago (Edited)

EvoRaat said:
Krem12, for my benefit, being new to FoMoCo ...can/how do you reset the EEC? Tnx.

For EEC-IV systems (basically everything pre OBDII) the failure modes will automatically reset once the ECU determines the fault over. Unplugging your battery for roughly 15min will "reset" all fueling trims and learned behavior.

New systems are a lot more complex, and i guarantee they can cycle through failures as needed as well.

To further explain what's happening to OP's car, The ECU looks for O2 sensor switching. If it switches from high to low (indicating exhaust is hot enough and its jumping above and below stoich) in a certain amount of time the Closed loop flag is thrown and the car switches from open loop. no switching, the car looks at the other O2 for the other bank. If that doesn't happen it stays in open loop, and reads from a certain tables for spark and fuel and enters a failure mode.

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LFugate

Discussion Starter · #8 · 4 mo ago

Thanks everyone.

evintho said:
What EvoRaat said^^^.

BTW....that's a sweet '69!!

Thanks evintho.

Krem12, you're talking way over my head with the adaptive fueling conversation. You're saying with a light cam, etc. that I should have a tune on the computer?

I Ohm'ed my plug wires and everything was good. I searched for a vacuum leak using two deferent methods and found nothing. I'm waiting for my son to bring his timing light over to check the timing.

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Krem12

· #9 · 4 mo ago (Edited)

LFugate said:
Thanks everyone.

Thanks evintho.

Krem12, you're talking way over my head with the adaptive fueling conversation. You're saying with a light cam, etc. that I should have a tune on the computer?

I Ohm'ed my plug wires and everything was good. I searched for a vacuum leak using two deferent methods and found nothing. I'm waiting for my son to bring his timing light over to check the timing.

With no O2 sensors installed yes. There is no feedback loop for the computer to adjust fueling. What you essentially have is a carb with the wrong jets in it and no way to correct it, basically negating the benefit of being EFI in the first place. With O2's in and working correctly the computer can probably compensate enough with the adaptive fuel trims for it to run fine at part throttle.

What ECU do you have in it?

For what its worth this wont cause actual backfiring, that's from the car being miss timed. ECU does control timing but unless you changed the heads itll stay roughly the same. Miss fueling will cause stumbling, general performance issues etc.

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LFugate

Discussion Starter · #10 · 4 mo ago

Krem12 said:
With no O2 sensors installed yes. There is no feedback loop for the computer to adjust fueling. What you essentially have is a carb with the wrong jets in it and no way to correct it, basically negating the benefit of being EFI in the first place. With O2's in and working correctly the computer can probably compensate enough with the adaptive fuel trims for it to run fine at part throttle.

What ECU do you have in it?

For what its worth this wont cause actual backfiring, that's from the car being miss timed. ECU does control timing but unless you changed the heads itll stay roughly the same. Miss fueling will cause stumbling, general performance issues etc.

Thanks.

It's the A9L computer.

If it was the timing, would it not backfire all the time? It will only backfire when I first get it up to about 35 or so. It will backfire 2 or 3 times most of the time, up to 6 times rarely and only when it's cold. Very light backfires at that, and it's only started back firing in the last 150-200 miles.

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Krem12

· #11 · 4 mo ago

LFugate said:
Thanks.

It's the A9L computer.

If it was the timing, would it not backfire all the time? It will only backfire when I first get it up to about 35 or so. It will backfire 2 or 3 times most of the time, up to 6 times rarely and only when it's cold. Very light backfires at that, and it's only started back firing in the last 150-200 miles.

Ill see if i can find a stock spark advance table and strategy. Theres generally a timing retard table based on air charge temp and other parameters.

Are you sure its a backfire and not just hesitation? What did the timing light say for the base timing?

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EvoRaat

· #12 · 4 mo ago

Is there any chance you pulled the distributor? If you did, there could be a chance you reinstalled and are off by one tooth...

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LFugate

Discussion Starter · #13 · 4 mo ago

It was originally my son's engine that he had in a Fox body that he drag raced. He wanted a 351, so I bought the 302 off him. After I did all the EFI lines/computer wiring and got it running in my car he did the timing again. I'm waiting for him to come over to check the timing on it now. He drives a semi so he's not always around.

It's a light backfire with a hesitation.

EvoRaat said:
Is there any chance you pulled the distributor? If you did, there could be a chance you reinstalled and are off by one tooth...

The distributor was pulled and I would say that would be a possibility, but the car ran fine for 800 miles with none of the problems I am having now.

Thanks.

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jon.young40

· #14 · 2 mo ago

I would say to check the vacuum going to the transmission. If it keeps up, you’ll be looking at a rebuild.

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