Running a ZZ4 crate motor with a Superram and accel computer in a sandrail. Has shorty headers and resonators. Engine has developed a popping/backfire in the drivers side exhaust on decel. Happens right after I let off the gas when accelerating. Only backfires once and becomes pretty consistent after the engine gets warmed up. I've learned to control the popping for the most part by carrying a little pedal thru the decel. If I need to brake right after accelerating, it backfires, I can get the backfire when stopped by revving hard, but not all the time. Buddy said it shot out a 3' flame when it happened sitting there.

I have already changed the exhaust manifold gaskets twice (no signs of leak), Changed plugs and wires and looked closely for cracks in the manifolds. I've also increased the throttle following to no avail.

Help appreciated........

 

That's excess fuel passing through the motor unburned, and igniting in the exhaust.

Not an exhaust problem, as such. It's in the tune.

 

Can you be a little more specific,,,,Which tables am I looking at.
Why only one side?

 

Look at the fuel tables. You're dumping in too much when the vacuum spikes high (sudden throttle closing).

No idea why only one side.

It's also not impossible that you have bad injector(s), specifically, leaking; since when vacuum is high, that could allow extra fuel to get "sucked" past the pintle. Or even, a bad FPR. But I'd suspect the tune first and foremost.

One way or another though, the only way for there to be flames, is for there to be fuel. Keep it simple. Flame = fuel. It's the only way. The exhaust can't cause that on its own. Not possible. You can replace the exhaust system every day, and twice on Sundays just for good measure, and if there's too much fuel getting dumped into the motor and getting ignited in the exhaust, it will still spit flames.

Find and eliminate the fuel.

 

Could be a bad injector or injector driver also. People actually ask for tunes like that - it's easy to do - on decel you yank out all the timing and dump in a bunch of fuel. The kids call it a "burble tune". It's stupid but we charge them for it.

GD

 

Was leaning in the leaking injector direction at this point as well. Even considered a tight valve...Dread removing the superram is not high on my list.
hadn't really considered tune since it hasn't changed in 15 years (other than minor idle adjustments),

appreciate the responses, they all help.

 

Do the shorty headers have air tubes on them?

 

No tubes

 

 

The tune has been mentioned a couple times. As I said before, tune has not changed in 15 years,

Assuming its the tune, could anyone tell me what I would be looking for, in what table(s) of the accel gen 7 program? I have limited tuning experience with this setup.

And yes, the FPR is stock type that is vacuum controlled and adjustable by screw on top of housing. running just under 45lbs, vacuum hose disconnected.

I've attached a picture of the engine and exhaust configuration.

 

Fuel is burning in the exhaust. Don't waste your time on the tune. Figure out why you're suddebly expelling fuel out the exhaust. Either the cylinder burn is incomplete now (not burning the fuel the way it used to), or an excess of fuel is getting dumped into the cylinder.

 

If I think really hard, I might be able to see a Thirgen F-Body like in the word "Thirdgen.org"

 

The more plausible explanations would be:

1. An injector (or injectors) in one bank streaming fuel instead of atomizing, thereby allowing fuel pooling at low airflow (once the throttle is closed);
2. A grounding injector in a batch-fire arrangement completing the circuit for all injectors and causing the batch to go static on;
3. An injector is mechanically sticking, dumping raw fuel. This could also be intermittent, and specifically after heating such as from running high pulse widths;
4. A control system not shutting off injectors on decel.

Without removing injectors and flow testing, it would be difficult to determine the injector spray pattern. If no other causes can be found, this may be necessary, but the easier possibilities should be checked first.

To determine whether an injector is mechanically stuck and/or leaking through, a residual fuel pressure test can be performed. If the system cannot maintain residual pressure after the fuel pump is shut off, the reason(s) for this should be found.

It may be helpful to measure injector coil resistances both cold and hot, and determine whether insulation may be breaking down, potentially causing a set to ground and go static. It may also be helpful to inspect the injector wire harness for the same potential grounding, but that would likely not create symptoms only on decel.

Does the Accel system also batch-fire the injectors like the stock ECM? I have no knowledge of that control, but it could be a clue to why only one bank is apparently going silly rich. Based upon the stock configuration with injectors wired in sets of four, a grounded injector can fire the entire set. However again, that would probably not occur only on decel.

It is also possible that the fuel map (tables) have somehow been altered, but the likelihood of that is also very low.

An additional possibility is that the control system is failing, specifically in its outputs. Again considering the stock ECM in which the injectors are operated via transistors, it is possible that a given output transistor is heating after running injectors at high pulse-widths, whereupon it could experience thermal runaway and become unable to shut down. This is more likely in the driver stages than the outputs themselves, but could explain the delay in shutting off injectors after a "spirited" run, thereby dumping lots of fuel into a intake tract starved by a closed throttle.

 

SbFormula...?

Vader, thank you for the informative response. Have to admit, allot of which is above my knowledge base.
From what I am able to determine from your response, I think removal and testing/replacing of the left bank of injectors would cover much of what you are listing as possible causes.

Note: The injectors were replaced a some time before the problem started when I was resolving a running issue. Turned out to be the MAP sensor.

 

Injectors could be tested in place if you can gain access around the plenum. The same applies to the wire harness, and also testing the residual fuel pressure. I would think that would be easier than pulling injectors on a hunch.

 

I can access at least some of the injectors wires....How do I to test and what am I looking for?
I can inspect the injector wiring harness...
The Accel system is Batch fire.
As I recall, the fuel pressure has always gone to "0" pressure shortly after shutting off engine. It never held the pressure for very long.

Weja

 

In random order...

If the fuel rails remain pressurized for even 45 seconds, it's probably not an injector leaking any in amount that would cause the symptoms you describe. Any running engine would likely tolerate such a small amount of extra fuel without dumping it raw through the cylinder and into the exhaust.

For checking the injectors and harness, disconnect all injectors to measure the resistances.

The injector harrness can be visually inspected for any chafing, bare wires, or inadvertent grounding, and tested with an ohmmeter to verify that the ECM side (switched ground) is not constantly grounded. Expect at least 200Ω if the harness at the ECM is still connected, more if it is not connected.

Injectors themselves can be tested individually with an ohmmeter from terminal to terminal. Typical DC resistance should be in the range of 12-16Ω hot or cold. Much below 10Ω could be an issue.

Due to the reported symptoms it would also be a very good idea to test each injector terminal to its case/ground. Any injector solenoid which measures any appreciable resistance to ground is highly suspect.

 

If the fuel rails remain pressurized for even 45 seconds, it's probably not an injector leaking any in amount that would cause the symptoms you describe. Any running engine would likely tolerate such a small amount of extra fuel without dumping it raw through the cylinder and into the exhaust.
Seeing how its done this through 3 sets of injectors. I doubt it is the current cause.

For checking the injectors and harness, disconnect all injectors to measure the resistances.
Cannot get plugs off injectors with intake plenum on.

The injector harness can be visually inspected for any chafing, bare wires, or inadvertent grounding, and tested with an ohmmeter to verify that the ECM side (switched ground) is not constantly grounded. Expect at least 200Ω if the harness at the ECM is still connected, more if it is not connected.
Have a good buddy that knows electrical stuff. Also sounds like a plenum off procedure too.

Injectors themselves can be tested individually with an ohmmeter from terminal to terminal. Typical DC resistance should be in the range of 12-16Ω hot or cold. Much below 10Ω could be an issue.
Same as above

Due to the reported symptoms it would also be a very good idea to test each injector terminal to its case/ground. Any injector solenoid which measures any appreciable resistance to ground is highly suspect.
This would be a intake off test as above

Seems I am gonna have to remove the plenum to get to any of the testing and diagnostic sttuff.

 

Likely with that intake.

I know the DFI 7 is *capable* of full sequential injection (we just ripped one of those awful things off a Cadillac), and if it's wired thus the injectors could be tested through the wires to the ECU and wherever they are wired for ignition switched power.

GD

 

I know its batch fire because when I pinned the injector harnesses out when dealing with prior issue, they were not even close to being on the correct cylinders and tune showed the toggle switch in the batch fire position. Unfortunately I do not know which plug go to which injector. might be difficult to trace them out.

I am not really pleased with the Accel Gen 7 DFI setup as well..

The backfire always occurs immediately after I let off the gas.

 

Yeah we ripped one of the DFI-7's off a Cadillac recently because it was not controlling timing.... basically at all. There are certainly alternatives that are much more modern and their software can run on modern laptops, etc. Personally I run a LINK on my 86 Trans Am. It can be configured to handle the Accel (or holley/MSD) dual sync distributor and it will run batch or sequential, etc. Must faster processing speeds, better modeling, and more features.

GD

 

Don't even get me started on the distributor. I cant even figure out how it works. instructions look like Chinese. This red light, that green light, turn back, line this up. Hope it never goes out...

When I get time at the end of the season. Think I'll just pull the intake and get a good look at everything.

 

You try checking temps on the primaries at idle? See if you have one that’s far off the other 7? May help narrow which cylinder it’s comin from. Although not likely, worth a shot and easy to do

 

Are you thinking a bad injector would manifest itself in a hotter or colder exhaust temp?

If so, it would be hard to recreate the conditions under which the backfire occurs, Engine runs great otherwise

Interesting thought. If the backfire actually occurs in the primary, it might cause an increase in temp.

That being said, if I determine which injector is dumping, I would still be pulling the intake to replace it. I will probably replace the 4 on the drivers side and send the ones removed in to be tested and evaluated.
I already have a spare set to install and hopefully would find out that one of the injectors is bad.

Will probably check the valve adjustments first just to rule that out. And haven't completely ruled out the FPR yet either. then on to the injectors.

Thanks for the input.

 

I was next to a purple BMW with dark tinted windows and fart-can mufflers the other day and every time he let off the gas, all you could hear was burble, burble, burble. Sounded so f______ stupid but not stupid enough to bother the cop in the car right behind him. All I could think was I want to kill GD for tuning cars like this on purpose. At least you got paid for it.

 

LoL. We Laugh at them and tell them it's hard on turbo's and sounds ridiculous (to us) etc - they still want it. And we charge them A LOT in comparison to the difficulty of the procedure. It's actually really easy - you just take the cells in the decel area of the timing map and put them at between 0 and -10 degrees (10 ATDC) depending on how loud you want the burble, and give it about 2-3ms of injector. It's literally a 5 minute change to the tune. Essentially we are lighting off the charge really late so it shoots the combustion event out the exhaust valve. We typically charge an extra $100 as a stupidity tax for the "burble tune" Ala-carte add-on to their tuning session.

GD

 

Not to mention it destroys catalytic converters too

 

Likely. All these cars have either high flow cats or none at all. Haven't seen it destroy one but I wouldn't doubt it.

On the one hand it's stupid, but whatever keeps the kids from buying electric cars..... Hopefully we can hang on building performance combustion engines till I retire and disappear down into my missile silo bunker.

GD