decel backfire

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blarson
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decel backfire

Post by blarson »

I am having a problem where if I rev the engine over 4k and then cut the throttle I get loud backfire through the exhaust. The engine in question is a 3.4 V6 from a 95 Camaro running MS2 with control of spark and fuel. I have checked for vacuum leaks and exhaust leaks and found none of either. I have had the injectors cleaned and tested good. I have confirmed that overrun fuel cut is on with default settings. I have reduced fuel to the bottom lines of the VE map and I have increased timing advance at the bottom of the spark advance map yet the problem remains. I have not loaded the engine yet. All of this is sitting in the garage. I expect the upper ranges of the maps to be a little wonky until I can get to a dyno.

Can one of you fine folks point me in the right direction? Based off forum searches it appears that I may need to reduce fuel further but I'm hesitant to go too far on that track for fear of causing damage. Is it possible that my exhaust is restricted? Immediately following the backfire rpms drop and the engine tries to stall for a few minutes before returning to normal idle.

I think the datalog I took last night is too big for this forum and isn't letting me post it. I'll run again tonight and post one that is more discreet. In the meantime, here is a screenshot of the behavior.

Thanks again for all the help.
Matt Cramer
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Re: decel backfire

Post by Matt Cramer »

Please post the complete log instead of a screen shot; this will give us more to go on.

Usually a decel backfire is a sign your high RPM / low load VEs are too rich, however.
Matt Cramer -1966 Dodge Dart slant six running on MS3X
blarson
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Re: decel backfire

Post by blarson »

sweet, I figured out how to trim the log file. Here we go.

It is not lost on me that the auto tune feature of tunerstudio pushed my VE high before I figured out how to set it to filter above that MAP position. I did move VE lower in that region and the problem was still there. How low can one go in the low map region? Also, wouldn't overrun fuel cut automatically resolve the issue in general?
blarson
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Re: decel backfire

Post by blarson »

oh, and I neglected to mention. The spark control is GM DIS. I'm not sure if that matters here but figured it made sense to disclose as fully as possible.
elutionsdesign
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Re: decel backfire

Post by elutionsdesign »

I think you're just too rich to start with at that point, not sure why your AFR table takes a dive at higher rpm at such low load (MAP) values. But that's just too much fuel and when you snap the throttle shut it has to backfire. Raise it to closer to stoic, low load does not need that amount of fuel.
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16vboost
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Re: decel backfire

Post by 16vboost »

In addition to what's been said, you could try to raise your MAP and RPM lag factors a bit higher like 95. With a low lag number, the sudden change in MAP isn't seen by the controller right away. So you're engine might go to a MAP of 10, but MS is still seeing a MAP of 40 with fuel and spark to match. See how your MAP signal lags behind your TPS signal?

-Alex
blarson
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Re: decel backfire

Post by blarson »

Thanks guys, the car hasn't been to the dyno yet so that sharp drop was my misguided attempt to be conservative while getting a base VE map. I adjusted the lag times as suggested and took out a bunch of fuel in both the VE table and the AFR table. That appeared to reduce a little bit of the backfire but it's still there. I took fuel out of the bottom three rows where it was trouble and watched the AFR. Now it's steady state lean at those low loads. Do I keep going reducing the fuel or should I be looking for something else? Attached is a new datalog and updated msq.
16vboost
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Re: decel backfire

Post by 16vboost »

Why not use VEAL (VE Analyse Live) to tune your VE table while you drive? You probably have too little fuel in those areas, but won't know until you start tuning. Have you driven the car yet? Oh and I have to ask, have you verified your timing is correct with a timing light?

-Alex
blarson
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Re: decel backfire

Post by blarson »

I would love to. Unfortunately it's a track car and I can't run it on the street. See: http://bangshift.com/general-news/event ... -buck-fun/

For a good laugh. I had it licensed when I lived in Washington but now that I live in Oregon it would have to pass emissions which is never going to happen with a V6 where an inline 4 used to be. Though I admit I'd kind of like to try.

Also, VEAL was pushing those low MAP VE figures high. That seems to be opposite of what we were looking for. Or am I wrong?
16vboost
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Re: decel backfire

Post by 16vboost »

Well then come move to Florida, where there's no emissions testing or any sort of vehicle inspection. Though a better disguise for the street might be in order :mrgreen:

I wouldn't do any VEAL unless the engine is running at a relatively steady state for a relatively long time (minutes not seconds). Idle is OK, but for everything else, you're going to have to load the engine. If you just hold the RPM at say, 4000 in neutral and let VEAL try to work at say, 20 kPa, you might be in a situation where your injector pulse is too short for any real fuel to come out (I'm seeing 1ms in your log) and VEAL may artificially be increasing the VE bins there due to the non-linearity of the injectors.

I find it best to tune everything cell that you drive in naturally. Usually from idle RPM/MAP to redline RPM/full MAP to mid RPM/low MAP (cruising). Then just extrapolate everything else.

So if it's a race car why do you care what it sounds like when it backfires? I thought you were trying not to upset the neighbors.


-Alex
Yves
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Re: decel backfire

Post by Yves »

FWIW, I have a decel backfire issue as well and managed to reduce it some but not completly. What I've done so far is put a fair amount of ignition timing into those decel areas. I mean 40-44°. I then made them a little richer as well.
Helped some.

I run ITB's so the change in map is really fast between different TPS positions and after closing the throttle during shifting. So in a way it has got too much fuel from the higher kPa areas when it almost instantly goes to an area with almost no air entering the engine (2% TPS). The amount of fuel most be a lot so that doesn't burn and ends up in the exhaust where it ignites.

What I first intend on doing is making the transition from those high kPa areas to those decel areas as smooth as possible in the AFR table ( I run incorporate AFR now), while at the same time not having a too low AFR value in those low kPa areas. I hope that will help even more.
After that I'm going to put the lag factor for the map sensor at 95. Currently it's at 60. Heard that could cause the engine to react too slow to map changes.

If the previous things don't help I think using the decel feature or really leaning out those bottom rows is in order.

Haven't been able to test the last couple of changes though, since my car is out due to a minor fuel leak on my surge tank.

Other more knowledgeable people may have better ideas though.
pit_celica
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Re: decel backfire

Post by pit_celica »

I think that you should reduce the delay before overrun fuel cut happen. It's currently set at 1.5s. I have mine set at 0.5s. Also, increase the timing at low load rows like Yves said is a good advice.

Sam
16vboost
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Re: decel backfire

Post by 16vboost »

EAE can also be used to reduce PW immediately as you let off the gas. In many cases, EAE sets my PW to zero during decell even before over-run kicks in. The funny thing is my AFR stays about the same for a half second or so as the "wall fuel" is sucked up.

I also agree that more timing at low MAP, higher RPM cells should help.

Also, please describe the exhaust system on your Chump Car. What are you concerned about damaging? What restriction could you possibly have in your exhaust?

-Alex
Yves
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Re: decel backfire

Post by Yves »

I went at it again. However the popping is still there. Decreasing fuel in the decel areas didn't help.

Can an exhaust header to pipe leak cause this type of behavior ?
nathanhardy
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Re: decel backfire

Post by nathanhardy »

Timing!!! I once set my timing low like less then 10 in my deccel area it backfired all the time, think I freaked someone's grandma out to when it lit off and flamed coming down a hill when I had fuel cut off aswell! :lol:
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Yves
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Re: decel backfire

Post by Yves »

nathanhardy wrote:Timing!!! I once set my timing low like less then 10 in my deccel area it backfired all the time, think I freaked someone's grandma out to when it lit off and flamed coming down a hill when I had fuel cut off aswell! :lol:
Not sure what engine you are running, but how much timing on decel ? My max is currently 42-44°

What you're describing seems what I see going on. It doesn't immediatly start to pop, but some seconds after it reaches it's lowest vacuum and vacuum drops again. Especially when getting back on the throttle. It's always accompanied by leans spikes which I think are misfires taking place.
Yves
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Re: decel backfire

Post by Yves »

I went over my logs and there seems to be a spike in map whenever I come from decel and go over to accel. The tps based accel doesn't kick in, although I have tpsdot set to 30. Would a % map ae help to prevent the lean spot I'm seeing in the area around the map spike ?
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