Over-run backfiring, what makes Subaru's do it so much?

[Phizinza]

Just wondering what people think makes Subaru's backfire on over-run so much? Mainly with larger exhaust systems I've noticed.

[zyewdall]

By overrun, you mean a situtation when you are engine braking down a hill? I actually haven't noticed it with subarus, but my old ford courier does it pretty bad -- bang, bang, bang, going down the hill.... I do wish I knew why....

 

Z

[subiemech85]

or do you mean the pop pop that happens when you let off the gas to shift

 

I have noticed that if the throttle is not completely closed, it does not happen

 

anti after burn something tells me this is a "feature" of the subaru boxer engines, as they tried to make it not happen with various "tricks"

[Loyale 2.7 Turbo]

Well... I`m not Sure on Subies, Might be Timin` or Fuel/Air Mixture, but in Old Ford Couriers (Made by Mazda) I repaired one just Changin` a ignition Part, I Don`t know its English name, but it looks almost like the same one that some Older Subies have at the Disty:

in Spanish they`re Called "Platinos" ... So Old School... ...

[subiemech85]

^ points

 

luckily we upgrade to electronic

[Loyale 2.7 Turbo]

So They`re Called "Points"? ... Thanx! ... ...

[Phizinza]

^^^^^

Yes

 

^^^^

Yes

 

^^^

Points

 

^

Yes

 

 

I do still run a points disty, never got around to getting the better one. But I don't think this is what causes it.

 

To me it seems like there is too much fuel while under deceleration. I guess if you have better spark burning that fuel before it gets to the exhaust then it might help?

 

I've read its common when you have an exhaust leak near the heads. It sucks air in and burns in the exhaust making popping sounds.

Could the EGR be failing and actually be letting air from the manifold to the exhaust?

[Greenley]

My 84 started doing this after i cut the muffler off. Never did it before

 

Just somethin' to ponder.

[Wirebrush]

My 85 Brat does it quite a bit especially when it's cold. It does have a glaspack and no cat though. I don't know for sure, but I think the ignition on these motors is pretty weak. I plan to install an MSD 6A ignition and blaster 2 coil when I upgrade to a 2.2. I had the same ignition on a hopped up Dodge Omni GLH with the high compression 2.2 and that thing would fire instantly every time even when it was -20 and I never bothered to even hook up the manual choke. It also got considerably better mileage than I'm getting with the 1.8 in my Subaru.

[TeamCF]

My 83 GL does it bad since I flattend the last CAT on a rock and removed it.

It also has a "turbo" style muffler. (one of the cheapo $20 kind) In the stock rear muffler spot.

Exhaust system seems to be sealed up good (no leaks) but it'll still give a shotgun blast when I let off the gas and slow down at the hwy first thing. That's only a cold thing. But it pops like mad all the time on engine braking. And gives a decent shot on every shift.

[Phizinza]

Yeah, like I said lots of Subaru's do it (all?). But what is it.

 

Ignition so far, I'm wondering how the ignition system could make a difference while your under deceleration with no throttle.

 

Hey, even my EJ22 done it, and that's fuel injection that is suppose to cut fuel all together while decelerating.

[zyewdall]

^^^^^

Yes

 

^^^^

Yes

 

^^^

Points

 

^

Yes

 

 

I do still run a points disty, never got around to getting the better one. But I don't think this is what causes it.

 

To me it seems like there is too much fuel while under deceleration. I guess if you have better spark burning that fuel before it gets to the exhaust then it might help?

 

I've read its common when you have an exhaust leak near the heads. It sucks air in and burns in the exhaust making popping sounds.

Could the EGR be failing and actually be letting air from the manifold to the exhaust?

 

That's a really interesting theory -- because the courier has an air injection pump that puts alot of extra air into the exhaust manifold intentionally. Did 70's subaru's ever have this? Seems like they stopped doing this by around 1979, and started putting cats on instead.

[hatchsub]

My 83 sedan does it. Weber with a straight pipe out to a high flow muffer. Never did it before when i had the dual cats on it.

[Skip]

zye

has hit the nail on the cranium.

 

They do this because of the AIR system,

(Air Injected Reaction).

 

A system to reduce pollution by burning unburnt hydrocarbons

as they exit the head.

 

On "cat cars" the extra 02 helps the reaction (burning)

that occurs in the cat.

 

Filtered air is introduced right at the output of the heads.

Lots of posts on meltdowns occurring when the reed valves do not close.

Exhaust gas backs up and melts the silencer.

The plastic it then ingested by the carb and the throttle plate

can be plastic welded open (don't ask me how I know).

 

There is an after burn valve fitted to reduce the effects but they seldom

work and are bypassed many times.

"In theory" this valve allows a slow controlled burn, when it fails

the result is the backfire.

 

I do not know if the Aussie version of the EJ22 uses a similar system

but

most of us seal the pipe to the output off and eliminate the problem.

[Subarule]

The anti-backfire valve is bad?

[The Beast I Drive]

I believe it also has something to do with ignition timing. Back when I had the stock EA82 in my car with straight pipe it didnt do it very bad, but my mom's car has done it really bad even with the new engine, and its got a full exhaust. When I swapped motors to my second EA82 it did it really badly at first, until I played with the timing a bit. It seems to me, the more you advance the timing, the more power you can get (to a point) but it is more likely to backfire and idle rough, and if you mess with the carb it will probably try to "diesel" when you go to shut it off if you run the mixture too rich. My setup now, EA81 w/ Weber 32/36 and dual straight pipe w/Cherry Bomb glasspacks does not back fire at all, EVER, and it only diesels just a tiny tiny bit if I dont let it idle down after a really hard run. I have no specs on the timeing because i have no timing marks to line up with, I am running an EA82 flywheel. I do it all by "feel", and it feels pretty darned good

-Bill

[Phizinza]

Skip, I can understand all that but it only applies to the EJ in this case where I'm leaning more towards my EA81 and why it does it.

My EA81 doesn't have air injection. It does have the exhaust gas recirculation (EGR) but that should only put exhaust from cyl 1 and 3 back into the manifold for burning. And I know this isn't what does it (or at least not all of what does it) as my brothers subie EA81 didn't have EGR nor a mates 88 subie.

 

I've seen many cars from the 70's and 80's with those air pumps. Lots of toyota's, even my 79 Merc had one. Most have been unbelted, blocked up, ripped out etc though. Maybe when you pump lots of air in there it gives it more of a "conrolled burn"?

 

I was speaking to my dad about it. He's thinking maybe its got to do with the vacuum advance. When your at high revs and you come off the throttle it has no vacuum advance?

[Skip]

Thomas, are you sure the AIR system is not installed on the EA81.

All of mine (again here in the states) have it.

 

Remember there is no air pump like the other cars of this vintage had.

The A.I.R. system uses the scavenging effects to draw

air into the exhaust.

 

It's is just a plate (maybe 25 mm thick) between the head output port

and the "J" pipe. (I believe EA82s have one on each side)

 

This plate has one metal tube running to it.

It is this tube that gets plugged to prevent the KaBoom.

 

Dad has an idea on the timing advance, our distys use a ported

manifold vacuum. Since this vacuum pick up point only sees

vacuum when the throttle plate is open.

 

The advance would fall as soon as the throttle was closed.

To test this theory, re-plumb the vac advance to a non ported

vacuum source.

 

The after burn valve (also referred to the anti backfire valve)

has been investigated many times to no avail.

The fix for most of us is the plugging of said air discharge tube.

[ihscout54]

In the case of the "smog pump" they will backfire if the diverter valve has failed, and boy do they backfire, I had a scout shoot pretty blue flames.

If youre idle is too low you will get this, or too rich.

All my non F.I. vehicles have done this after changing exhaust. Reduction in backpressure... I think some times the deceleration backfire was present just alot less noticeable with the stock muffler. I dont know just how harmful it really is, but if you advance youre timing a bit, or up your idle, I bet it will reduce that popoppop youre getting.

There will be no vaccume advance on decel.

[Phizinza]

Remember there is no air pump like the other cars of this vintage had.

The A.I.R. system uses the scavenging effects to draw

air into the exhaust.

 

It's is just a plate (maybe 25 mm thick) between the head output port

and the "J" pipe. (I believe EA82s have one on each side)

 

The after burn valve (also referred to the anti backfire valve)

has been investigated many times to no avail.

The fix for most of us is the plugging of said air discharge tube.

Every where in my manuals for EA81's that pipe is referred to as an EGR I believe. So I always thought it was what an EGR is and that is a device for taking exhaust and putting it into the intake manifold. Hence the name Exhaust Gas Recirculation. I have seen what you are talking about Skip on other cars but it plums into the airbox, not the manifold. Thinking of that, my airbox does have a unused port with the same rubber "valve" thing those other cars use? Maybe the original headers had ports coming off going up to that port on the airbox?

 

My EA81 goes not have a ABV, and I know they are a pain and pretty much useless anyhow.

 

I'm contemplating removing my EGR. I have the parts off another engine to plug it up. If it is what I tihnk it is and has failed then it could cause the problem. If its what Skip knows it as then it could also cause the problem. But my brothers EA81 subie used to do it and thats the one I have for the parts to block it off. So I can't think that it would be the whole problem.

 

Happy 25th of decemember anyhow.

[mountaingoatgruff]

fwiw, my hatch did this, too.

 

EA81 running SPFI (computer advance), d/r 4speed, no air injection system, functional egr, 2 cats, no muffler

 

it would pop like a firecracker shifting cold, but only between 1st & 2nd and sometimes 2nd & 3rd after it warmed up.

 

it did it pretty consistently while engine braking.

 

this is past tense cuz the hatch is getting an EJ22 & d/r 5speed as soon as i get caught up with my sidework (gotta pay the bills somehow). almost all the parts are sitting in the garage waiting!

 

phizinza, you said that you've seen an MPFI EJ backfire like this? was it completely stock, especially exhaust?

[Phizinza]

^ Yes, it was in my Brumby/Brat. Motor had a pod filter, exhaust was standard y-pipe into cat then 2" straight through pipe.