Turn ignition off = engine burps! Help explain!

[WydRyd]

Interesting phenomena I've experienced lately and am wondering if anyone knows what may be causing this: Everytime I switch off the engine, 2-3 seconds later, I can hear a faint (for the lack of a better word) burp or fart coming out the exhaust. It almost sounds like a flame is being put out and there's a soft back-fire out the exhaust. Anyone know what can be causing this? Only really started happening since I installed the Crane CDI ignition unit. I also installed new plugs, but with a greater plug gap than what I was running without the Crane CDI. I use to run stock 0.024"/0.6mm gaps, but now run at 0.040"/1.0mm. Any idea? Could it possible be unburnt fuel igniting in the headers? In which case, should I decrease my plug gaps a bit? Bizarre!!!

[Jim2]

Afterburner

[Larry_Ratcliff]

It does sound like unburnt fuel igniting. I agree with running a smaller gap to cure the problem ... the time frame of a couple of minutes is confuseing though.

[WydRyd]

Couple of seconds... not minutes

OK, I might try reducing the plug gaps I guess

[Larry_Ratcliff]

LOL sorry WydRyd I dont know where I got minutes now either ... guess the beer is getting to me. that kind of explosion can only come form unburnt gass though.

[WydRyd]

Yeh, ACTUALLY, if I blip the throttle just before shutting her off, she sets off a gun fire type of back-fire! Must be dumping way too much fuel in there I'd say

Only explanation I can think of is:

- when ignition is shut off spark plugs stop firing immediately
- engine is still turning a little bit after ignition is off and sucks in a bit of fuel from injectors
- since no spark is present to ignite the mixture, the mixture seeps into the hot headers past the hot turbine housing and is ignited
- ignited mixture lets off a subtle back-fire

... kind f makes sense to me now.

BUT, do any of you other guys experience this at all?

EDIT: JUst went and re-gapped my plugs to 0.035", so I'll see tomorrow if that makes any difference. Either the mixture is way too rich and I'm not burning the residual fuel fast enough, or my gaps were too large to begin with. See what happens I guess

[javadog]

I think what you are describing is fairly normal. When you turn off the ignition switch, it should cut off the fuel pumps and the ignition should still be firing as the engine revs drop. I seem to recall the ignition stays on for about two seconds. Both of mine pop a little bit as they wind down.

If yours pops a second or two after the engine quits spinning, I can't help you as to why but it would seem that any popping is a result of unburned fuel.

JR

[kellcats521]

Here is the deal - the factory CDI box allows the plugs to continue to fire after the ignition is turned off to burn any remaining fuel dumped in by the CIS system. (you know that CIS is always spraying fuel into every cylinder when the fuel pumps are running). So, on the OEM setup, when you turn the key off, the pumps shut down but the CDI will deliver spark if the distributor signals it for a few more seconds - that's why these cars seem to 'run on' for a second or 2 when you turn the key off. They're burning the last bit of fuel sprayed in. The Crane unit has no capability to do this (at least the Crane units that I'm familiar with - HI-6, HI-6S, HI-6N), so you're getting the pop when this fuel gets lit by some other hot component (prolly the exhaust as you've surmized).

With a well tuned EFI system, the pulsewidth should be 'right', and there shouldn't be enough fuel 'laying around' to create any kind of a burp. It might be possible to have 1 or 2 cylinders woth of fuel in there if your setup is a batch fire system (the ECU fires all of the injectors at the same time for each revolution and each cylinder's intake stroke pulls the fuel in) and thereby cause a burp. If you have sequential EFI, my only input is that your setup is not tuned correctly or optimally.

[Jim2]

Merv, a shot in the dark here, but do you think you might have a leaking injector(s) which is bleeding off residual fuel rail pressure after shutdown. At shutdown as the engine spins down the unburnt fuel from the leaking injector is cycled through into the exhaust system and eventually ignites from a hot component.

EFI fuel rail holds pressure after shutdown due to the diaphram in the fuel press reg and check valves in the discharge end of the fuel pumps.

Jim

[javadog]

I am not sure that gas will ignite from a hot exhaust. At least in my experience, it won't. There is a long story to illustrate this but I'll save it for another time.

I have zero experience with the Crane CDI on a 930. Does it kill the sparks immediately upon shutdown, as kellcats521 suggested?

JR

[WydRyd]

Originally posted by Jim2
Merv, a shot in the dark here, but do you think you might have a leaking injector(s) which is bleeding off residual fuel rail pressure after shutdown.

hmmm, possible... one plug use to always foul worse than the others... so it's quite possible I have a dud injector

I might have to buy a spare 951t injector and just swap it out to see if anything improves.

It does smell very rich at idle and my fuel economy sux. It's either a badly set Octane Quality switch on my Motronics, or a badly set AFM, or a leaking injector

[kellcats521]

Originally posted by WydRyd
hmmm, possible... one plug use to always foul worse than the others... so it's quite possible I have a dud injector

I might have to buy a spare 951t injector and just swap it out to see if anything improves.

It does smell very rich at idle and my fuel economy sux. It's either a badly set Octane Quality switch on my Motronics, or a badly set AFM, or a leaking injector

These other symptoms would be consistent with a bad injector or a bad fuel pressure regulator (keeping too much pressure in the lines) rather than a poor tune - I assumed that the burping was the only issue.....

[WydRyd]

kellcats521, all my other plugs have a nice tan color to the electrode, whereas this particular cylinder has a more black electrode, suggesting fouling and the A/F's are way out in that one cylinder

I guess I just have to pull that fuel rail off, energize the fuel lines and see if there's any leakage from that one suspect injector.

Other symtoms, at idle, I can hear a very faint missing/popping, possibly due to that one cylinder overloaded with fuel?

Apart from that, the car runs very strong, doesn't smoke, leak or anything.

[WydRyd]

UPDATE: I dropped my plug gaps down to 0.035" (from 0.040") and it seems to have cured the engine burp upon shut-down. I guess the mixture is being burnt a bit better with the slightly smaller gap

Still smells a bit too rich at idle though. I'm going to try replacing that suspect injector and see how it goes

[john walker's workshop]

the 2-3 second over run is due to the delayed ignition cutoff relay, not the CD unit. it keeps 15 (key on) power to the CD box after the key is shut off. if the crane has "key on" power to it, the relay could be made to work with that too i would imagine.

[WydRyd]

Thanks John. Might check it out, but since I dropped the gap to 0.035", the subtle backfire on ignition off has disappeared.

As for my overly rich idle, my injectors checked out fine. I suspect my AFM is on it's way out... that'll be the second AFM I guess they just aren't designed for *any* kind of boost, especially over 1.1bar Must get rid of that fast and go MAP sensing asap.

[kellcats521]

Originally posted by WydRyd
Thanks John. Might check it out, but since I dropped the gap to 0.035", the subtle backfire on ignition off has disappeared.

As for my overly rich idle, my injectors checked out fine. I suspect my AFM is on it's way out... that'll be the second AFM I guess they just aren't designed for *any* kind of boost, especially over 1.1bar Must get rid of that fast and go MAP sensing asap.

What type of AFM are you using? I have successfully cleaned many hot wire units - it's not hard to do if you're careful. Also, if you're using a K&N filter (or other brand air filter that is oiled), be very skimpy when you oil the filter. Oil and hot wire AFMs do not mix well.

Last, if you have the ability to alter your engine management program, you can start to trim the A/F ratios at idle speeds. The typical Bosch Mass Air function will have voltage readings that correspond to a given mass of air. If you use a voltmeter and record the AFM voltage at idle (for a 5V Bosch system it should be ~1V), you can go to the Mass Air Transfer function and lower the air mass values slightly (10% max) and see if this helps.

[kellcats521]

Originally posted by john walker's workshop
the 2-3 second over run is due to the delayed ignition cutoff relay, not the CD unit. it keeps 15 (key on) power to the CD box after the key is shut off. if the crane has "key on" power to it, the relay could be made to work with that too i would imagine.

Is this relay in the back, or under the dash? I've never seen it. Sorry for the mixup there.....

[WydRyd]

I'm using the OEM BOSCH AFM, found in the '84-'94 Motronic based 911's. It's not a MAF like the 993/993TT uses

These are notorious for prematurely failing in a boosted application... they don't like high pressures pushed through them I've already been through one!

[NathanUK]

Sorry for OT, but I would like to know how to clean my AFM on my 3.2 carrera ?
I use a K&N on it.

TIA