MFI and exhaust backfire -- how much is normal?

logician · 09:11 AM

Hi,

I know the subject of fixing backfiring in MFI equiped cars has been covered extensively in other posts, but I haven't seen much discussion about how much exhaust backfire is normal. From the Porsche "MFI Check Measure Adjust" booklet: "Backfiring in the exhaust will occasionally occur and cannot always be completely avoided, even by the most careful tuning."

In my case, my '73 911S still has exhaust backfires upon decleration even after a major tuneup. My mechanic set the microswitch a little more agressively, and that helped a little. I've tried modifying my driving style to try not to feather the throttle on the theory that I might have the throttle down just enough to deactivate the microswitch, and thus defeat the fuel shutoff mechanism. But it still backfires at least once every time I drive my car.

I've checked for proper operation of the microswitch, rpm transducer and stop solenoid -- engine speed correctly hunts between 1300 and 1500 RPM when microswitch is operated manually. The engine is a little tired, and I suspect the throttle bodies leak air, since the idle goes as high as 1200RPM until you kick the pedal a few times to settle it down to 900RPM. The engine runs great otherwise -- very strong. I believe that it had a top end rebuild about 10 years ago.

So, should I be concerned and continue to try to find a cause for the backfire? Is some amount of backfire normal? How bad is this exhaust backfiring for the exhaust system and engine? Do I have to change my driving habits even further, and if so how? Is the 2.4 911S more prone to backfiring than other MFI engines? What is the experence of other MFI owners?

-Juan

targa911S · 02-24-2003, 01:44 PM

I gave up years ago. My 70 2.2 S, w/ MFI does it just like yours. The micro switch is the key. You can adjust it to suit yor driving style. Factory has it set at around 1100 to 1400 RPM. Also taking your foot completely OFF the pedal on deceleration helps. Mine is a little less crabby after it warms up. I consider it the nature of the beast. Let's face it, it wants to run where the air is thinner!

BK911 · BK911's Garage

You may have sticking throttle linkage. Make sure the entire system works smoothly. If it's being bound anywhere the microswitch will never engage.

Good luck!
BK

Scott Clarke · Scott Clarke's Garage

My E has never, ever backfired (for what it's worth).
-Scott

Paul Thomas · 02-24-2003, 06:14 PM

Know anyone with an exhaust gas analyzer? Have you tried going up or down a few clicks on the pump to see if one way or the other fixes it? I would try that, just make sure that you record where you were so you can get back if it doesn't work.

Paul

tryan · 02-24-2003, 06:21 PM

'72 t does not backfire no matter how hard i try.

NYSCAR · 02-24-2003, 07:08 PM

Well it's kinda funny, I can't get mine to backfire either, and my rpm transducer doesn't even work

Make sure your ignition (points, plugs, wires, etc) are good, and check your exhaust for leaks...

Brad

FA-18C · 02-24-2003, 07:46 PM

My 70E used to backfire at every lift throttle. I worked tune, balance, and timing issues, but it took replacing the muffler to rid the backfires. I can now rev to 5+K and lift the throttle under load and no backfire - ever. You may wanna check for minor exhaust or intake leaks.....

IAN · 02-24-2003, 07:53 PM

I had the problem you are describing with my 72T and a very respected wrench in Oregon told me to take one of wires off the micro switch.

I did and my car no longer backfires.

logician · 07:10 PM

Hi,

So based on the responses, I decided to keep looking for a problem that might cause backfiring. What I found was that the throttle was sometimes hanging on the microswitch without fully depressing it. It wouldn't hang all the time, but would tend to do it if I released the throttle gently. It would seem that the spring in the microswitch was too strong -- stronger than the throttle return spring! The end result was that the fuel shutoff mechanism wasn't working all the time.

So I had my mechanic replace the microswitch. Actually, he used a non-Porsche microswitch which has a much softer spring. This would appear to have fixed the problem. The only backfire that I got driving it this afternoon was a very muffled one once, while the engine was still warming up. I did a lot of coasting in places where it would have previously backfired. So I'm pretty confident the microswitch was the problem.

Also, the engine now idles at 900RPM more reliably, although it still simetimes idles at 1200RPM. I guess the throttle hanging was one source of idle air leak. But there must still be one other air leak remaining -- perhaps worn throttle bodies.

Thanks to everyone that responded and helped solve this backfire bug!

-Juan

brokke · 02-26-2003, 01:39 AM

A very experienced wrench told me that in the early days they always disconnected the micro switch because the speedswitch/microswith setup in the MFI Porsches always gave more problems than solutions. My 73S runs without a microswith and does not backfire.

Peter
911 2.4S

logician · 09:08 AM

Hi Peter,

> A very experienced wrench told me that in the
> early days they always disconnected the micro
> switch because the speedswitch/microswith
> setup in the MFI Porsches always gave more
> problems than solutions. My 73S runs without
> a microswith and does not backfire.

Interesting. IAN and NYSCAR had similar reports. I am curious how that could be. Without the microswitch working, you would be sure to be throwing unburnt gas into the exhaust, and thus be prone to backfire. My experience would seem to confirm that. Did your wrench elaborate on what problems the microswitch caused?

I guess one possible clue in my case is that some additional air was being let through the system because the throttle was not fully seating due to the microswtich. Perhaps this additional air is sufficient and required to cause a backfire. On the other hand, the additional air can't be much compared to the regular idle air, as was only enough to raise the idle to 1200RPM.

I wonder if the problem that the old microswtiches presented was not electrical, but instead was mechanical -- that they would sometimes prevent the throttle from fully closing, like my expereince? Can you tell me in your car, whether the microswitch is still installed, and whether the trottle is still engaging it, even though it might be electrically disconnected? I'm thinking that the "old wrench fix" might be mechanical -- disengage the microswtich by backing off the adjustment screw so that it can't effect the throttle at all. TARGA911S: want to give that a try to see if it cures your backfire? I'd be curious to know.

In any event, a functional microswitch does have a big advantage -- gas milage. Without it, you are throwing away a significant amount of gas when coasting.

-Juan

Porschedude · 02-26-2003, 09:01 AM

Fuel in the exhaust system! Just that plain and simple.

It sounds like you have addressed the problem. (Throttle Bodies are worn out)

The slop in the shafts may lead to the microswitch no closing once in a while.

Keith

KS Carrera · 02-26-2003, 11:31 AM

Quote:

Originally posted by Porschedude
Fuel in the exhaust system! Just that plain and simple.

It sounds like you have addressed the problem. (Throttle Bodies are worn out)

The slop in the shafts may lead to the microswitch no closing once in a while.

Keith

So, as my '74 Euro Carrera has only done 28,000 miles from new, are you saying the throttle shafts are worn out because it backfires? No, this is a common MFI problem which frequently has little to do with mileage. Anyway, who cares about the occasional 'kaboom!'? It wakes up the guy in the next lane, if nothing else?