MFI troubles - backfires on deceleration

[derek murray]

My engine backfires (gurgles actually) upon deceleration. It will also do the same thing, occasionally, when idling, but only when the engine has warmed up...

Any ideas?

I have done a "search" but was unable to find anything specific to this condition... if I've missed a previous post .... sorry.

[Peterfrans]

You might look in the tech articles on the main pelicanparts site:

http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm

This will offer a wealth of information regarding MFI

Good Luck!

[pwd72s]

By gurgle, do you mean a minor popping? If so, that could be normal...a pollution control thing, designed to burn the fuel normally unburnt when decellerating.

[derek murray]

It could be called popping, but I wouldn't consider it minor. It is quite pronounced, and doesn't seem right. But then again this is my first Porsche, and I'm not very knowledgeable on what is "normal."

I have reviewed the tech article mentioned, thank-you, but it left my poor newbie head spinning.... lol

[Halm]

Is the backfiring through the exhaust or intake stacks?

[pwd72s]

Originally posted by derek murray
It could be called popping, but I wouldn't consider it minor. It is quite pronounced, and doesn't seem right. But then again this is my first Porsche, and I'm not very knowledgeable on what is "normal."

I have reviewed the tech article mentioned, thank-you, but it left my poor newbie head spinning.... lol

It could be a mere "adjustment" problem. If the tech bits are too confusing, I don't know if I'd suggest you trying to tune the injection yourself. I just hope there is a qualified guy near you who does understand MFI. Not a lot of today's younger mechanics do. Your post makes me grateful that a guy named Gordon is a mere 100 miles or so from me. AND I know his favorite brand of scotch! Be kind to your favorite mechanic..!

[derek murray]

The backfiring is through the exhaust. I'll confirm this tonight when I get home, just to make sure.

The tech article is wonderful, and I feel fairly confident that I could tackle an adjustment. But I want to take "baby steps", so a little help in the diagnostics is appreciated (focusing the newbie mind). I like to drink scotch...so maybe I'm halfway there

[Early_S_Man]

'Gurgling' is one of those auditory pleasures which delight the ears of MFI owners ... which I normally attribute to the proper operation of the Speed Relay (also known as the Speed Swich or RPM Transducer) and Microswitch and Cut-off Solenoid ...

However, if your engine backfires repeatedly after warming up full -- on over-run ... the Speed Swich is probably malfunctioning. This has been discussed much over the past year or so ...

Here is the explanation of how the Speed Relay, Microswitch, and Cut-off Solenoid should function in a properly-operating MFI system:

MFI microswitch-- Warren?

If you want to read a bunch of threads related to RPM Transducer problems, and fixes ...

http://forums.pelicanparts.com/search.php?action=showresults&searchid=72576&sortby=lastpost&sortorder=descending

[Halm]

Warren, Would you help me out and define "on over-run". I too am new to MFI but I think I follow most everything else being discussed.

Thanks.

[Early_S_Man]

Hal,

Over-run means, by my definition, that at some engine speed above 2000 rpm the throttle plates are closed at the idle position ... you backed off the throttle completely, so that the linkage returns to its' idle position AND the Microswitch will close and send +14 Volts to the cut-off solenoid on the injection pump ... assuming that the Speed Relay is healthy and doing its' job!

[Halm]

Got it. Thanks.

[jkeyzer]

As a side note... I disconnected my speed switch last time I had the system apart and haven't put it back on yet. I've been driving the car for a few weeks like this, and despite having it in a horrible state of tune part of the time, and never a perfect state of tune the whole time... It has never significantly backfired on deceleration.

I used to get BIG, frightening backfires when I had bad plug wires and warped throttle bodies (big air leaks) but since fixing those problems it has gone away.

Just my 2 cents. If it's doing it while idling also there may be a problem with a specific cylinder's airflow or ignition. Mine is doing THAT right now until it fully warms up and it has be perplexed... It is embarassing to sit in traffic with an intermittent POP POP behind you!

[derek murray]

Ok, so I spent the last few hours poking around. This is what I have discovered.

1. The microswitch adjustment screw does not enage the switch, in fact at idle it is sitting about 1/8" above the rubber contact. I'll adjust this tomorrow as well as test the switch, rpm transducer, and shut-off solenoid.

2. Three of the six throttle plates wiggle a little bit at idle. I can't tell if its a linkage adjustment issue, or a rebuild one (I am assuming this should not be happening).

3. I found a disconnected lead, wrapped in tape sitting on top of the alternator shroud. Its a blue green wire which my schematic (thanks guys) tells me its from the ignition transformer to the BHKZ unit? I dont think this particular issue has anything to do with my backfiring problem... but it would be nice to know if this wire should be connected to something (BHKZ Unit?).

Thanks for the help and I'll let you know how I make out.

[Early_S_Man]

Derek ... the lead from the Bosch CDI-unit (BHKZ) to the coil (ignition transformer) is normally color-coded Blue/yellow, and is essential or the engine won't start or run ... so, I assume it has been replaced by some other wire at the hands of a person with electrical system malpractice as a 'hobby' or profession!!!

Good luck with testing the Microswitch, Speed Relay, and Cut-off Solenoid!

[derek murray]

Warren, that makes sense. The previous owner replaced the CDI-unit and "rewired" it with standard, garden variety blue and red wire... now I know why the tach isnt working... or at least where to start looking... but thats for next week, lol.

Thanks for all your help.

[70flatsix]

You might consider an exhaust leak as a potential source of your exhaust backfiring. Until I plugged the holes in my heat exchangers, my mfi motor would backfire during deceleration all the time. Plugged the holes with exhaust putty and backfiring problem went away. Have recently purchased new ssi heat exchangers and dansk muffler from pelican to take care of the problem for good!

[9XI]

If you have sport exhaust, it may just be lack of backpressure. It's kind of annoying, but the positives of sport exhaust outweigh the neighbor complaints.