Back fire / plenum pop - Alfa Romeo Bulletin Board & Forums

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post #1 of 13 (permalink) Old 04-11-2010, 04:00 PM Thread Starter
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Back fire / plenum pop

I think title is the question. How common is it? What causes it? My engine sometimes backfires when I start, and I used to think that the problem is caused by Megasquirt which is not tuned correctly, but constantly I see people talking here about backfires and plenum pops. Is this really normal behaviour for an Alfa V6? I just can't believe that they would sell engines with such a serious problem.

I'm personally not that worried about the plenum popping, but as it causes the engine to stall so suddenly I always fear that the timing belt might jump...

Alfa 75 3.0 V6 America. QV cams, Megasquirt, Crane HI-6, CSC headers.
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Last edited by Tuomas; 04-11-2010 at 04:07 PM.
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post #2 of 13 (permalink) Old 04-11-2010, 05:43 PM
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Since you have aftermarket FI this may not pertain to you. For those of us still running the stock L-jet my understanding is that most backfire issues occur during startup. Any amount of throttle on start up is bad and can cause phenum pop. Well actually you can either have no throttle or pedal to the floor which is the "clear a flooded engine" mode. Apparently the phenum pop is semi-enginered into the system to save the AFM from being damaged in backfire conditions. I believe the rudimentary nature of the L-jet system is the underlying cause of the backfire-upon-startup-with-some-throttle-pressure problem. If, as it seems you are, you are having backfire issues at other time (maybe related to the MS) you are just having to deal with alfa's "solution" to the L-Jet start up issue.


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post #3 of 13 (permalink) Old 04-11-2010, 05:46 PM
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It isn't a problem unless you have air leaks or other issues. If your car won't start with just the key and no throttle you are a candidate for plenum pop. It is not an issue if everything is right. Backfiring is not good, especially if you're monkeying with the throttle pedal when trying to start. Something is out of whack.

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post #4 of 13 (permalink) Old 04-11-2010, 08:43 PM
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It is an unusual thing to happen and will not happen if things are correct under the hood. timing, vacuum leaks, mixture richness etc, etc will all effect cold start issues including plenum backfires.

specifically injector seal leaks, or vacuum leaks on the plenum-to-intake runner hoses will definitely make it worse. having timing advanced way too much can also cause this.

Normally there should be NO issue starting the car cold with slight throttle pressure. although it is not needed, doing it should never cause an intake backfire unless something is wrong.

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post #5 of 13 (permalink) Old 04-12-2010, 02:37 AM Thread Starter
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It has backfired two or three times, the first time I got really scared that it broke, but luckily nothing happened. I remember reading somewhere that one cause is the cold start injector, which is not needed with ms. To start the engine when it's cold does require some throttle, which you say increases the risk of backfiring. Perhaps I need to look into the cold start settings if it could be made to start more easily without any throttle...

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post #6 of 13 (permalink) Old 04-12-2010, 03:15 AM
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Does it back fire when cranking or when the engine is running?

I'm not at all familiar with the Megasquirt software, but surely they have a cranking set up as well as coolant temperature compensation for fuel?
If the Megasquirt is controlling ignition timing and the system has a coolant temperature compensation map for ignition timing, you could also try retarding the cold engine ignition timing a few degrees.

Also, air leaks on a MAP sensed engine management system won't have the negative effect that the have on the airflow meter equipped factory system.

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post #7 of 13 (permalink) Old 04-12-2010, 07:52 AM
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Tuomas, you should definitely be able to get it to start w/out throttle. My megasquirt'd X1/9 would start dead cold by reaching in and cranking the ignition from outside the car.

It took some playing, but I was very pleased with cold start.

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post #8 of 13 (permalink) Old 04-12-2010, 11:49 AM
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I never have pops with the VEMS installed but I also used a 164 type intake. but it sounds like you are getting trigger errors.
the old L-jet did not even have a computer at all. it is a wonder it works at all, is it super simple. just some op-amps one shots and a fancy wiper that selects tuned resistors for that motor. (yep the VE map is in the AFM)

on the bright side you should not have a flap to damage. So hard mounting/strapping will not hurt.

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post #9 of 13 (permalink) Old 05-06-2016, 01:55 PM
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Well this happened to me this week.

Big Bang.

My own fault, thought I was in the junior, pumped the throttle twice and started it cold with half throttle open.

It didn't blow the plenum off, because I had the throttle open. It traveled down the intake hose, split that, separated the hose from the AFM, and then blew the back off the AFM !!!!!!

Not sure if it's damaged the AFM, but ordered a new hose and will see from their.

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post #10 of 13 (permalink) Old 05-11-2016, 02:03 PM
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This burned my garage down last time it happened to me. Make sure to key off as soon as it happens and keep a fire extinguisher nearby whenever dealing with a no-start issue. Not to mention having one within reach in your shop at all times.

88 Milano Verde "Corvino", 91 164L 5spd, 79 Sprint Veloce, 85 GTV-6
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post #11 of 13 (permalink) Old 03-12-2017, 02:48 PM
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Since suffering the pop, it's been a *****.
Occasional difficult to start. Rough cold idle. Splutters on full throttle when driving.
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post #12 of 13 (permalink) Old 03-12-2017, 03:28 PM
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After verifying that everything is hooked up properly, dist/rotor are clean of contaminants and corrosion, the plugs are clean and adjusted, and fuel flow/pressure is within spec, it does sound like something was damaged during the backfire, most likely something in the AFM. I think I would search for a used AFM. Should be a bunch out there.



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post #13 of 13 (permalink) Old 03-12-2017, 06:43 PM
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Agree on likely AFM issue. If you can't find one cheap, see if someone's got a known good one you can borrow just for testing purposes.
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