Holy MF - fire in the airbox!! wtf

[Topocalm]

Wanted to start the car (2.4T) and the motor did not start properly. It ran two weeks ago after it sat for several month in the winter and it ran fine. Today sometimes it would catch, do some misfirings, but not run on its own.

I tried for maybe 5 to 10 min, and then I realised I had smoke coming out of the engine grill. Opened the back, smoke was coming out of the airbox.

Opened the airbox and the air filter was smoldering and burning right above the pop off valve (I think). WTF? On the inside of the airbox cover, i could see drops of oily / gasonline (?).

Any ideas what problem causes this to happen?

[proporsche]

1st your pup out valve is installed the wrong way..it should be facing back..then most likely your fuel mixture is not correct..check the basics for cold start Like points in distributor not too close ,set timing and then correct the fuel mix...
Ivan

clarification..of ....it should be facing back--(correction spring facing back)

Ivan

[Quicksilver]

Originally Posted by proporsche

. . .1st your pup out valve is installed the wrong way..it should be facing back.
. . .

It's obviously working since it let the fire out (as the burning filter attests to ). The popoff is glued in so there is no reason to do it over.

- Buy yeah, get the thing running right so it doesn't pop off again.

[Quicksilver]

It occurs to me we didn't answer the OP's simple question.

- What happened? - The pop off valve is an add on to the system so if you have a back fire the pressure in the airbox doesn't crack and destroy the airbox. Obviously you had a backfire and the pop off valve did it's job and let the pressure out saving the airbox.
And along with the pressure came some flame! Good thing you saw it in time to get that filter out of there. The only money you are out is a filter. (Don't buy a Fram) Now you need to go through the injection and the ignition and see what is out of spec. There are guides here on Pelican to help you. Just don't get the "replace things until it works" sickness. Test, test, test!

[proporsche]

quicksilver..it might be glued but when it backfires the flap will hit the filter(as you can see on the pic the fire was right there where the flap stayed open) and it will stay open so backfire again..too much air .....there is a reason it was designed facing towards the front of the car...

topo...
As i mentioned start with the basics points, timing and fuel mix..but the most i forgot to mention is ,with this backfire you have most likely knock off the sensor plate adjsutment.It has to be like this..

after you adjust it you have to adjust the fuel mixture...
If you are having difficulties i can take my 911 drive to you and fix it;-)

Ivan

[john walker's workshop]

After a good backfire, a sideways valve can get caught on the filter folds and stay open, and the amount it can open is limited also.

[wayner]

Isn't backfire by definition "fire coming back?"

As in back though the intake?

The better questions is why do so many of us survive a backfire without a bigger fire starting.
(one answer is that continuing to crank can such the fire back in.)

[proporsche]

wayner..it is all about the correct set up of your 911 fuel mix,timing airflow sensor plate e.t.c......it all has to be done perfect...sometimes the pup-off valve saves your air box,it is there for protection...
If you have the correct set up on your engine you do not even need the pup-off valve..

Ivan

[pmax]

Good thing the toilet seat aka pop off valve didn't lead to more damage, such as burning your car down, by porting flames into the engine compartment.

Were those airbox screws missing prior to this incident ?

[proporsche]

good point Pmax..blind me;-)

[Coastr]

Originally Posted by wayner

Isn't backfire by definition "fire coming back?"

As in back though the intake?

The better questions is why do so many of us survive a backfire without a bigger fire starting.
(one answer is that continuing to crank can such the fire back in.)

I guess it depends how much fuel has been dumped into the intake befor it goes.

[Topocalm]

Thanks for all your input!

My homework now is
1. Install the fire estinguishers I bought within 20min of this incidend teaching me that the "fire" in backfiring ist to be taken literal
2. Install the pop-off valve correctly. I understand I have to turn it by 90° so it does not open sideways. Above I see suggestions it should be facing front as well as back. both works, just not sideways?
3. Add the missing airbox screws. I had no idea.
4. Warn my buddies of the responsible Porsche shop who once more proved that they are all over the place
5. Learn to to set up the engine correctly

By the way: I tried starting the car again with the pop of closed and it ran fine. So it is a starting setup issue...

[john walker's workshop]

It opens toward the front of the car only.

[gomezoneill]

That cover on the pop up valve most likely got hung up on the air filter while you were cranking and backfiring.Meanwhile the csv is pumping gas into the box below the air filter atomizing the fuel then FIRE! Maybe your airbox doesn't have the spider to distribute the fuel to the individual intakes.

[porsche930dude]

mustie1 just did a video on a targa that had a weird little pop off valve at 17:00 is this something we should all be checking?
https://www.youtube.com/watch?v=lUGnge7uIJU

[gomezoneill]

Originally Posted by porsche930dude

mustie1 just did a video on a targa that had a weird little pop off valve at 17:00 is this something we should all be checking?
https://www.youtube.com/watch?v=lUGnge7uIJU

930 dude where you located in upstate ny? I'm near Saratoga.

[porsche930dude]

near binghamton

[E Sully]

Very valuable site. 911 CIS Primer - Index
You will have to check the air box to verify it is not cracked and leaking air.
Here is a picture of the proper position of the pop off valve.


The hand throttle should be working and set properly. Also the micro switch on the throttle body. This allows the start valve to activate when cranking the engine, which should also be checked to see if it sprays. The 2.4T CIS does not have a thermal time switch, and sprays when the engine is cranking with the throttle depressed by foot or hand lever. Later '74 on have a thermal time switch which limits the cold start valve. The spider mentioned by gomezoneill was installed on later cars to reduce chance of backfire. 1973 does not have it.

Of course all the basic tests should be done with a proper CIS fuel pressure gauge set. As already mentioned, basic tune of points, condenser, plugs and timing should be set.

The manual has a starting procedure. Cold or warm start, pull up hand throttle and slowly lower after starting, keeping rpm at 12-1400 rpm till engine is warm.
Hot engine, fully depress gas pedal, no hand throttle while cranking.

[Walt Fricke]

Ed - you caught the points I had noticed about the models, though I didn't know the 73.5 CIS T didn't have the thermo time switch.

Do you suppose one could be fitted? You could put a bung into a chain cover, or just get a left chain cover from a 2.7 or 3.0. It should be easy enough to route the CSV power through the switch. Seems like a very useful addition.

The cause of these backfires is an engine which won't start right up, leading to pooling of cold start fuel in the bottom of the air box/atomized fuel in the air box at some point while cranking over, with - I surmise - a backfire from the combustion chamber through an at least partly open intake valve. I don't think there is anything in the intake system/air box otherwise to cause a spark inside the box.

[boyt911sc]

Originally Posted by Topocalm

Wanted to start the car (2.4T) and the motor did not start properly. It ran two weeks ago after it sat for several month in the winter and it ran fine. Today sometimes it would catch, do some misfirings, but not run on its own.

I tried for maybe 5 to 10 min, and then I realised I had smoke coming out of the engine grill. Opened the back, smoke was coming out of the airbox.

Opened the airbox and the air filter was smoldering and burning right above the pop off valve (I think). WTF? On the inside of the airbox cover, i could see drops of oily / gasonline (?).

Any ideas what problem causes this to happen?

Topocalm,

You have two (2) missing screws for the airbox. This an obvious sign that your airbox has been compromised. Do yourself a favor and do a pressure or smoke test to confirm the absence of sources of significant air leaks. The accumulation of unburned fuel in the airbox because of the previous failed attempts to start the motor and the absence of the spider plumbing inside the air box eventually ignited the fuel. The POV (even incorrectly installed) saved you this time.

Next time you might not be as lucky. Keep us posted.

Tony