What is causing this backfire ?

[BalGlenn]

Hey Guys,

I’m Glenn from Belgium and own a ‘78 911SC targa for about 2 years now.

Car is completely repainted and it’s been a pleasure to work on.

Last year all the rides were perfect in any conditions.
But i noticed the fuel pump coming on, as soon as the key was turned to the first position.

Being an electrician with, and with a lot of experience in old cars too (Audi 100 C1 1973 , VW beetle 1973, both completely rebuild engine), i’ve redone the front compartment electricity this winter.
Followed to correct schematics and everything works like a charm now how it is supposed to.

BUT, i also came across a disconnected cold start injector, so i connected it like it should (checked with the air sensor plate, they are correct).
Since then, the car has left me stranded twice, everytime because the pop off valve opened and got stuck on the filter (PO mounted it backwards).


I’ve never had/heard a backfire before, also on the 2 breakdowns, i did not feel anything like a backfire.
Could the cold start injector be faulty and cause a backfire, and therefor it was disconnected maybe ?

Like mentioned, didn’t have any problem before the cold starts injector was connected again.

Hoping to learn

Grts,
Glenn

[ahh911]

Welcome Glenn,

The cold start injector electrical connector was originally disconnected at the injector?
The other end of that wire should connect to the the thermal time switch. tts.
If you disconnect the connector at the tts, it should permanently remove any voltage from the cold start injector, similar to how you received the car..
You can also check that the tts turns off within 8 seconds cold.


Phil

[BalGlenn]

The cold start injector, indeed was disconnected at the injector itself.
I can’t help but think that the problem is related to connecting it again.

Could the tts keep voltage to the cold start injector (not shutting off) ?
What caused a too rich mixture ?

[scarceller]

Did the valve pop open while driving? If so, under deceleration or acceleration or just steady state driving?

Fully warm engine or cold?

Would be helpful to know the conditions just prior to stalled engine.

What we know is that the intake somehow was pressurized for the valve to blow open, question is 'what could cause that other than a backfire'?

[BalGlenn]

Yes, both times i started the car without any problems, and it stalled in the first minute.
Let’s say 10s to pull out the parking spot, and then 1ste gear, 2nd, 3th and stall.

So cold engine, but warm weather conditions, normal driving/accelerating to get to 50km/h

[scarceller]

Seems to be a issue during warmup of engine. I suspect something is not correct with the cold start injector, that injector should only turn on at initial cold start during cranking. I believe once engine is running the injector should never be active. You need to verify the circuitry and the cold start sensor that triggers that injector. I suspect they unplugged it because something is/was messed up with the functionality of that circuit.

[PeteKz]

Reinforce what scarceller said.

Get a wiring diagram. You can find copies online and under threads in this forum. You can also find previous threads regarding this problem and how the cold start circuit works. Because you are an electrician, this should be easy for you.

[BalGlenn]

I have the diagram printed out 60cm by 3meterd, hanging against the wall
I will follow this TTS up and test it ����

[A930Rocket]

Unplug it and see what happens. If it runs good, leave it unplugged.

[BalGlenn]

I can confirm the TTS is faulty now, and it keeps power to the cold starte injector, thus floating the intake too much, what caused the backfires

[eastbay]

So the car does not run with the popoff open and jammed on the air filter. BTDT

Just like you I had a 911 that had the popoff installed backwards, I popped off the popoff valve with a big screwdriver and glued it back in the correct direction. It was easier than it sounds.

The correct fix is to buy a new airbox and not destroy it with a foolish popoff valve

[PeteKz]

Lots of blown airboxes indicate that a pop-off valve is a good thing to have. Just in case.