BACKFIRING 911sc

[andreas panayio]

Hello again everybody ....
there seems to be a problem with my SC.
the car has been backfiring ever since i brought it to the Europe.
Whenever its cold it backfires and it does let the car run propebly. I also notices that the car does not perform the way it used to.
My mechanic checked it and couldnt figure out exactly what it is.
At the begining he said it coule be the warm up regulator but then he said is not that.
Today he told me it could be the catalyt and he recomended removing it.

Please advise if you know something better before proceeding.

[Gunter]

Provide more details.
What year?
What type? US or Euro?
What maintenance/tune-up was done?
Cap, rotor, wires, plugs, timing, mixture?
Checked for vacuum leaks?
Take a few pictures of the engine bay so we can see the components.

[andreas panayio]

Its a 1981 911SC us model.
I had the car for 7 years and just shipped it Europe where i moved.
I just had a major tune-up done since the car was sitting for a while.
My mechanic checked all the adjusted all the mxtures, checked the timing, wired and plugs as well as the vacum leaks.
He couldn'd find the problem.

[Gunter]

OX sensor new?
Frequency Valve working? (You can feel it vibrating when running)
If not, check under the passenger seat, remove the relay for the Lambda brain, clean the pins and use dielectric grease. The fuse for the relay is the same as for the dome light.
Distributor serviced? (Cleaned/oiled weights & Springs?)
Vacuum advance working?
AAR working correctly?
Do you have the Bentley SC Repair Manual?

[andreas panayio]

Is is possible that the catalyt converter creates backfires and also affects the overall performance of t he car?

[andreas panayio]

Guys .... if there is anyone who knows more about this issue i would really appericate it ....

I am trying to figure out what is the reason the car is backfiring when cold and at the same time not performing as it used to.

If the catalytic is clogged, is it possible to cause both backfires and the car to under-perform?

[Ron.G]

Did this start happening after the tune up?
I know from my own experience that if it backfires when cold, your mixture is to lean at start up. for what ever reason.

You should have him check your cold control presure between the WUR and the fuel distributor.

This would be IF it is back firing through the intake, Not exhaust.
Back firing is not a good thig with CIS as it creates vacume leaks.

[andreas panayio]

No, it was the same even before the tune up.
The car was not being used for more that a year and I noticed the difference after i started to use it.
It backfires when cold but the car in general is under-performing even when hot.
My mechanic did all the necessary checks. No vacume leaks.

I just wanted to know whether the catalytic converter could relate to both problems.

[JohnKo]

The only catalytic converter issue I've seen that could be called at all common is blockage and high restriction, caused either by internal mechanical failure, or brought on by actions such as using leaded fuel. I doubt very much that it's your problem.

[Ron.G]

Check the little screen in the WUR. it may have some junk in it.
It's under the line that goes to the fuel distributor. Maybe blast it out with some carb cleaner. How old is the fuel?

I'd check for vacume leaks again. All around the air box seam and the intake runner boots.

Take the air filter out and see if the little screws that hold the air box together have popped up. if so, the seam under the air box my have opened up.
Good luck.

[foamy911]

KISS, I just had a similar issue. Turned out to be simply bad plug wires. She was a bugger to start backfiring and missing, when warm she ran OK, but had a loss of power. Grab a ohm meter and check the wires. It may be that simple !

good luck

[mack6820]

I had the same problem too. Since the car has been sitting, Id have the vacuum advance replaced and the distributer checked out.

[JP911]

Double check that the ignition rotor is in OK condition, that the cap is seated well, and that all of the plugs are in the correct firing order at the cap and then at the plugs. Backfire problems (on CIS cars) that I have encountered have had to do with ignition issues.

[andreas panayio]

Thank you guys.
I ve been discussing with my mechanic.
I will let you know how it will end up.

[diverdan]

All good ideas. Plug wires can sneak up on you. Do you know why the 930s had braided metal over the rubber? ...To prevent backfiring.

Good luck,

Diverdan