backfire on deceleration

[lbailey75@tampa]

My 2.7 911 is running rough at idle. when driving it seems like it wants to stall. This happens under normal or hard acceleration. when you back off the accelerator, it backfires.
checked points, timing, wire resistance, new plugs ( old plugs looked good ), new coil, new cdi unit.
could it have jumped timing ?
can it be a fuel pump or fuel distributor or injector problem?
professional advice please

[proporsche]

if it backfires several times remove the air filter and look(in bad case) if by any chance you do not have broken the air box.
The next would be to look at your sensor plate if it is not out of alignment-from the backfires.The fuel mix would off if the sensor plate is offset.The pic graff is taken in the opposite direction.
i assume the timing is correct on the ign.distributor?

check these things 1st.
And of course it is always the recommended to put on the fuel pressure gauges to see the system pressure and cold start pressure

[Cherry Bomb]

Exhaust leaks will also cause some noise on decel.

[lbailey75@tampa]

Yes, the distributer timing is correct. A bent or offset throttle plate would not cause random un sequenced sputters at different rpms and gears while driving. I installed a pop off valve in the air box years ago. It does backfire at the exhaust and pop off valve.
I will be checking fuel pressure once I can install a gage that can be read while driving. I will also check the fuel pump's volume flow rate.

[proporsche]

what is your fuel mix??? you did not say where does it backfire? exhaust or intake?

Ivan

[AndrewCologne]

@lbailey

Backfiring means typically that the combustion is in such a lean condition that by this the mixture can't get iginite the common way by the sparks from the plugs. So the unburned air/fuel mixture gets into the exhaust system where highest temperatures occur which let the mixture ignite in an arbitrarily way.
In combination with your rough idle I would follow Ivans suggestion and check your mixture for proper CO setting. Also check the deacceleration valve behind the throttle body – if your 2.7 engine model already comes with that one installed. This also should prevent backfiring by keeping the vacuum in the manifold when deaccel. as low as needed.

[fanaudical]

I'm assuming you mean "popping out the exhaust" and not a real backfire, which would involve combustion in the intake system.

I spent a bunch of time chasing this on my '75. I was simply getting too much fuel at idle and decel. Root cause for me was that the plastic CIS lines I had made from the wrong size tubing allowed the injectors to pulse, and so I had the fuel mixture cranked too far up just to get the thing to run.

If you have factory lines, you still might want to pull the injectors and ensure they are all spraying nicely and no solid streams of fuel are present. Other than that:

- Verify fuel pressures are correct (cold control pressure, warm control pressure, overall system pressure). This would be done with a CIS test kit and is not necessary to test while driving.
- Verify even fuel atomization and flow from all injectors
- If you have the -009 WUR, verify that the vacuum enrichment is working properly and no vacuum leaks in the WUR body.
- Verify the decel valve works correctly.
- Verify no air leaks in the intake system (smoke test the manifold)
- Verify ignition timing is correct.

[gduke2010]

Originally Posted by fanaudical

I'm assuming you mean "popping out the exhaust" and not a real backfire, which would involve combustion in the intake system.

I spent a bunch of time chasing this on my '75. I was simply getting too much fuel at idle and decel. Root cause for me was that the plastic CIS lines I had made from the wrong size tubing allowed the injectors to pulse, and so I had the fuel mixture cranked too far up just to get the thing to run.

If you have factory lines, you still might want to pull the injectors and ensure they are all spraying nicely and no solid streams of fuel are present. Other than that:

- Verify fuel pressures are correct (cold control pressure, warm control pressure, overall system pressure). This would be done with a CIS test kit and is not necessary to test while driving.
- Verify even fuel atomization and flow from all injectors
- If you have the -009 WUR, verify that the vacuum enrichment is working properly and no vacuum leaks in the WUR body.
- Verify the decel valve works correctly.
- Verify no air leaks in the intake system (smoke test the manifold)
- Verify ignition timing is correct.

mine does the same especially, down hill. not as bad as a backfire.

[7783911]

same a GDUKE...so did a smoke test, all is good. The shop thought everything was in order and suggested a baffle in the muffler may have corroded after 20 years (5 sitting in a warehouse). I havnt done anything to fix the heavy gurgle on downhill decel (decel valve btw is good) as i intend to convert to a street exhaust as the car is just too sporty (noisy) for my neigbhors liking :-)

[Dr J]

I will agree with Andrew that it seems too lean. When the exhaust is changed or a cat bypass is installed, it is not rare to get gurgle on decel. Given that timing was ok and there are no air leaks, I would richen it up a bit and check it out.