After months still small backfire at warmup.

[Nick Triesch]

Cannot figure it out. Car has great idle like it is supposed to at warmup with no pops. Then settles to 800 RPMs then after a few minutes it begins to pop. Will then pop for several minutes until all warmed up. Then an occasional pop while driving but very few. New tuneup, plugs wires, valves, fuel filter. All vacuum leaks found and fixed. At least all the ones I can find. I just do not know any other place to
Look. But it is interesting, before my big maintenance the idle would go down when I removed the oil cap. Now it does not seem to go down. I just do not get it.

[porsche930dude]

Id say richen it up a tad and see how it does. That fixed my backfiring completely

[Nick Triesch]

I tried that on my 3.2. Did not work. I’m thinking that it must be a vacuum leak because it gets better as the motor warms up and engine gets tighter. I just have no idea where! I have checked in all the places with starter fluid. Changed intake gaskets. Changed hoses. I have warmup idle backfire rich or lean so must be vacuum leak. Have new 02 sensor.

[juanbenae]

you really need to share greater details on the motor and intake system. if you have carbs it could be this, if it is CIS it could be that, if a 3.2 it could be the little box under the passenger seat?? popping generally indicates a lean condition, or an intake vac or exhaust leak. more specifics equate to more accurate could be's.

[Nick Triesch]

Sorry,85 3.2

[AndreasK]

Iīve got roughly the same symptoms as you. Mine is a 77 2,7 CSI. I also feel the throttle response is a bit sluggish at low RPMīs. (Below 2-2500) Feels almost like its flooding slightly when gently blipping the throttle. Higher RPMīs are fine.

Have you tried a new distributor? Mine got a lot better after changing, but the symptoms came back after some time.

Oil cap indicates an air leak. My older car RPMīs doesnīt change with oilcap removal. With yours it should.

[Nick Triesch]

yup, cannot find vacuum leak. After I found and repaired the leaks my car does cold start like it should at about 1200 rpms then it settles to 800. Was all
Over the place before the repairs.

[Nick Triesch]

So is this correct? When cold start, the CHT makes fuel more rich, then after a few minutes when 02 sensor warms up , the fuel leans out? Because when car first starts , no backfire for a few minutes. Then it begins to pop for several minutes, then it stops!!! Holy cow

[Nick Triesch]

I just did a test. I just did a cost start with the 02 sensor disconnected and there is no backfires! My 02 is very new. What the heck???

[proporsche]

disconnect the sensor and do not plug it in ..there is no reason for it....unless you go for smog test...problem solved

Ivan

[Nick Triesch]

Wow! Thanks. But if there is no reason for it why is it there and what does it do? Will motor run to rich or lean with it unplugged?

[proporsche]

The sensor is there to clean the emission going out of the muffler and trough catalyst..mainly for smog test purposes to be short.Let`s say you sensor is plugged the emission will read probably co.0,4 and Hc about 80 with unplugged you should have your 911 set up about 1,00 CO and Hc will be probably 80-120....
It does not change you fuel mix once you have it set up and runs great.
Most of the 911 in the rest of the world do not even have it..Do you still have an catalyst? If yes get your self a strait pipe...you will love the sound and a little more horsies...
Ivan