newmexico

Hello, I purchased a 1989 911 turbo Porsche that backfires and sometimes stalls and idles low. Where do I start? Sorry for dumb question but this is my first turbo Porsche

rbonome

Ignition system and AFR

Mark Houghton

When/under what conditions does it does it backfire, how low is your low idle? Is is hard to start when cold, will it hold a higher idle when cold or do you have to keep your foot on the gas to keep it running? Please be as descriptive as possible. Could be as simple as your mixture being off.

newmexico

It backfires a lot when letting off of the peddle while shifting up or down. Even if I stay in 1st gear on busy streets and letting off the gas coming to a stop sign. When I start car, after 2 minutes or so if I'm not giving it gas it will die as if it ran out of gas. The car sat for 4 years. Cleaned fuel tank and had it serviced. Ran great for a week or so. Car is from Dallas, Texas and I have it in New Mexico now. There are no Porsche experts that I am aware of in New Mexico.

md993a

In addition to the above recommendations, check you CDI and coil.

Mark Houghton

I would first look at fuel control pressures as controled by the warm up regulator. From your description it sounds as if your AAR is working when cold, providing the air bypass at the throttle body to sustain a high cold idle until the AAR warms up and closes off. You may be too lean as a result of the WUR not controlling properly or because someone has not properly adjusted the mixture screw.

After fully warmed up, try adjusting the mixture adjusting screw clockwise just about 1/8 turn or less (that will push down the control arm in the fuel distributor slightly and richen your overall mixture). If by doing that you find the idle going up, then indeed it was to lean. Or if you find that by doing that adjustment the idle gets even lower, then go the opposite direction (CCW) which will lean it out. Either way, you want to find lean best idle, then richen it up just a tad as most of these cars idle best when slightly rich.

None of those steps will tell you the condition of your WUR as to the control pressures (and thus the resulting air-to-fuel ratio) it's providing. You would need to procure the proper gauge setup to test it's function and really should do this before monkeying too much with other stuff....process of elimination.

Make sure your air cleaner filter is clean, and may also want to replace the fuel filter. And while you're at it, do a tune up (check plugs, check/adjust timing, check all vacuum hoses for leaks) so you don't have a bunch of contributing factors and just end up chasing your tail.

79turbo

Could be something as simple as a leak some where between the throttle plate and the metering plate, such as a cracked rubber hose on the intercooler, cracked old rubber connection on one of the many connecting hoses. All these will create a lean condition that would give the symptoms you have described and go bad with time and are easy to check. I would go there first.

paulleslie

Hi
If the problem is intermittent, then it could be one of the 2 fuel pumps ,usually the rear one. As suggested you could check the fuel pressures,but this may not show up the intermittent nature.
You can put your hand around the rear pump and feel for roughness in the running . Fuel pumps often work intermittently before they fail ,especially if you put them under a sudden load, that is when you can get the backfire and rough running and poor idle.
Just one of the many things that can cause your problem.
Good luck

MUSSBERGER

Don't forget to check the contacts on the fuel pump relays and contacts on the fuses.

Bruce P

Take it to Russ Kelso in Corrales. I'm sure he can find the problem. Do a Google search for PMCI in Corrales.

T930

not likely ignition.

follow what mark said. its likely to do with fuel, an air leak or the speed relay under the seat.

you can take the breather off and onec warm and idleing, lift or push down on the sensor plate. that can at least tell you is you are rich or lean. down=too lean, up=to rich.
dont make any mixture adjustments until you have fixed any air leaks and checked control pressures.

the speed relay is prone to bad solder joints. hittting all the joints with an iron is the fix