Another CIS Question?

Greg P · 07-08-2009, 01:30 PM

Hi All,

Long time lurker, first time poster here. I'm trying to get a 911SC back in the road after being parked for a few years. Long story short, I replaced the injectors, sleeves, filters, plugs, wires, cap, rotor, and installed an MSD unit.

Here's the symptoms: The car starts instantly and purrs like a kitten at idle. As you start to accelerate and get just above idle speed, the engine hesitates and backfires. If you keep your foot in it everything clears up and it runs great. Same thing happens as the engine is slowing down. Once you are back at idle everything is great.

I'm not sure how to diagnose the problem at this point. A friend who is not a Porsche guy but a really good wrench thinks it is the fuel distributor. Any thoughts?

Greg

htbetz · htbetz's Garage

Mine kinda does the same thing so I'll be curious as to what you come up with.
Harry

James Brown · James Brown's Garage

does this problem happen when warmed up all the way? sounds like an air leak to me if it runs good under full load cold.

longhornchris04 · longhornchris04's Garage

What year SC? The early ones have a vacuum controlled WUR that would do mixture enrichment under load. The later ones had an O2 sensor and computer that would keep the mix stoich at part throttle and go rich under more throttle. If either of these isn't working you could get a substantial transient.

Additionally, it could be vac leaks / unmetered air, or your fuel distributor could be clogged.

If you've replaced the injectors already, then you know they should flow evenly. With the car off, you should check the movement of the air-flow plate -- there should be no binding.

You could also check for matched flow rates by pulling all the injectors, placing them in glass jars, and manually lifting the air meter. You should get even flow.

James Brown · James Brown's Garage

also you need a pressure gauge and a pressure/temp chart for your model engine. the only way to properly adjust this system is to check everything! every component effects the rest and air leaks are the first thing to check (airbox, vent lines, AAR,AAV, decellvalve, ect) then if its all good and tight, check control pressure,WUR, lamboda system, fuel/air mixture. Then it should run good for a couple of days!!! There are many checks to do so others may chime in.