Strange ignition og injection problem

[cfeste]

Hi!

I have a 1978 911 SC, which has a strange problem. It runs very well on idle, and also well with full throttle while driving. CO readings are fine, it has newly been tuned. But when driving normally, it responds badly to the accellerator, hesitating and backfiring a little bit. It can run fine while accellerating, but it seems when it reaches a certain RPM, it starts hesitating and "jumping". If I then keep accellerating with more throttle, it runs fine until it again reaches an RPM level where it runs bad. So it seems something is off in the timing between accellerator and RPM. Could this be the distributor itself having problems? It has a new rotor and the distributor cap seems fine. Spark plug cables are new, coil seems fine. I am also wondering if it can be an injection problem, but all injectors are new and fuel flow seems fine.

All help appreciated!

[ossiblue]

What caught my attention in your post was the repeated use of the phrase, "seems fine."
CIS systems can display your symtoms for many reasons but unless you actually test the components that interact with eachother, you'll drive yourself crazy. Each of the areas you mentioned that "seemed fine" could contribute to your problem so you'll need to get started with some solid measurments on things.

Here's a start: You mention poor response to acceleration and backfiring. Could be air leaks leading to lean condition, faulty Warm up Regulator that isn't compensating for wide open throttle, or bad plugs, connectors or wires (even new wire sets can be faulty). Others here will offer plenty of suggestions and remedies. Be patient if you are going to DIY, or find a local Porsche expert who will help.
Keep us up to date as everyone's problem (yours in this case) becomes someone else's sometime in the future and it's nice to know how to solve them.

And by the way, if you don't post a picture of your car, agents of this forum will track you down, take your picture, and digitally alter it to put you in a compromising position with Rosanne Barr in the back seat of a Yugo.

[Paulporsche]

Boy I hate when that happens! Rosanne Barr?!

Seriously, were these symptoms there before the recent tuneup?

[cfeste]

Oh no, not a Yugo... Picture attached ;-)



The car was better when I got it. It had some issues with bad response around 2000 rpm, and I checked the distributor rotor and cap myself. I also did some checking for vacuum leaks without any luck. After that the car ran really bad, and a Porsche mechanic set the ignition timing correctly, adjusted the mixture and also changed the warm up regulator. This lead to the car running as I mention, very well on idle, both cold (1400-1500 rpm) and warm (950 rpm). Maybe a bit high when cold, I am not sure. When driving it has the described problems, ie it only really likes to drive with a lot of throttle, and seems to run best while the rpm's are increasing together with throttle. Easing off causes sometimes small backfires and jumpy behaviour, same does trying to accellerate slowly and in general drive in a civilized fashion.

Anyhow, thanks for replying! I found a Bosch mechanic who will test fuel pressure in all the ways he can, and I will get the ignition/distributor checked properly also. A problem is that in my country (Norway) everybody and their mother goes on vacation in July, so I have to wait until August.

BTW: The car did 130 mph today with no struggle, and had more to give, but the road ended... Of course with full throttle

[Paulporsche]

Having those fuel pressures is a good idea. Even though your WUR has been replaced, you still may have your mixture off once warmed up. BTW 1500 @ idle from cold isn't too high.

Are the backfires when you let off the throttle through the exhaust? If so, that is usually a sign of your mixture being too rich. A vac leak, often in the decel valve can also cause this.

Good luck and let us know what happens.

[Flieger]

Great looking car. I really like the color- Turkis Blau? The black center/anodized rim Fuchs are radiant.

[cfeste]

Hi! The color is supposed to be "electric blue", but it was a poor paint job and it is quite damaged from sun. I will have it repainted next year

[ossiblue]

Yes, very nice color--you couldn't tell from the pictures that the paint job is poor, your car looks great! Needless to say, you're now safe from the Yugo photos