SC poor performance
 

Hi
i have recently bought a 1980 3.0SC which has really poor accelaration, the car slowly builds up speed it never really pulls. The mileage is 107,000 with top end work done around 60,000.Th e oil pressre is good, over 1 bar per 1000rpm, there are no leaks, noises or smoke. I have changed the Dist. cap / rotor arm and plugs, accumulator, fuel filter, pump and check valve, the leads look quite new. None have made any difference. The car starts first time cold (around 900 rpm) but touching the throttle within the first few seconds would cause the car to stall. Warm tick over is around 800rpm (was 600 before i turned it up). Any likely causes for poor accelaration ?

Assuming your tune up included a new air filter, plugs, a check on timing, and a valve job? Did the dist. rotor snap back to position, esp. after you oiled the felt? Any bad gas in your area? Do you notice a drop in idle when you remove the oil cap when idling - if not be sure to check the pop-off in the airbox and the overall condition of the airbox (ie not blown). When you start the car, put your foot on the brake pedal and ensure it moves away from you as vacuum is applied from the crankcase.

Sorry not to be of more help but hard to spec. from your description. It sounds to me like timing or some sort of intake blockage?

Blocked cat? remove and see if that helps.

Cat's a possibility. I had some fuel injection work done last year & when I got the car back it was a full 40% slower than when I dropped it off (despite brand new fuel injectors). While diagnosing the problem the shop ended up melting the catalytic converter.

Needless to say I've got a new (awesome) shop. Took a broomstick through it & now everything runs great.

johntom,

The first thing to check is the throttle linkage. I am amazed at how many 911 owners want to modify cars that don't have throttles that fully open.... Have your lovely assistant Tabitha mash the pedal and check if you can open it more at the TB. Then start replacing the bushings. Once you have full throttle, then check high speed timing and the FV duty cycle.

Paul

I had similar symptoms and new plug wires cleared things right up. It is amazing how much more power a couple of extra cylinders provide. It was smoother too! :rolleyes:

This is a stretch. Make sure there is not a 3mm allen wrench in your CO adjustment screw that someone forgot to pull out.

Have you driven any other SC before? A serious question with no ill intentions.

BTW was your fuel pump issue resolved?

Yeah, I had the same problem on my SC. ;) Seriously, what are you comparing it to? A stock SC isn't the fastest 0-60 car if compared to any of today's sporty cars.

I would definitely check the mixture. Also make sure you have the timing right and fresh gas in the tank.

George

Hi I wasn't comparing the car to anything, it just feels lethargic, nothing really happens when you put your foot down, Even at 70 ish and 3000rpm it just builds up speed with no Urgency. I forgot to mention that the car also backfires when lifting off the throttle, but not all the time. The airbox looks good but there is a preformed hole, about 1/2", on the left hand side, should that be there or sealed? The car doesn't have a cat and the previous owner removed the air pump. I will try the idle with the oil cap removed.

It might be as simple as no power to the frequency valve control unit. The search feature could be your friend.

If it were me, I would:

Do a leakdown test first. This will determine if the engine is healthy.

Check ignition next. Condition of everything ignition related. Set dwell and timing.

Finally fuel injection. Check / adjust CO.

I do those steps whenever I get a new car, even if it is running well. And only after I check the brakes, fuel lines and filters, bearings and suspension.

Hi what and where is the frequency valve control unit ? Thanks

the preformed hole is most likely where the pop-off valve should be. I would test the car by using something to block the hole, something your car will NOT suck up the injection system. If the car runs better with the hole plugged then you will need to buy a new pop-off valve.

You might also update your information so we can see where you are located. Someone might be able to come out and help you if they are close by.

My bet is a clogged-up filter screen on the WUR intake line. Simple enough to take off the line and blow it out w/ some compressed air.

Hello,

I have an 80sc...went through something similar. Mine ended up being some broken head studs...I wasn't getting any compression...course a compression test would show you that. Or pull the lower valve covers off and look....

Alan

Sorry, I meant the small round hole is in the left side wall of the airbox, not in the pop off valve (which is in the bottom of the airbox). Unfortunately I'm in the Uk.

Frequency valve is part of the O2 system. It squeezes and loosens fuel pressure to feed less or more fuel to the engine based on what the O2 sensor is telling the control box. It is in the picture in the middle - not the blue one - just up to the right from that. The blue one is the cold start valve.

I would check the timing advance as suggested above. The snap back test.http://forums.pelicanparts.com/uploa...1282148026.jpg

Hi
just out of interest i have arrowed the hole in the air box where the pipe shape is. The hole goes through the sidewall into the box. Is this correct? I don't have a cat. or O2 sensor so i won't have a frequency valve?http://forums.pelicanparts.com/uploa...1282151757.jpg

That is where the hose from the carbon canister should go. The hose will wrap around from the canister on the fender wall behind the oil tank.

I think you still will have a working frequency valve it just wont vary pressure - However, the control unit can still control the frequency valve to a degree like on cold start where it tells the frequency valve to maintain a duty cycle of 65% which is rich. A temp sensor tells the control unit when the car is warm and the O2 system should then take over in closed-loop mode. You have no O2 sensor so I think there is a default mode of 50% duty cycle. However, there is also a wide open throttle circuit that overrides the O2 sensor input so yor frequency valve will help you there.

Suggest you read this CIS primer. It's pretty interesting. Go to CIS Components and then the O2 Sensor / Lambda SYstem.

CIS Primer for the Porsche 911