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930 3.5 Liter Twinplug Racing Engine....sputter then stall...Help!

Got an issue that is driving me crazy and to the poor house at the same time. Have a California trasnsplanted 1980 930 turbo slantnose twinplug 3.5 with mahle 100mm pistons....engine recently rebuilt with new pistons and barrels, new twin plug distributor cap,etc. Car had been very strong but all kinds of OEM stuff had to be reinstalled to pass inspection/registration in Canada. Leakdown test had revealed issues with rings and on inspection pistons had been beat up so entire engine was rebuilt. Recently had a new cold start system and injector installed to help with cold start issues. Had headers all heat wrapped to reduce under hood heat. Here are the symptoms of what my mechanics at Alpine Porsche in Calgary are unable to solve. There are several issues here.
The car now fires up Ok if I've had it on the charger overnight, but smokes like a mofo about 2 minutes after starting,,,,very embarassing. ...it then cleans up as you start to drive and runs fine for 30-45 mins until the temp guage just clears the cold line and moves 1 cm above it.......at this point the engine will stutter/miss several times and within 2 minutes it will backfire repeatedly and I am forced to pull over or it will stall out ....I then sit for 1 hour to allow the engine to cool....the engine can sometimes be a hard start but then it runs fine and I head for home before it leaves me stranded again. Could it be the fuel is being vaporized by the heat in the lines even though the guage is not even close to half way???? I am desparate and after 3 or 4 visits over the last year am discouraged. The smoke issue on initial I think is from the valves....or is it a worn seal in the turbo?

Last edited by spanky930; 08-10-2011 at 06:30 AM..
Old 08-10-2011, 06:24 AM
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Hey there,
Well it sure sounds like you have multiple issues going on here, all of which are driving you nuts & poor.
I have never experianced worn valve guides that smoke 2 minutes after start, they either cause smoke at start up and then quickly clears up, or they cause smoke on heavy deceleration. The first from oil pooling in the cylinders after shut down from seepage around the very worn guides, the second from oil being pulled through the worn guides via vacuum during deceleration.

If it is a true backfire (popping back through the intake) sounds more like fuel starvation than anything else. So possibly fuel pressure deviation from errant WUR??
If is popping through the exhaust, sounds more like an over rich mixture situation...hummm...same cause, errent WUR??

Of course ignition timing changes can also cause the popping, stalling... what are you running for ignition/timing controls??

I hope folks with far more knowledge than me chime in here to help you out!!!
Old 08-10-2011, 06:57 AM
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Welcome

There are lots of pros here that can help you, but I'm not one of them sorry. It does sound to me like two separate issues though and the fuel issue does sound like a vapor lock much like I used to experience back in the day with my 1977 MGB. I loved that car when I wasn't pushing it
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Old 08-10-2011, 06:58 AM
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Not enough information to make worth while guesses because you don't go into any detail on what you had done to the motor or if the muffler may be full of oil from a previous sick motor.

If your guys rebuilt the top end the smoking may be from piston rings that havn't seated to the cylinder walls yet but it sems that would be the first thing they would tell you.

There are so many things that could cause the poor running and you don't say what has been looked at and crossed off the list of things your guy's have checked already.

If they know old mechanical CIS fuel injection, have basic tools like a CIS fuel pressure gauge, some way to check the exhaust AFR's and a simple inductive timing light and they still have no idea whats wrong then maybe you need a porsche mechanic that knows the older cars.
Old 08-10-2011, 11:07 AM
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Quote:
Originally Posted by JFairman View Post
Not enough information to make worth while guesses because you don't go into any detail on what you had done to the motor or if the muffler may be full of oil from a previous sick motor.

If your guys rebuilt the top end the smoking may be from piston rings that havn't seated to the cylinder walls yet but it sems that would be the first thing they would tell you.

There are so many things that could cause the poor running and you don't say what has been looked at and crossed off the list of things your guy's have checked already.

If they know old mechanical CIS fuel injection, have basic tools like a CIS fuel pressure gauge, some way to check the exhaust AFR's and a simple inductive timing light and they still have no idea whats wrong then maybe you need a porsche mechanic that knows the older cars.
+1, I had a large big city Porsche dealer/shop tell me they don't work on air cooled pcars cuz they don't know how...RC
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FS 1987 930 DTAFast EFI w/Electronic Dash, 17x9x11" Lindsey Racing Fuchs, Andial 8.39 R&P, C2 Intake, SuperCup Cams, Flame Ringed Heads & Cylinders, TurboKraft Full Bay Intercooler, RearlyL8 Twin Scroll Headers, Big Bullseye Twin Scroll Turbo...SOLD
1999 Harley Davidson Custom 1250 Hardtail Bobber..SOLD
2014 BMW X6M, 2012 BMW 128i, 2014 GMC Sierra, 2015 Cobia 237 Center Console 300hp Yamaha
Old 08-10-2011, 11:13 AM
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is it efi or cis? did i miss it? efi sounds like map sensor gone south
Old 08-10-2011, 11:27 AM
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Gotta good CIS understanding shop with a dyno in the area?
Old 08-10-2011, 11:57 AM
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The smoking sounds like valve guides to me (already mentioned). Fueling may be the other culprit (definitely find someone that knows how to use a CIS gauge to test system and control pressures to rule out the fuel head and/or WUR), possibly fuel pump relays drawing too many amps and overheating, possible CDI overheating and causing ignition issues. Lots of possibilities, but as Jim said it would help to have more information as to what has already been ruled out by your mechanic.
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Old 08-10-2011, 02:24 PM
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You don't mention but is car EFI, if so i'd check your cam sensor. it might be malfunctioning.
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Last edited by EmptyGarage; 08-10-2011 at 07:19 PM..
Old 08-10-2011, 07:17 PM
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