911SC Turbo

[tsuter]

David, On the SC there are two different oil pressure relief valves. The main one opens at 6bar or close so around 90psi plus or minus maybe 10psi.

The second relief valve opens when the main one is stuck or has failed. It opens at 8bar or around 120psi plus or minus maybe 10psi.

Given your readings I would say your main pressure relief valve is inoperative and you are on the safety valve.


The safety is on the left side of the crankcase and the main one is on the bottom. The main one had some failure problems and is typically replaced with an update kit.

[sammyg2]

I did some more tinkering with my engine and stumbled across the best running combination so far for my car.

I was not happy wityh the way the O2 sensor was controlling the fuel injection. it was swinging wildly, often causing a noticeable surge. I adjusted it every way until sunday with little improvement. My guess is the sensor is tired and reacting too slow. I have a new one ready to install but I tried something else first.
After disconnecting the O2 sensor I hooked the sensor wire that goes to the lambda brain to the hobbs switch. The switch grounds the lambda brain at 3 psi increasing.
I adjusted the fuel distributior so that the engine runs slightly lean at idle, near stoichiometric under steady part throttle cruise, and it goes slightly rich under full throttle and boost acccording to my a/f meter.
Results: The car runs very smooth under all conditions, but under boost it pulls way harder. I'm at 7 psi boost but the power increase made me wonder if my boost gauge was broken. Lots more power now.

I posted a question last month about the difference in the lambda brain signal to the frequency valve when grounded vs. ungrounded, never got a response. Now it appears that ungrounded/unhooked defaults to around a 50% duty cycle and grounded goes way higher increasing the fuel pressure to the injectors. Anyone have any hard numbers to back this up?
Next is to creep up to 8 psi and see how it goes.
Thats about as high as I'm going until I rebuild the engine and install an intercooler.

[iamchappy]

I may try a different sender as the one on the engine is a 79 and the dual, temp, oil pressure instrument gauge I have is from an early 911 so I am unsure if the oil pressure readings are correct, although the temp readings seem to be right on. It sounds like a simple fix either way if the pressure is to high.

[tsuter]

Originally posted by sammyg2
I did some more tinkering with my engine and stumbled across the best running combination so far for my car.

I was not happy wityh the way the O2 sensor was controlling the fuel injection. it was swinging wildly, often causing a noticeable surge. I adjusted it every way until sunday with little improvement. My guess is the sensor is tired and reacting too slow. I have a new one ready to install but I tried something else first.
After disconnecting the O2 sensor I hooked the sensor wire that goes to the lambda brain to the hobbs switch. The switch grounds the lambda brain at 3 psi increasing.

Sam, That is correct. you definitely want to disable the O2 sensor under any boost. Using the fuel enrichment Hobbs control switch is OK but later. I would add a second switch for the O2 sensor itself at 1.5#s.

This is covered on page 2 of this thread.

Here is a schematic also from page 2.

[svandamme]

small question on the thermotime switch move earlier in this thread :

does anybody know if it would be possible , to place a TTswitch on my chain covers, and wire that up to my MFI pump in some way
so i can ditch my heat exchangers and run headers instead?
effectively doing away with the air hose going from the left HE to the mfi pump for cold start control?

[iamchappy]

I have tried just about everything to bust loose the slotted cap to get in there to check out the relief springs, by the way those caps are torqued in there those pistons may be the problem, I want them off. Heat maybe my last resort. I tried my impact driver, a visegrip, I even ground down a 1/2 extension to a slot tip and tried my impact driver on it to no-avail. Any ideas?

[BGCarrera32]

Any chance you could find a small chunka dry ice to drop on there and then try and bust it loose again...? (i.e. shrink it all instead of heat)

[iamchappy]

Maybe, It is hard to believe that a slotted screw could be frozen in that tight. I at this time believe the high oil pressure is the problem with the turbo leaking all along. I tried a small pipe wrench but still no luck, I will remove my shift rod to get a better angle on it and try again.

[tsuter]

David, I never had trouble getting out the slotted screw but often had trouble getting the stuck piston to drop down after taking out the spring.
I hope you're not just attempting the first hurdle.
I did look and there are a few threads on oil pressure check valves.

[iamchappy]

Thanks Thad, I will look them over, I want to get it on the road again.

[iamchappy]

I got it off with the vise grip method upon inspection the piston looks a little shiney in a few spots, I ordered up new kit just to
eliminate this as a problem. I will let you know when and how I solve my turbo oil problem.

[iamchappy]

I have a front mounted oilcooler, is it possible the the external pressure relief valve could be the problem with the high oil pressure.

[MichiganMat]

what weight oil are you running? My pressures went up considerably when I switched from 10-40 to 20-50.

[iamchappy]

20/50 but my pressure is always reading 100 up to 140 cruising and 60 at fast idle.

[sammyg2]

My guess is a sender/gauge problem. Have you tried hooking up a mechanical pressure gauge to verify your electronic guage is reading correctly?
You wouldn't have to run it all the way to the cabin, just mount it in the engine compartment and compare it to the electronic guage at idle and when raising the rpm to around 3000 and holding it. They are only about $15 at the FLAPS.

[iamchappy]

I think that is the only way to know for sure, I have been wondering if I could have my external oil cooler thermostat, oil pressure relief union hooked up wrong, the oil cooler thermostat must work because it passes oil to the front cooler when the oil gets up to the 180 to 190 range. but maybe the oil pressure relief side is not right. I guessed when I originally hooked it up. Im going to look over some pictures and crawl around under the car to check if the hoses could be hooked up wrong.

[iamchappy]

thermostat

[iamchappy]

Thad I have had a few nice sized backfires lately when I close the throttle quick after a WOT run, it wont happen if I am able to back off slowly . Do you ever experience this , is the system being overloaded with the unburned fuel enrichment.

Also I've sourced out a 964 oil scavenge turbo pump, do you know if they are different from the 930's?
I'm still smoking and believe it may be the electric Tilton pump not keeping up with the flow.

[tsuter]

I'm guessing you mean exhaust backfires. Try unhooking the Tial BOV control line (plug it) and see if you get the same results.

I used to use a 914 CSV as the supplemental injector but the current manufacturing standards of them don't support the fuel pressures run so I got leaks. The 911 CSV is more fuel for sure and maybe too much.

But the first thing I would check is to disconnect the BOV.

I have always got some exhaust backfiring but can't say whether it's more than usual as the car has been a garage queen for a while.
Harley time of the year now.


edit: Don't know about scavenge pump differences.

[iamchappy]

On one run I ran it up to redline in 5th and had to back it down real fast. Huge exhaust backfire, my hunting dog who was along for the ride was looking around for the falling bird.
An exhaust backfire shouldn't harm anything but the muffler right?
Since I am running the Flowmaster with little restriction and they are very heavy duty, chances are that it should hold up to the abuse.