CIS to ITB EFI conversion

[dannichols1474]

My 1975 911 Carrera is an October 1974 build and was equipped with the 1974 federal mandated starter seatbelt interlock system where the starter was prevented from cranking the engine if the seatbelt wasn't fastened.

That system had been defeated long before I bought the car (in 2006) by disconnecting the seat sensors - if the seat is empty, then the seatbelt doesn't have to be latched for the starter to crank when the ignition switch is turned to the start position.

But my car is now 50 years old and I thought if that interlock relay fails, I won't be able to start my car. So first I removed the interlock relay and confirmed the starter doesn't crank when the ignition key is turned to the start position.

Next I reviewed my wiring diagram to see that on the interlock relay pin C goes to the ignition switch and pin 50 goes to the starter. Then I made a wire jumper out of two 16 gauge yellow wires (correct color, wire size I had in my tool cabinet) with a 1/4 inch male blade connector on each end (I used two 16 gauge wires to approximate the current capacity of the 12 gauge wires used up to the pin C and pin 50 terminals of the interlock relay socket). Finally, I plugged the wire jumper into pin C and pin 50 of the interlock relay socket and verified the starter cranks when I turn the ignition switch to the start position.

Interlock relay and wiring diagram:



Wire jumper in place on the interlock relay socket:

[dannichols1474]

It has been 15 months and 6200 miles since I rebuilt the 2.7L engine in my 75 911 Carrera. The car runs great and is a lot of fun to drive especially since I had the suspension rebuild in October.

The one "but" that I have been working on throughout 2025 is a small persistent oil leak from the 4,5,6 bank that manifests at the back of cam tower / lower valve cover.

I have discovered that both my upper and lower valve covers have warped over time with the ends bowing up away from the cam tower sealing surface. The upper valve covers which I believe were original, are magnesium units (.0R casting number) and were warped badly (over .060 inch from flat) and even doubled up valve cover gaskets would no longer seal. The upper valve covers were replaced with OEM units made from aluminum (.2R casting number), p/n 901-105-115-11. The lower valve covers were also replaced with OEM units, p/n 930-105-116-05.

Even after replacing the upper and lower valve covers, I was still seeing some oil leakage on the 4,5,6 bank, so I decided to replace all 12 of the original rocker arm shafts with new OEM units from Mahle. The new rocker arm shafts (p/n 901-105-342-05) with the RSR style seals (flat o-rings, p/n 911-099-103-52) fit more smoothly into the bores of the cam tower bosses than the used shafts had. I used one of the old shafts as a gauge to set against the outside of the cam tower shaft bore bosses and verified correct placement of the rocker arm shaft in the cam tower was with end of the rocker arm shaft flush with the outer surface of the narrow cam tower shaft bore boss - that put the rocker arm shaft seals equal distance from the inner surfaces of the cam tower shaft bore bosses. I torqued the rocker arm shaft bolts to 17.5 ft-lbs.

If I do another engine rebuild, I would definitely replace valve covers and rocker arm shafts from the get go.

In order to perform the rocker arm shaft replacement with engine in the car, I removed the exhaust headers, the engine tin pieces mounted on the cam towers, the 2 interlocking engine tin pieces on the back of the engine, and the engine mounted oil cooler to gain sufficient access to the back of the cam towers. I removed the #3 rocker arm / rocker arm shaft first by rotating the engine to TDC of the compression stroke, then repeated for #6, then #2, then #5, then #1 and finally #4. I performed the installation in the new rocker arm shaft / existing rocker arm beginning with #4, then #1, then #2, then #5, then #3, and finally #6.

While I was in the neighborhood, I replaced the original Behr engine mounted oil cooler with a new CSF replacement unit which fit perfectly.

To remove left side rear interlocking piece of engine tin, I had to disconnect and remove the throttle linkage that runs through the slot in the engine tin piece down to the bell crank on the side of the transmission. For simplicity, I removed the screw securing the linkage to the cross bar between the banks of ITB's then removed the nuts holding the bell crank to the top of the crank case, disengaged the rod at the lower bell crank and lifted the entire throttle linkage assembly off and out of the engine bay.

When I was done installing rocker arms / shafts, adjusting the valve lash, installing the valve covers, oil cooler, engine tin and exhaust headers, I re-installed the throttle linkage, installed a new oil filter and added 10 quarts of 20W-50 oil.

Next I connected my notebook with the TunerStudio app running to the MS2 ECU and turned on the ignition so I could adjust the TPS for zero and full throttle positions. Sure enough, the throttle position was reading -5.5% instead of 0.0 at the throttle closed position and I performed the TPS calibration by pressing the "Get Count" button for the thottle closed position, then press the "Accept" button to make the new ADC count active which now made the throttle position reading 0.0. Then I repeated the procedure for the full throttle position and verified I was now reading 100% at full throttle.

When I released the throttle back to the closed position, the throttle position reading wasn't 0.0% , but 1.0%. I repeated the throttle closed calibration, followed by the full throttle calibration and back to throttle closed and the closed throttle position moved again. I checked the TPS mounting screws for tightness and found they weren't snug, so I tightened them up - it doesn't take much movement in the TPS position to throw the throttle position reading of the ECU off. I repeated the TPS calibration at closed and full throttle positions and now the reading returned to 0.0% after the full throttle calibration.

I started the engine and let it run to warm up the oil and check the oil level. Once the oil was at 180 deg F on the dash gauge, I checked the oil level and added another 1 1/2 quarts of 20W-50 to bring the oil level just below half way between the min and max marks on the oil dip stick.

I have made two short drives of 35-40 miles each and the oil leak situation is looking promising, but only time will tell....

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