'73.5 CIS gurus: troubleshooting "Throttle Valve Control Pressure Regulator" ?

[BottleNose]

This "Throttle Valve Control Pressure Regulator" device, located between the throttle body and the sensor plate, really killed the idle when I sprayed carb cleaner directly at it:



>Description:
>"Early CIS only ('73 - '75) Operated by a cam driven off >the throttle shaft. Enriches the mixture at full throttle and >at idle by decreasing the control pressure.

>Failure mode: Leaking diaphragm keeps fuel mixture too >rich at part throttle"

Can these have vacuum leaks or is this a sign that this unit is faulty? How do you test and troubleshoot this part?

My '73.5 911T CIS starts and idles fine, but stumbles and backfires whenever you try to accelerate. If you hold the throttle steady at higher RPMs it will be smooth, and decel is OK, but any acceleration causes severe bucking and backfires.

So I read here that the first thing to do was to check for vacuum leaks by spraying carburetor cleaner around the engine and seeing if the idle increases (indicating a vacuum leak.

I sprayed at all the possible vacuum leak areas, and nothing affected the idle except spraying under the CIS intake boot (the big one on top of the fuel distributor and airbox). When sprayed under there it LOWERED the idle and the car almost stalls. From there I determined the above device caused this.

Thanks for any advice.

[echrisconnor]

I belive it just controls system pressure manually based on throttle position. It has a physical link from the device to the throttle valve. I don't belive there is a vacuum component to it...? If you look on the Fuel diagrams for 73.5 it is shown. It leaks fuel when it fails. It does not appear on any of the vacuum diagrams that I have seen.

So, if you're getting a vacuum leak at this location, I'd say chances are that it's a leak in your boot, or other vacuum line which branches off from the same relative area. The area you're spraying is the vacuum heart of the system, and any number of things right in that area could be leaking. However, I don't think it's the Throttle Valve CPR.

--chris

[Jim Sims]

throttle position control pressure regulator

[BottleNose]

Thanks for the advice and the link. Wonder why that link didn't show up when I searched. What else in that area is prone to vacuum leaks? Everything seems to be tight and leak free in there. The car has an aftermarket aluminum airbox, so I doubt the box itself is the culprit.