‘77 911S - finding weird stuff!

[grillage]

Hi,

I’m chasing down a couple small issues with a mostly well running car and am finding a few things that has me confused.

The car runs really well for the most part. System pressure is good, the WUR is recently rebuilt and seems to be functioning correctly. 2.1bar at CCP, 2.75 at warm pressure.

The car has lots of miles but a rebuilt engine 60k miles ago
EGR is disabled, air pump removed, the thermal reactors are long gone

The symptoms I’m chasing down are these:

Rough running at start that seems like not firing on all cylinders. That self corrects in 30 seconds of running or so. This is only after sitting for a bit

Idle when warm is different at different times. Sometimes it’s high - 1000-1200 rpm, sometimes it revs itself from 600-1500 over and over, sometimes it’s dead on 800.

When the car is warm, any small revs (such as a little bump in the throttle to pull into my driveway, is met with 5 seconds of 1500 rpm idle - then back to normal

Throttle seems really touchy. It’s easier to stall this car than any other of my manual cars (924S and 928)

Otherwise, it’s really running pretty well!

I started digging in to the vacuum lines to try and find an obvious leak or something and found that the cold start valve is unplugged - could easily explain the hard starts

Also, the air meter switch wire harness is crumbly and broken completely. The plug still in the socket but no wire. The wire is loose, corroded, and in the engine bay.
Pictured here:


These cars are supposed to have a FP relay that only fires the fuel pump if the meter is open right? Is that the only thing that switch is for? My fuel pump relay fires the pump at the turn of a key so someone has “fixed” this problem in the past.

Is there any other purpose for this switch?

Also, on the vacuum lines, there is no line to the top of my WUR - in fact, there is no thermal switch either. I can buy one and add it as the drawings show but want to make sure this car is supposed to have one first.

Thanks alll!

[grillage]

Also, and this one’s got to be unrelated, my tachometer can’t go above 4200 RPM. Just stops there, works fine below that, but not above. Weird

[Gonbad]

Welcome to 77 911S’s!

[jma0506]

Welcome to the '77 911S!

So a few things you mentioned could all be contributing to your issues:

1) Broken air meter plug means your fuel pump running when you turn the key vs. cranking the engine which could be contributing to your hard start issues

2) Same can be said for the cold start valve being disconnected, probably to avoid over fueling on start up and compensating for point 1

3) Your issue "That self corrects in 30 seconds of running or so." could possibly be tied to your TTV (vacuum valve) causing enrichment issues or general vacuum issues

4) Definitely look into the vacuum lines, I had a similar issue where i'd get erratic idle speeds and it was a combination of bad vac lines, bad fuel injector seat seals and improper vac line routing


good luck!

[EC900]

“ When the car is warm, any small revs (such as a little bump in the throttle to pull into my driveway, is met with 5 seconds of 1500 rpm idle - then back to normal”

- I had recent similar issue. Found the 2 throttle bell crank bushing (901-423-253-00) completely disintegrated, also all 3 throttle rod bushings (901-423-193-00] in the tunnel just as bad. This situation cause the throttle linkage to bind up with the clutch rod linkage and kept my idle in the 2000’s until I blipped the throttle pedal to free it up. Something to consider.




Also I had cracks in the brake booster hose at the pedal cluster, also check on the same line at the engine where it come out of the tub. Had an injector seal go bad too. Seems prev owner may have had CIS tuned with these vac leaks, plus it was a Calif car, high altitude and strict emissions which the mixture might still be a bit off that my thermal reactors are gone and backdated heat exchangers.

[grillage]

Good call on the pedal cluster suggestion. I actually have one of the bronze bushing kits on the way.

I've ordered a thermal valve as well so I can get the vacuum lines to be as they should be

[EC900]

PP didn’t have the bronze in stock so I opted for the nylon, should last another 40 yrs. I read here that the bronze may require to pressed in, not sure if that poster had the proper bushing though. The nylon slide right in, required a little manipulation of the linkage (also left in place) to get into the first bushing.
At any rate, no need to pull cluster, it’s best to remove the seat and have plenty of light to get at these. The cotter pin is the most difficult for me to see and bend straight since the bend is out of sight and needle nose couldn’t bite the loop end.
Throttle response now is definitely different and tighter, no slop.

[PeteKz]

Your WUR should be an 033, which does have a vacuum nipple on the top, and a nipple on the outside (which connects to a hose that goes to the top of the throttle body, above the throttle plate--it is an atmospheric vent line, but goes there to get filtered air). The number is on the top front edge of the WUR and will read something like 438 140 0XX, with the last three digits being stamped.

Post a picture too.

As jma0506 suggested, you very likely have air leaks. Chase those down, either by spraying starting fluid or propane around the injectors and other spots, or by doing a smoke test.

The fuel pump running all the time will not make it hard to start. My 1973.5 car did not have a FP relay at all, so it runs as soon as I turn on the key. The fuel just recirculates back to the tank.

As this age, it almost certainly has a bad fuel accumulator, unless that has already been replaced. The quick test is to disconnect the return line from the FA, plug or pinch the line, then run the pump and look for gas leaking out the bottom of the FA. Any leaks mean it's bad. That will contribute to hard starts, especially warm restarts.

The plugs to the CSV ad the air metering unit are easy to get swapped. They are slightly different colors, one green and one blue, but over time these dade and get dirty and hard to tell apart. Make sure you have the right plug on the right device.

The thermo time switch and thermo valve are different things. The thermo valve (TV) holds off vacuum for 10 seconds or so from reaching the WUR, in order to decrease the CCP, to provide a richer mixture immediately after starting; then it opens and allows vacuum to reach the WUR. The thermo time switch (TTS) is what controls operation of the cold start valve (CSV). It allows the CSV to operate to provide a short squirt of additional fuel into the intake manifold only when the engine is cold. The TTS is on the back of the left timing Chain cover. See if it is plugged in.

If you don't have the diagrams of how the vacuum lines and other parts of the CIS are installed on your year engine, find them in these threads and print them out so you can see them while looking at/working in the engine bay.