possible thermotimer dead?

[leebu]

I was following another thread here about cold start problems and how a replaced thermotimer did the trick...

I'd like to run my symptoms by this august group of minds and see if my thermotimer might be dead as well...

My 76 911S started running really badly during startup and for several minutes afterwards. It starts fairly quickly and usually on the first try. Then it usually dies at least once and always has minor backfiring. After about 20-30 seconds of running it suddenly starts to rev higher and smooth out. If I try to drive off within the first 2 minutes (or so) I'll get backfiring as I give it gas. After 5 minutes or so the engine is running fine with no backfiring. I'm only having a minor surging problem between 2100rpm and 2500rpm (which I'm told is normal, right..) so I don't think it is a fuel or pump issue...

I'm kinda hoping that it is the thermotimer. Otherwise, I'm afraid that it's something much deeper in the system...

Is there a way I can check the thermotimer? Like bridging the two connectors when starting cold and disconnecting them manually after 2 minutes?

TIA! Lee

[Early_S_Man]

The Thermo-Time Swich only has an effect while the starter motor is turning the engine over! Any running or warmup issues after the starter is disengaged ... have NOTHING to do with the Thermo-Time Switch!

[leebu]

Thanks. Guess it's time to move onto other troubleshooting..

[bigchillcar]

Leebu,

I had similar problems in my '74 with CIS last December. In my case, the problem got worse until a final 'backfire' and it refused to start again. The problem was that the points had become dirty and finally closed...backfiring is often a symptom of this. The good news for me was points are cheap, but actually, I just re-opened and cleaned mine and it's been fine ever since. Anyway, that's a possible cheap fix to check out IMO.

Regards,

Ryan


1974 911 2.7 CIS coupe

[RarlyL8]

Warren - is the thermo time switch you are referring to the same thing as a thermo valve? The device I am speaking of is located in the vacuum line between the distributor and the vacuum source (manifold). The function of this item is to alter the timing during start up. Are we typing about the same part?

Lee - it sounds like you have a cold start mixture problem. You don't say what the temperature is when these symptoms occur.

[leebu]

The thermotimer I orginally asked about is the electrical device attached to the timing chain cover. I haven't looked at the vacuum advance yet..

This all happens when it's cold. It's worse after an sitting overnight, but it will do it even after siting for a 1/2 day (4 hours) in a parking garage at work...

Thanks for the tip on the points. They look OK, but I'll check closer.

Lee

[RarlyL8]

OK, I know what you're talking about now. These names are very confusing. Different schematics call the same item by different names.

You would need to look at an electrical schematic for your particular engine/car, but I think the temperature probe you're talking about may be in the same electrical path with the 7th injector and the WUR. If it isn't working correctly the others won't either. Pull the connectors off the 7th injector and WUR, turn the key to the aux. position, and check for power.

[wckrause]

The 7th injector (cold start injector) only gets power when directed by the thermotime switch. If the car starts up quick then dies or runs rough then it's neither the thermotime switch or the cold start injector. The WUR should have power, but the lack of power to the WUR will not cause a cold start problem. It only needs power so that it can heat up and lean out the mixture.

The WUR has no interaction at all with the cold start injector.

You should get a CIS pressure tester to make sure your WUR is operating within specs.

I don't believe '74 CIS cars have a thermovalve that alters ignition timing. I don't think any N/A cars have that.

[leebu]

Thanks for the pointers everyone. Turned out to be a shade-tree mechanic created problem with the points.

Looks like they were slightly dirty and they were no longer in adjustment. Either I didn't adjust them correctly last time I was in there, they slipped or they just wore faster than I was expecting. Last time I touched them was about 4k miles ago.

Might be time for the Pertonix Ignitor upgrade from PelicanParts...

Thanks again! Lee

[RarlyL8]

Glad you found the problem Lee. I tend to forget that some of these cars have points distributors. That would be something to ditch when the funds allow.

Bill - could you explain in more detail, the workings of the WUR?
Is the default position (no electrical power) rich? Does the WUR then lean the mixture while the car is running?

There are a gazillion different CIS component configurations for these cars. Every country, year, state, etc, has there own rules for smog crap. That is why it is imperative to check the wiring diagrams for YOUR car specifically. On top of that, these cars are 20+ years old. Almost none have survived that long without some sort of tinkering done by a PO along the way. Really gets messy trying to track down a CIS related problem. The turbo models add even more layers of complication. Then there is California..........................

[bigchillcar]

Rarlyl8,

Right after the starter, thermotime switch/cold-start valve do their job, the wur takes over. It has a thermal switch inside (bimetal strip) that senses the car's block (where its mounted) is cold. It reduces the fuel pressure to lean the engine from the 'rich' start. As the engine warms, the wur senses and raises the pressure. It actually senses temp. changes and responds by varying fuel pressure accordingly. Happy CISing!

Ryan