Originally Posted by
NorthernThrux
A lot of hacks to get around what is a pretty elegant electromechanical beast (911.617.117.00). Nowadays some programmer put in several hundred lines of code in an ECU to do the same thing. There are a number of visual and electrical checks that should be made on these switches to ensure they are correct and can work as intended.
The bi-metallic switch in the brass housing responds to temperature and disables the cold start enrichment if above a certain temperature. That temperature is stamped on the thermo-time switch and should be 45 C (113 F if you don’t speak metric) at least for the 70- MFI cars. There is also a heating element that is driven by the wire to the starter. The longer you crank, the more you heat the bi-metal switch. When the bi-metal switch opens (at 45 C !) it prevents further cold start enrichment by interrupting the circuit from the relay to ground. The colder the motor, the longer it takes for the circuit to get interrupted and the more fuel is dumped into your intakes. Also the colder the motor is, the sooner the thermo-time switch cools off and again allows cold start enrichment. Somewhere in the neighbourhood of 10s in the temps most of you drive. So you have to wait between tries of the starter if it doesn’t catch the first time if you want the benefit of enrichment.
Even here there is a catch. Original switches are marked 3W, 8W or 40W. Depends on year of manufacture and MFI, CIS etc. The 40W one heats up faster and hence stops enrichment faster. I think it is the one that is correct for MFI cars. It’s also what the new ones are marked as. I think the faster heating ones were used to prevent oil dilution with gasoline, but may compromise starting by limiting the cold squirt duration. I haven’t timed how long it squirts in my car, but the engine catches in 1-2 s, so it is adequate.
Now here’s the ole timey mechanic checks. Often thermo-time switches have never been removed from the motor, even during rebuilding. The switch has a brass housing, an aluminum or copper sealing washer and is screwed into the magnesium breather cover. All are subject to contact corrosion from the dissimilar metals or oil crud in the valley of leaks in that area. The switch should be removed and all the surfaces cleaned to ensure continuity to ground. And speaking of ground, the transmission braided ground should be checked as this is part of the path to ground for the switch and that braid and its attachment points are on the bottom of the car and really get disgusting. I changed mine and it made a difference to cold starting and smoothness of the idle. Also check that the fuel console is grounded to the body stud just behind it or your cold start solenoid won’t open.
Now when the car is warm, the recommended procedure is to depress the accelerator fully and crank. The extra fuel from WOT should be enough to start the car with a warm motor. But if you have vapor lock, you may not get fuel to the injectors. This is where using the cold start enrichment injectors helps as they aren’t in direct contact with the hot engine and aren’t so prone to fuel vaporization. The delay relay above can be used to fire the cold start solenoid for a second every time the starter is turned and that may help starting if your fuel system is not tuned perfectly. Probably no point in adding a thermostatic switch to that as all you are doing is effectively recreating the thermo-time switch but with a limited squirt duration and no waiting between subsequent starting attempts. Plus without any temp control you have a chance to defeat vapour lock on hot starts.
If everything is working properly as when the car was new, the original system is all that is needed and works perfectly. I haven’t had issues with hot starting in 34C weather after 15 minutes or 2 hours on a fully warmed up engine. Of all the hacks, the delay relay is probably the one I’d chose as it requires no switches. You just pick the initial squirt duration that works for you and forget it. If you are a fair weather driver and don’t need to start in cold temps, the fixed duration squirt will be fine and will probably help your hot starts to the point that you don’t need to depress the accelerator pedal either. You could wire the delay relay in parallel with the thermo-time switch to get enrichment on hot starts but preserve the proportional enrichment duration with lower temperature that the stock switch offers in colder weather.
If you really wanted to be fancy, replace the trim pot that adjusts the duration on that relay with a thermistor with the right characteristics and then squirt duration would be a function of temperature. But then all you’ve really done is build your own thermo-time switch! Some kind of plug-in replacement for the relay in the engine console that deals with all this in a logical manner would be perfect.