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I have a 1987 S4 N/A FC3S with cold start issues. I've been hesitant on creating a new post about this since there are literally thousands of threads about cold start. I have searched on this forum for months and tried everything that I could find but nothing has worked so far. Anyways here it goes:
Symptoms:
When my car is dead cold, I have to crank it for about 30 seconds for it to start. I'll crank it for about 10 seconds at a time and when it starts catching, I have to modulate the throttle and give it more air until it finally starts up. Then I have to manually idle the car with my foot for a couple minutes until the car is warm enough to idle on its own. My car hot starts with no issues at all. It makes plenty of power and is smooth and idles beautifully once it is fully up to operating temperature. (Sounds like a water thermosensor issue, right? Keep reading.)
Car's current state:
Compression test numbers: Front rotor (93, 87, 90psi), Rear rotor (83, 86, 87psi)
Brand new battery from Autozone
Alternator upgraded to FD alternator
Fuel injectors recently cleaned and tested by RC Fuel Injection
Brand new fuel filter and fuel pulsation damper
Replaced all electrolytic capacitors in stock ECU
My car has all emissions related components removed, OMP removed, throttle body mod (thermowax system also removed).
The car also has a street-port and has the 5/6th actuators completely removed.
I have redone my entire intake system because there were vacuum leaks everywhere. Replaced all vacuum hoses (only ones I'm using are 1 for FPR, 1 for pressure sensor, and 1 between UIM and LIM for fuel atomization) and plugged all the rest of the unused vacuum nipples. To put it together, I used all brand new gaskets and even sprayed them with copper gasket spray. So I'm fairly certain that I have no vacuum leaks.
Testing:
Based on my cold start symptoms, everyone seems to have fixed their issue by fixing the water thermosensor on the back of the water pump housing.
Since mine was old and falling apart, I replaced it with a new one. This didn't fix the issue.
So I tested the sensor by probing the back of the sensor and got the following resistance values:
1.96 kohm at room temp
1.1 kohm at 100F
260 ohms at 180F
So I thought maybe its an issue with the wiring. So I did a continuity test:
Green/white wire to pin 2I - connected
Black/brown wire to pin 2C - connected
Then I back probed the connector at the ECU to test voltage:
At 100F, my ECU sees 1.6V
At 180F, my ECU sees 300-500mV
My water temps come from a Autometer water temperature gauge that is getting water temp from the back of the water pump housing, right next to the water thermosensor.
I have tried my stock ECU (N326) as well as an ECU from a vert (N338). Switching ECUs did nothing to fix the cold start. I swapped out the electrolytic capacitors in both ECUs and tried again, but I still have the cold start issue.
Conclusion:
I have tested that my water thermosensor is working properly and the ECU is receiving the correct signals to cold start. I have a new battery and alternator (runs at ~14V) so the issue shouldn't be related to low voltage. My fuel injectors are newly rebuilt and I replaced the fuel filter and pulsation damper so I shouldn't have fuel issues. And I re-sealed my entire intake system, so it shouldn't be related to a vacuum leak. It makes sense that the car has issues idling when cold since I have the thermowax assembly removed, but I have no idea why it has such a hard time cold starting.
Any ideas? I'm almost at the point where I want to buy a full standalone ECU just to fix this cold start issue lol. And if you made it this far, thank you for taking the time to read my long explanation.
I just changed my temp sensor behind the water pump today because of cold start issues.
Did you see these values?
Here are the things I've had to deal with lately to get it to run semi decently. .. .Nothing made a real change in my cold start up issues until I dealt with the temp sensor.
I have a 1987 S4 N/A FC3S with cold start issues. I've been hesitant on creating a new post about this since there are literally thousands of threads about cold start. I have searched on this forum for months and tried everything that I could find but nothing has worked so far. Anyways here it goes:
Symptoms:
When my car is dead cold, I have to crank it for about 30 seconds for it to start. I'll crank it for about 10 seconds at a time and when it starts catching, I have to modulate the throttle and give it more air until it finally starts up. Then I have to manually idle the car with my foot for a couple minutes until the car is warm enough to idle on its own. My car hot starts with no issues at all. It makes plenty of power and is smooth and idles beautifully once it is fully up to operating temperature. (Sounds like a water thermosensor issue, right? Keep reading.)
Car's current state:
Compression test numbers: Front rotor (93, 87, 90psi), Rear rotor (83, 86, 87psi)
Brand new battery from Autozone
Alternator upgraded to FD alternator
Fuel injectors recently cleaned and tested by RC Fuel Injection
Brand new fuel filter and fuel pulsation damper
Replaced all electrolytic capacitors in stock ECU
My car has all emissions related components removed, OMP removed, throttle body mod (thermowax system also removed).
The car also has a street-port and has the 5/6th actuators completely removed.
I have redone my entire intake system because there were vacuum leaks everywhere. Replaced all vacuum hoses (only ones I'm using are 1 for FPR, 1 for pressure sensor, and 1 between UIM and LIM for fuel atomization) and plugged all the rest of the unused vacuum nipples. To put it together, I used all brand new gaskets and even sprayed them with copper gasket spray. So I'm fairly certain that I have no vacuum leaks.
Testing:
Based on my cold start symptoms, everyone seems to have fixed their issue by fixing the water thermosensor on the back of the water pump housing.
Since mine was old and falling apart, I replaced it with a new one. This didn't fix the issue.
So I tested the sensor by probing the back of the sensor and got the following resistance values:
1.96 kohm at room temp
1.1 kohm at 100F
260 ohms at 180F
So I thought maybe its an issue with the wiring. So I did a continuity test:
Green/white wire to pin 2I - connected
Black/brown wire to pin 2C - connected
Then I back probed the connector at the ECU to test voltage:
At 100F, my ECU sees 1.6V
At 180F, my ECU sees 300-500mV
My water temps come from a Autometer water temperature gauge that is getting water temp from the back of the water pump housing, right next to the water thermosensor.
I have tried my stock ECU (N326) as well as an ECU from a vert (N338). Switching ECUs did nothing to fix the cold start. I swapped out the electrolytic capacitors in both ECUs and tried again, but I still have the cold start issue.
Conclusion:
I have tested that my water thermosensor is working properly and the ECU is receiving the correct signals to cold start. I have a new battery and alternator (runs at ~14V) so the issue shouldn't be related to low voltage. My fuel injectors are newly rebuilt and I replaced the fuel filter and pulsation damper so I shouldn't have fuel issues. And I re-sealed my entire intake system, so it shouldn't be related to a vacuum leak. It makes sense that the car has issues idling when cold since I have the thermowax assembly removed, but I have no idea why it has such a hard time cold starting.
Any ideas? I'm almost at the point where I want to buy a full standalone ECU just to fix this cold start issue lol. And if you made it this far, thank you for taking the time to read my long explanation.
Sounds like a thermowax issue. Have you checked it?
rlynchster:
My water thermosensor was at around 2kohms at room temperature.
The FSM states that the thermosensor should be about 2.5kohms at 68F, so I got a 2.4kohm resistor and stuck that into the thermosensor connector. This didn't make a difference.
Because of this, it seems like my cold start issue has absolutely nothing to do with the water thermosensor (which doesn't make any sense!)
Cardinell:
Yes, my thermowax is removed. It makes sense that I have to manually idle the car while it's cold with the thermowax gone, but I just want it to fire up right away. I've had my battery die multiple times because it got drained trying to start the car. I've also had my starter motor get stuck and become unresponsive from using it so much (a light tap with a hammer on the starter housing fixed this).
I just pulled out the emissions harness out of the car and unwrapped the entire wiring harness. I will be testing for shorts / open circuits in the harness and will clean the crap out of all the connectors. Hopefully my issue is in this harness because I don't know where else to look. This is the last unknown variable.
If the issue isn't in my wiring harness, the only thing I can think of is to tear apart the engine and do a full rebuild (my compression numbers are on the lower side) then go with a full standalone ECU and build my own wiring harness....all just to fix my cold start issue. I might as well go 6-port turbo while I'm in there lol
Some extra stuff that I forgot to add to my original post:
Spark plugs and spark plug wires are all brand new. I bought a bunch of extra spark plugs since people say they foul easily. Even with brand spankin' new spark plugs, it doesn't cold start.
I also followed AaronCake's grounding procedure and cleaned up all of the engine / chassis / sensor grounds. I took each ring terminal to a wire wheel and chased each hole with a tap. So my grounds are for sure grounded!
I even took apart each spark plug ignitor and wire wheeled the grounds on those too!
11-26-21, 01:56 PM
