Thread: scary noise
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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
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How do you know the cold start injector is staying open after the car starts?
There's no visual on that, and the only test I can imagine is to check for voltage going to it.

The cold start injector is powered/opened by the current going to the starter solenoid low amperage 12volt primary wire on the starter motor, and then only when the thermal time switch thats mounted in the driver side cam chain box is sensing it's below a certain ambient temperature.

Because of this the cold start valve is open only while you are cranking the starter motor.
So once the car starts and you release the starter the cold start valve no longer has a source of 12 volts to open it's solenoid valve and let fuel spray into the intake manifold underneath the throttle body.
The thermal time switch also controls how long the cold start valve is energized or open depending on ambient temp should the car not start and you keep on cranking it so too much fuel does not get sprayed in flooding the motor...
The colder it is out, the longer it will spray fuel into the intake manifold until the car starts.

Also, if you have knocking at low idle speeds like you describe there is excessive mechanical clearence inside your motor somewhere... I've listened to my '87 930 motor when the aux air valve sticks shut once in a while and the motor starts really slowly but I don't hear any weird noises, and I have a good ear for that.
Or you may have forged pistons which expand more than cast pistons so they have to be installed with a larger cylinder to piston clearance at the piston skirt around 3/4 inch up from the bottom of the skirt and 90 degrees to the wrist pin than cast pistons.
Once they get up to operating temp they have expanded and taken up that larger clearence... but they can make some piston slap noises while cold.
In some engines you can actually hear forged pistons ringing a little bit as they rattle and rock against the cylinder walls when lightly reving the engine cold, if you know what to listen for.
After a couple minutes the noise goes away as the pistons heat up and expand.

The ONLY engine I've ever heard make ALOT of rattle and knocking noise while idleing and it was considered normal is the old 4 cylinder turbocharged Offenhauser engine last used by car and driver editor Patrick Bedard at the Indy 500 in 1981... I was there.
Those things sounded like they were about to come apart till they got really hot, and were running fast.
Old 12-17-2007, 06:31 PM
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