Knock sensor...
07-27-2019, 09:26 AM
#1
Burning Brakes
Thread Starter
Knock sensor...
Can a bad knock sensor prevent it from starting? I was getting a knock sensor SES light code. But now it won’t even start. As soon as it fires up, it kills itself.
if the knock sensor isn’t the problem, what else gives this type of symptom?
if the knock sensor isn’t the problem, what else gives this type of symptom?
Last edited by 91 black convert; 07-27-2019 at 10:09 AM. Reason: Want to add to post.
07-27-2019, 10:15 AM
#2
Safety Car
No, the Knock sensor can be totally dead and the car can run. I found this out on my 1988 C4 as it had a bad knock sensor when I bought it. I drove it over 100 miles with a defective knock sensor.
You say the car won't start after getting the Knock sensor SES light code? Does it start in open loop mode (Cold) or not at all? Have you checked your fuel pressure while starting the car? Does the fuel pump prime the engine like it normally does on startup? There is a circuit that has to see oil pressure (above 4 psi) to continue the starting process (as the engine is aware that it is turning). If that sensor does not see oil pressure then you would have a no start/ no run condition. The sensors/switches are near the end of the intake system near the distributor and the one with three prongs (connectors). Check in this area for loose, disconnected or broken wires.
Can you please describe the problem a bit better for us. Like does it run at all now or is it just able to crank, start and shutoff? If this is what is happening then you need to check your fuel pressure, the fuel pump and the fuel pump relay. Hook a pressure gauge to your shrader valve on the fuel rail on the passengers side in the rear section. Put the gauge on the windshield and watch it while you try to start the car. With the key in the "Start" position the system should charge right up to about 42 psi. After turning the key to run and the car starts up what happens to the fuel pressure?
What you are describing sounds like a fuel related issue and those are fairly common on these old Corvettes. I have removed the tank filler cover and exposed the wiring harness to the fuel pump. The harness has three wires going through it. One is the Fuel Pump power (12 Vdc), the middle one I believe is for the sending unit and the last is the ground for the fuel pump. If you are comfortable doing this please apply 12 volts to the pump after identifying the wires in the harness. If the pressure comes up to 42 then it is likely okay. If the pump is bad you will know by the pressure it is delivering. I keep a spare pump around for testing.
We will go from here once we get a couple answers regarding the fuel system. I think your problem is likely to be in the fuel delivery system. Have you checked the Fuel Pump relay to see if it is sending the voltage to the pump? These relays have been known to fail and cause major starting issues.
Your fuel filter is clean I am assuming here, if not you might want to replace it to prevent problems. It is inexpensive and easy to change..
Are there any modifications on this car? Can you tell us if there are, please? I spent a couple days helping a guy only to find out in the end that the car was equipped with a supercharger which changes a lot of things around on us. The more we know the better chance that one of us can help you with your beautiful Corvette!
Look forward to hearing from you again! Have a great day!
Best regards,
Chris
You say the car won't start after getting the Knock sensor SES light code? Does it start in open loop mode (Cold) or not at all? Have you checked your fuel pressure while starting the car? Does the fuel pump prime the engine like it normally does on startup? There is a circuit that has to see oil pressure (above 4 psi) to continue the starting process (as the engine is aware that it is turning). If that sensor does not see oil pressure then you would have a no start/ no run condition. The sensors/switches are near the end of the intake system near the distributor and the one with three prongs (connectors). Check in this area for loose, disconnected or broken wires.
Can you please describe the problem a bit better for us. Like does it run at all now or is it just able to crank, start and shutoff? If this is what is happening then you need to check your fuel pressure, the fuel pump and the fuel pump relay. Hook a pressure gauge to your shrader valve on the fuel rail on the passengers side in the rear section. Put the gauge on the windshield and watch it while you try to start the car. With the key in the "Start" position the system should charge right up to about 42 psi. After turning the key to run and the car starts up what happens to the fuel pressure?
What you are describing sounds like a fuel related issue and those are fairly common on these old Corvettes. I have removed the tank filler cover and exposed the wiring harness to the fuel pump. The harness has three wires going through it. One is the Fuel Pump power (12 Vdc), the middle one I believe is for the sending unit and the last is the ground for the fuel pump. If you are comfortable doing this please apply 12 volts to the pump after identifying the wires in the harness. If the pressure comes up to 42 then it is likely okay. If the pump is bad you will know by the pressure it is delivering. I keep a spare pump around for testing.
We will go from here once we get a couple answers regarding the fuel system. I think your problem is likely to be in the fuel delivery system. Have you checked the Fuel Pump relay to see if it is sending the voltage to the pump? These relays have been known to fail and cause major starting issues.
Your fuel filter is clean I am assuming here, if not you might want to replace it to prevent problems. It is inexpensive and easy to change..
Are there any modifications on this car? Can you tell us if there are, please? I spent a couple days helping a guy only to find out in the end that the car was equipped with a supercharger which changes a lot of things around on us. The more we know the better chance that one of us can help you with your beautiful Corvette!
Look forward to hearing from you again! Have a great day!
Best regards,
Chris
07-27-2019, 10:22 AM
#3
Burning Brakes
Thread Starter
It cranks real well, but as soon as it fires, it shuts down, and doesn’t really get to run at all. With the very first sound of it starting to run, it shuts down. Which makes me thing it’s an electrical shutdown. No more cranking, no running on. It just quits without out even running.
07-27-2019, 01:00 PM
#4
Burning Brakes
Thread Starter
My mechanic was able to get it to start, but it would not idle. He hooked up the new knock sensor without installing it, and it ran and idled. So he replaced the knock sensor, and all seems well.
07-29-2019, 10:58 AM
#5
Safety Car
That is great that you have a now functional Corvette!
I would have never believed it was possible that a dead knock sensor could cause a no start condition. I learn something new every day!
Great work and thanks for the update!
Best regards,
Chris
I would have never believed it was possible that a dead knock sensor could cause a no start condition. I learn something new every day!
Great work and thanks for the update!
Best regards,
Chris
07-29-2019, 12:40 PM
#6
Burning Brakes
Thread Starter
I guess it wasn’t really a “no start” condition, because my mechanic got it to start with much pumping of the gas pedal to keep it running. It would not idle by itself, however.