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Knock sensor question

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Old 09-04-2017, 08:53 AM   #1
luvmy88
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Default Knock sensor question

First let me say hi as I am a new member and appreciate all your help and expertise.
I have a C4 88' all stock with 98k miles. I was getting the dreaded SES lite, it would come on for a few minutes and then go out. It seem to come on just as the engine bucked or missed (Slightly)... a few times and then would smooth out. I had it looked at by a local trusted mechanic and the didn't get any codes. I had a fuel injector system flush done thinking it was carboned up and I had not had one done in a few years. They did see evidence of carbon build up. But a few days later the SES can on again and this time stayed on, AND the engine was running rough, like it was only hitting on 7 cyl. A code check showed 42 & 43.
From what I've read, code 43 could be a bad knock sensor. My question, would a bad knock sensor cause the engine to run rough?
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Old 09-05-2017, 07:31 AM   #2
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Hello and welcome to the forum!

best of luck with your C4
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Old 09-05-2017, 09:25 AM   #3
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Welcome to the Forum!
If I remember correctly, Code 42 is the Electronic Spark Timing. Several things could affect this. Check to see the EST wire (tan and black, I think?) is connected at both ends. Also the PROM not fully seated in the ECM can give this code.
You are correct; Code 43 is your Knock Sensor. Several things can set this code including a defective Sensor, the Sensor is loose, the EST line has an open/short/or disconnected (which could result in getting both 42 and 43 codes). Also a bad plug wire (or wires) can make the engine run rough enough that it sets off the Knock Sensor.
Some of the experts in the C4 section of the Forum are better equipped to help you with this.
Good luck curing this problem.
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Old 09-05-2017, 12:26 PM   #4
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Welcome to the Forum. Check with the folks on the C4 forum. A lot of good info there. Good luck!
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Old 09-05-2017, 12:44 PM   #5
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Welcome to the Forum. Check with the folks on the C4 forum. A lot of good info there. Good luck!
Welcome to the CF!
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Old 09-05-2017, 01:14 PM   #6
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Welcome to the forum!
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Old 09-05-2017, 01:28 PM   #7
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Moving this over to C4 Tech.
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Old 09-05-2017, 05:26 PM   #8
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I had a fuel injector system flush done thinking it was carboned up and I had not had one done in a few years.




Which does your injector look like? Multec or some other?
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Old 09-05-2017, 07:13 PM   #9
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Welcome to the Forum!
If I remember correctly, Code 42 is the Electronic Spark Timing. Several things could affect this. Check to see the EST wire (tan and black, I think?) is connected at both ends. Also the PROM not fully seated in the ECM can give this code.
You are correct; Code 43 is your Knock Sensor. Several things can set this code including a defective Sensor, the Sensor is loose, the EST line has an open/short/or disconnected (which could result in getting both 42 and 43 codes). Also a bad plug wire (or wires) can make the engine run rough enough that it sets off the Knock Sensor.
Some of the experts in the C4 section of the Forum are better equipped to help you with this.
Good luck curing this problem.
I will have to check on that wire connection when I get a chance to look at it this weekend. I was also told it could be the main connection to the ECM.
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Old 09-05-2017, 07:16 PM   #10
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Which does your injector look like? Multec or some other?
I'm not sure what they look like? I will have to check when I get a chance this weekend. I got a lot going on during the week and little free time. I will keep you posted.
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Old 09-05-2017, 08:10 PM   #11
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I'm not sure what they look like? I will have to check when I get a chance this weekend. I got a lot going on during the week and little free time. I will keep you posted.
Just take a peek. It shouldn't be too difficult, I think. I don't remember what mine looked like stock.

http://www.ebay.com/itm/Corvette-TPI...-/282037142507

Picture 1: the little black things on the left of the pair of runners.

If you have Multec, whomever told you to do the injector flush is doing you a disservice. The coils are cooled by fuel and ethanol and the solvents used might not created a good environment. IIRC the TSB said not to flush it. Flushing is a guessing game anyways. I find some build up every 3 years. Depending on how tolerant you are, you can stretch it out but I remove them for proper testing and cleaning. I can't afford the cheap way which is to hook up some snake oil to the fuel rail and cut off the pump while the engine runs.
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Old 09-06-2017, 03:46 PM   #12
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Originally Posted by luvmy88 View Post
My question, would a bad knock sensor cause the engine to run rough?
To answer your question, yes a bad knock sensor will cause the ecm to pull timing and cause the engine to run poorly. A good knock sensor will do the same if the engine is pinging.
Your timing being off can also cause pinging which will set off the knock sensor and pull timing to save the engine.
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Last edited by Dt86; 09-06-2017 at 04:01 PM.
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Old 09-06-2017, 07:44 PM   #13
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Default Very good information

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To answer your question, yes a bad knock sensor will cause the ecm to pull timing and cause the engine to run poorly. A good knock sensor will do the same if the engine is pinging.
Your timing being off can also cause pinging which will set off the knock sensor and pull timing to save the engine.
Thanks Dt86 for all the info, however it's about a foot over my head. That's going to take a while to digest.
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Old 09-06-2017, 07:54 PM   #14
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Default Your attached link to EBAYs Corvette TPI is exactly like mine.

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Just take a peek. It shouldn't be too difficult, I think. I don't remember what mine looked like stock.

http://www.ebay.com/itm/Corvette-TPI...-/282037142507

Picture 1: the little black things on the left of the pair of runners.

If you have Multec, whomever told you to do the injector flush is doing you a disservice. The coils are cooled by fuel and ethanol and the solvents used might not created a good environment. IIRC the TSB said not to flush it. Flushing is a guessing game anyways. I find some build up every 3 years. Depending on how tolerant you are, you can stretch it out but I remove them for proper testing and cleaning. I can't afford the cheap way which is to hook up some snake oil to the fuel rail and cut off the pump while the engine runs.
This is an all original, I've owned it for 20 years and never done anything to it other than reg maintenance and of course the fuel system cleanings about every other year. I had read "someplace" that cleaning the fuel injectors regularly was recommended because the corvette is a car that is made to drive hard and fast, and operating at highway speeds 60/65 all the time, will allow a carbon build up.
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Old 09-06-2017, 08:44 PM   #15
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Originally Posted by luvmy88 View Post
This is an all original, I've owned it for 20 years and never done anything to it other than reg maintenance and of course the fuel system cleanings about every other year. I had read "someplace" that cleaning the fuel injectors regularly was recommended because the corvette is a car that is made to drive hard and fast, and operating at highway speeds 60/65 all the time, will allow a carbon build up.
When you clean your house, do you clean it with your eyes open or shut? If shut, how do you know what was cleaned? Same thing. Fuel injection cleaning and the reasons you mention is recommended because it is a "Dr Feelgood" thing. Makes you feel like you have done something good and the SOTP dyno also tells you the car runs better. I'm not sure where, besides the snake oil seller's site, it documents that it has carbon build up.

AFAIK, what happens is that it sits and the fuel varnishes up the pintles. They sell you a $5 mechanic in a can, you do it, feel good about it and that is about all it does. Whether it works or not, you don't really have a clue. However, it is a few bucks so you don't question. A few bucks here, a few there and they make great money.

If you really want to know, send the injectors out for cleaning and testing. I do it every 3 years since I am picky about the build up and I want to KNOW, not GUESS, KNOW that the volumes are consistent and do not trust the SOTP dyno. I can see the "before and after" results. BTW, I have occasional access to a test bench so I can see what I am sending and getting back every so often.

Reason I asked is that if you have Multecs, they cause problems when the ethanol in the fuel. I would go with getting a set of reman units that are Bosch 3 design. They don't have fuel cooling the coils so there is no issue of ethanol (which will be present in fuel usually) eating the coils. If you have an old set that is Bosch, you are fine but the cost of rehabbing them, you can get reman and be happy.

https://fuelinjectorconnection.com/c...3-injector-set
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Old 09-11-2017, 05:35 AM   #16
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Update from this weekend. First, I found a loose spark plug wire??? Plugged in in and took it for a spin. Of course it ran better... not like before hitting on 7 cyl. But it was still missing or bucking. I jacked it up and tested the knock sensor using a multimeter. The plastic connector crumbled in my hand, so now I have that to deal with. The sensor did registor as working when I tapped on the block, but thinking I will replace anyway.
I did the darkness test. Started the car in dark garage and looked for sparking. HOLLY SH%T!!! There were sparks jumping across wires on both sides in multiple places. These were good quality AC wires that were only a year old.
Any suggestions on cause for that and better wires to use?
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Old 09-11-2017, 08:34 AM   #17
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I did the darkness test. Started the car in dark garage and looked for sparking. HOLLY SH%T!!! There were sparks jumping across wires on both sides in multiple places. These were good quality AC wires that were only a year old.
Any suggestions on cause for that and better wires to use?
What is the definition of "a year old"? That it was made a year ago or that it was put on the car a year ago? You see, it could have sat at a warehouse for 20 years before being sent to you. So while you have 1 year on the thing, it has 20 years of the ozone screwing up the rubber.

I had a set of MSD which was universal length. I also got a crimper and made my own length from that. It has held up for 4 years so far.
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Old 09-12-2017, 08:44 AM   #18
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Default What if the wires are good

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What is the definition of "a year old"? That it was made a year ago or that it was put on the car a year ago? You see, it could have sat at a warehouse for 20 years before being sent to you. So while you have 1 year on the thing, it has 20 years of the ozone screwing up the rubber.

I had a set of MSD which was universal length. I also got a crimper and made my own length from that. It has held up for 4 years so far.
Is there another cause that would make spark jump between wires?
This is not just in one place, but multiple places on both sides of the engine.
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Old 09-12-2017, 08:50 AM   #19
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Is there another cause that would make spark jump between wires?
This is not just in one place, but multiple places on both sides of the engine.
Wires gone bad. Maybe like I said, they are new to you Sat on the shelf. Replace

Last edited by aklim; 09-13-2017 at 03:08 PM.
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Old 09-13-2017, 01:01 PM   #20
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Default can all this be cause by bad plug wires?

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Wires gone bad. Maybe like I said, they are new to youth Sat on the shelf. Replace
Just to go over the happening of the last week or so and review my original issue.
88' vet with 98K miles and stock.
Had an occasional mis that would trigger a SES light but did not produce any codes.
Thought it was clogged fuel injectors so had the cleaned (flush)
Drove home 35 miles and ran fine no issues.
Then two weeks ago driving to work I got a SES that stayed on and ran rougher, when check got code 42 & 43. Ran like only hitting on 7 cyl.
Drove home 35 miles, ran rough but NO SES light.
When inspecting it I found a loose spark plug wire, reconnected it and it ran smoother BUT still missed.
Replaced the coil, no help.
Tested Knock sensor with multimeter and it registered when I tapped on the block.
Looked at the wires in the dark while running, seen several places on each side of engine where spark was arching across wires.
I got new wires on order, waiting delivery.
SO... last night I started it, it idled fine and no SES so I backed out of the garage and it stumble and SES light. I imediately drove it back into the garage where it stumbled several more time. This was the worst it has run yet.

Would this have to do with the new coil? Does the ECM have to learn the new coil?
Bad ground on the coil?
Bad ignition module?
All of the above? None of the above?
Or could all of this just be bad wires?
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