Knock retard at highway speeds.
02-09-2019, 08:23 PM
#22
Safety Car
Car is a 90 L98 completely stock save the injectors. When I bought the car I had to replace the entire fuel system and bought 24lb injectors based off my LT1. Didnt find out till after they are 2 lbs too heavy. Car runs a little rich but not too bad. Cooling isn't an issue, maintains about 195-200 once warmed up.
02-10-2019, 01:13 AM
#23
Racer
Thread Starter
Base timing was reset late December when I replaced the head gaskets due to a leak. Found a coolant leak to the combustion chamber allowing burnt gasses out and a little coolant in to the chamber. WOT power is better every time I find and fix an issue but still not what I would expect from a 350. I do not know if the balancer is slipped and I'm not exactly sure how to check. I looked at the price and due to it looking like its original I may just buy a new one. The O2 sensor was replaced last year when I found the old one broke off during a test drive.
Thanks a lot guys, I'm still learning my way around the older engines. The last 2 I worked on where from 94 and both used reluctor wheels and sensors for setting spark advance so I have never had to rely on the balancer for accurate timing.
Thanks a lot guys, I'm still learning my way around the older engines. The last 2 I worked on where from 94 and both used reluctor wheels and sensors for setting spark advance so I have never had to rely on the balancer for accurate timing.
02-19-2019, 09:32 PM
#24
Racer
Thread Starter
So I have not been able to source a new balancer due to work and sickness but feeling better today I went and did some data logging. The first one was taken earlier today on my drive back from town and the knock retard doesn't seem as bad but the power loss is real. About 9 minutes in you can see I floor it in 4th gear and it falls on its face. Full rich but 17* spark advance and no power what so ever. When the car is cold and during the warmup phase it runs well but once warmed up its got nothing. The second data log is from this evening when I dropped the fuel pressure from 50psi base to 35psi base to try and correct for the poor economy and overall stupid rich condition. Ran lean at first while the ecm figured out something was different.
At this point I'm not sure. The max 43* spark advance isn't making it knock when cold so I cant see it being such a difference to warrant pulling up to 12* advance at times. What are the symptoms of a bad knock sensor and a bad knock module in the ecm?
At this point I'm not sure. The max 43* spark advance isn't making it knock when cold so I cant see it being such a difference to warrant pulling up to 12* advance at times. What are the symptoms of a bad knock sensor and a bad knock module in the ecm?
Last edited by Space387; 02-19-2019 at 09:35 PM.
02-20-2019, 05:51 AM
#25
Melting Slicks
Looks like prom bcc is ARFP 7185 based upon 5801 scan id.
90-91s are around 10:1 compression with the flat-top pistons.
High load (kpa), Low rpm, and hot coolant temp. Could be actual knock based on the conditions, but it doesn't recover when it is pulling timing, so hard to say based on the data alone.
Verify the initial timing as already discussed and confirm that the mark on the damper aligns with the #1 crank throw or try and determine tdc and compare with the timing tab. Also try and verify that ecm commanded timing is what you get with a timing light.
You could try running a tank of 100 octane unleaded race gas for a test in an attempt to rule out actual knock vs false knock.
Then you can always retune the ecm accordingly to retard the timing or reduce the knock sensor's authority depending upon the conclusions of actual knock vs false knock.
90-91s are around 10:1 compression with the flat-top pistons.
High load (kpa), Low rpm, and hot coolant temp. Could be actual knock based on the conditions, but it doesn't recover when it is pulling timing, so hard to say based on the data alone.
Verify the initial timing as already discussed and confirm that the mark on the damper aligns with the #1 crank throw or try and determine tdc and compare with the timing tab. Also try and verify that ecm commanded timing is what you get with a timing light.
You could try running a tank of 100 octane unleaded race gas for a test in an attempt to rule out actual knock vs false knock.
Then you can always retune the ecm accordingly to retard the timing or reduce the knock sensor's authority depending upon the conclusions of actual knock vs false knock.
02-20-2019, 02:59 PM
#26
Racer
Thread Starter
So... yea, its all my fault. Instructions unclear, toaster in the bathtub. It seems that what I through was the control line was actually the tach signal and I set the car to 6* advance with the ecm playing with the spark. If I understand this correctly I set a base timing of almost 10* ATDC not the 6* BTDC. Or I had a base timing of nearly 20* either way, Damn I hate distributors. So a quick jog around the block at 70 and yea the issue is gone. Still have a vibration above 4500 but I think that is either the rotating assembly up front or maybe the flywheel/clutch pack. I do need to add a little more advance than 6* but right no w there is no knock detected.
Last edited by Space387; 02-20-2019 at 03:01 PM.
02-20-2019, 04:15 PM
#27
Racer
Thread Starter
After reviewing the data log gone may be too strong of a word for the knock issue. More like unnoticeable. I didnt feel the timing pull but there is a few events where up to 8* timing is pulled at low RPM loads with the timing set correctly this time. Not feeling it is good but still having it makes me wonder if there isn't still an underlying issue.
02-21-2019, 03:02 PM
#28
Tech Contributor
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Sorry for the delayed response life got in the way of trying to fix this issue. So as of right now this is what I know, spark retard sets in at 1600 and 2000 now. I can feel it in gear 3-6. As you go to a higher gear the effect of the power loss is exaggerated, I suspect this is simply due to gearing. I have tried blocking off the EGR at the inlet to the manifold, no change. Tried unplugging the EGR activation solenoid, SES light without improvement. disconnected the EVAP solenoid, no change. After data logging All I see is the knock retard set in, spark advance drops to about 10* and without moving the throttle plate the MAP pressure rises from 70kpa to 101( ambient pressure). Ego2 does not indicate the car is leaning out and my injector pulses are increasing to match the increase in manifold pressure. The engine does not sound or feel like it is knocking at these rpm's. There is still a vibration above 4k but it is relatively subtle. At this point i'm unsure what is causing this issue or where to go next. tia
edit: Forgot to mention tried the car without the knock sensor and it ran like poop. Had a bit too much advance at cruising rpms and just felt unhappy the rest
edit: Forgot to mention tried the car without the knock sensor and it ran like poop. Had a bit too much advance at cruising rpms and just felt unhappy the rest
I'm trying to recall the different ECM changes over the years to detect a broken/open sensor. The ESC system likes a 2k ohm input impedance to operate correctly. IIRC your vintage used a 4k ohm sensor in parallel with the 4k ohm input impedance of the ESC module. If the sensor resistor fatigues, the sensor signal level rises, potentially being read as (false) knock. (In later Corvettes, an A/D circuit reads the DC voltage on the sensor line to check for sensor integrity.)
The harness connector to the sensor is a delicate piece. After just a few connects/disconnects the integrity of the connection sometimes goes to ****. An intermittent connection causes step changes in the signal level received by the ESC, and these step changes look like bursts of knock, causing the ECM to react and retarding the timing.
02-22-2019, 02:35 PM
#29
Melting Slicks
With the Fixed_timing log, it still knocks a little, but seems to recover now. I'd consider this normal behavior for the conditions. You could always run a higher octane fuel, retune the ecm's spark advance and knock retard parameters or retard the initial timing a bit to compensate...or use 5th gear instead of 6th below 75 mph, or drive faster.
02-22-2019, 04:00 PM
#30
Racer
Thread Starter
With the Fixed_timing log, it still knocks a little, but seems to recover now. I'd consider this normal behavior for the conditions. You could always run a higher octane fuel, retune the ecm's spark advance and knock retard parameters or retard the initial timing a bit to compensate...or use 5th gear instead of 6th below 75 mph, or drive faster.
02-22-2019, 04:02 PM
#31
Melting Slicks
If you have been running "a little rich" for awhile, maybe you have carbon buildup in the combustion chamber that is causing early detonation and the knock sensors are picking that up...? You said it's only in certain gears/conditions, so maybe carbon buildup plus EGR function equals excess detonation? Just thinking out loud. Good luck.
02-22-2019, 04:34 PM
#32
Melting Slicks
Hey just caught this: Appears that you have an auto trans bin, based on the scan id.
8D ARFP 32.00 KB 1990 32647 16137184 12277277 1855801 YB L98 5.7 MD8 AUTO GM1
Manual trans bins typically have less low rpm/high load timing for this very reason...
Interesting to note that Chevy Thunder shows this as both an auto and 6 speed bin: http://chevythunder.com/
Screwy...Maybe worth moving to a 91 manual trans bin.
8D ARFP 32.00 KB 1990 32647 16137184 12277277 1855801 YB L98 5.7 MD8 AUTO GM1
Manual trans bins typically have less low rpm/high load timing for this very reason...
Interesting to note that Chevy Thunder shows this as both an auto and 6 speed bin: http://chevythunder.com/
Screwy...Maybe worth moving to a 91 manual trans bin.
Last edited by tequilaboy; 02-22-2019 at 05:04 PM.
02-22-2019, 10:50 PM
#33
Racer
Thread Starter
Thats weird. The past owner was very good about documenting any changes they did and a new ecm was not on the list. The car is also bone stock so I dont believe there was ant reason to change the chip
02-23-2019, 12:28 PM
#34
How hot was the engine when it pulled timing?
02-25-2019, 03:08 PM
#35
Safety Car
Hello again,
From what you are describing it sounds like maybe you are experiencing some pre-ignition that falsely triggers the knock sensor in your Corvette.
The knock sensor can be tested while in place on the engine. I have a harness from Mid America that came with their EFI setup tools that hooks directly to the stock sensor and allows you to see the voltage output from the knock sensor. I have rapped the engine block and or cylinder heads with a metal wrench and it shows a spike in voltage. I suspect that your knock sensor is over-reacting as that is one of the failure modes of the older style knock sensors like the ones used on L98's. An over-reacting knock sensor will come trigger the timing retarding more often and cause a serious power loss to occur. The only way to learn if your knock sensor is bad and over-reacting would be to replace it with a known "good" unit and drive your car and load it up.
I run 87 octane in my 103,000 miles old 1988 C4 and have no problems, I have never HAD to use premium gasoline to squelch the noise of the pre-detonation. I run the stock timing but the 700R4 transmission won't load up your engine like going slow in 6th gear with a manual transmission. If 93 octane does not stop the pinging then you have to stop the root cause of the pinging or detonation. Hot engines are more likely to ping from the pre-detonation, make sure your cooling system is up to the job of keeping it around 200* with a good pressure radiator cap.
Keeping the fuel systems clean on the old C4 is a bigger task. I try to use only one brand of fuel and I add a can of BG 44K Fuel System Cleaner every few thousand miles or as needed by the car. The BG 44K is a very good cleaning agent, it removes carbon and all kinds of junk from the combustion chamber as it gets burned up with the fuel. The lower Compression engines of the 1980's and 1990's had a bad habit of getting carbonized if they are not pushed hard every once in a while.
If the knock sensor hears a noisy lifter it can trigger retarding the ignition, any metallic noise generated by an older engine can be mistaken as knocking inside and cause the timing to get pulled back. I used hydraulic lifters in my 427 to keep the noise level down, my 427 wears a knock sensor hooked up to an MSD device that tells me if and when the car is detonating. I like the dial that allows me to set the desired level of sensitivity of the Knock sensor.
I have a big Block 427 in my 1968 L71 Corvette and it has 12.25-1 Compression ratio. I have been trying to source an ignition system device that will use a knock sensor and automatically retard the particular cylinder that is knocking by 2* per revolution until the noise stops and then it will try to turn the timing back up to the original settings. This type system was developed by Bosch to help the late model Corvette race cars survive the race. GM kept having their cars get DNF's because they were breaking pistons and rings from detonation (most likely) so they approached Bosch who made them just what they needed to allow our Corvette racers to finish the races and to win races.
There is a company who makes such a system for us out here. It is made by and called J&S SafeGuard and they are available now. I am going to use one on my 500 HP+ 427 to allow me to run as much timing as I want without destroying the engine. The system will piggyback into your original electrical system or in my case it works with all my MSD devices already operating in the car. I have a Snow Performance Water/Methanol injection system to protect me from detonation and make my combustion temperatures drop a bit. With the J&S SafeGuard my engine will make even more power and my timing will be "Under Control" thanks to this new technology.
Good Luck and let us know what you find that was causing you all this grief!
Chris
From what you are describing it sounds like maybe you are experiencing some pre-ignition that falsely triggers the knock sensor in your Corvette.
The knock sensor can be tested while in place on the engine. I have a harness from Mid America that came with their EFI setup tools that hooks directly to the stock sensor and allows you to see the voltage output from the knock sensor. I have rapped the engine block and or cylinder heads with a metal wrench and it shows a spike in voltage. I suspect that your knock sensor is over-reacting as that is one of the failure modes of the older style knock sensors like the ones used on L98's. An over-reacting knock sensor will come trigger the timing retarding more often and cause a serious power loss to occur. The only way to learn if your knock sensor is bad and over-reacting would be to replace it with a known "good" unit and drive your car and load it up.
I run 87 octane in my 103,000 miles old 1988 C4 and have no problems, I have never HAD to use premium gasoline to squelch the noise of the pre-detonation. I run the stock timing but the 700R4 transmission won't load up your engine like going slow in 6th gear with a manual transmission. If 93 octane does not stop the pinging then you have to stop the root cause of the pinging or detonation. Hot engines are more likely to ping from the pre-detonation, make sure your cooling system is up to the job of keeping it around 200* with a good pressure radiator cap.
Keeping the fuel systems clean on the old C4 is a bigger task. I try to use only one brand of fuel and I add a can of BG 44K Fuel System Cleaner every few thousand miles or as needed by the car. The BG 44K is a very good cleaning agent, it removes carbon and all kinds of junk from the combustion chamber as it gets burned up with the fuel. The lower Compression engines of the 1980's and 1990's had a bad habit of getting carbonized if they are not pushed hard every once in a while.
If the knock sensor hears a noisy lifter it can trigger retarding the ignition, any metallic noise generated by an older engine can be mistaken as knocking inside and cause the timing to get pulled back. I used hydraulic lifters in my 427 to keep the noise level down, my 427 wears a knock sensor hooked up to an MSD device that tells me if and when the car is detonating. I like the dial that allows me to set the desired level of sensitivity of the Knock sensor.
I have a big Block 427 in my 1968 L71 Corvette and it has 12.25-1 Compression ratio. I have been trying to source an ignition system device that will use a knock sensor and automatically retard the particular cylinder that is knocking by 2* per revolution until the noise stops and then it will try to turn the timing back up to the original settings. This type system was developed by Bosch to help the late model Corvette race cars survive the race. GM kept having their cars get DNF's because they were breaking pistons and rings from detonation (most likely) so they approached Bosch who made them just what they needed to allow our Corvette racers to finish the races and to win races.
There is a company who makes such a system for us out here. It is made by and called J&S SafeGuard and they are available now. I am going to use one on my 500 HP+ 427 to allow me to run as much timing as I want without destroying the engine. The system will piggyback into your original electrical system or in my case it works with all my MSD devices already operating in the car. I have a Snow Performance Water/Methanol injection system to protect me from detonation and make my combustion temperatures drop a bit. With the J&S SafeGuard my engine will make even more power and my timing will be "Under Control" thanks to this new technology.
Good Luck and let us know what you find that was causing you all this grief!
Chris
02-25-2019, 07:10 PM
#36
Melting Slicks
ARFP (90) and AXCN (91) share the same spark advance table settings, so disregard my previous comment about trying a 91 specific 6 speed bin. It appears to be knocking just a bit in the mid load (map) region.
Main spark table shown below for reference. Sorry if it is difficult to read in this text format.
TABLE: Spark Advance Main
RPM MAP (kPa)
20 30 40 50 60 70 75 80 85 90 95 100
4800 37.97 46.05 46.05 46.05 37.97 34.10 31.99 29.88 28.13 26.02 23.91 23.91
4400 37.97 46.05 46.05 43.95 40.43 35.16 31.99 28.13 27.07 24.96 22.85 22.85
4000 40.08 40.08 46.05 46.05 40.08 34.10 28.83 27.07 24.96 23.91 21.09 21.09
3600 37.97 46.05 46.05 42.19 36.91 30.94 28.83 27.07 24.96 22.85 22.85 22.85
3200 37.97 46.05 46.05 40.08 36.91 30.94 29.88 28.13 24.96 23.91 22.15 20.04
2800 37.97 46.05 46.05 46.05 40.08 36.91 33.05 29.88 27.07 24.96 22.15 22.15
2400 37.97 42.89 46.05 46.05 40.08 37.97 34.10 29.88 28.13 26.02 23.91 22.15
2200 37.62 43.95 46.05 46.05 40.43 37.97 36.91 33.05 29.88 28.83 23.91 22.15
2000 39.02 41.84 46.05 46.05 40.08 40.08 37.97 34.10 30.94 28.83 24.96 22.15
1800 36.91 37.97 43.95 43.95 41.84 40.08 34.10 34.10 29.88 27.07 23.91 22.15
1600 37.97 43.95 43.95 43.95 41.13 37.97 35.86 33.05 29.88 26.02 22.85 22.15
1400 35.16 39.02 41.84 41.13 37.97 36.91 33.40 29.88 27.07 23.91 20.04 17.93
1200 26.02 28.13 35.86 40.08 37.97 35.16 30.94 27.07 24.96 20.04 17.93 17.93
1000 23.91 23.91 29.88 35.86 35.16 34.10 30.94 28.13 23.91 15.12 11.95 9.84
800 20.04 20.04 24.96 29.88 28.13 23.91 21.09 17.93 14.06 11.95 9.84 9.84
600 20.04 20.04 20.04 20.04 20.04 17.93 14.06 11.95 9.84 9.84 9.84 9.84
400 20.04 20.04 20.04 20.04 20.04 20.04 17.58 16.17 14.06 11.95 9.84 9.84
Main spark table shown below for reference. Sorry if it is difficult to read in this text format.
TABLE: Spark Advance Main
RPM MAP (kPa)
20 30 40 50 60 70 75 80 85 90 95 100
4800 37.97 46.05 46.05 46.05 37.97 34.10 31.99 29.88 28.13 26.02 23.91 23.91
4400 37.97 46.05 46.05 43.95 40.43 35.16 31.99 28.13 27.07 24.96 22.85 22.85
4000 40.08 40.08 46.05 46.05 40.08 34.10 28.83 27.07 24.96 23.91 21.09 21.09
3600 37.97 46.05 46.05 42.19 36.91 30.94 28.83 27.07 24.96 22.85 22.85 22.85
3200 37.97 46.05 46.05 40.08 36.91 30.94 29.88 28.13 24.96 23.91 22.15 20.04
2800 37.97 46.05 46.05 46.05 40.08 36.91 33.05 29.88 27.07 24.96 22.15 22.15
2400 37.97 42.89 46.05 46.05 40.08 37.97 34.10 29.88 28.13 26.02 23.91 22.15
2200 37.62 43.95 46.05 46.05 40.43 37.97 36.91 33.05 29.88 28.83 23.91 22.15
2000 39.02 41.84 46.05 46.05 40.08 40.08 37.97 34.10 30.94 28.83 24.96 22.15
1800 36.91 37.97 43.95 43.95 41.84 40.08 34.10 34.10 29.88 27.07 23.91 22.15
1600 37.97 43.95 43.95 43.95 41.13 37.97 35.86 33.05 29.88 26.02 22.85 22.15
1400 35.16 39.02 41.84 41.13 37.97 36.91 33.40 29.88 27.07 23.91 20.04 17.93
1200 26.02 28.13 35.86 40.08 37.97 35.16 30.94 27.07 24.96 20.04 17.93 17.93
1000 23.91 23.91 29.88 35.86 35.16 34.10 30.94 28.13 23.91 15.12 11.95 9.84
800 20.04 20.04 24.96 29.88 28.13 23.91 21.09 17.93 14.06 11.95 9.84 9.84
600 20.04 20.04 20.04 20.04 20.04 17.93 14.06 11.95 9.84 9.84 9.84 9.84
400 20.04 20.04 20.04 20.04 20.04 20.04 17.58 16.17 14.06 11.95 9.84 9.84