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C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

Knock retard at highway speeds.

 
Old 02-09-2019, 06:30 PM
  #21  
RWDsmoke
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Check base timing? How old is the o2 sensor?
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Old 02-09-2019, 09:23 PM
  #22  
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Originally Posted by Space387 View Post
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.
Are you positive the balancer has not slipped and you have way too much base timing? Nothing with this makes sense. A 1990 should work fine without knock at cruising speed. How is the wot power?
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Old 02-10-2019, 02:13 AM
  #23  
Space387
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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.
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Old 02-19-2019, 10:32 PM
  #24  
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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?
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Last edited by Space387; 02-19-2019 at 10:35 PM.
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Old 02-20-2019, 06:51 AM
  #25  
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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.
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Old 02-20-2019, 03:59 PM
  #26  
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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 04:01 PM.
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Old 02-20-2019, 05:15 PM
  #27  
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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.
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Old Yesterday, 04:02 PM
  #28  
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Originally Posted by Space387 View Post
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
Did the ECM throw a code and light when you disconnected the sensor?

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.
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Old Today, 03:35 PM
  #29  
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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.
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Old Today, 05:00 PM
  #30  
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Originally Posted by tequilaboy View Post
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.
I agree it feels more like it should. I always run 93 octane in my v8's as its the highest I can get without buying race gas at $7.50 a gallon. Buying the tools to burn chips is next on my list of purchases for this car so a retune will probably be in the near future. And about the Knock sensor, with it unplugged I do get a CEL for it.
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Old Today, 05:02 PM
  #31  
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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.
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Old Today, 05:34 PM
  #32  
tequilaboy
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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.
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Last edited by tequilaboy; Today at 06:04 PM.
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