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

Knock retard at highway speeds.

 
Old 11-29-2018, 06:21 PM
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Space387
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Default Knock retard at highway speeds.

I will preface this with, if I can find them I will post the datalogs to show what's happening. So for some reason only in 6th gear when accelerating between 65 and 75 my L98 adds in the maximum spark retard and I can feel the power loss at highway speeds. No other noticable symptoms are coming up and it's only at that speed in 6th gear. Am I safe to assume it's the knock sensor or likely something else?
Thanks in advance
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Old 11-29-2018, 07:10 PM
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What kind of RPM do you have at that speed?

I'd consider dropping to 5th then back to 6th. Might be loading the engine up with high load/low RPM
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Old 11-29-2018, 07:31 PM
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About 1500rpm. The weird part is that I upshift 5-6 at about 50 mph and it has no issues until 65mph. This is all when i am going gentle on the car. If I want to accelerate with any haste I hold off the down shift. The only reason i bring this up is because the mass Kr event is right at cruising speed for most highways here and its killing my fuel mileage. I'm ok with bad mileage if it's for power but when it's bad just to be bad it needs to be fixed.
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Old 11-29-2018, 08:11 PM
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Is it just lean right in that rpm range ?
It seems like if it were the knock sensor, then the problem would exist in a much wider operating range.

Last edited by PatternDayTrader; 11-29-2018 at 08:16 PM.
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Old 11-29-2018, 08:21 PM
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Possible but unlikely. I am running 24lb injectors on a stock tune. I'm in the process of getting the chip and burner to remedy this and get ready for the LT1 top end sitting in storage.

I'll do some more logging Monday when I have a day off and the car is back together. Clutch master and slave are out, waiting for a new hose
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Old 11-29-2018, 08:32 PM
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Can you hear any spark knock or preignition when its backing the timing off ?

Last edited by PatternDayTrader; 11-29-2018 at 08:33 PM.
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Old 11-29-2018, 08:53 PM
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No sound of spark knock or feeling of breakup only a smooth gentle loss of power
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Old 11-29-2018, 08:55 PM
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Are you running premium gas?
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Old 11-29-2018, 09:04 PM
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Interesting.
Is the egr system still intact ?
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Old 11-29-2018, 09:46 PM
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93 octane every time and yes the EGR is still functioning.
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Old 11-29-2018, 10:03 PM
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1500 rpm you say. I wonder if its hooking on to a harmonic in the driveline. See if you can replicate it in the same rpm band at lower speeds. You also may be able to hear something if it is a harmonic...
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Old 12-01-2018, 12:24 PM
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Is it possible that you might have a "fuel flow" issue that is causing a leaner mixture by not supplying the volume of fuel that the engine needs at the speeds you described. It could be as simple as a dirty fuel filter that could cause the symptoms like the ones you are experiencing. On our 1988 Coupe the fuel line rusted shut internally and reduced the volume of fuel but kept the pressures in the normal range.

I thought I had a pump failing and after swapping out a new pump the same thing happened. It was not until I measured the amount of fuel that would flow in 15 seconds through both the supply line and the return line that it became obvious what was going on. My return line flows fine but the reduced flow from the supply line became apparent at higher fuel consumption times like accelerating hard from a high speed. After the new supply line was installed the car acted perfectly normal and had no more hesitation at any speed.

I suspect that you would see a pressure drop when you get to highway speeds or at the point between fifth and sixth gear. Have you tried the old Fuel Pressure Gauge taped to the windshield and watch what happens at speed? The other way to find out if a fuel line is causing problems is to measure the fuel that comes out of the pump during a fixed period of time. Do both lines the supply and the return, I did this by disconnecting the fuel lines at the engine and putting in a calibrated 1 quart container and running the pump through the supply and then through the return lines.

If you have not changed the fuel filter in a while it might be a good thing to do. One more thing to check, people crush the fuel lines by improperly lifting Corvettes, probably not done by you the owner, but by a gas station mechanic or somebody not watching closely.

Good Luck and be sure to let us know what you find that was causing the problem. You can help us learn something from your experience, that is the great thing about this Corvette Forum!
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Old 12-01-2018, 03:30 PM
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The fuel issue would be a better candidate if I was seeing it consistent from say 3k to redline and under heavy loads. I had that exact issue about 8 months ago from contaminated fuel plugging the filter. That issue was resolved with the method you described, currently I see the following pressures

Pump bypass car off 50psi
car idling and hwy cruse 42psi
WOT 48-50 psi

As of this time last year the entire fuel system was replaced and a second filter was changes this past spring. It could still be a fuel issue but only if the ecm is pulling fuel at more than 20% fuel load from the o2 sensor adjustment. Between the 10% larger injectors and the inadvertent +5 psi of fuel pressure I don't think the ecm is capable of pulling enough fuel to make the car lean at any point.

Thanks for ideas, keep them coming.
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Old 12-02-2018, 01:11 PM
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My thought is disconnect the knock sensor and drive the car in the same area where you are seeing the knock retard. Listen for any knock, it is possible that a harmonic at that RPM is causing the knock sensor to pull timing when there is no actual knock. If you have actual knock vs fake knock the fix for this will be entirely different.

Let us know what you find.
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Old 12-02-2018, 06:53 PM
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If the rain let's up tomorrow I'll surely give it a try. The sensor is at the base of the block passenger side right?
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Old 12-02-2018, 09:22 PM
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Originally Posted by Space387 View Post
If the rain let's up tomorrow I'll surely give it a try. The sensor is at the base of the block passenger side right?
Correct
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Old 12-02-2018, 09:28 PM
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Originally Posted by Space387 View Post
93 octane every time and yes the EGR is still functioning.
Disconnect or otherwise disable the egr from opening. Just to make sure. It seems to me when the issue is occurring, is also the same throttle and load conditions as when the egr opens. So somthing weird might be happening with that.
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Old 02-08-2019, 10:33 PM
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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

Last edited by Space387; 02-08-2019 at 10:35 PM.
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Old 02-09-2019, 08:17 AM
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What year is the car, I assume 92 or 93? Is the car stock? How hot does it run when this happens?
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Old 02-09-2019, 04:00 PM
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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.
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