I had some work done last year, cooling system flush being one of the jobs. Before I put it away last year I noticed that a lot of pinging was going on during partial acceleration. Even with a just filled tank of hi-test at partial throttle.
1) After sitting all winter would you add a bottle of octane boost to the full tank. I never did before.
2) With the flush needing to remove the knock sensors for draining, could they (or one) be disabled, broken or not reconnected. Of course I’ll check for wire disconnection as I’m bringing it out of hibernation but how can I test them for functionality. 91 L98 ZF Thanks, Frank

 

Your L98 has a single knock sensor, and while that could cause the problem I think the check engine lamp would illuminate if it was disconnected. Check the diagnostic procedure in the correct Factory Service Manual to test the circuit properly.
A common cause of spark knock is a non functional EGR valve, that is another system I would check carefully.

 

I removed my EGR valve and my car does not knock. I also removed all the polution pump system.

 

Tap the block or heads around the knock sensor on the right side of the motor. I recommended tapping on the actual knock sensor a while back and got yelled at by our forum brothers, but that is what I used to do. If the circuit is functional, your motor will decrease a hundred rpm or two when idling and the block tapped sharply with an object (wrench, etc).

 

Entirely possible, but that does not mean your experience applies to all vehicles. A tune or other modifications can eliminate knock that would typically occur when the EGR is not working.

 

I do have a aftermarket chip!! My service engine light comes on ever sense I removed my EGR but the car still runs great.

 

If the Knock sensor was removed, its very important that it be replaced properly. The most common problem is over tightening. Its only 14lbs torque and any more makes it harder for the sensor to operate. Since the tip lives in the coolant its possible that its fouled or dirty like low coolent sensors will get.

Knock needs to be defined. Its different than Ping. Ping is Pre-ignition meaning the spark or something ignites the fuel charge too soon,. where knock is the incomplete burning of the fuel charge and can come from bad wires, dirty ignition parts,m weak module, cheap gas.

Define the actual condition and then you can address the cause. I had a knock issue and chased it for months....finally discovered it was the new set of wires that I had installed. never in a million yrs would I have suspected NEW wires to cause that. Put the OLD wires on and my problem went away..

 

EGR, interesting. When I got the car back last year after the work, the check engine light set. I found that "they" had forgotten to attach the vacuum line to the EGR valve. After I reattached everything was fine and the light has stayed off. The EGR valve appears to be working.
Is there a suggestion that if the knock sensor isn't operating correctly that the check engine light will be set? Because so far it hasn't.
I know the same shop checked my timing during repair as they found all my cats were clogged. They also pulled a plug to check the current tune, looking for the power loss and prior to doing a exhaust pressure check.
The cars never run better just that I can hear that cackling or light pinging during acceleration. So I'm looking for suggestions.............Thanks, Frank

After reading leesvet response I'd say it's pinging, if that helps...............

 

It is certainly possible for the knock sensor to malfunction without setting a code, notice my earlier post said I thought it would turn on the MIL if disconnected.
The EGR solenoid and valve can both function, but if the passages in the tube or intake are plugged with carbon it may still knock because the exhaust does not reach the cylinder to reduce combustion temperature which is the function of the system.
Other possible causes would be running to hot or too lean.

 

EGR is simple...
it adds cooler exhaust gasses that are burnt and without o2 so they will cool down the current combustion when it reaches an excessively high temp...
Pinging can come from excessive combustion temps that make a plug tip stay red hot like a glow plug or carbon build-up or even a scar in a piston dome..anything that acts like the filement in a light bulb causes "pinging"...

if the EGR valve is stuck closed, it can;t do its job. The knock sensor could care less about your "ping" problems...its tuned for a specific frequency and ignores everything else.

If you can access the vac line that is to the EGR valve, you can suck on it (apply 10 second rule as needed) and use a small mirror under the edge of the EGR diaphram housing to look thru the small holes and see if the valve is reacting to the vac that you apply. It should move a little. If not, the EGR is suspect and needs further investigation.

As for removing the EGR, sure, we lived without egrs for many yrs. EGR is used to cool combustion for 2 reasons...current tuning makes knock more likely, and current fuels make for some nasty exhaust gasses when combustion temps go too high. You can tune a motor to run without the EGR, thats easy enough,. just a matter of timing/temp & tune. Just don;t get caught....It IS a federal offense to mess with smog devices UNLESS the car is for "off-road use ONLY". Thats a very very important term in the aftermarket business...
They still get away with it in Texas...for now, but with Houston being the 2nd most polluted city in the nation only 1/2 a percentage pt behind L.A....the messing around and backdoor inspection stickers will be going away real soon. Good news for that state is that at 25 yrs you;re a classic and get a free pass...only safety inspections...