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1991 L98 Pinging on Shell 93 Octane

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Old 09-12-2017, 01:15 AM   #1
koritz123
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Default 1991 L98 Pinging on Shell 93 Octane

1991 Coupe

Runs great purrs like a kitten but a mechanic friend riding along asked about the pinging.I would rate it as 3 or 4 on a 10 point scale, 10 being the loudest.

Engine has been rebuilt in the last 500 miles with a lot of new parts especially tune-up and electrical parts.

The prevouis owner used 91 octane plus 104 octane booster.
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Old 09-12-2017, 07:08 AM   #2
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Engine has been rebuilt in the last 500 miles with a lot of new parts especially tune-up and electrical parts.

The prevouis owner used 91 octane plus 104 octane booster.
Sounds like the rebuild has raised the compression requirements a bit requiring more octane boost.

So, you have three choices
1) live with it (not advised but a choice)
2) continue to buy best grade possible plus octane boost.
3) double check and/or retard timing. Assuming that your electrical is functioning if you go back to stock (6 degrees) you should eliminate the ping. I run 8 degrees on my stock L98 all day with 91/93 gas- no issues.

If your are more of a "Cruiser" and not a "racer" start with the timing. Cheap and easy.
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Old 09-12-2017, 10:55 AM   #3
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is the egr and the switch functioning?

that 104+ stuff in the auto parts store is junk marketed to kids...if you need a real octane boost look at Torco they have non leaded stuff first find your problem and fix it.
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Old 09-12-2017, 12:30 PM   #4
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Hello there,

Can you do a compression test and give us an idea of the numbers you get. It would be best done if the engine was warm but not hot. You might even do a second test after shooting a bit of light oil in the cylinder.

Did you mill the original heads when you did the rebuild? What type of head gaskets did you use in the rebuild? Stock type or aftermarket? Were they the original thickness or are they thinner than the original?

What kind of plugs are you running in the motor when it starts pinging?
Are you using the original heat range plugs from the factory setup?
Is your car running the complete ignition control system as it came from the factory?
Knock sensors installed and operating normally?

Like billschroeder5842 suggested go to the original timing setting of 6 degrees. I have never heard my L98 engine ever ping. I even experimented with using regular gas and had no problems on my car.

Pinging is the fuel exploding before the proper time (spark plug firing) and is caused by multiple factors.

Pinging is frequently caused by lower grade or low quality fuels, certainly should work okay on 91 octane.

Do not let it continue to ping or you will be rebuilding the engine again soon. This really sounds like there should be a code in your OBD system.

Are you really sure that it is in fact pinging? There are other noises that could be mistaken for pinging. Pinging should only happen when the motor is under a heavy load like a long grade or when you accelerate. Do you have a FSM for your car?

I have never put any kind of octane boost in my '88 Coupe. It has never needed it.
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Old 09-12-2017, 12:49 PM   #5
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My 91 knock sensor worked but was not sensative enough. I compared another sensor by tapping with a hammer while hooked to a millavolt meter. changed sensors and no more knock! good luck
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Old 09-12-2017, 02:36 PM   #6
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Engine has been rebuilt in the last 500 miles with a lot of new parts
Were there any mods to the engine during the rebuild which might have increased the compression ratio? Does the pinging increase when you accelerate, especially when you're going up a hill?

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Old 09-13-2017, 02:20 PM   #7
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Hello there,

Can you do a compression test and give us an idea of the numbers you get. It would be best done if the engine was warm but not hot. You might even do a second test after shooting a bit of light oil in the cylinder.

Did you mill the original heads when you did the rebuild? What type of head gaskets did you use in the rebuild? Stock type or aftermarket? Were they the original thickness or are they thinner than the original?

What kind of plugs are you running in the motor when it starts pinging?
Are you using the original heat range plugs from the factory setup?
Is your car running the complete ignition control system as it came from the factory?
Knock sensors installed and operating normally?

Like billschroeder5842 suggested go to the original timing setting of 6 degrees. I have never heard my L98 engine ever ping. I even experimented with using regular gas and had no problems on my car.

Pinging is the fuel exploding before the proper time (spark plug firing) and is caused by multiple factors.

Pinging is frequently caused by lower grade or low quality fuels, certainly should work okay on 91 octane.

Do not let it continue to ping or you will be rebuilding the engine again soon. This really sounds like there should be a code in your OBD system.

Are you really sure that it is in fact pinging? There are other noises that could be mistaken for pinging. Pinging should only happen when the motor is under a heavy load like a long grade or when you accelerate. Do you have a FSM for your car?

I have never put any kind of octane boost in my '88 Coupe. It has never needed it.
The mechanic friend I had riding along thought it sounded like a ping I do too and it is at acceleration even mild acceleration on a flat surface again it's a three to four on a ten-point scale I'm not sure it is a ping how do I determine that it is or isn't a pinging?
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Old 09-13-2017, 05:29 PM   #8
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When you set timing did you find true TDC or just assume that it's 0 on the balancer?
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Old 09-13-2017, 05:42 PM   #9
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The mechanic friend I had riding along thought it sounded like a ping I do too and it is at acceleration even mild acceleration on a flat surface again it's a three to four on a ten-point scale I'm not sure it is a ping how do I determine that it is or isn't a pinging?
For a 91, you should be able to hook up the scanner to see if it is pinging or not, I would think. Have you done anything to modify the motor? Has the ECM been reprogrammed to take that into consideration? Do you have accurate timing? What injectors are you using?
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Old 09-13-2017, 06:41 PM   #10
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When you set timing did you find true TDC or just assume that it's 0 on the balancer?

The seller set the timing.
I've invited him to join in but who knows
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Old 09-13-2017, 06:46 PM   #11
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The seller set the timing.
I've invited him to join in but who knows
Trust but verify
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Old 09-13-2017, 06:51 PM   #12
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For a 91, you should be able to hook up the scanner to see if it is pinging or not, I would think. Have you done anything to modify the motor? Has the ECM been reprogrammed to take that into consideration? Do you have accurate timing? What injectors are you using?
Pic of injector can you tell what kind of injector it is by the color?



I asked the seller and engine Rebuilder to join into this thread don't know if he will though only he can answer these other questions I am non-mechanical and need to depend on other people to do anything that's significant.
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Old 09-13-2017, 07:01 PM   #13
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Pic of injector can you tell what kind of injector it is by the color?



I asked the seller and engine Rebuilder to join into this thread don't know if he will though only he can answer these other questions I am non-mechanical and need to depend on other people to do anything that's significant.
Look at this on eBay http://www.ebay.com/itm/370963131683

Those looking like 30 pph which are not stock. What fuel pressure? ECM has to be adjusted CORRECTLY for that.
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Old 09-13-2017, 07:08 PM   #14
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Look at this on eBay http://www.ebay.com/itm/370963131683

Those looking like 30 pph which are not stock. What fuel pressure? ECM has to be adjusted CORRECTLY for that.
The previous owner did tell me the previous owner that's two owners back put oversized injectors in the car I can take it to the Corvette mechanic in St Louis and have him take a look at it not sure what other path to follow the mechanic friend I have really doesn't want to fool around with things unless it's real significant and interesting to him.
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Old 09-13-2017, 07:50 PM   #15
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The previous owner did tell me the previous owner that's two owners back put oversized injectors in the car I can take it to the Corvette mechanic in St Louis and have him take a look at it not sure what other path to follow the mechanic friend I have really doesn't want to fool around with things unless it's real significant and interesting to him.
Don't know what to say. You cannot simply stick an injector in going from 22 to 30 without changing the ECM programming. BTDT and had issues with getting it up the tow trailer
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Old 09-13-2017, 08:21 PM   #16
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Those are Bosch injectors.if you get the number off of them you can look them up and get the flow rating.There are many parts # with different flow ratings and bosch injectors are used in just about everything.Make sure your fuel pressure is correct while accelerating and does not fall off.I don't know how a 91 ecm is set up but my 87 runs a lot of timing advance at part throttle cruise and depends on the knock sensor to retard it after it gets more throttle.You can confirm a lean or rich condition by wiring up a wide band o2 meter.If the oem program in the ecm has been rewritten it might have too aggressive of an advance curve burned into it.Make sure the timing is set per the service manual.My 87 has the knock sensor wired to a knock module -that module is an input to the ecm.A lot of wiring in an old car makes for a lot off troubleshooting-continuity testing of wires.Very few people have the software to actually tune these things and the only place I have found any kinds of tuning software/cables is on a website called Moates.net.Knock/preignition is very hard on the piston and rod bearings-the piston is actually forcing the engine to try to go backwards.

Last edited by odxr; 09-13-2017 at 08:34 PM.
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Old 09-13-2017, 08:52 PM   #17
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Those are Bosch injectors

If the oem program in the ecm has been rewritten it might have too aggressive of an advance curve burned into it.

Make sure the timing is set per the service manual.

My 87 has the knock sensor wired to a knock module -that module is an input to the ecm.

A lot of wiring in an old car makes for a lot off troubleshooting-continuity testing of wires.Very few people have the software to actually tune these things

and the only place I have found any kinds of tuning software/cables is on a website called Moates.net.Knock/preignition is very hard on the piston and rod bearings-the piston is actually forcing the engine to try to go backwards.
I agree since it looks like what I put in the 383 with the 74219 cam, TFS heads and Superram manifold. It didn't run right even just getting it onto the tow trailer to bring to be tuned.

Well, that is kinda what I suspected.

Possibly the OP was an idiot and tried to ramp up power by cranking the timing so I would check it. I never trust the OP of anything to tell me anything.

IIRC, the later ones like his don't have the module.

And that is a very real issue.

IMO he shouldn't run the car until he has someone dyno tune the thing.
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Old 09-14-2017, 01:50 PM   #18
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I agree since it looks like what I put in the 383 with the 74219 cam, TFS heads and Superram manifold. It didn't run right even just getting it onto the tow trailer to bring to be tuned.

Well, that is kinda what I suspected.

Possibly the OP was an idiot and tried to ramp up power by cranking the timing so I would check it. I never trust the OP of anything to tell me anything.

IIRC, the later ones like his don't have the module.

And that is a very real issue.

IMO he shouldn't run the car until he has someone dyno tune the thing.
I'll wait till Bill is in the shop at Elco Chevrolet on Monday and explain the situation to him and see what he recommends if he wants it Dyno tuned then I'll have it Dyno tuned.

Thanks all for the feedback I wish I was more mechanically inclined even though I'm the son of a mechanic in situations like this I want to make sure everything is right and it's better to be safe than sorry so I'll just pay for what I need to have done to make sure that I'm not risking ruining a perfectly good engine.
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Old 09-14-2017, 03:08 PM   #19
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I'll wait till Bill is in the shop at Elco Chevrolet on Monday and explain the situation to him and see what he recommends if he wants it Dyno tuned then I'll have it Dyno tuned.

Thanks all for the feedback I wish I was more mechanically inclined even though I'm the son of a mechanic in situations like this I want to make sure everything is right and it's better to be safe than sorry so I'll just pay for what I need to have done to make sure that I'm not risking ruining a perfectly good engine.
If they do dyno tuning there, I'd be surprised. I doubt they know how to tear apart the program and see what is going on which is why I suggested getting a tune that is known and not guess at what is happening and was done to the engine.
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Old 09-14-2017, 03:15 PM   #20
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If they do dyno tuning there, I'd be surprised. I doubt they know how to tear apart the program and see what is going on which is why I suggested getting a tune that is known and not guess at what is happening and was done to the engine.
Bill is an expert 35-year Corvette mechanic and he's mentioned on the Corvette forum over and over and over again people come from all over the United States to see him I was only going to ask for his opinion and he will recommend garages and places in town that he knows that are reputable and do great work I'm just trying to get some feedback from him he might even be willing to test drive the car for me to verify that it is in fact pinging it doesn't sound like knocking it just sounds like high pitched metallic pinging about a 4 on a ten-point scale.

When I had an inch play in my steering wheel vertically and he failed it on inspection he told me what shop to take it to however a mechanic engineering friend of mine fixed it but he has a list of people from the 35 years that he's dealt with people in St Louis of shops that are competent so that's what I'm hoping for.
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