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At 243,000 my LT1 has started to misfire and buck. I have totally replaced the entire ignition system including the PCM. I had no codes and with all the changes now it shows code 43. At 60 mph the spark advance is 30-34. When it starts misfiring it drops to 16-19. At idle the spark is 19. At 60 the engine is running good. I have replaced the knock sensors, ignition module, Electronic spark control(ESC), Optispark, and finally the entire computer. The knock sensor on the OB1 should read yes or no but it give a number. Any ideas what I'm missing?
 

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On a running sensor scan, you get the knock counts, which is a normal reading. What you need to do is to disconnect the knock sensor connectors and read the ac voltage being output by KS with engine running. Your service manual will give you the normal voltage range in the code 43 page.

If you get normal readings from the knock sensors themselves, then you will need to reconnect the wires and take a voltage reading at the other end of wires. Service manual wiring diagram would be helpful here.

What your looking for would be a voltage degrade at end of wire do to bad wire or connector. I can't help much with b body KS wiring as f-body cars only have 1 KS.

Unfortunately, KS problems usually don't cause missing & bucking. I've run my 90 v6 car with no KS connected for years. Other than retarding the advance of ignition timing, it has no effect on how engine runs.
 

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When you replaced opti, if you bought any opti besides GM, Delco, or Delphi, I would suspect problems there. Unfortunately, to 100% check low & high resolution signals from opti, you need an oscilloscope. You can buy a PC based scope for around $70 on ebay, amazon, etc.

With it you can see if low & high res signals from opti are the proper square wave. And when car acts up, you can see if signals change wave form or drop out. Scope also good for checking 50 hertz fuel enable signal that theft deterrent module sends to PCM. It's also a square wave.
 

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Discussion Starter #4
On a running sensor scan, you get the knock counts, which is a normal reading. What you need to do is to disconnect the knock sensor connectors and read the ac voltage being output by KS with engine running. Your service manual will give you the normal voltage range in the code 43 page.

If you get normal readings from the knock sensors themselves, then you will need to reconnect the wires and take a voltage reading at the other end of wires. Service manual wiring diagram would be helpful here.

What your looking for would be a voltage degrade at end of wire do to bad wire or connector. I can't help much with b body KS wiring as f-body cars only have 1 KS.

Unfortunately, KS problems usually don't cause missing & bucking. I've run my 90 v6 car with no KS connected for years. Other than retarding the advance of ignition timing, it has no effect on how engine runs.
Yesterday I had to use the wagon. At 60mph it runs smooth as it aways has. At 61mph it starts the misfire and bucking. I can stomp it and at 70 it seems to even out but not really sure. I keep my OB1 connected all the time and the spark advance stays at 32-34 running smooth. When the misfires start it drops to 20 and under. The knock sensor reading is supposed to be on or off as per the service manual and the knock retard should show various degrees. My knock sensor reads various numbers, different at each start up and the knock retard has never given any reading. The oscilloscope is out of my field of expertise. I have in the past bought a pricy Summit Racing opti-spark and it wouldn't even fire. Sent it back and put in the original with 100,000 on it and it has been running just fine. This time I bought a Dura-last new, not remanufactured opti. It wasn't a cheapy but of course not a Delco. I replaced the original that I thought was the problem and it didn't change anything. Can I just disconnect the knock sensors and test drive the car?
 

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Yes, you should be able to disconnect knock sensors and still drive car. SES light will be on and spark will be retarded. Since my 90 f-body is about 2000 pounds lighter than your roadmaster, I can't predict how well the car will respond.

Learning how to use an oscilloscope is not rocket science. With a little experimenting you will use it like a pro. The hardest part is setting time and voltage settings to get signal displayed correctly.
 

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Yes, you should be able to disconnect knock sensors and still drive car. SES light will be on and spark will be retarded. Since my 90 f-body is about 2000 pounds lighter than your roadmaster, I can't predict how well the car will respond.

Learning how to use an oscilloscope is not rocket science. With a little experimenting you will use it like a pro. The hardest part is setting time and voltage settings to get signal displayed correctly.
I have been on you tube trying to learn how to use the oscilloscope. It is ridiculous. There are more videos on how to unpack the thing than how to use and where and what the probes are used for. I will buy one if I can see how to use it.
All the reviews of the scopes on Amazon are as confusing as the you tube videos. At 75 I guess my brain isn't working any more.
I have disconnected the knock sensors and the ohms on the sensor are within limits. The voltage to the leads are supposed to be 5 volts key on. Mine read 4.79. I have to lower the car and test drive it with knock sensors disconnected.
 

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I have been on you tube trying to learn how to use the oscilloscope. It is ridiculous. There are more videos on how to unpack the thing than how to use and where and what the probes are used for. I will buy one if I can see how to use it.
All the reviews of the scopes on Amazon are as confusing as the you tube videos. At 75 I guess my brain isn't working any more.
I have disconnected the knock sensors and the ohms on the sensor are within limits. The voltage to the leads are supposed to be 5 volts key on. Mine read 4.79. I have to lower the car and test drive it with knock sensors disconnected.
I have just returned with the Roadmaster with the knock sensors disconnected. It has a constant spark advance of 19 or less. Used to be 32-34 at 60mph. Doesn't buck as bad but not right. I will reconnect the sensors when it cools down and try again. The knock sensor was reading 9. It is showing no knock retard, but it never has. Are there any mechanics in the Central Florida area that could do further testing? I hate to take it to any garage as most don't even know what an LT1 is.
 

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As far as the oscilloscope leads, they are positive & negitive as all DC electrics use to make measurements. The leads of a scope are connected just like your multimeter leads when measuring DC voltage

As far as using the scope the voltage and time knobs are use to get voltage being measured into center of scope.
 

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At 243,000 my LT1 has started to misfire and buck. I have totally replaced the entire ignition system including the PCM. I had no codes and with all the changes now it shows code 43. At 60 mph the spark advance is 30-34. When it starts misfiring it drops to 16-19. At idle the spark is 19. At 60 the engine is running good. I have replaced the knock sensors, ignition module, Electronic spark control(ESC), Optispark, and finally the entire computer. The knock sensor on the OB1 should read yes or no but it give a number. Any ideas what I'm missing?
With all the money you spent replacing components you could have gone with TorqHead 24X system, 8 coils over plugs and a new OB11 computer. It's plug and play for under $1,700.00.
 

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Discussion Starter #10
As far as the oscilloscope leads, they are positive & negitive as all DC electrics use to make measurements. The leads of a scope are connected just like your multimeter leads when measuring DC voltage

As far as using the scope the voltage and time knobs are use to get voltage being measured into center of scope.
I was nervous about getting and using an Oscilloscope for one engine problem. I was told about an old shop in my area called Dyno-tune. I knew they had been around since the old Sun tune up console was used. I am embarrassed to say what they found was the only thing I didn't change. Until the engine got on the scope and I saw the readout there was no way to see or know the problem. #8 plug wire was totally dead. I was running on 7 cylinders. It is a bear to replace those wires. Half the accessories have to be removed to snake the plug wires through. After a long struggle I got them in. I now have no codes and my Lt1 is purring as always. Thanks to all for the help.
 
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