Franko6
Vendor , w/Business number
Several years ago, an engine came into the shop that had a VERY bent #4 rod. It looked like it should have broken.
Anyway, the rest of the rebuild was fairly normal. We replaced the damaged rod and trashed the piston. Balanced the new set against the #3 piston and rod and slapped it back together.
We could not get the engine to time. It was way up or way down off the timing graph. When you raised the engine above idle, it would smooth out and seem to run ok. The I.Q. was all over the place at idle. Idle balance numbers were way off. For a couple of days, we were stumped. No part we changed made any difference.
With the broken parts of the engine strewn before me, I sat like Hamlet, with Yorick the dead piston in hand..
Act 5, Scene 1:
Alas, poor Piston! I knew it well Randy(as I spoke to my workmate):
an engine of infinite spirit, of most excellent quality:
it hath borne me down the road a thousand times; and now, how
abhorred in my imagination it is! my bile rims at
it. Here hung those piston rings that I have revved I know
not how oft. Where be your gibes now? your
gambols? your tune? your flashes of merriment,
that were wont to set the engine a-roar?
So, while examining the piston thus, I realized an oddity. The bottom of the piston was damaged at the skirt. Quickly,I assembled the piston and rod and clamped it onto the #4 journal of an available crankshaft. I'm sure you would have done the same...
The piston skirt with the rod so bent would not pass the reluctor ring without bending it. The reluctor wheel is the steel dish that is read by the engine speed sensor on an ALH or AHU engine. The reluctor wheel has 4 cutouts on the wheel set 90 degrees apart. The 1/4" gaps in the wheel are read by the Hall's sensor and the information sent to the ECU. Along with several other inputs, the ECU uses the speed sensor information in the idle compensation block. This can be read in the VCDS, engine module, block 13. This block will tell you how much an injector is adding or subrtacting fuel in relation to the other pistons.
At idle, if a piston is determined to be slow by the timing in the engine speed sensor, more fuel is added to speed it up, or conversely, fuel is removed to slow it down. That is the only compensation that the ECU has to vary rotation speed between each piston, with the goal to make idle rotation as smooth as possible. However, this is ONLY an idle compensation. Once the engine gets above 1500 rpm, each cylinder is fueled as determined by the injection pump. There is no fueling compensation above idle. It's complicated and gets worse...
If then, the reluctor wheel is bent more than about 1/8", one of the timing marks cannot be read by the engine speed sensor. The ECU gets a read that makes it appear it took a LONG time for the next mark to come up, so the ECU makes the #1 injector go to maximum fueling to speed that cylinder up. The whole model for idle compensation is then screwed up.
I dropped the oil pan and with a screwdriver, I straightened out the reluctor wheel. The car tuned and the idle compensation module worked correctly again. Problem solved.
This problem can happen if the crank is dropped. The conditions of the error read will change if another segment of the reluctor wheel is bent, but it will make an error read. The best thing to do is make sure on any assembly to check the reluctor wheel is spinning with little variation.
Once installed the easiest way to determine if there is a reluctor read error is with a oscilloscope. There should be 4 distinct spikes of equal height and distance. The other way to check is by removing the engine speed sensor and measuring the distance to the reluctor wheel. It should not vary in depth more than a 1/16". The plate should not swing outside of the window laterally. Gaps in the plate should be equal sized. You will need a mirror to see it.
Anyway, the rest of the rebuild was fairly normal. We replaced the damaged rod and trashed the piston. Balanced the new set against the #3 piston and rod and slapped it back together.
We could not get the engine to time. It was way up or way down off the timing graph. When you raised the engine above idle, it would smooth out and seem to run ok. The I.Q. was all over the place at idle. Idle balance numbers were way off. For a couple of days, we were stumped. No part we changed made any difference.
With the broken parts of the engine strewn before me, I sat like Hamlet, with Yorick the dead piston in hand..
Act 5, Scene 1:
Alas, poor Piston! I knew it well Randy(as I spoke to my workmate):
an engine of infinite spirit, of most excellent quality:
it hath borne me down the road a thousand times; and now, how
abhorred in my imagination it is! my bile rims at
it. Here hung those piston rings that I have revved I know
not how oft. Where be your gibes now? your
gambols? your tune? your flashes of merriment,
that were wont to set the engine a-roar?
So, while examining the piston thus, I realized an oddity. The bottom of the piston was damaged at the skirt. Quickly,I assembled the piston and rod and clamped it onto the #4 journal of an available crankshaft. I'm sure you would have done the same...
The piston skirt with the rod so bent would not pass the reluctor ring without bending it. The reluctor wheel is the steel dish that is read by the engine speed sensor on an ALH or AHU engine. The reluctor wheel has 4 cutouts on the wheel set 90 degrees apart. The 1/4" gaps in the wheel are read by the Hall's sensor and the information sent to the ECU. Along with several other inputs, the ECU uses the speed sensor information in the idle compensation block. This can be read in the VCDS, engine module, block 13. This block will tell you how much an injector is adding or subrtacting fuel in relation to the other pistons.
At idle, if a piston is determined to be slow by the timing in the engine speed sensor, more fuel is added to speed it up, or conversely, fuel is removed to slow it down. That is the only compensation that the ECU has to vary rotation speed between each piston, with the goal to make idle rotation as smooth as possible. However, this is ONLY an idle compensation. Once the engine gets above 1500 rpm, each cylinder is fueled as determined by the injection pump. There is no fueling compensation above idle. It's complicated and gets worse...
If then, the reluctor wheel is bent more than about 1/8", one of the timing marks cannot be read by the engine speed sensor. The ECU gets a read that makes it appear it took a LONG time for the next mark to come up, so the ECU makes the #1 injector go to maximum fueling to speed that cylinder up. The whole model for idle compensation is then screwed up.
I dropped the oil pan and with a screwdriver, I straightened out the reluctor wheel. The car tuned and the idle compensation module worked correctly again. Problem solved.
This problem can happen if the crank is dropped. The conditions of the error read will change if another segment of the reluctor wheel is bent, but it will make an error read. The best thing to do is make sure on any assembly to check the reluctor wheel is spinning with little variation.
Once installed the easiest way to determine if there is a reluctor read error is with a oscilloscope. There should be 4 distinct spikes of equal height and distance. The other way to check is by removing the engine speed sensor and measuring the distance to the reluctor wheel. It should not vary in depth more than a 1/16". The plate should not swing outside of the window laterally. Gaps in the plate should be equal sized. You will need a mirror to see it.
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