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Testing for a worn timing chain

General engine tech -- Drag Racing to Circle Track

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travis
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Testing for a worn timing chain

Post by travis »

Testing for a heavily worn timing chain on a ‘99 L31 Vortec engine...

Roughly how many degrees of back and forth movement at the balancer before the rotor or cam position sensor (in this case) moves is considered too much? I think the timing set has been replaced before judging by the globs of clear silicone around the bottom of the timing cover. Cranking compression is very good at 180 psi. I put a mark on the lower pulley and hit it with a timing light, and the mark moves back and forth over about a 1” range. I’m not sure what kind of movement range would be normal for an EFI/computer controlled engine that is constantly adjusting things
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Re: Testing for a worn timing chain

Post by PRH »

If the timing is read off a crank trigger, the slop in the timing chain should have no affect on ignition timing.

I’ve had a timing light on very very few factory engines running a crank triggered ignition, but the ones I did check have always been rock steady if the engine speed is held constant.

The tune up specs on the tag under the hood of my old Dakota listed the timing as “not adjustable”.

My recollection is that my old Neon didn’t even have any way to check the timing.

Have you verified the timing jumps around if using other than the #1 plug wire?
Last edited by PRH on Sun Oct 11, 2020 6:11 pm, edited 1 time in total.
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Re: Testing for a worn timing chain

Post by BOOT »

I know you posted other threads bout this truck, so you may have mentioned it already but did you check the distributor gear?
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Re: Testing for a worn timing chain

Post by turbo camino »

The cam sensor in the distributor is only a 1x sensor, it's only used for the computer to tell if #1 is at TDC compression or TDC exhaust. It does not have anything to do with ignition timing.
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Re: Testing for a worn timing chain

Post by travis »

turbo camino wrote: Sun Oct 11, 2020 7:50 pm The cam sensor in the distributor is only a 1x sensor, it's only used for the computer to tell if #1 is at TDC compression or TDC exhaust. It does not have anything to do with ignition timing.
I understand that. I was going to rotate the engine back and forth to see how far the balancer moved before the CPS moved
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Re: Testing for a worn timing chain

Post by travis »

BOOT wrote: Sun Oct 11, 2020 6:08 pm I know you posted other threads bout this truck, so you may have mentioned it already but did you check the distributor gear?
Yes...it looks brand new
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Re: Testing for a worn timing chain

Post by travis »

PRH wrote: Sun Oct 11, 2020 6:07 pm If the timing is read off a crank trigger, the slop in the timing chain should have no affect on ignition timing.

I’ve had a timing light on very very few factory engines running a crank triggered ignition, but the ones I did check have always been rock steady if the engine speed is held constant.

The tune up specs on the tag under the hood of my old Dakota listed the timing as “not adjustable”.

My recollection is that my old Neon didn’t even have any way to check the timing.

Have you verified the timing jumps around if using other than the #1 plug wire?
The timing isn’t adjustable...I was just looking to see if the mark was steady. I had to make a mark on the pulley since you can’t even see the balancer on this thing from up above. I will try some other plug wires and see if I see the same thing.

I don’t even know if this is a valid test...
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Re: Testing for a worn timing chain

Post by BillK »

Travis,
The big single roller chains in those engines seem to last forever without stretching. My 99 Tahoe has 280K miles on it with no issues at all. I have had some marine engines with over 1000 hours of run time and the chains still look about the same as new.

Are you having an actual problem or just checking stuff for grins ?
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Re: Testing for a worn timing chain

Post by dannobee »

You can look at the cam sensor offset with the engine off idle and see if it dances around or if it's pretty steady. IIRC, at idle the PCM doesn't give a live reading to the scan tool, but it does at 1500rpm or so.
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Re: Testing for a worn timing chain

Post by travis »

BillK wrote: Sun Oct 11, 2020 9:18 pm Travis,
The big single roller chains in those engines seem to last forever without stretching. My 99 Tahoe has 280K miles on it with no issues at all. I have had some marine engines with over 1000 hours of run time and the chains still look about the same as new.

Are you having an actual problem or just checking stuff for grins ?
It was something I noticed when troubleshooting another issue that has finally been resolved. It runs and drives fine now, but now I have this kind of gnawing at me.

The cranking compression numbers make me think that everything is ok. And...I really don’t want to work on this thing anymore
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Re: Testing for a worn timing chain

Post by rebelrouser »

10 degrees of movement was what we used for years, as a standard for chain replacement.
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Re: Testing for a worn timing chain

Post by MadBill »

To expand a little on RR's post, it's much easier to rotate just the crank than it plus the cam and valve gear, so if you bar the engine CW until the effort suddenly increases and then ditto CCW, the 'low resistance' range is chain slop. E.g., the CW turning effort suddenly spikes at 6° ATDC and CCW at 4°BTDC, so the slop is 10°, which is more than I would want for a performance engine.
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