Timing chain wear on engines with non-adjustable ignition timing

[travis]

On engines with non-adjustable ignition timing, what happens as the timing chain wears and gets slack in it?

In this particular case I’m referring to a stock L31 vortec. The distributor only has a very narrow range of error that it can be installed in without throwing a code. So, as the timing chain wears, does that installation window move?

[BillK]

Travis,
I cant answer you exact question but if it helps any my 99 Tahoe is getting ready to turn 280K miles with the engine never having been touched and it still runs basically the same as it did when new. The original plugs rotor cap and wires were not changed until about a year ago and probably had 250K miles on them. It would set a code if the timing was off far enough for the ecm and it has never done that. I have never touched or even checked the timing. I have had the Tahoe since new.
I have taken quite a few engines part with the big single roller "motorcycle" chain and to be honest with you they have never been much looser than they were when new. This is true for both small and big blocks.
Hope this helps

[Lizardracing]

Chain tension, Cam twist, Dist gear lash, Dist end play lash all effect the timing so typically the more clearance there is in the system, timing will retard.....Usually the clearances close and open rapidly and the effects seen is is the damper mark dancing around. 2-4 degrees or more.

[Tuner]

The L31 engines have a crank sensor for ignition timing and the distributor is just a cam position sensor with a cap and rotor.

[travis]

Travis,
I cant answer you exact question but if it helps any my 99 Tahoe is getting ready to turn 280K miles with the engine never having been touched and it still runs basically the same as it did when new. The original plugs rotor cap and wires were not changed until about a year ago and probably had 250K miles on them. It would set a code if the timing was off far enough for the ecm and it has never done that. I have never touched or even checked the timing. I have had the Tahoe since new.
I have taken quite a few engines part with the big single roller "motorcycle" chain and to be honest with you they have never been much looser than they were when new. This is true for both small and big blocks.
Hope this helps

Bill, what does your oil pressure look like with 280k miles...particularly idle and highway speeds? My truck has 300k on it and still has excellent oil pressure (55 cold idle, 45-50 at highway speed fully warmed up), but I’m not sure if the engine is original or not.

I’m just grasping at straws at this point, as I have been troubleshooting a major drivability issue on this thing for way too long.

[travis]

The L31 engines have a crank sensor for ignition timing and the distributor is just a cam position sensor with a cap and rotor.

I know, I just suspect that as the timing chain stretches the crank and cam position sensor phasing will move, possibly outside the narrow window that these run in. I dunno at this point...again just grasping at straws. Nothing looks out of whack on the scan tool, but drivability is terrible unless the MAF is unplugged

[naukkis79]

I know, I just suspect that as the timing chain stretches the crank and cam position sensor phasing will move, possibly outside the narrow window that these run in. I dunno at this point...again just grasping at straws. Nothing looks out of whack on the scan tool, but drivability is terrible unless the MAF is unplugged

Distributor is adjustable. There's not much of range where distributor has to be set as there's 720/8 - 90 degrees of timing for distribute spark to each cylinder, and if there's about 15 degrees air gap to each direction and spark advance variation is about 50 degrees distributor has to set within few degrees to be able to distribute spark to correct cylinder.

If distributor is in wrong position ecu sees that, gives error code and limits it's spark advance tables so spark is still distributed to correct cylinder.

and your problem is probably broken maf.

[dannobee]

On the GM SI2000 trouble tree for code P1345, Cam/Crank Correlation Problem (the one that usually sets when you install the distributor wrong)

Diagnostic Aids

Check the following items:

A loose CMP sensor causing a variance in the sensor signal
Excessive free play in the timing chain and gear assembly
Incorrectly installed distributor - 1 tooth off in either advance or retard positions
A loose distributor rotor on the distributor shaft
A loose or missing distributor hold down bolt

I've replaced a LOT of intake gaskets on L31's and as long as they're not a tooth off, you can reinstall the distributor to where the cap fits back on and the code won't set. If you're really paranoid, you can use the scan tool to check the cam retard offset and adjust it close to zero.

[travis]

There is no difference in drivability between the old MAF and the new one. So, I don’t “think” the MAF is the issue.

Since these things have no timing marks or pointers (not that I can see anyway), I just made a mark on the water pump and the crank pulley as close to each other as I could get, and put a timing light on it. The mark is moving a good 10+ degrees when checked on several different plug wires. I think the timing chain is shot, or else there is something wonky going on with the cam and crank position sensors (both of which have been replaced as well).

In any case, I’ve reached my limit with this truck. Even though I like it (it’s super handy especially with 5 kids), I have too many vehicles, and I haven’t really driven it in a couple years (don’t need it anymore), and I’m tired of it taking up space in the driveway. It’s sitting in the front yard with a for sale sign on it now

[rebelyell]

Travis,
I cant answer you exact question but if it helps any my 99 Tahoe is getting ready to turn 280K miles with the engine never having been touched and it still runs basically the same as it did when new. The original plugs rotor cap and wires were not changed until about a year ago and probably had 250K miles on them. It would set a code if the timing was off far enough for the ecm and it has never done that. I have never touched or even checked the timing. I have had the Tahoe since new.
I have taken quite a few engines part with the big single roller "motorcycle" chain and to be honest with you they have never been much looser than they were when new. This is true for both small and big blocks.
Hope this helps

^^^^THIS^^^^ and (in most sbc) I prefer the HD 1/2" pitch single roller "cycle" chain ... even over a double roller. Single ALWAYS fits without block clearancing.
Consider how all L30 & L31 OE Roller motors' Vortecs came equipped w/ 80 lb seat single springs as most prior sbc also have OE 80 lb on seat but w/ a flat damper as well.

FWIW ... Late great Smokey Yunick is said to have preferred the OE-type nylon-toothed cam sprocket TS (that so many loathe) in some race motors because they transmitted less harmonics to valve train and he changed them out with regularity. Who am I to disregard such experience & intellect.

[naukkis79]

There is no difference in drivability between the old MAF and the new one. So, I don’t “think” the MAF is the issue.

Since these things have no timing marks or pointers (not that I can see anyway), I just made a mark on the water pump and the crank pulley as close to each other as I could get, and put a timing light on it. The mark is moving a good 10+ degrees when checked on several different plug wires. I think the timing chain is shot, or else there is something wonky going on with the cam and crank position sensors (both of which have been replaced as well).

As ignition is triggered from crank timing chain stretch has absolutely no effect on ignition timing.

Have you read MAF-live data? There might be burnt intake valve which makes MAF-reading to oscillate which makes ecu to change timing. Removing MAF will put ECU to use calculated load-values instead which masks that mechanical failure.

[travis]

There is no difference in drivability between the old MAF and the new one. So, I don’t “think” the MAF is the issue.

Since these things have no timing marks or pointers (not that I can see anyway), I just made a mark on the water pump and the crank pulley as close to each other as I could get, and put a timing light on it. The mark is moving a good 10+ degrees when checked on several different plug wires. I think the timing chain is shot, or else there is something wonky going on with the cam and crank position sensors (both of which have been replaced as well).

As ignition is triggered from crank timing chain stretch has absolutely no effect on ignition timing.

Have you read MAF-live data? There might be burnt intake valve which makes MAF-reading to oscillate which makes ecu to change timing. Removing MAF will put ECU to use calculated load-values instead which masks that mechanical failure.

A burnt valve was an early suspicion that I haven’t ruled out yet. I don’t remember the MAF readings moving around but I can certainly recheck it.

I certainly don’t fully understand how all the different sensors and what not interact with each other. However, if the timing chain is flopping around, would that not still affect the cam position sensor? So even if the crank position sensor triggers the ignition when to fire, if the rotor isn’t in the right place...

But...then why would unplugging the MAF make it run better if it actually is a timing issue, because the timing should still be moving around...

A burnt valve seems to make sense, but then again so has a bunch of other things I’ve tested or replaced.

I’ll go hook up the scan tool again to refresh my memory

[travis]

Travis,
I cant answer you exact question but if it helps any my 99 Tahoe is getting ready to turn 280K miles with the engine never having been touched and it still runs basically the same as it did when new. The original plugs rotor cap and wires were not changed until about a year ago and probably had 250K miles on them. It would set a code if the timing was off far enough for the ecm and it has never done that. I have never touched or even checked the timing. I have had the Tahoe since new.
I have taken quite a few engines part with the big single roller "motorcycle" chain and to be honest with you they have never been much looser than they were when new. This is true for both small and big blocks.
Hope this helps

^^^^THIS^^^^ and (in most sbc) I prefer the HD 1/2" pitch single roller "cycle" chain ... even over a double roller. Single ALWAYS fits without block clearancing.
Consider how all L30 & L31 OE Roller motors' Vortecs came equipped w/ 80 lb seat single springs as most prior sbc also have OE 80 lb on seat but w/ a flat damper as well.

FWIW ... Late great Smokey Yunick is said to have preferred the OE-type nylon-toothed cam sprocket TS (that so many loathe) in some race motors because they transmitted less harmonics to valve train and he changed them out with regularity. Who am I to disregard such experience & intellect.

I don’t doubt this at all...it’s just that what I am seeing looks like a heavily worn timing chain

[travis]

The MAF reads .70-.71 lb/m with random spikes up to .75-.80 at warm idle. At a steady no load 1500 rpms it reads a steady 2.39 lb/m +/- .02.

MAP reads a steady 9.7” HG at idle, no spikes at all. About 8.9” at a steady no load 1500 rpms, again no spikes.

Ignition timing at idle jumps all over the place between 20* and 24.5*. It’s a rock solid 28* at steady no load 1500 rpms.

[Roundybout]

I don’t see the ECU commanding such a weird scheme at idle but who knows. I’d suspect it’d throw a code if the timing was so far out of whack than what the computer is expecting. Small wear/tolerance stack up contributing to mechanical wear seems plausible for erratic idle timing I suppose. The secondary ignition system would be suspect. Coil(s), rotor, cap, wires, plugs any modules ect.

If any sensors were responsible they’d throw a code. Coolant sensor going from cold to hot quickly over and over for example. This seems to be happening without the ECU knowing lol. Which I suppose could be a good thing.