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Cam gears off two teeth... what's the technical impact?

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Cam gears off two teeth... what's the technical impact?

 
Old 04-24-19, 07:07 PM
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sbagdon
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Default Cam gears off two teeth... what's the technical impact?

As posted elsewhere, did the timing belt today on the '98 GS4. Both the left and right cams were off by two teeth, to the right (so it was counter-clockwise two teeth...?)... the last mechanic had manually timed the cams to a "T" casting, not to the timing line/protrusion. If you use an oem or oem-quality belt, the belt will have letters on the belt "face", and lines on the teeth of the belt, so when you follow the factory directions, you can time the crank and cams on the first try, by aligning the side-lines to the timing marks on the crank and cam gears... which is helpful, with the grenade-pin tensioner and the effort to re-compress it, if you get the timing off. Can't imagine how anyone can do this job, without an oem/quality belt, and the factory manual.

So... the timing was off by two teeth, performance stunk, and has been throwing odb-ii codes since we got it. Now, it launches like a scalded-cat, and after clearing codes, none have come back, after a sizable drive.

What does all that mean...? Not an "engine person"... was the timing advanced, retarded, or something else? And how did this impact performance?

The last time we saw this, it was on a Gen3 Camry with a 5S engine, which has only one cam... the intake cam drives the exhaust cam with a gear, and when it was off one tooth on the exhaust cam, it was un-drivable... it had to be re-timed, then when it started, a huge blast of white-smoke came out, then after a few minutes, it ran normally. This time, it just felt like an under-powered 6-cylinder.

Edit: guess it was time to figure this out, and did some math. From pics, there appears to be 48 teeth on a cam gear, so 2 teeth would be 1/24th circumference rotation, or 15deg of the cam gear, or 7.5deg of crankshaft rotation. So, effectively, as the cam gear was mis-timed two teeth to the right, for the cam gears to be timed correctly, you would have to move the cam gears back 15deg, or move the crank gear back 7.5deg, resulting in... 7.5deg of retarded timing (please check that math!)? If the engine was really running 7.5deg or timing retard, that would explain the idle and torque issues...

The 1UZ-FE VVTi compression ratio is 10.5:1 (which presumably would be at TDC) with a stroke of 82.5mm. With 7.5deg of ignition retard, can only imagine what that would have done to effective compression, with ignition occurring with a 7.5deg retard...

So it appears should just be glad they didn't mis-time the cam gears two teeth to the left. Image the 1UZ engine with 7.5 degree of ignition advance...


EDIT 2: thought it through again... the cam gears are 48-tooth, and the crank gear is 24-tooth. Moving the cam gears two teeth in one direction (2/48th or 1/24th) would move the crank gear 1 tooth in that direction (1/24th), which would appear to have the mis-timed cam gears give 15 degrees of retard, not 7.5 degrees.

Thx.

Last edited by sbagdon; 04-29-19 at 10:29 AM.
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Old 04-25-19, 07:08 PM
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Old 04-26-19, 05:03 AM
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Not really, after reading the entire thread... this is a great thread for doing the job,and what happens if you do the job incorrectly (bent valves!), not necessarily about engine-design, and impact on incorrect crank/cam timing (which should really be generic).

Yet disagree on some things:

* remove the radiator. Since you're draining the coolant (and you are, when you remove the water pump), taking out the radiator is two upper coolant hoses, two lower p/s lines, three sensor connectors, and two bolts holding the radiator on... maybe 15min of work. Since you're also removing the p/s pulley, alternator and a/c compressor, plus removing all the long-bolts for the pulleys, it's just easier to do all this work, with the radiator out.
* if at all possible, use the crankshaft pulley SSTs: ok, yes... we've been doing Toyota/Lexus timing belts for 25yrs, and "happen" to have a nice collection of SSTs. We already had the compatible crank-pulley puller kit, and the stabilizer arm, yet not the stabilizer (for the 1U motor), which (amazingly!) we found on eBay, for $50ai.
* tensioner: eghads, they didn't replace it, and re-used it...?! Would never have done that.
* use an oem/oem-quality timing belt: it includes the CR/LR/CR markings.
* at least on our 1U engine, the cam gears moved: some say going to 50deg ATDC will stabilize the cam gears, yet our our motor, it didn't... the cam gears moved about 5-6 teeth. Yet, by following the FSM notes, and using the oem markings on the timing belt, we were able to time the crank/cams correctly, the first time... all of those manufacturing markings mean something, when doing maintenance.

Yet amazing that someone would re-use a part, on a timing-belt job... it just seems odd to go this deep in to the engine, and spend so much time doing the job, and not replace all the wearable parts... water pump, idler pulleys, tensioner, etc. An oem-quality timing-belt kit can be had on eBay for <$300, and a factory set (with Lexus part numbers) for around >$425. Imagine going through all that work, then having a $50-$75 part going bad, having to do all the work again.

Anyway, thanks for the link, and just some thoughts.

Last edited by sbagdon; 04-26-19 at 08:19 PM.
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