timing belt alignment, again

[myhui]

For NA2 engine, service manual says to put pin punches through camshaft to check alignment (page 6-19). The rear bank's camshaft pulleys are also aligned differently than the front's (page 6-18), in that the exhaust pulley is half a tooth below the TDC mark.

Has anyone tried that and can confirm that it is exactly the right procedure? I ask that because I remember someone posting that they can't get the one half tooth below TDC and have the pin punches line up.

Is there a different alignment one should look for when installing a new belt versus inspecting an old belt that's already installed?

 

[Daedalus]

It sounds similar to the NA1 procedure, which works fine. The pins are there to get you close and to prevent the cams from jumping suddenly due to valve spring forces, but they won't get you dead nuts. For the NA1 there are marks on the cam sprockets that need to be lined up. Front bank is a straight shot. Rear bank is a little trickier. I wouldn't think the NA2 would be much different. "1/2 tooth below TDC" could mean 1 of 2 positions. Make sure it's 1/2 tooth forward, not backward. Most people use a paint pen or similar to mark teeth on the old belt relative to the 5 sprockets (4 cam, 1 crank), and then transfer these marks to the new belt to locate it correctly. Nothing wrong with this approach, but for peace of mind it's nice to verify it against the factory method IMO.

 

[Larry Bastanza]

NA1 and NA2 engines are identical. The procedure for installing the belt is to ADVANCE (clockwise) the rear exhaust cam 1/2 tooth so you can slip the belt on, then you reposition the cam EXACTLY aligned. The rear exhaust cam pulley is the last pulley you put the belt on. Remember to follow the installation sequence EXACTLY as it says in the manual.

LMK if you have any questions.

HTH,
LarryB

 

[drew]

Reading between the lines it seems he is trying to verify if a timing belt has been installed correctly?

If so: a good indicator of a successful timing belt install is to do a compression test (preferably: before and after)

drew

 

[myhui]

Reading between the lines it seems he is trying to verify if a timing belt has been installed correctly?

That is correct. I am diagnosing a problem.

If so: a good indicator of a successful timing belt install is to do a compression test (preferably: before and after)

Thanks for the tip.

I can envision these scenarios, and their effect ("early" is same as "advanced"):

• intake cam early: valve hits piston, so engine would never run.
• intake cam late: loss of compression, as drew pointed out, but engine would run, poorly.
• exhaust cam early: engine would run, poorly, but with no effect on compression.
• exhaust cam late: valve hits piston, so engine would never run.

Do I have it right?

I suspect there is a subtle effect with the very long timing belt used on the NSX: if tension is not set just right, using the proper procedure outlined in the manual, the rear exhaust cam will have an imprecise angle (i.e. "exhaust cam early" above) since it is the last gear on the belt before it goes back to crankshaft, and that can affect engine balance. I suspect numerous people have made the mistake of installing a new belt and get engine vibration afterwards that wasn't there before with the old belt. Yet TDC was perfectly aligned! Or does the cause lie somewhere else?

The camshaft position sensor is located on the front bank only, so the ECU has no way to cross-check whether the rear bank's camshafts are properly aligned.

 

[drew]

Do I have it right?

Almost, it is the degree (literally) of early/lateness.

If the timing belt is off at all: a compression check will tell. I cannot fathom a scenario where misaligned belt does not show up on a compression check.

I believe the NSX can have any one cam off by +/- 2 teeth and not have valve interference. It might be any or all cams can be off by +/-2 teeth...but this would be a miserable case.

BTW: The car can run pretty darn good if the exhaust cam is off by a tooth.

Do the compression check. The bank with the cam off will show up quick.

What are your symptoms?

Drew

 

[Daedalus]

Depending on what is and isn't on the car, a vacuum check may be easier to do than a compression check.
Working out the slack by making minor adjustments as you go may be required, but in the end, the marks on the cams still have to line up before the belt is looped over each sprocket, and before the rear exhaust cam is rotated the 1/2 tooth. Use zip-ties and/or clamps to secure the belt to each sprocket once its located correctly to keep it from slipping. Start with the crank sprocket, then go fwd exhaust, fwd intake, rear intake, rear exhaust.

 

[myhui]

BTW: The car can run pretty darn good if the exhaust cam is off by a tooth.

Drew, are you absolutely sure of that? Do you have personal experience on that? It could be the cause of my problem. I don't have it apart, so no visual confirmation yet.

I'm missing the crankshaft pulley holder shown on page 6-12. Is the crankshaft so hard to turn that using the tool is preferred?

You guys want to lend me yours (like, tonight)? I can offer a generous cash deposit.

( I guess working on your own car can be a lot of fun ... but if it turns out to be a day job, the fun dies pretty quickly )

 

[Daedalus]

If you don't have the car apart yet definitely do a vacuum check.
The crankshaft bolt will take at least several hundred ft-lbs of torque. The only other way to get the bolt off besides using a holder would be to use a chain wrench on the pulley. Common on other cars, but I wouldn't dare try it with that kind of stress. You're welcome to borrow mine, but I'm way out near LAX. If I had the time I'd lend you hand. Unfortunately I'm working a FT job and in school FT, have an exam later this week.

 

[Larry Bastanza]

Drew, are you absolutely sure of that? Do you have personal experience on that? It could be the cause of my problem. I don't have it apart, so no visual confirmation yet.

I have had about 3 cars come to me with timing belts off a tooth on the rear cam. Distinct V8 rumble at idle.

I'm missing the crankshaft pulley holder shown on page 6-12. Is the crankshaft so hard to turn that using the tool is preferred?

If you want to just turn the crank to check the alignment marks you can use a 19mm socket on the crank bolt, out of gear, plugs out, of course.

HTH,
LarryB

 

[nis350]

So a vacuum test would indicate if any of the cams is off?

Is there a reason why the rear cam always seems to be the one that is off?

I am also wondering why the car would run OK with the cam off? would you lose compression?

 

[Daedalus]

So a vacuum test would indicate if any of the cams is off?

Is there a reason why the rear cam always seems to be the one that is off?

I am also wondering why the car would run OK with the cam off? would you lose compression?

Either a vacuum test or a compression test will indicate if something is amiss. The vacuum test takes more interpretation but is an excellent quick check of an engine's internals. It can indicate whether one cylinder is out of whack or if one head is mis-timed, but it won't say which cylinder or which head like a compression test will.
The rear cam is the one that tends to be off because it's the one that doesn't match up to the belt "just right" like the others do when the belt is taut, the engine is TDC, and all sprocket marks are properly aligned. The easy thing to do, without knowing better, is to add a bit of slack to the belt to get it to drop into place. This effectively retards the rear cam 1/2 tooth rather than advances it. Being off 1 tooth means being retarded by over 8* on all exhaust valves on the rear head. I don't know what values you would get for compression and vacuum, but 8* off will show up loud and clear.

 

[myhui]

The vacuum test takes more interpretation but is an excellent quick check of an engine's internals.

Mityvac's instruction manual http://www.mityvac.com/user_manuals/07000-06820.pdf has an excellent tutorial on how to interpret engine vacuum to diagnose problems.

 

[Larry Bastanza]

This is very simple. The rear cams are off because timing belts are installed incorrectly, because techs do not follow the procedure exactly. The marks on all the cams, and the crank, are perfectly aligned when it is done correctly, and the belt is tensioned properly.

You advance the cam 1/2 a tooth so the belt will go on, once on, you retard it back and it aligns perfectly. Then you set the belt tension per the manual.


HTH,
LarryB

 

[myhui]

Another counter-intuitive design is the tensioner pulley.

Design intent is to only let the spring supply the tension, but during adjustment only. Once tension is set, tighten the pulley bolt so the pulley's position won't move, and the spring action is defeated. That is quite unconventional. That's why the spring action must be allowed to operate at two different angles of revolution of the crankshaft, where the off-TDC angle you check last has maximum slack on that part of the belt, so that the spring will pull it tight at that point and that's as tight as it'll ever need to be.

I hope I have that right. I tend to study the manual 4 times and study the car 3 times before I take apart anything.

 

[nis350]

This is very simple. The rear cams are off because timing belts are installed incorrectly, because techs do not follow the procedure exactly. The marks on all the cams, and the crank, are perfectly aligned when it is done correctly, and the belt is tensioned properly.

You advance the cam 1/2 a tooth so the belt will go on, once on, you retard it back and it aligns perfectly. Then you set the belt tension per the manual.


HTH,
LarryB

I think I got it now. So the main reason to advance the rear exhaust cam by 1/2 tooth is just to install the belt properly.

The final result is that the timing marks on both cams on the rear bank would line up just like the front bank.

I guess it would not be possible to put the belt on properly if you start with both rear cams having the timing marks line up?

Thanks for the clarification. :wink:

 

[drew]

MyHui's you never broadcasted what your actual symptoms actually are?

Remember: A well defined problem is half-solved.

One of my main hats is software engineering, I tend spend most of my time defining what the problem actually is (the process drives the customer nuts), but you cannot give a solution until you know what you're solving for.

You keep asking about "solving" the timing belt, which might mean you might have already jumped to a solution without really defining the problem.

Drew

 

[Daedalus]

The final result is that the timing marks on both cams on the rear bank would line up just like the front bank.

Not with the belt taut, no. If they did you wouldn't have to advance the cam.

 

[Larry Bastanza]

I think I got it now. So the main reason to advance the rear exhaust cam by 1/2 tooth is just to install the belt properly.

YES THAT IS CORRECT. You cannot slip the belt on, if the rear exhaust cam is at TDC position, you must advance it first, slip the belt on, then retard it back. When it is set correctly ALL MARKS LINE UP PERFECTLY. Without even setting the tension I get the rear cams to line up at this point of the procedure.

Belt tension is set by the spring, follow the book, turn the crank to the blue mark, and set it as stated. After that, turn the crank CLOCKWISE to TDC #1 and check the marks. All this should be done with the plugs out, so there is NO compression and the engine will turn smoothly and easily.


HTH,
LarryB

 

[Larry Bastanza]

Not with the belt taut, no. If they did you wouldn't have to advance the cam.

Yes they will.

LarryB

 

[Zennsx]

This is very simple. The rear cams are off because timing belts are installed incorrectly, because techs do not follow the procedure exactly. The marks on all the cams, and the crank, are perfectly aligned when it is done correctly, and the belt is tensioned properly.

You advance the cam 1/2 a tooth so the belt will go on, once on, you retard it back and it aligns perfectly. Then you set the belt tension per the manual.


HTH,
LarryB

What exactly retards the belt? After thoroughly reading the manual I was always under the impression that tensioning the slack out is what accomplished this. Please clarify.

 

[Larry Bastanza]

What exactly retards the belt? After thoroughly reading the manual I was always under the impression that tensioning the slack out is what accomplished this. Please clarify.

What retards the belt is putting it on a tooth off, because the rear exhaust cam was not advanced as specified in the procedure.

HTH,
LarryB

 

[Zennsx]

What retards the belt is putting it on a tooth off, because the rear exhaust cam was not advanced as specified in the procedure.

HTH,
LarryB

Larry ,you misunderstood my question. After advancing the rear exhaust cam .5 tooth or so does tensioning of the belt then cause it to then align perectly. It sounded as though I was missing a step

 

[Larry Bastanza]

The cam gears are about 1" thick, the same width as the belt. If you thread the belt in the sequence specified the belt will not go on the last gear(rear exhaust cam gear), because of the angle of the belt since it is completely installed on the other gears and pulleys. Advancing the rear exhasut cam gear 1/2 tooth, allows the cogs on the belt to align and you can slip the belt on. Once it is on, you retard the rear cam pulley to tension the belt in-between the rear intake cam gear and rear exhaust cam gears and the marks will all align nicely.

The most important thing is to set the tensioner pulley to FULL LOOSE, tighten it, install the belt on ALL pulleys, then unlock the tensioner(spring loaded) and let it take up the slack. I like to pull the tensioner as tight as possible at this step. Then tighten the tensioner, then rotate the crank 720 degrees, and check the marks again. They should be all aligned at #1 TDC. Then perform the tension adjustment as stated in the service manual.

HTH,
LarryB

 

[Zennsx]

The cam gears are about 1" thick, the same width as the belt. If you thread the belt in the sequence specified the belt will not go on the last gear(rear exhaust cam gear), because of the angle of the belt since it is completely installed on the other gears and pulleys. Advancing the rear exhasut cam gear 1/2 tooth, allows the cogs on the belt to align and you can slip the belt on. Once it is on, you retard the rear cam pulley to tension the belt in-between the rear intake cam gear and rear exhaust cam gears and the marks will all align nicely.

The most important thing is to set the tensioner pulley to FULL LOOSE, tighten it, install the belt on ALL pulleys, then unlock the tensioner(spring loaded) and let it take up the slack. I like to pull the tensioner as tight as possible at this step. Then tighten the tensioner, then rotate the crank 720 degrees, and check the marks again. They should be all aligned at #1 TDC. Then perform the tension adjustment as stated in the service manual.

HTH,
LarryB

Oh ok ,now I see what your getting at. I was always a little foggy with the manuals explanation or lack thereof as to how this
works. Thanks for the clarification Larry.