Cam Slip

[Tobal]

I was curious if the NSX engine tend to have Cam gear slips. Because when i bought my car a year ago. I noticed that the timing was off by one tooth. Just wondering if it was because someone installed installed the timing belt wrong or Did it actually slip.

 

[Jimbo]

I bet someone installed it wrong.

-Jim

 

[Larry Bastanza]

I agree with Jimbo,


Installed wrong. Rear exhaust cam?? I think the belt will only slip if the tension is too loose, and I would have a hard time believing it would slip only one tooth.

It would slip over the rear intake cam MANY teeth

My $.02
LarryB

 

[daveh]

It's not uncommon for it to slip a tooth if you turn the motor the wrong way(anticlockwise) while doing a valve adjust .Especially if it's an old ,loose belt .

 

[DanO]

There are certain cam/engine positions where the t-belt is slightly slack at the rear cam and if the car is on a severe slope and in gear and the parking brake not on and the car pointing down the slope in such a way that the transmission is in the wrong gear direction (so that if the car rolls the transmission will want to spin the engine backwards) and if the driver is using the engine as a brake and just shutting off the engine when parking and releasing the brakes so that the car rolls a bit and gathers some inertia before the car gets a chance to stop itself from rolling down the hill by the tires engaging the transmission which engages the crankshaft which is connected to the crankshaft t-belt pulley which is connected to the cam gears which eventually keep enough valves closed so that there is enough compression to keep the engine from spinning and the car from rolling down the hill. If the car has enough inertia, it could skip a tooth.

For example: If the car is being parked on a slope such that the rear of the car is facing down the slope and the driver wrongly selects 1st gear (instead of reverse or vice versa) to hold the car in place and does not elect to use the parking brake (wrong too) and abruptly lets off the brakes after turning off the ignition and lets the engine/drivetrain absorb the inertia of the roll, the engine will spin backwards (whoops!) and if the cams are in such a position so that the t-belt is somewhat slack on the rear cam, skip, skip…it’s just like spinning the engine backwards on the crankshaft pulley.

Happy parking,

DanO

 

[daveh]

damn

 

[Larry Bastanza]

Hey DanO,

Do you think this could happen if the belt is tensioned properly?


Thanks,
LarryB

 

[docjohn]

On a related note guys I just had a valve adjustmnt at 53K miles,~20k after the IEM and the exhaust valves on the rear bank were tight as per my tech ,,My ? is would this have degraded performance in any way or led to any > stress if not corrected?

 

[DanO]

Hey DanO,

Do you think this could happen if the belt is tensioned properly?


Thanks,
LarryB

Hi Larry

Yes indeed, I believe it could happen (I’m about 85% sure) but the cams would have to be in an exact position and the speed of the car and/or slope would probably have to be radical. There is a cam position where the rear exhaust cam is sprung so that it wants to rotate counterclockwise such that there is some relative slack in the t-belt between the rear exhaust cam pulley and the crankshaft pulley. Next time you’re doing a t-belt, watch the tension in the belt in this position as you rotate the crankshaft and notice the tension variation—it varies quite a bit at this spot. Side: there’s something specially related to that 9th tooth (on the crankshaft pulley) past TDC mark and either max tension or min tension on the belt at the tensioner (see SM pg. 6-22 step 5)—wonder what exactly is happening there? Anyway, the tension varies between the pulleys at different times depending on the direction and orientation of the spring force on all four cam pulleys.

From personal experience in trying to remove my crankshaft pulley bolt, there are certain cam positions that will certainly skip a tooth on the crankshaft pulley without too much counterclockwise torque on the wrench (under 200 lb ft, probably less than 100 lb ft), so it shouldn’t take too much force at the wheels to make enough torque at the crank to skip teeth.

One question I have: can the engine naturally shut off in such a way that the belt is in this “loose” combination or will it always shut off so that this portion of the belt is tight. If this portion is naturally tight when the engine shuts off then it would probably take a pretty severe act to skip the belt…like trying to stop your car from rolling down the hill backwards at 25mph (when the ignition is off) by putting it in first and popping the clutch.

.01

DanO

 

[DanO]

On a related note guys I just had a valve adjustmnt at 53K miles,~20k after the IEM and the exhaust valves on the rear bank were tight as per my tech ,,My ? is would this have degraded performance in any way or led to any > stress if not corrected?

From an extreme perspective, if the valve clearance were WAY too “tight” this would mean that the valves may not entirely seat when in the closed position and the valve train components would experience a high degree of wear—clearly this was not the case with your car. Most likely the valve clearance was a very little off (or you tech’s measurements were), like .001” and that it resulted in negligible performance and/or wear differences. I suppose the tighter things are, the more wear and tear and heat and friction the valve train experiences, and the looser things are then the valve train may become a little louder and the valves will have less lift. My guess is there is no measurable difference in performance or wear&tear on the valve train as long as the clearances are within the service limit. In you case, I’d expect there to be more error introduced in terms of setting/measuring clearances then there is in actual wear and tear as wear and tear would most likely result in a looser clearance no a tighter clearance.

.02
DanO

 

[Larry Bastanza]

Hi DanO,

I am familiar with the procedure and use it always. Ins't it the 9th tooth on the "Camshaft" though . Which mean it is 18 teeth on the crank, I think. (I know when I do this the blue mark seems pretty far away from TDC). When I do this procedure, although it is not specifically mentioned, I have all the plugs removed. I do this to insure an even steady "pull" on the belt as I bring it to the blue mark. I think this rotation from the white to the blue mark is to insure all the slack is between the rear cam gear and the tensioner/crank pulley. So when you release the tensioner, the spring can take the proper slack out. I will check next time to see the variance in the slack.

I wish we had this conversation last week, since I just did one.

Hi John,

I agree completely with DanO on the valves. Typically I have seen valves both tight and loose, by .001-.002. It is when the clearance is gone that you can have trouble. Dan mentions about "less lift" when loose, and it also makes sense "more lift" when tight. But this is all negligable if we are talking about .001 or so. I think the key is even settings to insure the smoothest balance of air flow between cylinders, therefore the smoothest running engine

My $.01, so between Dan and I, you have "our" $.02 LOL.....

(Maybe $.04 who's counting!!!!!!!!!!!!!!)

HTH,
LarryB

 

[DanO]

Hi DanO, Ins't it the 9th tooth on the "Camshaft" though . Which mean it is 18 teeth on the crank

Dang, now that's twice I made that mistake

minus .01

DanO

 

[docjohn]

Your 4 cents are worth a million to the rest of us!Thanx for always helping the hapless!