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  #1  
Old 01-18-2002, 10:00 PM
jerryrigged
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Question unsteady valve clearances

I've recently bought a 1983 300D with (allegedly) about 140,000 miles. Upon checking the valve clearances, I found that there were none. All the valves were tight.

After adjusting the clearances, some of the valves' clearances would actually go tight again upon rotating the valve a bit. Do the adjusting cap nuts and/or rocker arms need replacing? Any advice will be most appreciated. Thanks!

Jerry
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  #2  
Old 01-18-2002, 11:30 PM
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Join Date: Jul 2001
Location: San Antone
Posts: 408
Since you say this happening when you turn the valves after adjusting them, I would think the valves have worn in the one position and when you rotated them the valve face to seat contact point is changing. You may need to adjust the valves slightly loose doing the go, no go method (adj. the valve with the correct size feeler gauge slightly loose, and then check with the next thicker feeler gauge so it will go in with some force - but not too much force). Then, take the car for a spin (say 10-20 miles) and re-adjust the valves after the engine has cooled off for the cold adj. clearance.

The valves on my non-turbo 300D have a device called valve rotators and I believe the turbo 300D's have them also. The valve rotators do just that, they rotate the valves so they wear evenly and maintain a good seal. Since your valves had no clearance, there is a chance the valve rotators were not able to do their job and this is causing the change in valve clearance when you rotate the valves after adjusting them.

I don't know how you are adjusting the valves, but when the adj. nuts are tightened the clearance will change so you have to compensate for this. You will have to get a feel for this, sometimes tightening the bottom nut up will tighten (or reduce) the clearnace, tightening the cap or top nut down will loosen (or increase) the clearance, and tightening both about the same will keep about the same clearance.

You may need to adjust the valves again in a few hundred miles (or sooner if they start making noise) because they may wear in a bit changing the clearances.

Also, look to see if there are grooves worn in the rocker arms. If there are grooves, then you need to make sure you are measuring the area where the cam shaft contacts the rocker arm and not the the non-grooved area. I've looked at several non-turbo 300D's and have noticed that the rocker arms for cylinder's 1 & 2 will get grooves, but cylinders 3, 4, & 5 will not have grooves. I have looked at turbo 300D's and the valve train assembly looks similar to the non-turbo - so the turbo rocker arms may get the same grooves, but I have not done a thorough examination so I am extrapolating. I saw the same grooves on some Honda motorcycles (overhead cam and rocker arms) and all of a particular model would get grooves in the rocker arms - even if the valves where adjusted before the engine was ever started and then adjusted per the interval required and, in some cases, more frequently. The Japanese engineers could not or would not answer why this happened. Has anyone else seen this and, if so, did you ask the dealer, the M-B Rep, or say a M-B engineer why this happens?

Good Luck!
Tom
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America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
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Supervisor of Maintenance largest tree care co. in US for offices in Tex.

Last edited by tcane; 01-19-2002 at 09:58 AM.
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Old 01-19-2002, 11:06 AM
jerryrigged
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Hi Tom. Thanks for the info. I didn't consider the possibility of worn valves/valve seats, but was thinking along the same line concerning the adjusting cap nuts/rocker arms.

If the valves/valve seats are worn in the one position and assuming there is a valve rotator device, can maintaining proper, or even loose, clearances eventually correct this?

I'll recheck the clearances soon, and also take a close look at all the external parts. BTW, the car ran well before the valve adjustment and continues to do so along with an easier start. All the best!

Jerry
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  #4  
Old 01-19-2002, 12:17 PM
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Location: Evansville, Indiana
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Jerry:

Addjust valves 5-10 thousandths loose and drive a bit, then adjust again, still loose. It will take some time to get the valves seated again, and if they are really bad, you will need to do a valve job .

If the clearance changes with valve rotation (check to make sure you are not just screwing the cap nut!), set them at the point where you have the least clearance.

Neglected valve adjustments are hell on these engines -- allows the valve to burn, causes excessive wear, etc. I've heard of W123s going 300,000 miles without having the head removed, but also have seen valves completely shot at 60,000 from no adjustments!

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #5  
Old 01-24-2002, 12:16 PM
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Jerry:

I looked at my CD-ROM 300D engine manual and found that your turbo engine does have the rotating device for the valves. M-B calls the device a Rotorcap.

With luck, the valves may work OK so you don't have to R&R the head to do a valve job. Sounds like you adjusted them slightly loose and that will help a bit, but if they were tight without any clearance for a long time then who knows. The Rotorcaps should work better now that there is valve clearance and that should help to regain valve to seat sealing as the Rotorcaps rotate the valves.

Peter's suggestion to use really large valve clearances (up to 3 times intake valve clearance and nearly twice for the exhaust) to let the valves pound themselves into the seats to regain sealing integrity could be an option. However, you have already adjusted the valves with more clearance and I would allow some time to pass (several hundreds of miles) with 2-3 or more valve adjustments to see if they are OK. On racing motorcycles and some cars with solid lifters there were cams made to operate with greater than normal valve clearances, these engines all had two valve springs per valve to control the valve when accelerating off the valve seat when opening and deaccelerating when closing - and these were racing engines using camshafts designed for greater valve clearances.

The M-B diesel engines have one spring per valve to control them (this same spring is also used on some M-B gas engines as the outer spring in a two spring per valve set-up to control the valve). The suggestion to use much greater than normal valve clearances means the valves will be accelerating and deaccelerating much faster than normal without the camshaft helping to control the valves during a good portion of the valve's travel. The lack of valve control will mean the valve will accelerate much faster than normal and I am concerned about uncontrolled acceleration causing the single valve spring to momentarily lose control of the valve's speed resulting in possible contact with the top of the piston (there are really tight piston to head clearances to achieve the high compression ratio of 21.5 to 1 in your turbo engine - about 0.50 to 0.90mm). Similarly, on deacceleration the camshaft will not control the valve for a longer time and the valve will have greater speed than normal when it contacts the seat. The valve spring will probably allow the head of the valve to bounce off the valve seat upon initial contact (perhaps more than one bounce). At low engine RPM's these valve control concerns are less, but as engine RPM's increase these control concerns will increase - even if the engine is only operated for a short time or driven a short distance.

I would do a compression test before using much greater than normal valve clearances (and after runing this engine for several hundred miles and doing several valve adjustments). With the engine at normal operating temperature, compression should be 350 to 400 psi (24 atu or more) with a minimum of about 300 to 325psi for decent engine operation (M-B info and comments by Steve Brotherton the moderator for the Tech's Only forum). Compression reading may be affected by the compression tester used, so some leeway should be considered. If the compression is low, then a pressure/leak-down test can be done to see if the problem is with the valves (listen for air movement when listening at the intake manifold/air cleaner and exhaust pipe, rings listen at the oil filler opening, head gasket leak into the coolant jackets listen at the radiator filler). If you have low compression and/or the pressure test shows that the valves are not sealing, you could then try the much greater than normal valve clearances to try and get them to seal by pounding in an attempt to avoid taking the head off to do a valve job. You should realize that more damage could occur if you use much greater than normal valve clearances. So, you will need to weigh the risks vs. benefits vs. costs.

Hopefully, running the engine with slightly loose valves with several valve adjustments in the next few hundred miles will restore valve to seat sealing and the above info will not have to be considered and/or done. However, you will not know until you put some miles on this engine and see what happens.

My $0.02 worth!
Good Luck!!
Tom
__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.
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  #6  
Old 01-24-2002, 09:55 PM
jerryrigged
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Hi Tom. Hi Peter. Again, thanks for the insight and also for the warning. The car has been running well, so maybe there won't be a need for a valve job anytime soon.

Haven't rechecked the valves yet, but when I do - quite possibly this weekend - I'll determine the difference between points of least clearance and most clearance, and then adjust for normal clearance at the point of least clearance. Hopefully, there won't be any unpleasant surprises in any of the least/most differences.

Oh well, if a valve job is needed, a valve job is what she'll get. Take care all.

Jerry
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