About half way through my stem seals, I'm changing my air hose/spark plug adapter to the next spark plug hole. Except I'm having a problem keeping air pressure up. It's bleeding off through the air intake of the fuel distributor. So I rotate the engine thinking I will find a spot where all the valves are closed. I rotate that engine about 3 turns and never got to a point where I wasn't losing air. I know I need 100 psi or so to keep that valve up, don't want it to drop into the cylinder. But I'm dropping pressure, 80 to 60 to 40. I can maintain 40 but I know this isn't enough. Somebody please ring in and call me the dumb ass I feel I am. What in the world am I missing? Right now I've stopped in the middle until I can clear this mystery or until I put it back together and just run it. Thanks in advance.
Jet

When the cam lobes for the cylinder you're working on are not positioned to depress either of the valves, then your cylinder should hold pressure. If the 420 top end is anything like the 560 top end, then getting air through your fuel distributer means your intake valve for that chamber is open. If the lobes are pointing away from the engine (ie not pressing down on the valves) then you should not be losing much air (if any, at least from the top end) and your guage should be pretty steady.

Good luck!

It is important to get the engine to TDC compression stroke for the cylinder you are working with otherwize you will blow the piston down to a point where the valves open. Bring the piston to TDC compression stroke using your distributor or you thumb on the plug hole as a guide. If you can see the piston get it all the way op top by eye. You will have to feel for the exact TDC by rocking the crank back and forth to feel for the piston going over center. It's a very suttle feel. At this point the piston will have no leverage to rotate the crankshaft when you apply pressure. Also 100 PSI is high, you might be better off reducing pressure below 40 PSI just in case you are slightly off TDC.

John Roncallo

Czar, I've rotated the engine so many times it's crazy. I've had the camshaft with neither lobe pushing a valve in, and I still get air leakage. I have bumped the engine around degree by degree and I can almost get it stopped but still have a little bit of an air leak.
Jet

The cylinder is not perfectly sealed an there will be leakage. You need enough compressor flow/stored air to maintain some pressure, but you don't need 100 psi, and 20 psi may be enough.

If the engine is at TDC and a valve drops you should be able to pick it back up by the valve stem. Mark the balancer in 120 degree increments, then starting with #1 TDC, move the crankshaft in 120 degree intervals and change the seals in the firing order.

Prior to compressing the valve, place about a 3/4" socket on the valve retainer and give is a tap with a hammer. This will break the retainer loose from the locks and allow the spring to be compressed without taking the valve with it.

Duke

Duke is right. No way air is going to stay perfectly sealed. One of the compressors jobs is to keep it at a given level throughout the task.

I may stand corrected, but I could swear I recall the pressure spec being 5 bar(approx. 75 lbs).

You may have seen this, but here's a thread that may help. There are many. Search for the word "stem" and "titles only".

Valve stem change in 300E

Could the intake valve be leaking?

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2012 E350
2006 Callaway SC560

jamesjetton:
I just did my 91 560 SEL about a month ago. Changed stem seals but did not have to use air pressure. All I did was make sure the cylinder I was working on is at TDC. I used the TDC whistler to determine TDC, this way I did not have to remove the fan. I rotated the crank from below.

How did you keep the valve from dropping?

when you are at TDC, the most that the valve would move downwards is about a 1/4 of a inch. I had plenty of room to compress the spring and be able to drop the keepers in place. Actually, turning a cylinder to TDC on one head would also set another cylinder to TDC in the other head, so you can actually work on 2 cylinders(one at a time) with one TDC turn. I used a plastic piece of stick to push( on the metal sleeve) the seals down in place. I realize that having compressed air to keep the valves up would make it a lot easier to put the keepers back in, but in my case, it worked just the same. I just made sure that the car was secured not to move. Battery was also disconnected.

If you're using compressed air, you should leave the breaker bar/socket on the bolt and bungee them up so it wont move.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

What does this have to do with stem cells?

What engine is this? Duke2.6 gives good advice for marking the crank pulley if you have a 6 cyl engine. A 4 cyl engine requires marks at 180* and an 8 cyl engine requires marks at 90*.

Sixto
95 S420
87 300SDL

Could it be a burned valve? I know that doesn't usually happen on the intake side, but it sounds like it. Also, it's very possible that a spec of carbon or something is preventing a good seal.

I just did my valve stems but I pulled the head out. I did find that on a couple of valves I had to put a piece of pipe over the valve spring and smack it gently with a hammer to unstick the valve keeper.

You could also try spraying some oil down into the cylinder in the hope that you might get lucky and get some oil into the valve/seat area and help make a better seal. Can't you just unbolt the rocker arms, then it wouldn't matter where the engine crankshaft is? (admittedly I've got a different engine).

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89 300te 222,222
92 300e 190,000

Thanks to all who replied. I retraced my steps and on cylinder #2, the valve stem seal was not seated. It had actually moved up to about half way up the valve stem. I could have sworn I seated it well. I had purchased a madrel from Performance Products and it works well. Anyway, I replaced the seal on #2 and the car runs much better. I still have a slight miss, I believe may be vacuum related. So I continue....
Jet

Did you do a compression check on this car. You have a misfire and you cant get the valves to seal. Maybe you have a bad valve.

John Roncallo