W108 engine help

[R72benz]

I finally got around to adjusting the valves in my 280SE, and Tomguy was correct. I was able to make the adjustments with a good crow’s foot attachment so gave up my search for the special tool. Some minor adjustments on all but two valves. Two required a good quarter turn, so glad I made the effort. Easy job, and car ran great afterwards.

However, when I had the air filter housing off I noticed the vacuum hose to the manifold pressure sensor was not clamped to the sensor. An easy tug and it came off. I figured that can’t be right so I clamped it. When I started the car it ran higher rpm than normal. Thinking this was just the cold start valve, I drove it a few miles until up to temperature. It still ran significantly higher idle rpm. Maybe 1200 rpm idling.

I went back and looked at Jerhofer’s pics since his engine was the same and he did such a great job of documenting his work. His car did not have a clamp on the MPS device as best I could tell.

Can anyone explain what caused this condition?
Was the clamp a bad idea?
Put it back the way it was and leave it alone?
Never touch the MPS?
Does not seem a clamp should cause such a change (see pic).

[Idle]

The clamp could not have made any difference. The reason there is no clamp from the factory was because it was not needed. The connection is a vacuum connection. It is constantly sucking the hose so the hose is constantly under pressure to stay on the connection point.

Later 450 and 280's, the 110 were full of vacuum connections. None of them ever had a clamp. When the engine is running they are difficult to pull off.

So the lack of a clamp was a cost issue.

But back to the idle situation: I think after the valve adjustment the engine ran correctly. But previous owners, who didn't want to take on the task of adjusting the valves, just tweaked the idle adjustment to compensate for an engine that was not idling as it should. They thought adjusting the idle would fix the 'problem'. No, you fixed the problem. Now you must correct their short-cut moves.

And using the crow's foot is the way to go. The special tool you mentioned is supposed to be used with a torque wrench when doing the adjustment. If moving the adjustment screw doesn't take enough pressure then the screw is not going to hold. That way you know that screw needs replacement with an oversized one that, at least at one time, was available from Mercedes.

I doubt they sold many of these replacements. I don't know of anyone who ever had an adjustment screw work loose. I guess they could.

And the special tool? I have seen a few of these. They have always been ground down a bit as the original tool is a bit too fat to fit. So mechanics will grind off a few mm from the outside face. It makes all the difference in the world.

[R72benz]

Thanks Idle, I think you are spot on. After pondering it some I could not imagine how the clamp would cause this change in performance. I know I had one exhaust valves not closing completely with two significantly out of adjustment. I had it running fairly well, but now feel like I need to go back to square one.

And I agree after adjusting 16 valves that all took effort to turn, I can’t see how they could work loose under normal operating conditions. But there is a reason MB recommends valve adjustment every 10k miles...wear on the rockers maybe?

Anyway another challenge ahead and thanks again.

[cth350]

The EFI system is very sensitive to vacuum issues. If you don't know how old the rubber vacuum lines are, start replacing them; especially the MPS one. It's possible it is a bit porous.

If you should happen to be able to get spares easily, you can discover the joys of adjusting the MPS setting. You can make your 4.5 run like a scalded cat with zero fuel economy that way. A more careful approach requires a CO% gauge.

Search the forum for "D-Jet", "EFI", "Jetronic", etc and you'll find lots of good tech threads.

-CTH

[Tomguy]

The valves require adjusting on cars without hydraulic lash adjusters because of not just cam/rocker wear - the primary cause is actually valve seat wear. The valves themselves wear as do their seats, which is why they usually wind up "Tight" on the rockers (open slightly when they should be closed).

If you had valves not closing, you were losing a fair bit of power, as the combustion was leaking out. If it was leaking into the exhaust it's power loss, but if your compression or combustion leaked back into the intake manifold, you wound up with higher manifold pressures as well, which really throws DJet for a confusing loop. Now that your valves are set properly, check your timing, dial it in right if needed, and THEN adjust your CO mixture at the tailpipe with an old-school sniffer or a wideband O2 sensor (there are DIYs on how to make your own sniffer with one, but it probably costs more than finding an old mechanic to take 30 mins to dial you in).

[R72benz]

Thank you. I have set the timing at 6 degrees BTDC not as specified but that seems to be the smoothest setting. When you say dial in Tomguy are you referring to the MPS adjustment as CTH suggests? Turning the idle adjusting knob on the ECU does not make a lot of difference. I have it set at mid range. I have access to a CO meter, so dialing in will be easy. Just want to make sure I am on the right track.