the blues

[tom 114-75]

I'm new but have been reading your threads, so I'm not too new and ignorant.

1975 280 gas 114. car runs great but if I try to get on the freeway and go for a few miles at 60 mph or higher, after about 3-4 miles big intermittent plumes of dense blue smoke come out. I pull off the freeway asap and especially if its a downhill ramp I'll get more smoke. Then from that stop accelerating out - more smoke - then driving along and no smoke, like everything is ok. Another minute goes by and it does it again.

If I'm driving in town this doesn't happen at all. I need to be sustained 50+mph to get it to happen and the car has already reached operating temp.

I see talk about vacuum box (modulator), diaphragm, etc. If there is someone out there that recognizes this could you please respond and explain. Thank you if you can.

Tom

[BWhitmore]

From your description it seems that the problem occurs when the engine has high vacuum (50 mph and going down hill). Blue smoke generally means oil is burning. Does the car smoke after idling at a stop? I would suspect worn valve guides or valve guide seals which allow oil to be drawn into the cylinders and burned during times of high vacuum (idling, 50 mph, going down hill). Also, do you have unusual oil consumption? The other possibilty is a faulty transmission modulator where the engine is drawing trans fluid into the engine. Trans modulators are fairly cheap and easy to change. I believe you can disconnect the vacuum hose (plug the hose) from the modulator and drive the car (shifting will be pretty harsh). If the blue smoke stops then you have found the problem. The only proper way to check valve guide wear is to remove the cylinder head and remove the valve springs. Valve guide wear normally occurs near the head of the valve and is checked by wiggling the valve in the guide. The valve guide seals are checked with the valve spring removed. Valve guide seals should be flexible and have no cracks. Finally, if your master cylinder is faulty and allowing brake fluid to be drawn into the power brake booster this fluid will then be drawn into the engine and burn like oil. Check the fluid in your master cylinder and also check the inside of the vacuum hose going from the engine to the power brake booster for any signs of brake fluid (it should be dry).

[tom 114-75]

Hmmm, very interesting. There is another problem with the operation that I did not know was related, but it might be.

The brakes are a bit squishy at the top but work well at 30% depressed or more and do everything right - but sometimes when I'm warming up (I always try for at least 2 minutes) I touch the brakes to go from 'park' to 'drive' and the car stalls and it appears that all the gauges shut off at the same time. Most of the time this doesn't happen. It appears random. I don't need to depress the brake to shift, like newer cars do so I don't touch the brakes if I'm going forward, but if my plan is to reverse out of my spot it just stalls and I can't do anything about it. I actually have to push it out. Once I get the car going down the road the problem immediately is usually gone.

I am pretty sure that every time I experience the blue smoke thing, before
I can get home, this dying thing will happen when I hit the brakes.

I will repeat the one thing that is always true though, the blue smoke never happens, not at all, if I'm just driving around town. The only thing that will trigger it is a few minutes of freeway driving over 60 mph. Usually 3 miles. The car always runs perfectly, even when smoking. Just purrs .. well for the loud 110 that it is it may not quite be a purr.

I will do the test you suggest. Do you know about the vacuum box or trans modulator and cracking diaphragm allowing trans fluid spray to siphon back into the combustion chamber? Someone mentioned this.

Thanks for responding.

Tom

[tom 114-75]

And sorry, yes I have watched fluid levels and it doesn't appear that oil, trans, or brake fluid is being unusually drained. They could be a little

Also, this is my first time writing in any forum so I didn't know I could scroll down to review what you wrote before I sent - and I see you did suggest the trans modulator, excuse me. I hope that is it. Test coming this weekend.

Thanks, Tom

[Frank Reiner]

tom:

If the engine is equipped with a vacuum pump for the brakes (mounted at right front of cam box), look to it as a source of the oil that is being burned. The diaphragm in the pump eventually ruptures, allowing engine oil to be drawn into the intake manifold.

[tom 114-75]

Yes, it does have a vacuum pump. I see it with the clear/yellowed hose going in and black one going out. It looks like easy access with 2 flat screws to get in. I would assume that I should not see any fluids in either of these hoses as the pump should be pumping air only? If I do I'm getting blow-back which ends in the manifold for burning?

Thanks, BWhitmore and Frank.

Tom

[kraakevik]

Tom

Frank has almost certainly hit on the problem--it's likely to be a leaking vacuum-pump diaphragm.

If you want to repair it get a diaphragm kit from Mercedes or aftermarket vendor--you'll need to unfasten the circular housing with the eight or so flat-head screws to get at the diaphragm--the valves protrude from the housing and are unlikely to need attention.

The pump can be removed entirely--Mercedes did so in 1976--with the cam-tower opening covered with a stamped-steel plate. The plastic piping from the intake manifold is then routed to the brake booster with an in-line check valve.

Let me know if you decide to remove the pump entirely--there are a couple of precautionary measures necessary--good luck



Tim
kraakevik@voyager.net

[tom 114-75]

Thank you very much. I'm liking the consensus, and that it is an easy to get at item for me. I'm on it!

Tom

[Idle]

Don't drop any screws when removing the vac pump. If they land behind the crank pulley it is a time consuming job to fish them out.

A little care at that point will save a lot of time later.

And these pumps are actually rather easy to rebuild. Once you get it off you will see there is not much going on in there. As repair jobs on a Mercedes go this is not one of the harder ones.

[tom 114-75]

Great. thanks. I usually lay an old white towel down under whatever I'm working on.

[tom 114-75]

Well gentlemen, I got a new diaphragm from Pelican and installed it Sunday. Everything went well - I think. The housing would not come off the body easily. I figured that either it's been on a long time and is 'time-glued' on, or it is actually glued on, or inside the little top parallelogram box where the valves are there's another screw holding it on. I know you said those valves are rarely troublesome but I didn't want to begin to force it until I knew there were no more screws so I opened that up too. I do not have a shop manual yet and would have looked in it if I had one but figured this was going to be real easy.

So anyway, when I opened it up of course I saw it spring loading open and applied back pressure with my hand and everything opened slowly. But one of the valves toppled out of its bay when I lifted the spring out and I'm pretty sure that one valve is inverted and one not. I put it back as I thought I saw it come apart. So first question is; are these valves installed opposite (one right side up and one down) in order to create this vacuum?

Next, the diaphragm was hard to get out from between the 2 small metal plates that sandwiched it. Once I got them apart I realized I did not keep track of how they were flipped over. You see one of the 2 plates is smaller than the other but both have flared beveled edges all around their margin. I looked at the set up and reinstalled them with the bevel up against the soft diaphragm. This way the square edge would not start working a groove into the diaphragm. Second question; does it matter which way these plates go on?

So I go for a test slow test drive and everything is fine but the motor sounds a little louder. I'm worried that I didn't do one of these things right and I'm applying pressure or vacuum in the wrong place - or those little plates are touching something.

Ideas?

Tom