1973/450sl Testing vacuum advance - PeachParts Mercedes-Benz Forum

zeggate · #1 12-26-2003, 08:00 PM

I have solved my central locking system problems. Now I want to tackle my poor gas mileage and miss. I sprayed carb cleaner on the injector hoses while the engine was running with no change in idle. I unplugged the vacuum line from the distributor and the car seems to run smoother. I used a mityvac and applied vacuum to the distributor advance ,it would not hold vacuum . Is this asign of a faulty unit .Don't laugh folks I am not a mechanic just a committed DIY-ER. Thanks for any help in advance

Tomguy · #2 12-26-2003, 09:22 PM

Not really (I'm saying this because that's the way mine is).
It should have some, but not too much resistance to it (if it held vacuum indefinetly, when the advance is supposed to switch off, it'd keep its vac and not turn off). If your idle changed, it's working properly.
Did the car run smoother with it pulled, or seem to because it idled faster? Yoru timing may be too retarded (most American cars tell you to set w/vac line pulled, so if a mech didn't read/believe the sticker and set it to 5°ATDC with the line pulled, it'll be a PITA with it connected - 12°ATDC is WAY too late!).
If you have a timing light, set it to 5°ATDC with the car warm and at idle. Disconnect the line, you should see the timing jump (to about 7°BTDC). If this cures poor mileage and the miss (and I bet it would) then bingo!

zeggate · #3 12-27-2003, 05:34 AM

You may be on to something because the car ran smoother because she idled faster. I'm a bit confused because another member advise that the advance unit should hold vacuum.

ctaylor738 · #4 12-27-2003, 10:39 AM

Just because it won't hold vacuum doesn't mean that it isn't doing at least part of its job. But no way is the leak going to get better, and you are not going to be able to get the system to spec with it leaking.

Also - try spraying some carb cleaner around the injectors and see if you have some vacuum leaks there.

Second also - once you veriify the vac on - vac off settings, bring the RPM's to 3000 with vac on and check that you get 18 -22 degrees BTDC. Those settings will verify that the vacuum and advance mechanisms are working correctly. Then I would try playing with the advance and see how the car runs. I would guess that premium fuel and another 10 degrees advance would put some zing in the old guy.

CT

Tomguy · #5 12-27-2003, 10:59 AM

Chuck - mine doesn't really hold any vacuum. It works fine, but just doesn't hold it for any time - which I would think is how it's supposed to be, it cuts off instantly when it's supposed to. The way it is now, with the vac, it causes the ignition to be fully retarded - in other words, it won't move any further by hand than it does with the vac.
You sprayed it on the injector hoses? That wouldnt' change idle (AFAIK) if you did have a leak. Spray it around the bases of the injectors and around the manifold seals (the intake is 2 manifolds placed together with 8 plenum seals - spray around these seals or, if you can't see them, spray blindly in the areas of the manifold). Any change indicates a leak (as would a hissing sound!

)

psfred · #6 12-27-2003, 11:25 AM

Things I've done:

New cap and rotor. Make sure wires are good and correct for the plugs.

Pull plugs and inspect for condtion -- dark is too rich, white is too lean, should be light tan or gray. Ash means too much oil, need to find and fix the issue (normally valve guide seals).

Verify that the mechanical advance works -- you must be able to turn the rotor clockwise about 15 degrees and have is snap back on a spring -- you will hear the advance weights swing out. If not, get some PB blaster into the center of the shaft after pulling the rusty felt out and work the shaft back and forth until it moves freely. Add a drop or two of ATF, then a drop of engine oil every 3000 miles. Clean the felt and re-install.

Install new points, set to correct dwell (30 plus or minus 2 degrees), set ignition timing to 7 BTDC without vac or 5 ATDC with vac, or a couple degrees faster.

Must have engine rpm at 800 to get correct base timing.

Check for vac leaks and repair any.

At this point, it should start almost instantly, have tremendous power, and get around 14 mpg. If milage seems poor, wipe out the tailpipe and then check for soot after 100 miles. Large amounts of greasy soot means running too rich -- adjust MAP 1/4 turn clockwise. If there is no soot, you have white plugs, and it knocks on premium and surges cold, adjust MAP 1/4 turn counterclockwise.

If you have access to a CO meter, you want 1-1.5% CO at idle (adjust at ECU by the radiator, toothed wheel under the rubber cover) and 1-1.5% CO at load (3000 rpm at about half throttle in gear. Be VERY CAREFULL setting the full load CO-- car will blast off if the brakes slip.

This should get you 15-17 mpg in mixed driving, about 18 on the highway. City will be around 14. The only way to get higher milage is to set the timing another 5 degrees faster, but you will need to be very careful -- only the highest octane premium you can find, and no trailers or full weight high speed driving unless you reduce the timing some and richen the mixture up. Less of a problem on the SL than the sedans, but you don't want to melt the pistons or anything.

Peter

Tomguy · #7 12-28-2003, 12:11 AM

Peter: Does the 4.5 acutally have the ability to burn a piston?
I know NASCAR engines can - but they turn 9000+ RPMs and are under constant stress and torture. They also produce 800+ HP. Can a 4.5 actually burn a piston if set too lean? Wouldn't it stall first?

psfred · #8 12-28-2003, 10:42 AM

MB engines are fairly high performance for their size, and will suffer terribly from detonation at high speed and load if the mixture or fuel isn't right.

You won't hear it, unlike "spark knock", but the fuel is burning VERY hot, and there is usually a exeedingly hot "flame" protruding straigh out from the spark plug. On a Benz, this gets the valves more than the piston (on Alfas in the 70's, a common complaint was a suddenly dead engine -- neat little holes that looked as if they were drilled and reamed directly under the plugs....), but you can indeed melt the piston tops.

The only indications you will have are too much "rattle" and low speed and lower than normal performace at high speeds until either it starts to loose power dramatically when the valves no longer seal, or a spark plug insulator cracks and drops pieces into the cylinder and you bend a valve or chew the liner up.

Peter

Litton · #9 12-29-2003, 10:39 AM

Regarding the vac. retard module on the dist......this is what I found on mine.

Held 20" of vac for 15 minutes without any drop.

Began moving at ~ 2" and was fully open (retard) at ~ 6".

Vacuum on the line was 15" at 800 rpm and goes to 0" when the throttle is opened.

Tomguy · #10 12-29-2003, 12:35 PM

Wow, I suppose mine is faulty then? Taking this into concern, should I plug the line and set my timing to 7BTDC? Or is any loss of this vac minimal - it does "work" (retards my timing).

Should the MAP be 100% leakproof as well?

Litton · #11 12-29-2003, 01:13 PM

Tom, when I went through the excersize of trying to locate any vacuum leaks, I tested my MAP with the 'ol mity mite and found it to also hold 20" for 15 minutes.

I did find one leak in my central locking system where it won't hold vac more then 30 seconds but the leak is still small enough that it has no effect on manifold vac. (measured at the back of the intake manifold) Therefore I believe that a small leak does not effect engine performance.

My guess....if the retard works.....no harm because you only need 6" to achieve full movement.

Tomguy · #12 12-29-2003, 01:19 PM

My MAP definetly did not hold 15" for 20 min, IIRC. I think it dropped from 15" to about 10" in 20 seconds... I'll check it again after I am done cleaning the carbon deposits from my manifold.

psfred · #13 12-29-2003, 01:28 PM

MAP leaking vac is probably why you have such terrible milage. I would toss it, although you could attempt to adjust it. They are expensive ($500?), but it ain't gonna go right with a bad one, and if it cannot be set to provide adequate fuel control, it's replacement time.

Verify that the hose is good first.....

Vac retard diaphram must hold vac indefinitely -- it's vented at the switchover valve. If it leaks, you are pulling dirt into the engine. Ditto for other leaks.

Peter

Tomguy · #14 12-29-2003, 01:34 PM

Well, Peter, adjusting my MAP most definetly does change my engine's mixture.
I see 4 slotted screws on the MAP (or have seen them, if memory serves me) and wonder if it's possible to take it apart and attempt to repair dryrotted rubber on the leaking diaphraghm(s) with silicone?
Is it going to break it if I take it apart (I WANT my car to run) or should I just leave well enough alone and try to find one off a D-Jet car in the junkyard?

What other cars HAVE D-Jet?

psfred · #15 12-29-2003, 01:56 PM

Don't take it apart unless you have a spare....

Cars that used D-jet:

VW Fastback and Squareback, 411, 412 from 69 or 69 to 73.

Volvo 163E from 70 (I think) to 74 or 76

All MB v8s 71 to 76

Some other Volvo 240s, I think same years.

If adjustment works, try this:

With throttle off idle (so that you are running just the MAP control, not the idle mixture on the ECU), adjust MAP for max idle speed, then turn 1/4 turn CW. Clean tailpipe and drive for 100 miles or so, and examine for soot. If black and greasy, or soot present on outside of pipe, take another 1/4 turn CW. If gray or brown, probably OK. If engine stumbles badly cold (as opposed to surging from dirty or worn tirgger contatcts), go 1/4 turn CCW to richen it up. Adjust ECu for best idle after setting MAP with throttle closed.

If you have access to a CO meter, set the MAP to 1.5% CO at load (half throttle in gear against the brakes -- be carefull!) and idle mix to 1-1.5% CO. Should get you 15 to 18 mpg.

Peter