'74 450SL manual needed for hot start prob

[halw540]

I am new to the forum and Mercedes. I just acquired a '74 450SL with 93K miles from the original owner's family. Its a nice vehicle but it has a hot start problem that I need a manual to help with. Mercedes no longer offers the factory CD. There is a site, Mercedesmanual.com, that may be legitimate but looks flaky as a source of what I assume is a copy of the Mercedes manual. Does anyone have experience with them or can recommend another manual source?

The problem I am chasing: It starts cold fine and runs great (although on the rich side). After heat soaked on a 90 degree day, when I stop and let it set for 10 minutes or so it would not start immediately. After cranking it several times for no more than 5 seconds each time it finally tried to fire, then died. After a few more tries it caught and ran as though flooded or hitting on 5-6 cylinders and died when put into gear (with increased rpm). Finally, I got it to catch, ran the rpm up for perhaps 15 seconds, then although still missing (but not as bad) I put it into gear and by the time I had gotten up to street speed it settled down. I drove it home (30 miles) let it set and successfully restarted it after 5, 10, 20, 30 minutes!

This sounds like a problem others in the forum describe but I do not know the differences in models/year part designs - or where the parts discussed are located - or how to test.

Thanks in advance for any recommendations! Hal

[jjewell]

Replace the fuel accumulator. My '79 450SL did the exact same thing when I bought it. Its located next to the fuel pump. The internal diaphragm gives out & causes this problem.

[slowride]

If upon restart warm it blows black smoke, I would check the injectors for leaks. If not, it seems like the dreaded D-jet vacuum leak. Take this with a grain of salt as my experience with D-jet so far is theoretical and I have yet to fire my '72.

[halw540]

No noticeable black smoke at restart. I am new to these vehicles - what is the"dreaded D-jet vacuum leak"?

[meltedpanda]

ANY vacuum leak will give these D jet cars grief. All the lines to and from the injector rails should be checked as well as the main vacuum line from the
MAP sensor , a small can-like thing attached to the driver side fender well between the coil and brake booster. It is about the size of a medium jelly jar and made of aluminum. There is a major electrical connection from it to the ECU and a large diameter vacuum line to the rear of the manifold. There should be a black plastic cap on one end. The MAP senses manifold vacuum and converts this to an electrical signal for the ECU to regulate fuel flow to the injectors.
Valve lash is another item to check and get within specs, easy to do on this car.

Get the book "How to tune and modify Bosch Fuel Injection" by Ben Watson. Its very good.

[halw540]

"Replace the fuel accumulator" - I admit to not being the brightest guy on the block, but my '74 doesn't appear to have one. From the tank the fuel line goes to the suction damper then to the electric pump then to the fuel filter and then off to the engine - what am I missing?

Ron, thanks for the suggestion on the MAP sensor vacuum line - it was deteriorated.

[ScottinSoCal]

Originally Posted by halw540

"Replace the fuel accumulator" - I admit to not being the brightest guy on the block, but my '74 doesn't appear to have one.

It doesn't. That's a common issue on the K Jet engines, but not the D Jet.

[meltedpanda]

that line can cause big issues, crazy thing is every hole you plug you may have to adjust something up or downstream
Once you get it you'll know these gals run like crazy when in tip top shape.
Have you done your valve lash?

[halw540]

"Have you done your valve lash?" - no, need to get some valve cover gaskets first and I live hours from a major city (internet time, source recommendation for parts?). Education time - what does the engine do when they are out of adjustment?

Progress - Just got back from a 30 minute drive heat soaking it thoroughly on 85 degree day. After ten minutes of setting it started, but had a little initial miss/flutter for a few seconds (nothing like before replacing the MAP vacuum line). I took it back out and once it got into 3rd gear it smoothed out and never missed again - until I stopped it and let it set for a few minutes. Then it was an exact repeat - missing/flutter initially until reaching road speed. Clearly something else heat related is involved (compression is good and consistent across all cylinders, have done full tune up and replacement of plugs, wires, dist cap, etc), don't see any other vacuum lines deteriorated. Any suggestions??

[meltedpanda]

valves out of adjustement makes for hard hot starts, some irratic running at cold idle. but depending on how they are "off" could be just the opposite. I look to injectors next, have them pop tested and rebuilt. may be leaking down after hot