W126 Ignition Issue
 

1990 300SEL

Well, I'm almost done renovating this car, but I can't figure out the hard starting problem. New plugs, air filter, all the obvious things. Here's what happens:
The engine needs to crank between 12 and 15 times before it will start. Otherwise, I can turn the key on and let it crank 2 or 3 times, turn it off, and then turn it on and it will start after about 4 cranks.

1. Which is harder on the starter - one long attempt or two shorter ones?
2. Whichever way I do it, after the engine starts, and with the heater/AC OFF, what I'm guessing is a heater flap will slam by the front passenger foot area. Often I will hear it slam again when I turn the engine off. Doesn't matter whether or not the heater or air conditioner is on.

Any ideas greatly appreciated.
Thank you.

Though checks for other components,pipes etc should be made Hard starts on the 300sel are invariably tracked down to two relays not functioning correctly. fuel pump relay and the overvoltage relay. these are in a row with the Klima relay.
remove relays and clean the pins ,or resolder the joints .
Its Best to replace both for long time reliability as they are at the end of their life cycle in these models . The OVP malfunction does not let the cold start voltage through hence the long cranks.
regards
30se

Thank you very much - I'll get on it tomorrow.

I did replace the fuel filter, but not the accumulator, and I never heard of the Klima relay - can you give me a tip where to look?.

klima is the A/C relay . all these are in a row on the back firewall ,left of the Battery .in my car its OVP then Klima and last Fuel pump relay . the OVP relay has a fuse on its top.
mak

Thank you so much.

The slamming noise you are hearing from the passenger side footwell area is actually coming from behind your glovebox. It is the outside air flap to the heat/AC system. It has two vacuum pods on it, a short throw and a long throw pod. One of those pods has a leak in the diaphragm and when you shut the car off it is supposed to slowly close the outside air flap. When it leaks vacuum too quickly, the door slams shut. I replaced both pods on my 86 300sdl this summer and it is very quiet now. I have not changed the ones on my 85 380se and it is very loud!

Thank you very much.
I'm still searching to find out exactly where the fuel pump relay and overvoltage relays are on my car, as advised by responder "mak".

Based on your info, that will be my next step. I'm a little concerned about taking too much apart to get to things because no matter how careful I am it seems that the disassembly always results in something getting broken, cracked, etc.

Parts Identification
 

I took some pictures and circled the parts that I need to identify. If anyone can help me, PLEASE PLEASE be sure to tell me the picture number and which circled picture so I can be sure to know what you are telling me!!
Thank you.

In the first picture, the long black box is the heater fan fuse, rated at 30 amps.
I'm not sure about the relay to the right of the fuse holder, though.

The left item in the third picture is the OVP, or Over Voltage Protection relay, and usually has a 10 amp fuse under the cover.

I'm not absolutely sure on the rest of them. I *think* that the right item on the 4th picture is the fuel computer, but I'm not sure on that one.

photo #3 the left most item is the connector for the ABS system ,in the center is the OVP relay with a red fuse in the plastic cover , the last relay not in my car.easy to pull out get the # on it and check in buymbparts or other ordering catalog.

#4 the last one is the ABS connector not a relay
mak

If the mods let me, I want to delete the pictures and post them in another post titled "W126 Relay Identification" in hope that it will attract more viewers who can answer but wouldn't see the question in this thread.

Turn the key on for 3 seconds, then off. Repeat 3 times. If it starts quickly, you're losing fuel pressure when the engine is off; could be a faulty check valve in the return line or regulator.