73 MB 450SL stalls after warmed up and will not start - PeachParts Mercedes-Benz Forum

Jonchip · #1 05-25-2020, 02:29 PM

I am hoping that someone might be able to shed a a little light on my situation.
I have 73 450SL that I inherited and have been trying to get back to running condition. I am a retired Toyota tech of 23 years so theses European vehicles are kinda fun to work on. Unfortunately I am at a point of frustration right now... Let me tell you what I have done. The vehicle had been sitting for a while when I got it and did not run. I have put a new fuel pump and filter with new gas to at least get to run. I then did the normal maintenance spark plugs, wires, cap and rotor with points. After that I noticed that it had a little stumble in it so as rotted as a lot of stuff was I checked for vacuum leaks and ended up replacing all injector seals and some vacuum lines. The vehicle was and is running great! Fires right up every time.
The problem I am having now is after it sits at am idle for about 10 mins or so it starts to stumble and them stalls. It used to be that it would start after about 5 - 10 mins but now has got to be 1 - 2 hours before it will start. And sometimes longer. This is what I have done for diag when it will not start back up.
I know it is not a spark problem because it will run on alternative fuel source. I have check fuel pressures and it's at 31psi and does not bleed down for several hours. I have check the wiring to the temp sensors and ohm'd them and they seem to be good.. I pulled the trigger points and checked them and they were in great shape.. All relays are good and fuses. I put a stethoscope on the injectors when it is warm and they are not firing when trying to start it. Also when it cools down the starting injector is not kicking in (there is no power to it) but if I supply power to it the vehicle tries to start but doesn't. I am probably wrong but I keep thinking it's temp related but the funny thing is when it cools down it still will not fire up sometimes.
Any help will be more than appreciated if anyone can give me something to try next. I know it's going to be something simple in the long run..
It always is! Thanks

Frank Reiner · #2 05-25-2020, 03:37 PM

Jc:

Look first to the main relay for the ECU, and the fuel pump relay (which receives energizing power from the main relay).
Power to the main relay comes directly from battery (always hot). The connection may be a separate wire on the + terminal of the battery, or on a multi-terminal strip on the inner wheel well.

Jonchip · #3 05-25-2020, 07:41 PM

Thanks for the reply Frank, I have been out actually working on it today. All power and grounds are good. I checked them before I started the vehicle and after it stalls out. Fuel pump is working great and has never shut off since I installed the new one. I checked all temp sensors and they are within specs. The only thing that I found that was at aspects was the map sensor second coil. I had 0 ohms when I tested it. So I remove the plug clean the terminals and plugged it back in and now I am showing just about 500 ohms which I think is pretty close to specs. One thing I did notice is it is not holding vacuum though. Not knowing these vehicles very well I'm not sure where the map sensor could be causing it to stall when it warms up and not let it start back up for a while?

Jonchip · #4 05-25-2020, 08:18 PM

Quote:

Originally Posted by Frank Reiner

Jc:

Look first to the main relay for the ECU, and the fuel pump relay (which receives energizing power from the main relay).
Power to the main relay comes directly from battery (always hot). The connection may be a separate wire on the + terminal of the battery, or on a multi-terminal strip on the inner wheel well.

Thanks Frank for the reply. I have checked all power and ground readings at ambient temperature and operating temperature when it quits running and I am not losing any power or ground to the ECM, fuel pump relay or main relay. The only thing I have found out of the ordinary was the map sensor was reading 0 ohms on coil number 2. I remove the plug and cleaned the terminals and then I was receiving little over 500 ohms which I think is in specs. But the map sensor is not holding vacuum. Could that possibly keep the vehicle from starting when it is at operating temperature? Shouldn't it just run bad?

Frank Reiner · #5 05-25-2020, 08:57 PM

Jc:

That the map sensor does not hold vacuum would cause mixture problems once off idle.
If the throttle valve switch/sensor is signaling the ECU as such when the throttle is closed, then the idle mixture is controlled separately from the map sensor, via an idle mixture pot on the side of the ECU. The suggestion to check the relays was prompted by the statement that the injectors were not being pulsed when attempting a hot start. You have probably discovered that the polarity of the injectors in a D-jet system is reversed from all other pulsed injection systems.

Jonchip · #6 05-26-2020, 12:29 AM

Well I figured it out. I started going after the map sensor since it was the only thing not checking good. So I ran the vehicle until it stalled again and unplugged the ECU and check pins 8 and 10 and I had 0 ohms again. I unplug the map sensor and plugged it back in and there was still 0 ohms and the vehicle would not start. So I removed the map sensor and tested the connector directly and it was at 0 ohms and then all of a sudden started showing continuity but it was very glitchy. Finally after 10 minutes or so it calmed down to 390 ohms. I checked the vacuum when it was out of the vehicle and it is actually fine. It held vacuum for over 30 seconds and never dropped. I must not have been getting a good connection with my vacuum line when it was in the vehicle. so when it came back to the correct ohms I put it back in the vehicle and it fired right up again. I let it run until it stalled out and would not start and it was at 0 ohms again. I did this two times just to confirm that the ECU must need to receive a signal from both of the coils in the map sensor in order to fire the injectors. Looks like I am going to be put in a map sensor in the vehicle.

Tony H · #7 05-26-2020, 01:38 AM

I would recheck your trigger points. They are the cause of many obscure problems. They can get oily film from crankcase vapors. Try cleaning them with achocol