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I have a 1974 Mercedes 450 SL that runs for exactly 2 to 5 seconds and then stalls consistently… Any thoughts?

 

[rowdie]

Do you hear the pump run for a few seconds every time you turn the ignition to on?

 

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Do you hear the pump run for a few seconds every time you turn the ignition to on?

Rowdie-

Thank you very much for the prompt response. Yes, I do hear the engine run for approximately three seconds each time the key is turned to the ignition position. I

 

[1025MB]

Rowdie-

Thank you very much for the prompt response. Yes, I do hear the engine run for approximately three seconds each time the key is turned to the ignition position. I

Also, the car has eight new injectors and have followed Djet checklist religiously...The resistance and continuity values are within spec.

 

[MBGraham]

When did this start happening?

Did you do any work on car before?

Could be so many things so some history would help.

Maybe for a start, ensure you have 30psig of fuel pressure.

Then see if car keeps running if you (carefully) spray fuel into the intake once it gets started.

This might tell you if it is a fuel supply problem.

 

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Thanks for the response. I replaced all eight injectors ( previous were either clogged or solenoids locked up). The car started up immediately with 3 bar oil pressure, I let it idle for about 25 minutes (it took a great deal of time to reach operating temperature - 2.5 bar oil pressure) pulled over into a parking lot and measured the vacuum by way of a tee into the MAP sensor hose, adjusted the timing for optimum vacuum at idle. As a caveat, the car was a complete basket case (non running order) when I received it. The body had received a $10,000 paint job, new interior and countless new parts.

Fuel pressure - 34 psi

 

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Perhaps I have a high threshold of pain, but I recently restored a Rolls-Royce silver shadow rebuilding the engine with new cylinder liners, 3000 psi braking system, etc. By comparison, the 450 SL has been a dream to work on.

 

[MBGraham]

Sounds like you are quite capable of doing what may be needed.

Check the stickies (EGV107) under sections 07.4 and 07.5. In particular,
the Comprehensive guide posted by Proctologist (who we don't see here lately!)

The on-line FSM hosted by oncebitter (Open under engine type)

Bosch EFI training manual

Maybe think about adding Pertronix as points replacement or both points and switchgear (ign module)

 

[Jfheflin]

I believe car would act as you describe if it was running for a few seconds off of cold start valve/injector then stall because fuel injectors aren't firing. You mentioned checking vacuum at MPS. The 4-prong fuel injection harness plug on MPS must be plugged into MPS for car to run; otherwise, car will fire up for a few seconds on cold start valve then die because ecu won't send signal to injectors with MPS unplugged. Perhaps you knocked MPS plug loose when messing with vacuum hose?

 

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Thank you again for the very prompt response, really appreciate it. I agree with you, it is an iterative process and I am obviously aware of most of the resources you shared. However, thank you for the Bosch injection link, however, the FSM link is of great interest to me however appears to be broken.

 

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I am quite confident that the car is not running on the coldstart injector valve because there is no coldstart injector valve in the circuit. I removed it last year.

 

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As for the four prong harness connected to the mass pressure sensor, that has been connected and continuity verified back to the ECU

 

[MBGraham]

However, thank you for the Bosch injection link, however, the FSM link is of great interest to me however appears to be broken.

Sorry about that. I fixed the link in previous post. But here it is again anyway: Model 107 Maintenance Manual Index

Have you checked the fuel flow? You may know this, but some new owners have hooked up the fuel lines incorrectly. The supply line from the pump finds it's way to the front of the fuel rails (diferent cars have different rails, but in principle they are all like this) From there, flow splits and passes to rear of rails where they recombine through Fuel Pressure regulator . The fuel passes through a pressure damper and then flows back to tank.

If this is connected correctly, you might measure the fuel flow you get on the line leaving the damper. (You will have to jumper relay, or provide 12V to pump directly. You should get 2L/min (1L/30 sec is the spec)

While you have that line disconnected, make sure you can blow through the return line to the tank (there is a nozzle inside tank that gets clogged)

If you are not getting flow, common problem for cars that have been sitting, is sludge in tank, clogged tank screen and plugged up fuel filter. It could be that you just get enough fuel to start the car, but not to keep it running.

 

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MBGraham-

Thank you so much for correcting the hyperlink, that is a tremendous resource for I’ve been looking! It appears to be a far more organized compilation of another resource.

Thank you very much for sharing your thoughts on the fuel system, specifically fuel flow. I’m sure many of the readers on this forum is familiar with fuel pressure and how it differs from fuel flow. Concentrating on fuel flow, the specification that you shared is as a result of measuring flow after the fuel regulator AND the dampener (1L/30 second) prior to the return to the fuel tank. Incidentally, you referenced owners reversing the order of the supply and return the fuel lines. Do you mean that owners inadvertently connect the supply line to the dampener, Which passes through the fuel regulator then on through the rest of the rail?

 

[MBGraham]

Do you mean that owners inadvertently connect the supply line to the dampener, Which passes through the fuel regulator then on through the rest of the rail?

Yes, you would be surprised how many times that has happened here! Some thought it should be like that. Others inadvertently connected the hoses that way because at least on some cars, they are close to each other.

 

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There’s one phenomenon that I noticed last night, I was using one of the noid lights to verify an injector (#6) was firing and the light remained on. It’s counterpart (#3) was also on.

 

[1025MB]

SUCCESS!

Well, at least the car is starting and running. Unfortunately, I am receiving absolutely no contribution from cylinders 1,2,5 or 8. The other four cylinders are running perfectly with even contributions. Each of the eight of the cylinders has 150 psi and new injectors.

 

[MBGraham]

SUCCESS!

Well, at least the car is starting and running. Unfortunately, I am receiving absolutely no contribution from cylinders 1,2,5 or 8. The other four cylinders are running perfectly with even contributions. Each of the eight of the cylinders has 150 psi and new injectors.

That is odd. The injectors fire in pairs 1&5, 4&8, 6&3, 7&2. So if 7 fires, so should 2. Same if 4 fires, so should 8.

Make sure you have the numbering correct and that the injector wires are going to the correct injector (it is easy to reverse some of them. If you find an error and it is injector groups that are not firing, then check trigger points. First just rotate and using ohmmeter make sure each contact opens and closes.

Otherwise, check continuity of injectors back at the ECU plug under the dash. If you disconnect one injector bank (left or right) and measure resistance between terminals 11 (ground) and pins 3 to 6 in turn, the injectors should each measure about 2.5ohms. Then do the other bank. (If you leave injectors connected, you should read about 1.25ohms for the pair in parallel. This will tell you whether the harness from ecu to injectors and the injectors are OK.

 

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Thank you again for the response. I actually tested the triggerpoints by holding the distributor in hand and turning it clockwise. I verified that each set of the four sets of two paired injectors is firing. With respect to the wiring to each injector, that has been verified exhaustively. In fact, while the engine is running, I can take the injector plug from the non-running number five and connect it to cylinder number six and achieve combustion (obviously not optimal). However, that is not the case vice a versa. Keep in mind, these are new injectors with less then 40 miles on them. I plan on removing the driver side injector rail and swapping the injectors on contributing cylinders six and seven with those on the non-contributing cylinders five and eight (Probably evaluate spray pattern and 30 second volume test of 220 cc) By the way, the easiest way to verify that the injectors are indeed “firing” is to use the throttle switch after bumping the starter in order affect different iterations of the injector contact switches.

 

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By the way, the car runs absolutely beautifully on starter fluid.