I have a 91, 420 SEL with 197Kish on it. It always starts quick when it is bellow 40 degrees F, but after it has run for a bit, is shut off for a while, it "can" be cranky to fire up. Some times it will catch right away, and some times it takes a couple of tries and some foot work before it fires up.

The car was neglected when I got it. It needed a fuel distributor, accumulator, Injectors, etc. I have found that this warm/hot start issue is very common with these cars.

I have been collecting the parts to make a "Black Box" solution and today finally got around to building a proof of concept. Actually, yesterday I tried a quick and dirty version that didn't leave me all warm and fuzzy in side, so today I built what I had in mind and I am very fuzzy.

I still need to put some more validation time on the system and clean up the construction design before I will claim victory, but so far I it is doing exactly what I had in mind.

The "Black Box" goes between the cold start injector and the Computer. It has male and female injector connectors so there is no wire cutting. Only one wire needs to be inserted into the Starter Relay wire connector next to the brake booster, and that is it. No tools required to install!!

The Black Box pulses the cold start injector for an (adjustable) amount of time while cranking and allows the computer to pulse the injector normally.

After I am comfortable there are not any gremlins in the system, I could be persuaded to make some more if anyone had the same starting issues.

I am not familiar with the various (Gas) models but I imagine that this system could be installed on just about any model with a cold start injector.

That sounds like an interesting project.

The problem with the cold start injectors for those years was that their activation temperature was a bit too low. Often, at around 65 degrees ambient, it won't come on even though the motor would really like it to. I was told by my MB mechanic that this was a typical issue for these cars.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

I just got back from a friends wedding and for the first time I was not embarrassed from having to try a couple of times to get the engine started.

The car, (91' 420SEL), always gets attention as it looks very presidential, that is why I drive it to social events, and now that it starts I will be driving it more often.

If you have a MB with a cold start injector, that in some conditions is reluctant to fire up on the first crank, I highly recommend using the cold start injector for every start. (providing everything else is working correctly in the fuel system).

Right now I am pulsing the injector for 1 to 1.5 seconds while cranking. If you have the means to build a timed injector driver it will be well worth the effort. The connectors for the fuel injector are pricey but they allow access to the injector circuit without hacking into the wiring harness.

I am kicking myself for mot doing this sooner!!

OM616, consider replacing the fuel pump check valve. If the system pressure bleeds off it's possible the hot engine compartment could cause vapor lock in the fuel line.

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Fred Hoelzle

Tested that when I went through the fuel system. I was actually surprised how long the system will hold pressure. I let it set for 6 hours and there was still 75% of the running pressure present. I did mention above that the fuel system should be working correctly first. The needs of these cars like people change as they age. The minor electrical, mostly mechanical system does was not designed to adapt to the changing needs as the result of age and use.

At its best, 9 times out of 10, the engine would start the first try, however they often were 3 to 4 second long tries. I mean right when you would normally give up and stop cranking, if you waited another second it would catch. If it didn't, it would on the second try usually with some petal input. Sometimes it will sputter enough for you to let off the starter, but will not keep running. In all cases where starting is marginal, the fuel mixture is too lean IMO.

Starting usually requires a slightly richer fuel mixture which depends on the amount of remaining fuel vapors and the temperature of the block, heads, and the intake air temperature.

Additionally, the engine is worn and dose not pull as low of a vacuum during cranking as it did when it was new. The fuel system has to be compensated for the intake manifold vacuum trying to suck the fuel out of the injectors. Changes in vacuum levels affect the pressure drop at the injectors which has an effect on how willing the injector is to pass fuel in a sufficient quantity.

My mom often likes to take the car to outings and even though she is checked out on the starting procedure, I still worry if it will start for her. Now it starts like an EFI system, every time, with out hesitation. I checked and I am only rapid firing the start injector for 750 milliseconds. Just enough to give it a shot in the butt.