HARD START MID-RANGE TEMPERATURE

[Doubleclutch]

Hi ALL I'm a long term MB owner with a 1971 280sl and no experience with mechanical fuel injectors. My car received a rebuilt injector pump about 15 years ago and about 10k miles. Lots of sitting and even today our driving is very occasional. New injectors 5 years ago as well as a major tune up.
 
The car has a pressure loss in the injection system since we bought it that leads to a 20 sec starter turn and a very rough start over 2-3 hour cooling. Shorten the time frame and the turning time decreases. The car was tuned on ethanol and is now running on non-ethanol. Cold and hot starting is fine.

I have two questions??

I understand the pressure check valves in the  injector pump are ball and seat. With all the sitting the ball and seats may be varnished and that's causing the leak down. Any negative thoughts on running a light amount of fuel injector lub and cleaner for a while??

I have found the mechanical injection cars have this problem and the factory electric fix that triggers the coldstart valve for approx. 1 sec. I have a friend that worked for a dealership back in the day and had 2 Pagodas as a personal drivers. Both cars had this starting problem and the service dept wired a under dash switch to trigger the cold start valve. I'm leaning toward this solution as the impairment on this car is probably greater than most.

Thanks for any thoughts and help.   

[wwheeler]

The factory fix fires the cold start valve warm and hot. So if your hot starts are good now, they won't be with the fix because you will then have too much fuel. You need to determine if you have too much fuel or not enough during that time with issues. You can wire a temporary switch to the CSV to fire it as you desire. If it starts better with the manual injection, then you need more fuel and you can dive into the reasons why you are not getting enough.

[Benz Dr.]

I agree but I would also check to see if the starting solenoid on the IP is working, assuming it has one. Plugged fuel lines running back to the tank are also a common problem.

[Shvegel]

You seem to have a pretty good grasp on the issue for not having much experience with the system. I would try either Techron or BG44k injection cleaner and drive it for a while and see what happens. Throw a couple dollars at it and see what happens. It won't hurt anything and it might just fix your problem.  Pretty sure your car doesn't have a starting solenoid on the pump. If it does there will be 2 cylindrical things on the firewall side of the injection pump rather than 1.

[Doubleclutch]

Thanks to Wwheeler,Benz Dr, and Shvegel

Good point on the excessive fuel--the manual trigger is best.
Will check out the IP manual start sequence.

[ja17]

With a 71 280SL you may have no solenoids on your injection pump? Could also have just the deceleration solenoid? Anyway, make sure your colds start valve on the intake  is functioning correctly.  The intake fitting of the valve has a fine screen built in it, that can be clogged. The fuel jet on the CSV can also be clogged or varnished. You might try removing it and hook it up externally to watch it function. If yours is the late "Version 5" (starting aids), then there is no injection pump starting solenoid on your IP. With this version the intake starting solenoid is the only starting device used. It fires at eng. temperature under 95 F. and increases in duration as the engine temperature decreases. The correct function of the intake solenoid is critical on this version.

[Shvegel]

It is interesting seeing how Mercedes and Bosch struggled with something that wasn't an issue until they used the piston pump to push gasoline instead of diesel fuel.  I think they just didn't have any reserve capacity in the fuel system to allow for any leakage at the injectors or delivery valves when the car was warm so virtually any leak in the system caused the fuel in the lines between the delivery valve and the injector to vapor lock.  Bosch finally solved this in the next generation of Mechanical fuel injection (K-Jetronic or CIS) by adding a pressure check valve  at the return fuel line and a fuel accumulator somewhere between the check valve at the pump and the pressure which would hold pressure on the entire system and still maintain residual fuel pressure until the engine cooled down.  I installed just such a system on my 280SL before I took it apart for restoration and it worked perfectly.  Unfortunately, i was using an non-stock fuel pump at the time and i don't think the stock pump will reliably push the 40psi necessary to  overcome the pressure check at the pump outlet.

[wwheeler]

Interesting thought. If you put together the fact that they have numerous starting devices and then also numerous revisions on these mechanical injection systems, sort of tells you they had starting issues all along. My '60 W128 220SE coupe has a two plunger pump and has a completely different set up from the '68 280SE only eight years later.

As these injection systems wear, it appears they wear differently on each car requiring individual "adjustments". I am in the same boat as well. I ended up disconnecting the starting solenoid and using a manual CSV switch from inside. The start solenoid was over-fueling during a hot start. But I need the shot of gas at warm starts. I like your solution if you could get it to work on a stock system.   

[Shvegel]

I bet one of these tuned to 15 pounds and plumbed into the return line after the pump would fix it provided the outlet check valve of the fuel pump still worked.  The idea is to hold around 15PSI in the system for 45 minutes or so.  15 PSI is about the vapor pressure of "Gasoline" so when it vaporizez it will push at about 15PSI trying to push the fuel out of the lines. I have to do a little research but I think on the Bosch D-Jetronic system this regulator is from the regulator also acted as the accumulator.

https://www.summitracing.com/parts/bch-0280160001?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-bosch-automotive&gclid=Cj0KEQjwk-jGBRCbxoPLld_bp-IBEiQAgJaftbrAkGboJUH3yDJxUefiQhLw65rxSoueR3Oj4uQ2US0aAgxn8P8HAQ

[Doubleclutch]

I think we are on our way. The fuel injector pump was installed by a rural general garage by the former owner. It was never tuned properly. I brought in a expert that leaned out the overall mixture a little and cut back the activity of the WRV warm run valve. The car runs much better and therefore starts better (not perfect). We agreed to try a couple of tanks of gas treated with fuel injector additives to take a stab at dissolving any varnish on the check valves in the pump.  If not will rig the CSV with the manual switch. 

Thanks to all who participated---Chuck