Hi. I am new here. I am an Auto Mechanic for 30 years, and am only slightly familiar with mechanical fuel injection. Long story short. A car in My shop a 1985 Benz 380 gas no turbo engine towed in. It will start for a few seconds and stall out. If I add supplemental fuel into the throttle body it will run, and strangely, after about 30 seconds, if I stop adding xtra gas it will run rather well on its own with plenty of power, but a rough idle. After engine is turned off, upon restart, the same condition exists all over. The fuel pump was relaced, the filter is new, and the vacuum lines appear to be all connected and ok on visual inspection. Anybody got an idea what it may be the cause?
I would be most grateful for an answer, and maybe even give a reward. Also My many years of experience may hopefully help someone on this forum . Thanx, David W Choate.... My email is planecrusher@gmail.com

 

Hi David. I am new to this forum as well. Most of my limited experience is with gas models 1990 and newer. I assume you have already verified fuel pump operation when key is on or when engine is cranking. Also, if you have not done so already, crack open the fuel supply line at the fuel distributor housing (Continuous Injection System) to verify whether or not it has fuel pressure when the problem is occurring.

 

Aside from the typical tune-up checks, there are also some other things to keep in mind concerning rough idle, stalling, or no-start conditions.
Common component failures for Mercedes include the fuel pump relay, over-voltage relay, and even the voltage regulator mounted on the alternator (on some models) can wreak havoc on the electrical system due to improper voltage signals to sensors or computer. The voltage regulator could have a worn or broken carbon brush causing voltage concerns. These things can cause the very same symptoms you have described, which can make finding the cause of the problem even more elusive on these fine vehicles if they are not taken into consideration.
Talking about tune-ups....observe the ignition spark using a spare spark plug or spark checker. If it looks weak, inspect the cap and rotor system closely for arcing, rotor button burn-through, excessively high terminal resistance, etc., or just go ahead and replace them if there are any doubts.

 

Thanx for replying. I have been aware of the relay problems from research, but not the voltage regulator stuff. Thanx. I did not yet check the relay. thats about the only thing left that I'm going to do. The customer luckily does not use it as primary transpo and can leave it long, but I'm getting to the point where i'm used up so much diag. time that it's getting to be i'm making no money. But I hate when I cant figure something out, and will pursue the answer just to satisfy My own curiosity at this point.

 

Believe me, I understand and empathize with your situation. That would be the curse of a good tech.... can't let a mystery remain a mystery for long, especially when it's about to eat your lunch and is looking for dinner too.
Besides, three more of the same will show up next week.

BTW: one clue about a possible voltage regulator problem would be if the ABS light flashes or other dash lights start flickering at random intervals. Could happen in conjunction with hard start/rough running but not always. Would like to read about your fix if successful and you have time to post it later.