First I want to apologise, if my reaction on comments is a little delayed. I live in Germany, and when you post a comment - let’s say in late afternoon - I may already have gone to sleep.
Next I want to thank you for your comments, which sound very experienced.
Maybe the injectors are the reason for all of the 3 above mentioned problems.
“…Have you used a smoke machine to rule out the false air? …”
I must admit, I can only rule out false air according to the test method I used. I did not use a smoke machine.
I took off the air filter housing, let the engine run in idle and - equipped with a vaporizer filled with brake cleaner fluid – I thoroughly checked every corner of the hole intake system. Small hoses which are connected to it which use vacuum like the lock of the backrest of the passenger seat, I took off and used my finger to plug the hole. It certainly took me half an hour to do the false air test. The use of a smoke machine is probably more proper for a false air test.
“…Fuel pump relay . . . intermittently just quit, then work again for awhile….”
I can not rule out, that problem no. 2 is caused by the fuel pump relay.
A month ago I connected a small 12 volt lamp to the wire which supplies the fuel pump with voltage. I placed it on the center console (with a switch). So next time when the engine stalls I can check whether the fuel pump relay shows voltage output when I switch on the ignition again (it’s supposed to do that for 1 or 2 seconds).
But so far it never happened again yet.
As for the rough idle the fuel pump relay can be ruled out, because there is no change if I pull it off and use a bridging wire instead, so that the fuel pump runs as long as ignition is switched on.
“…only the genuine Benz parts are any good…”
“…Accumulator . . . leak down and cause hard starting…”
Ok, a leaking accumulator can of course cause starting problems. But wouldn’t it do that then all the more after the car has not been used for a week or two!? But as I said, after a longer shutdown time the engine starts immediately within a second – and stays on!
“…Fuel pressure regulator . . . leak down…”
Since the fuel pressure test showed flawless results (also 30 minutes after engine shutdown) I think that fuel pressure regulator is also not a suspect anymore.
“Coil … tend to fail when hot”.
I should check that.
The plugs and wires I installed 2.800 miles ago are Bosch.
But here is a confession: The distributor cap and rotor are not Bosch, they’re “Bremi” (one of the German aftermarket ignition components supplier).
I attached pictures of the current plugs, cap and rotor.
I bought new injectors already last week, but haven’t installed them jet. They’re Bosch (with the original Bosch-number and “Made in Germany” engraved on them.
I got them from an aftermarket dealership which offers them for 27 Euro ($ 37,50 acc the current exchange rate). That is half of the regular price at the local Bosch or Mercedes dealer.
The seller accepted that I connect them one by one to a test pipe which I connected to the fuel distributor (injector aiming into a big plastic bottle. (I did that outdoors with the wind blowing any escaping fuel vapor away from the engine – but to all readers: better don’t do that, pulverized fuel is highly explosive!). The seller said I could bring them back, if they don’t spray properly – as long as they haven’t been used in the engine yet.
The spray pattern looks ok, but what puzzles me is that when you release the throttle linkage at higher rpm and the rpm drops back to idle the injectors drip a little. I don’t know – is that normal? Or should I bring them back?
Back at idle they spray ok and atomize the fuel again, but not as nicely as at higher rpm.
“…slightly rough idle simply because whoever checked the engine oil level did not fully seat the oil dipstick in the tube …”
Never thought about that, but come to do that I think that could be an issue especially for CIS cars with injectors which have air ducts around them which are connected to the valve cover via breather hose. That way of course air can enter the crankcase and cause false air to the intake system. The engine of my Mercedes (M103) has that design.
I checked the pressure under the valve cover of my car this morning. I let the engine run at idle and took off the breather hose between the valve cover and the air filter housing. Then I plugged the breather neck on the valve cover with my finger. Result: There is a slight vacuum under the valve cover! Obviously that is caused by the intake vacuum through above mentioned breather hose between the valve cover and the intake manifold.
I only checked that at idle, I don’t know whether there is vacuum or positive pressure at higher rpm!
Anyway, that test better be done carefully, since air (possibly mixed with dirt particals) could enter the engine through some other spot (maybe through a rotary shaft seal) due to excessive vacuum!
Next week I will also install new motor mounts and a new transmission mount. I ordered them yesterday at the local Mercedes dealer (price incl. tax: Euro 260 / $ 360).
Warm gretings from Aachen, Germany