Fuel Injection diagnostic 103 engine. Need advice.
 

I am usually pretty good with repairs. I just re-did my own head gasket on this car. I have always had some troubles with this car's injection, but lately, it's become intolerable.

The residual fuel pressure appears to drop to near zero fairly quickly after shut off. I say this because cold or hot, the engine takes from two to 30 seconds to start. The time seems random.

I do not have a fuel pressure guage, but I come to the pressure drop conclusion by doing this layman's test. Run car. Shut off. Loosen a fuel distributor fitting. --squirt-- Run car again. Shut off and wait five minutes. Loosen a fuel distributor fitting. -a drip-

Some months ago, I replaced one of the two fuel pump check valves (it has two fuel pumps connected in series). No cure. I then replaced the other one, which in hindsight was a little silly.

I tested the cold start valve by removing it with the steel line attached and re-attaching it to the valve body, but pointed up in the air. I plugged the hole in the rubber boot below the CSV and ran the car. No leak. No seepage.

Tested the accummulator for a blow-out by pinching shut the fuel return line. Ran the engine. Shut it off for ten minutes. Started after ten-30 seconds cranking.

OVP is fine, because all systems operate fine and I get no engine light. The car once started idles and runs fine.

I tested the resistance on the (name forgotten) on the side of the fuel distributor, and it is exactly on spec at 19.5.

Tested the fuel pressure regulator by a tech CD recommended procedure of removing the fuel return line side and looking for more than a drip. I didn't get more than a drip...

I can't imagine that this has anything to do with the O2 sensor, but I'm ready to try anything.

I need to further narrow this b@$tard problem down, and need some professional advice before I hand over my wallet to the dealer.

Many, many thanks in advance.

STEVE I DON'T UNDERSTAND HOW YOU TESTED THE ACCUMULATOR BY PINCHING RETURN LINE!
ON THE BACKSIDE OF THE ACCUMULATOR IS A SMALL HOSE (5MM I.D.) REMOVE THIS HOS FROM THE MAIN FEED LINE AND PLUG THE NIPPLE ON THE FEED LINE. LEAVE THE HOSE UNPLUGGED AND START THE ENGINE,IT IS NORMAL FOR SOME FUEL TO RUN OUT FOR A SHORT TIME BUT IF IT IS STILL LEAKING AFTER TWO TO THREE MIN. THE ACCUMULATOR IS BAD!
KEEP A FIRE EXTINGUISHER NEAR BY AND DON'T USE A DROP LIGHT NEAR GASOLINE!!!!
MF

Thanks for a note on the accummulator test. The Popular Mechanics CD for this car recommended clamping that 5mm i.d. hose to prevent a failed accummulator from dumping its pressure back out into thenon-pressure side of the pumps.

That hose can only be disconnected at the acummulator side.

I should disconnect this hose from the back of the accummulator and plug the hose, right? Then look for continual leakage at the back of the accummulator to indicate it is bad, right? That makes sense.

Did you offer this as a lilely solution, or a different test for an unlikely culprit?

Thanks.

SOUNDS LIKE TOU UNDERSTAND THE PROCEEDURE JUST FINE.
THIS TEST WILL NAIL A DEFECTIVE ACCUMULATOR EVERY TIME!
MF

Tested the accumulator. It wasn't a problem. Thanks for that suggestion though.

Still looking for a solution.:rolleyes:

IF YOUR FUEL PRESSURE ISN'T DROPPING ACROSS THE PUMP/ACCUMULATOR IT HAS TO BE AT THE ENGINE SIDE OF THINGS.
YOU CAN CLAMP THE FUEL RETURN LINE IN THE ENGINE COMPARTMENT (THIS IS TOUGH ON FUEL LINES THOUGH) IF PRESSURE STILL DROPS YOU KNOW IT'S NOT THE PRESS REG.
MAKE SURE THE AIR FLOW SENSOR IS NOT HANGING UP AT THE REST POSITION(THIS WILL ALLOW FUEL TO DRIP FROM INJECTORS WHILE ENGINE IS OFF).
IF SENSOR IS OK HAVE THE INJECTORS CHECKED FOR OPENING PRESSURE AND LEAKDOWN.
MF

Seems logical that the accumulator, injectors and regulator would be the only three possibilities for allowing the pressure to leak down.

Now wouldn't I have other problems if I had an injector stuck open?

Is there any way for the layperson to test injectors for maintaining pressure?

Not necessarily. It could operate properly when running but have a problem closing completely.

It would be nice to have flexible fuel lines so the injectors could be removed with the lines attached. Then you could watch for leakdown.

Sixto
91 300SE
81 300SD

I have seen the situation where the fuel leaks out of the control piston in the fuel distributor. There is no seal on the piston other than very close tolerances and after many miles the piston can leak - which is not really a problem since the fuel simply evaporates and gets inhaled by the running engine. It might explain the leakdown pressure problem.

However - I dont think the leakdown problem is causing your long cranking times. When the fuel pump is started, fuel pressure is almost immediately at maximum. It does not take more than 1 second to reach maximum even if the accumulator is not pressurized.

Can you verify that the fuel pumps are indeed running when the problem happens? I had a similar problem and it turned out to be the fuel pump relay. There were micro-cracks in the solder joints on the board. It could have caused low voltage at the fuel pumps. Anyway, I resoldered the board and everything is fine.

You can build a poor mans fuel pressure tester by getting an old fuel line with the appropriate fitting and welding a pressure guage fitting to the other end. Install a 0-100psi guage and you have it. There are two places on the fuel distributor that can be monitored for pressure. Buy a fuel line from the local MB place and modify it.

Proceedures for testing leakdown rate are on the CD.