I just picked up a 1990 300E (M103 engine) with 188K on the clock at a local estate sale. According to the people I bought it from, the PO "worked on Mercedes for over 30 years". The car appears well-cared for for the age and miles, and it started and ran good this morning when I was eyeballing it. I bought it and came back this afternoon to get it - started up fine and drove it about 7 or 8 miles when it suddenly (and I do mean suddenly) died at 70 mph while crossing the "mighty Mississippi" river. Coasted across the bridge, fiddled with the car a few minutes, then called a wrecker and had it dropped at the shop. Did the usual test to separate fuel issues from ignition issues - sprayed with starting fluid and it starts on starting fluid and runs until starting fluid is removed. Pulled the hard line between the fuel distributor and the fuel pressure regulator and hit the key and I get a strong flow of fuel. I pull a couple of the injector lines and they are bone dry. I can have someone operate the starter and press the air flap all the way down while cranking and get no fuel at the top of the fuel distributor with the injector lines removed. I've unplugged the electrical connector to the EHA and I can still get no start. I have plenty of fuel pressure to the cold-start valve. My experience with the KE-Jet is somewhat limited. When the car was running, it was very well-behaved, easy to start and plenty of power. I can get it to start on starting fluid without a hitch, but as there is no fuel in the fuel injector lines it won't continue to run. Any helpful hints?

Fuel pump (one of two)

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
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The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.

what about fuel pump relay... mine failed recently on the 86 560SEL. thankfully i was just pulling out of the driveway when the car just shut off. i took the fuel pump relay apart and resoldered a broken point. so far so good.

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

Fuel pump is definitely running (seating and unseating the MAS relay was the first thing I did and then checked the fuse on the OVP). Its delivering volume, but maybe not enough pressure. I've got a Matco fuel pressure tester, and buried away in the case is a hose for testing CIS. I guess I need to hook it up and find out what I'm actually getting at the unit. My boss said he has seen the fuel pressure regulator diaphragm break and contribute to messing up the pressure at the EHA.

Fuel pressure is definitely where I would start. Had a 96 Blazer that just quit while going down the highway. Drove me crazy till I got a fuel rail gauge and found out pump was about 5psi too low for the injectors to work. New pump and it was back on the road. My 93 300e used to have intermittent pump problems, but turned out it was the OVP, but the fuse was good, just the relay went bad and would sometimes keep the pumps from kicking in.

I would go with the fuel distributor first if you have fuel all the way up to the engine compartment.

Could be the fuel pressure regulator but I would vote for the fuel distributor.

If the cylindrical part in the centre of the image is leaking fuel out of the plastic hose on the engine side it will most likely be it - The fuel pressure regulator.

I don't have any fuel leaking out of the vent hose on the FPR, so I'm betting that the FPR hasn't ruptured. I had forgotten about this car having two pumps (whenever you have to change fuel filters its always a fight to keep the banjo connection at the pump closest to the filter from leaking). I've changed a couple of the MAS units on cars that came through the shop, and I have one MAS unit that has a good fuel circuit but a bad A/C circuit sitting around to test with. I've got to find a good connection diagram and hook up the fuel pressure test kit and see how my pressure is running, and check the two pumps and see if both are running if my pressure is low. I lucked out in all the nearly 5 years I owned a 190E and never had CIS issues other than the hoses cracking on the idle air circuit and the IACV sticking and giving me all kinds of heck. I really want to get this car whipped into shape - the body is clean with no rust, all the power windows work, the sunroof works, even the power seats work good. 188K is "young" for these cars. We've got a customer with a M104 W124 with over 340K on the clock with the original engine and transmission.

It sure sounds like a fuel pressure/blockage problem. I think a key symptom is the suddenness it came about with, although you might not know if there had been similar occurrences like this prior to you buying it that would make it a more gradual failure. Wouldn't hurt to ask the folks you got it from if they know anything about it.
Crack a few lines on top of the fuel distributor. If you have fuel there it may indeed be a pressure problem. If the regulator seems okay there may be a problem with the fuel distributor. For a start I'd check for blockage in the fuel inlet. In the picture above you'll see the line leaving the distributor to the regulator. On the distributor opposite that line, above the throttle linkage (not visible in the picture) is the fuel inlet. The large nut that the line tightens into is has a fine mesh metal filter built into it that can get clogged somewhat easily with fuel line gel and varnish from sitting or bad fuel, reducing flow and killing pressure before it even enters the distributor. A cheap part at the dealer. The parts guy at my local dealer said he didn't even know there was a filter there, it was the first he'd ever sold.
Might not be the problem but worth a look, especially if the car has sat.
Doesn't seem likely though that the car would suddenly die with a problem like that.

this is something else....

First thing in the morning I'm grabbing the old MAS relay I have on the shelf that has no A/c function but good CIS control. I did a lot of research online, and the 1990 MAS has a "rev limiter" function. The rev limiter kills the second fuel pump and kills injection pressure at the upper chamber of the fuel distributor. Its the only electronic function that appears to kill injection pressure entirely.

I would warn against assuming the fuel pump is good despite it running.

I had a 190e that had an audibly running pump.

Turned out the FP relay would die as it got hot. We replaced this.

This didn't appear to fix the issue. So put the old FP relay in and replaced the pump. No go.

Turned out the pump had been putting more load on the relay - getting it hot and making it crack.

Replaced both at the same time. Problem solved.

There is a test you can do - From memory you can un do the hose from the fuel pump and put it in a bucket. It should pump 7 litres in 1 minutes or something like that. Can't recall - Ill have a look in my workshop manual later.

From my experience cutting out randomly is generally the fuel pump relay or fuel pump.

This '90 300e - would it be a 24valve model or 12 valve?
The 24 valve models have a MAS. The 12 valve models have a Fuel Pump relay and Klima (AC) relay..

You keep mentioning MAS in your posts rather then FPR.

Hooked the gauges up today, 75psi upper and lower chamber - which is why it wasn't injecting. I had a good working FPR handy and tried it first, no dice - still no start. I had an old EHA that I had set aside that came off of a car running too rich due to a bad fuel distributor. I put the EHA on and the car came to life. Its base mixture is off a bit, but it was good enough to start and run the car. I'm ordering a new EHA in the morning.

EHA's are expensive...I would first do an adjustment to do it to bring to pressure spec before you spend several hundred on a new EHA.

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