Car suddenly would not start!? M103 W126

[86560SEL]

VIDEO of the car trying to start below....

Drove my moms car today (1988 300SEL) because my work car is still in the shop.

Anyway I drove the car to work, then to my grandmothers after, then to the supermarket after that, it ran perfectly. Came out of the market and drove across the parking lot to get gas and I filled it up... then I went to start it and it only cranks and will not start! Toward the "end" of when I am turning the engine over, it will occasionally and very momentarily sputter and stall.

I am guessing fuel pump, fuel pump relay or? I have no idea. I know I have owned the car nearly 3 years (except the past 3 months she has had it) and its never gave me or her a problem until now. We had to push it away from the pumps and to a parking space in the store lot until we can try to figure out where to start or a shop to take it to, but trying to get some ideas. Oh, also the check engine light comes on momentarily when first started after you have been driving for a minute and goes off after 2 minutes, but not sure if the problem is related or not?

Thanks in advance for any help!

[86560SEL]

More research on other sites and most say it sounds like the FPR (fuel pump relay). A new one is $87. I am wondering if a used one would be worth trying to get cheaper and wondering if this was the same on all W126, or did it vary by year/engine size? Meaning would one from a 1985 380SE work? Or would it have to be from another M103 or at least another 86-91 W126 model, or?

Also going to mention that when the car was turned off, the engine was in normal temperature range and there were no odd noises or warning lights illuminated.

Thanks

[Rahulio1989300E]

I think an easy way to increase the confirmation that it is indeed the fuel pump relay is to try to start the car from a stone cold engine. If it starts and runs well until it heats up in the engine bay, then it could in fact be the FPR. A used relay should be fine, but that could have limited life...

I believe what happens in the relay is that the solder joints develop cracks. As the reply gets warmer from the engine bay heat, the cracks expand and eventually severe the connection thus terminating the fuel pump. You could buy a good used unit, fix the car, then take apart the old one, re-solder it, test it and keep as a spare in your trunk.

[86560SEL]

Thanks. Just got back from trying it again (cold engine, set all night) and it ran for about a second and stalled and would not start back.... ???

Originally Posted by Rahulio1989300E

I think an easy way to increase the confirmation that it is indeed the fuel pump relay is to try to start the car from a stone cold engine. If it starts and runs well until it heats up in the engine bay, then it could in fact be the FPR.

[S-Class Guru]

It's really easy to check if the fuel pump is running.
Turn the key on, and you should be able to hear the pump hissing with the driver's door open. Or get a friend to stand by the right rear and listen for it. The pump is in the rear to the right of the gas tank.
It should hiss for about 3 seconds, then stop. just turn the key off and on, and each time the pump should hiss for another 3 seconds.

Along those lines, if you hear it run, then turn the key on/off a few times; this will prime the system, and may overcome a clogged filter enough to get it to fire. If indeed it does fire and run a bit, then the filter will be the prime suspect. But for that big ol filter to go from fine to totally clogged is unlikely.

Another member put it quite well the other day - make sure the ignition system is functioning before messing with the fuel system. So, if the pump hisses, try looking for spark. Easiest way is to use an old plug - pull a wire and plug in the old plug and lay it on the exhaust manifold. Turn the engine over and have someone look for spark.
But, you say it did fire just a bit - so may indeed be fuel.

If the pump does not hiss, then the relay is probably the culprit, as suspected. I can't remember if the '86 has the separate relay, or uses the MAS relay. The MAS is easy to spot. You can do a search here and see a photo. You can pull the MAS out and short the fuel pump wires to get the pump going - I can't remember the two terminals to short (old timers' disease), but I'll look it up.

DG

[S-Class Guru]

Okay, the '86 560 has a separate fuel pump and cold start relay - N16.
you can pull it, and then short terminals 30 (12V) and 87 (fuel pumps, you have two) to "fire" the pumps (pardon the pun, be careful).

DG

[86560SEL]

ok. here is a short video of the car "trying" to start. Maybe someone can tell by the sound of what it may be. The battery on my camera was dead, so I had my neighbor (who also has a W126)take this video for me... thats who you see running around. lol.
just click on the pic and it will load from photobucket... you can hear the car will try to start at the end of the cranking, but it stalls back out...
remember this car was starting perfectly until I stopped last night at the gas station and filled it up with gas.

We are going to check for "spark" tomorrow and go from there.

[Cal Learner]

I don't find any fault with S-Class Gurus advice for the M103 motor. Your car does have the separate fuel pump relay and one fuel pump (unless retrofitted with 2). MAS started with MY90 or thereabouts. Since we don't know the condition of your ignition system, plugs, wires, cap/rotor, ignition coil and crank position sensor are at the top of my list of suggestions. If those turn out to be OK, a weak pump, intermittent FPR, clogged filter or even a collapsed fuel line are possibilities, before getting into the big $$ items like fuel distributor.

[tjyak50]

I had an identical situation a month ago. However in my case I had been replacing parts (Fuel dizzy, EHA, injectors, etc). Drove it for a few days and then one day just wouldn't start.

Its pretty much gotta be 1 of 2 things: Fuel or Ignition.
Assuming your airflow plate is moving freely?

I confirmed the fuel pumps were running and even jumped the relay to manually keep the pumps on with no change in starting.

I pulled a spark plug and it was BONE dry.
This indicated to me there was probably spark, but no fuel going to the cylinders.

I Jumped the fuel pump relay to engergize the pumps and depressed the airflow plate and nothing happened. The tell-tale "singing" of the injectors wasn't there. They make a distinct sound with the pumps on and the more you deflect the airflow plate (you will be flooding the cylinders).

I opened the fitting on the side of the fuel dizzy for testing fuel pressure with a 12mm socket, relieved all the pressure, closed it and tried again. Fired right up.. Never done it again.

Maybe the EHA was stuck? or the DPR?
Got me, never done it again, but I re-adjusted fuel pressure differntial after that.

Pull a plug first: Wet = ignition or flooding. Dry = fuel flow.
Wish I could be there to help.

[86560SEL]

Originally Posted by tjyak50

Fuel or Ignition.
Assuming your airflow plate is moving freely?

He took the air filter cleaner off and he said it was like it was spring loaded... it went down, but came back up.

[86560SEL]

Update - We checked and its getting fire to the plug wires. I dont know if he removed the plugs to see if they were gassy or not... I will check.

Nope, dont have a fuel pump jumper or any wire... maybe I can find some.

edit - I have discovered that this is actually the A/C relay by another forum and the poster below - thank you.
Anyway, is this the fuel pump relay? I just removed it and took a pic of it. (It was the one closest to the brake booster). Appears its been tested/replaced before... is there any way to test these?

To recap... we have fire to the plugs, fuel pump is "hissing" when switch is on... car will "try" to start, but will not. Maybe at this point replace the fuel filter and see if thats any help?

Thanks once again!

[mak]

most likely it will turn out to be the fuel pump relay if its still the original old one.
For testing i had replaced mine ,a 9 pin with a !0 pin one .it worked perfectly except it will have a lower cut off point of 4500 RPM ;4 & 6 cylinder variations
mak
300se

[tjyak50]

A Fuel Pump Jumper is a must-have part in your trunk. Do you have one?
I use a piece of good heavy wire with about 1/4" of exposed strands folded back againt the insulation.

See below.
With this you can do some more manual troubleshooting of the fuel system.

This piece of ware is about 3" long.

[86560SEL]

Thanks for the replies. I will relay all of this to my dad. I am not much of a mechanic and though I have owned 4 Mercedes over the past 10 years, I have really never had any problems until now, so not much experience with them in this area.
Thanks again.

[Rob Pruijt]

If you engine has no big mechanical failure, it either has no good ignition or no fuel.

1 Ignition.
The best way to check the ignition is with a stroboscope lamp (oscilloscope is even better, but very expensive).

The second way is;
- find about 5 feet of metal wire
- take all plug out
- Put the caps back on
- fix one end of the wire to the engine
- wrap the wire around the metal part of the plugs (upside down)
- Make a string of all six plugs with the wire and fix the other end
to the other side of the engine.
- Pull the fuel relay
- Have someone crank the engine and watch the sparks. They
should all fire (powerful spark)

Do not stand to close to the engine, some fuel/air mixture may come out and ignite!!

Put the plugs back in and reinsert the fuel relay.

2 Fuel delivery.
- Take two open end wrenches and loosen the fuel lines from the
injector (just a little bit loose).
- Have someone crank the engine and see if fuel seeps out of the
lines.


Rob