Car suddenly won't start!
 

So I was driving my 84 500 SEL one night and it was driving perfectly fine, nothing seemed different (it felt just as powerful as any other day), then I got home and parked it. The next morning, I tried to start it. The engine still cranked strong and roared to life, but only for half a second, and then promptly died. If I try to start it again right after it does this, it will only crank and will not sound as if it is roaring to life again. But if I wait about a minute or two and crank it again, it will roar to life again, but only for that half a second. So what could be the problem here? The things I have checked already that are fine include the distributor cap and rotor, fuel pump relay, spark plugs, and idle control valve. What do you guys think?

Fuel pump or coils most likely if nothing has changed.

What symptoms would point to the fuel pump as opposed to the ignition coil, and vice versa?

Well you need air, spark, fuel to run.

Pull a plug wire and stick a spark plug in it, then ground it on something and see if you have spark. Easy. Or safer is the inline plug tester.

OR

Spray some starting fluid down the CIS flapper and see if it fires.

When was the last time you changed the fuel filter? A big chunk of something could have gotten lodged in it--and if it has been a while, good idea to replace it anyways.

I don't have that car but it sounds like fuel to me. I like the clogged filter idea. Also, see if you can verify that the fuel pump is getting voltage the whole time and not just at start up. There might be a safety cutoff circuit acting up. Just a guess. Maybe wire a volt meter right to the pump connections.

Can you hear the fuel pump kick on? Sounds like a classic fuel pump relay failure to me, even though you've checked.

it is the fuel pump relay.

Is there a way I can temporarily jumper and bypass the relay to see if the car will start? My relay has 7 pins (4 on one side then 3 on the other).

yes look at bottom of relay and you will see numbers.right in the middle there will be two pins opposite each other numbered 87 and 30 use a pair of needle nose pliers and stick ends in female holes for those pins.you should hear fuel system running even with key off.

I tried jumping the female holes for pins 87 and 30 on the relay, but no luck. The car still didn't start, although I did hear the fuel system running as you said. So does this mean it's most likely the fuel filter then?

Sounds like you already checked for spark and it passed, if you have pressure gauges I would check the pressure at the fuel distributor, that would tell you where/if the fuel is losing pressure somewhere in the line. If your car has dual fuel pumps (I'm not sure on the w126, I know on the w124 they switched around 1990) perhaps one of them has gone out and the other is still working, giving enough pressure to start but not enough to sustain the engine running, and you would hear the system kick in still from the one pump. I caution you, that is just me thinking off the top of my head - I would still perform a pressure test.

If your fuel filter hasn't been changed in recent memory (say 15-20k miles), its probably a good idea to put in a new one anyway, since it is niether expensive nor difficult and is a maintenance procedure.

P.S. are you in my neck of the woods?

go buy a can of carb cleaner,take off air cleaner,have someone start car and spray carb cleaner at the plate.see if you keep spraying judicially that the car keeps running and when you stop spraying the engine dies.now you have proved that fuel is the problem.

Would it be alright to use starting fluid instead of carb cleaner?

same objective of fueling the engine but carb cleaner will clean the intake bore as well,a better choice .
mak