300E won't start. Fuel pump? How to check??
 

My 88 300TE wouldn't start yesterday after running fine earlier. Turns over fine, I put a spare spark plug in #1 wire and laid it on the manifold to ground it and it is showing a good spark. The fuel pump is getting power and it runs for a second when you first turn on the key (then shuts off - I assume it has reached full pressure) When cranking, the fuel pump runs, but no start!

For some time now, I have been hearing a slight backfire on starting. It starts, little backfire and dies, then a bit of cranking and it starts and runs fine. Same thing this time, a little backfire, but this time it won't start. I have at least a half a tank of fuel.

It appears to me that fuel is not getting to the cylinders. Is there a filter or trap that could be clogged? Any suggestions for a fix would be very warmly welcomed.

You should also check the rotor button and distributor cap. Because you see spark doesn't mean it is on time. Also, it could be extinquished by the cylinder pressure, IE a coil.

Hi Ron,
I had a similar problem b4 and guess what? the fuel pump relay was bad. Try shorting pin 30 and 87 on the connector(female) plug. You should get fuel pressure on the fuel line. The fuel pump relay is behind the battery..(can't remember the part number). Plug it out and find pin 30 and 87 on the connector. If you do get fuel pressure, the relay is most likely fried. You still can drive the car when you short the pins, but do remember to remove the short once the car is stopped, else you'll fry your fuel pump.
Regards

Update
 

Update to my post: I pulled a plug last night and it was clean and dry. The car also actually started for about a half second then died, so timing is (and has been) o.k. Sure looks like no fuel is getting to the cylinders.
I'll try the relay trick tonight. Many thanks.

I had this problem with my car. I had a spare relay that I swapped in and it started right up. I took the old relay cover off and found one of the pins in the middle of the circuit board was burned. It must have compromised the solder joint. I will be re-soldering the pin and then retesting since I read that others have done this with some success. Evidently, if too much current goes through the relay, the pin solder can melt. If it becomes chronic, then the fuel pump(s) could be bad.

My '88 recently stranded me. It was the coil, at 176k miles. It took the distributor cap & rotor with it; apparently if the spark's not real strong it over-works the cap & rotor, frying them

Just replacing the fuel pump relay is not a direct fix. What made the relay fail? Was it a fuel pump drawing too much amperage? Be sure to check these things before just replacing relay. Its easy to check amperage draw with the proper equipment. Cover the bases and make sure .:D

The fuel pump relay behind the battery is the one with the RPM number on it. You'll probably have to remove the Klima Relay to get a grip on it. However, If you can hear the pump run on its timed function then I doubt the relay is bad. A problem that I've seen several times is one where the hose from the idle control valve to the housing under the fuel distributor ages and falls off. This creates a massive vacuum leak and when cranking the air flow sensor plate stays closed and then of course injects zero fuel. The easy way to tell if your lacking fuel and not ignition is to spray a little carb spray into the air flow sensor below the plate. It should run for a second or two and then die. The car and rotor on these engines are usually done before 100K miles and the plug wires usually won't last beyond 120K. Use only a Bosch cap and rotor, but I find that the Beru wires are the best by far. Good Luck

Another update
 

Another update: Thanks very much for all the excellent help. Much appreciated!

Donnie and Rocket, I should have mentioned that I replaced the cap and rotor at 120K. Yes, I used Bosch. Plug wires are o.k.

Terryloh, your suggestion of shorting 30 and 87 worked. I did so and the car started instantly and ran fine. I’m obviously going to have to replace the relay.

VR, I checked the board with a magnifier, but no solder defects, although that technique does indeed sometimes save you!

Euro, good idea to check the amp draw. I assume that you would run leads first between 30 and 87k and then 30 and 87v to an amp meter. Right? What is an acceptable range of amperes? I don’t want to fry another relay, although from looking at the board, nothing looked burned so it just may be a bad chip or just old age.

By the way, Michaels good comment about the coil going bad begs the question, how do you check it and/or what interval should you change it? Any of you folks know?

Yes you can check amperage draw by jumping 30 and 87. Connect ampmeter, draw should be between 6-10 amps.If it exceeds 10 amps unplug 02 sensor heater filament g3/2x1 and then check. If it is out of spec replace fuel pump and filter, clean strainer on fuel distributor also. Good luck.:D

Checked the amperage draw last night and the highest I got was 8.2 amps, so the failure of the relay was an old age thing rather than being fried.
I ordered a new relay from our PartsShop yesterday and should get it today. Incidentally, Partsshop had the best price on the relay without even a core charge and Phil is certainly a good guy to talk to.
Thanks for all your help, guys and thanks to this great web site for facilitating this forum. It certainly saved me a bunch of money and taught me a few tricks.

Ron,

I sent you an unrelated private message.

Sixto
91 300SE
87 300SDL
81 300SD

Dumb, dumb, dumb!!! Crow is a miserable thing to eat, even with good beer to wash it down.

I did something a bit stupid. I simply didn't notice the terminal marked only 87, I only saw 87K and 87V, so those were the ones that I jumped from 30 to get the car to start.

When I got the new relay, the car still didn't start, so I went back and checked it with a magnifier. The car actually will start when I jump terminals 30 to 87V, but not 30 to 87. It runs, although it won't idle without stalling. I can't find what 87V goes to in my CD Rom schematic.

Since it starts, the ignition and the fuel pumps, filters, etc. must be o.k.

Any constructive suggestions would be very warmly appreciated.

A kind soul told me that 87V is the 12v+ to the cold start valve. Assuming that this is so, tonight I'll check to see if there is voltage to the valve and if it is operating.
If no voltage to it, what condition would cause the fuel pump relay to not send power to it?? (What signal is missing to the fuel pump relay, that causes it to not send power to 87V?)
Any ideas or suggestions would be warmly welcomed?

This is from the wiring diagram for an 88-89 300SE/L:

15 -- +12V from fuse 7, hot in run and start
50 (60?) -- start/cold start (signal?)
81K -- transmission kickdown
31 -- ground
81V -- cold start valve
30 -- +12V from battery
TD -- to AC control relay and CIS control unit from ignition control unit
TF -- CIS control unit
87 -- fuel pump/s and O2 sensor heater

It seems to me that you should be able to start, albeit difficult when cold, and run by jumping 30 and 87. If you don't hear the pumps humming when you jump 30 and 87, your pumps or the wiring to the pumps are bad.

Sixto
91 300SE
87 300SDL
81 300SD