'88, 300TE Won't Start. Need relay help!! Desperate!!! - PeachParts Mercedes-Benz Forum

Ron Johnstone · #1 05-02-2002, 02:38 PM

The car was running fine, just wouldn't start after I parked it for 10 minutes. Problem seems to be in the Fuel Pump Relay input signal as I bought a new relay and it still won't start. Signals on pins are as follows, with key on. 15=+12v, 60= ground, 31= ground, 30=+12V, TF=+1.75V, 87= ground w/key off and open w/key on. 87V=open

If I jump 30 to 87V, the cold start relay is activated and the car will start. Obviously the ignition and fuel pumps are therfore o.k., but something is not signalling 87V to send voltage to the cold start relay, and perhaps elsewhere. I can't see any vacuum hose that is loose or any connection that is questionable.

My CD Rom schematics are not very readable, so I would greatly appreciate any assistance with this nasty problem

sixto · #2 05-02-2002, 04:41 PM

It should start, run and stay running when you bridge 30 and 87.

Does your tach move when you crank and when it runs briefly on the cold start valve? The diagram suggests that if the relay doesn't see an rpm signal, it turns off the pumps. I don't know how to check the TD connection since it's probably a frequency signal. If the tach is working, I imagine the relay is getting the right signal.

Since the power feed to the pumps also powers the O2 sensor heater, how about trying with the O2 sensor disconnected. I think there's a connector under the carpet ahead of the driver's seat. Just a thought.

Sixto
91 300SE
87 300SDL
81 300SD

stevebfl · #3 05-02-2002, 06:07 PM

Let me clarify one of your observations. By jumping the cold start valve you are allowing fuel into the manifold no matter what the pressure. Your system needs at leat 50psi just to open an injector.

If jumping 30 to 87 doesn't cause the fuel pump to reach at least that pressure the engine won't run. By allowing 20psi pressure through the coldstart the engine will fire, probably not run. Without a gauge its tough because you can pump the tank through the system at 45psi and it will never run (unless you jump the cold start).

As a Floridian (transplanted) I never look to the cold start. Cars don't see cold start for at least 2/3 of the year here. Cold start is usually under 59deg F. It IS NOT a part of starting the car above 59 deg F.

Ron Johnstone · #4 05-03-2002, 01:44 PM

If I jump 87 to 30, both pumps do run, but the car won't start. If I also jump 87V momentarily to shove a bit of gas into the engine, it will start.

It seems clear that Steve is right and the pumps are simply not putting out enough pressure to activate the injectors.

Since one pump feeds the other, and they are damn expensive, I would hope that only one has to be replaced. Any educated guesses as to which one to try first? Or, do they kind of wear out together and I should replace both at once.

What kind of life should one expect from these pumps?

Many thanks for the help!

jcyuhn · #5 05-03-2002, 03:35 PM

Well, don't forget to check the basics, such as the fuel filter and the tank screen.

If you're a real cheapskate, you can try the "Geoge Murphy" fix. Remove the fuel pumps from the car. Using a battery, some tubing, and a pan of solvent, jog the pumps _backward_ while running solvent through them. George recommends diesel fuel. I suppose mineral spirits would work OK too. No guarantee, but I have read of this method curing a number of pumps - they spit out lots of nasty black crud, then run fine. Perhaps worth a shot if you have more time than money.

ke6dcj · #6 05-03-2002, 05:39 PM

We had this problem with our 1988 wagon as well at 150K miles.

I replaced BOTH fuel-pumps for about $300. One was definitely not making enough pressure, and I didn't want to worry about the other one, so I replaced both.

I also changed out tank screen (the old one was clean) and fuel-filter so I KNOW everything is clean.

:-) neil
1988 360TE AMG
1993 500E

sixto · #7 05-03-2002, 05:48 PM

A question out of pure ignorance:

Will a ruptured accumulator cause steady state reduction in pressure?

Sixto
91 300SE
87 300SDL
81 300SD

stevebfl · #8 05-03-2002, 06:39 PM

No, the accumulator is a reservoir of pressure, when it leaks it looses its ability to hold the pressure (if the pump isn't running)but does nothing to the pressure if the pump is running.

Ron Johnstone · #9 05-10-2002, 12:39 PM

Update: I pulled the main fuel pump assembly, ran the pump in a bucket of diesel fuel, both forwards and backwards, and got no crud out of it. The fuel filter, however, seemed to be clogged as there was a lot of black debris coming out of it when I drained it.
I got a new filter from Phil (Partsshop) and installed it. The car started right up, stalled a few times, but ran for a while if I kept it about 3000 rpm. (yes, the tachometer works), but finally stalled and now it won't start again, so I'm back to the original condition. I pulled the fuel line from the tank and the fuel flows nicely and it is clean.

I'm about ready to simply install two new fuel pumps, unless someone has a bright idea about something else that could be causing this condition. Thanks for all your help!

Ron Johnstone · #10 05-12-2002, 12:43 PM

I now have two new fuel pumps and a new fuel filter installed and it still won't start!!!
There is fuel in the tank, it will start if I activate the cold start injector, and it started for a minute when I installed the new fuel filter, but then it died. The ignition appears to be o.k. or it wouldn't start at all. It has to be in the fuel system someplace.
Any suggestions?

stevebfl · #11 05-12-2002, 12:57 PM

Test!

What else can one say. Fuel pressure is all you get from fuel pumps. But fuel pressure does not come from the pumps. It comes from the regulator.

I had a 380SL about 6 months ago that had the internal feed to the tank output plugged and fuel couldn't be pumped fast enough. This can not be guessed at. You need to test pressure. If its alright when the car dies or won't run, then go somewhere else.

Ron Johnstone · #12 05-15-2002, 12:34 PM

Unfortunately, I don't have a pressure tester and am working alone, so I kind of have to throw parts at the car. I just bought (from Parts shop) and installed a new "fuel accumulator," which I thought was the pressure regulator. Now I see that there is a real pressure regulator listed under fuel injection instead of fuel delivery.
Question: Is this a part to simply replace or is there any cleaning or adjustment on it? It costs a lot of $.
Exactly where is it located on the engine? There is no photo of it in the parts listing.

Ron Johnstone · #13 05-17-2002, 12:39 PM

Steve,
Quick question. I've located and pulled the pressure regulator. When I mouth blow into either of the side connections, (Fuel pressure line or return from the fuel distributor,) there is a slight air flow between them.

Should there be or is this a leak that is causing the lack of pressure problem?

Many thanks for your help and patience.

Ron Johnstone · #14 06-10-2002, 06:10 PM

For the record, I finally gave up on fixing this myself and had the car towed to a garage. The mechanic worked on it for a week and finally got it to run by pouring gas in it. The car let out a massive backfire, lots of black smoke and it has run fine ever since. The mechanic has no idea of what was causing it, other than perhaps something got stuck "down there".