78 w116 280SE, cranks, won't start - PeachParts Mercedes-Benz Forum

davestlouis · #1 04-04-2006, 11:11 PM

OK folks, I'll have the 1978 280SE in my possession this weekend...the seller, who is on the forum, states it used to run OK, in the past 2 years, got to the point that it has gas at the engine, and cranks, but won't fire. I looked the car over, heard it turn over, but it would not start. New fuel filter and pump, as I recall, and the seller seems mechanically adept. What steps would you suggest in chasing down the problem? I really want to see it run successfully before I move ahead with any rust repair.

ctaylor738 · #2 04-05-2006, 01:35 PM

1. Make sure the pump is running while cranking.

2. Be sure you have fuel to the engine. Crack a fitting on the FD with the engine cranking and see if there is fuel.

3. Pull a plug out, reconnect it, ground it, crank the engine, and see if you get spark.

4. Put a light on #1 wire and check for somewhere around TDC when cranking.

If you have gas and spark at the right time and it still won't start

- use one of those $4.95 siphons to empty the tank and add some fresh gas
- try disconnecting the safety switch on top of the FD
- go a couple of quarter turns richer on the mixture

davestlouis · #3 04-05-2006, 10:14 PM

Thanks much...I plan to play with it over the weekend and see what develops. I need to pick up the 1978 450SE in the next 10 days or so...that one apparently is in a similar condition...cranks but no start. The seller on that one thinks it needs fuel filter/pump and other fuel-related work.

Tomguy · #4 04-05-2006, 11:17 PM

I always pull a plug after making sure I'm getting spark. It tells so much. If it's wet with gas, there's too much fuel in the engine to fire. If it's bone dry, there might not be any fuel in there at all. You want a consistent dampness or gas smell, but not a drenched plug.

davestlouis · #5 04-05-2006, 11:26 PM

Guys, I really appreciate the input. I have only been accumulating these Mbz cars for less than a year, and prior to that, hadn't done more than an oil change in 20+ years. I am feeling my way through diagnosis and repair, and your help is the only thing keeping me from pulling my hair out. I'm proud of my budding mechanical abilities, limited though they may be. Again, thanks.

cth350 · #6 04-06-2006, 06:56 AM

one step at a time. You've got a few dozen arm chair mechanics here to walk you through it.

Anybody with basic mechanical skills can take things apart. With some care and precision, you can even put things back together again. However diagnostics is all about thinking, asking questions and figuring out the answer.

Think "simple". An engine needs four things to run, air, fuel, spark, and compression. Make sure it has each of the four.

If you haven't invested in a dwell meter, timing light and compression gauge, now's the time. Get a digital meter if you can afford the 50$. It's sooo convenient. Hitting a sears would do fine.

Before rushing to the store, pull a few plugs as other fellows described. Covered with soot is just about as bad as being soaking wet.

Take off the air cleaner and make sure the mechanism for the FI air flap moves smoothly with a gentil touch of your finger. It's one side of a level arm. The other side controls the amount of fuel into the engine. It's possible the mechanism is broken. But, we're not going to care about that for a moment. All we want to know is that it's not blocking air getting in the motor.

Spray a shot of ether (starting fluid) down the throat of that air flap and see if that makes it start for a moment. Nothing like hearing an engine roar for 2 seconds to know that you've got spark & compression, but fuel's weak or missing. Your gas might be too old to work well.

To test for spark, either hold a plug wire close to the motor somebody turns the key, or hook up the timing light to a wire. One needs an assistant, the other doesn't. Also check battery voltage and ignition voltage while cranking. You should see both stay at or above 9v.

Crack open a single fuel line at the fuel distributor mounted on that gizmo. Be sure to use a flare wrench or a box end (preferrably 6-point) on those nuts. They are hollow and don't stand up well to abuse. When retightening, don't over-do it. Fuel should be present at the line.

-CTH

rfitts46 · #7 04-06-2006, 05:17 PM

Turn ignition switch to on but dont turn motor over. Pull the plastic connector off of the safety switch on the fuel distributer. It is in the center of the top right next to the fuel distributer. This will allow fuel pump to run continously. Thisfirst thing pull the return line off of the fuel distributer. When looking at the fuel distributer from the drivers side of the vehicle it is the second metal line from the left ...its on the side of the distributer. This line allows gas to recirculate back to the gas tank. If gas pours out of there...then most likely everything from the gas tank to that point is ok. If ok pull the cold start valve. This is an extra injector that only injects gas for a couple of seconds at start up. If this valve is not working the car will not not fire. Actually if you pour a small amount of gas into the air intake if you have spark the motor should fire. If car does not fire check for spark. Put an inductive timing light on any wire and look for the timing light to strobe. No strobe no spark.

davestlouis · #8 04-06-2006, 08:25 PM

I went back through emails exchanged between the seller and myself...he states that with a shot of ether it will run until the ether runs out. Guess that narrows the problem area.

rfitts46 · #9 04-07-2006, 10:52 AM

thats great u have spark. Go thru the motions I previously suggested. Make sure the gas is getting to the fuel distributer. Check fuel return. If so check the cold start injector. That is most likely not working because the motor would fire if the valve is injecting. Once the cold start valve is working the motor will fire without ether. But most likely not run for long 10 -20 seconds maybe longer. Check cold start valve operation...remove valve and power connector. put 12vdc to the valve if it sprays gas good sign. Once you know the cold start injector is working we have to work on checking your control pressure on the fuel distributer head. Let me know when u get to this point. Bob

davestlouis · #10 04-07-2006, 08:51 PM

OK, let's say for a moment that I need to dispose of the old gas...what the heck do I do with it? We have a waste oil tank at the office, but surely gas shouldn't go in there. I guess I could pump it into cars on the lot that are total losses and scheduled for towing to salvage yards...

davestlouis · #11 04-07-2006, 08:52 PM

next thought, which is worse, a full tank, or a nearly empty tank? I would have to assume that a full tank would prevent most of the rust and condensation that would accumulate in an empty tank, but would leave more bad gas to dispose of.

davestlouis · #12 04-09-2006, 02:14 PM

The car needs a battery...the one that was in it was smaller than I'm used to seeing under the hood of a Mbz. I ran through Walmart, and the battery they show as being correct isn't huge either...did Mbz not use the huge rectangular battery on these gas cars? The insulation on both battery cables is cracked...anything special about the cables that I need to know about?

Larry Delor · #13 04-09-2006, 05:29 PM

If you are still not getting fuel, I would eyeball the plunger in the fuel distributor. A teesny weensy bit of surface rust will keep it from...well, from plunging. I was able to clean one up once (very, very carefully) and that took care of my lack of fuel problem.

good luck!

-Larry

davestlouis · #14 04-11-2006, 09:23 PM

I was puzzled about the small-ish battery in Walmart, so I sat down with my trusty Mbz parts guy...seems the battery tray is sized for the huge 100 amp unit, but the 74 amp smaller unit is correct for my car. Is there any purpose in using the bigger battery? I will order the 74 amp unit tomorrow unless someone gives me a good reason not to...$72 my cost, through the dealer.

psfred · #15 04-14-2006, 10:43 AM

Two problems here, one easy to fix, the other expensive.

The fuel distributor is full of varnish and crap, so no fuel is being delivered to the cylinders. Drain off as much of the old fuel as you can, put 4 or 5 gal of fresh premium in there, and add a whole bottle of techron or Redline injector cleaner. Other brands don't work as well, stick with one of those two.

Loosen the injector lines on the fuel distributor. Let the fuel pump run at least four hours, or overnight (you may need to put a battery charger on it). You must get some fuel leakage. Caution, explosion/fire if you have an ignition source, do this OUTSIDE! When you get fuel leakage, tighten the injector lines back up and loosen the injector end and bleed the lines of crap and slime. You may get fuel moving at once, may take a while, so test occasionally, dont' leave it dripping unattended!

From this point, you should be able to get it started.

If you can't, likely the internal diaphram on the fuel distributor is corroded through -- this is VERY common when these cars sit for years, and a rebuilt one is something like $1600 now, and many of the rebuilts are flaky. If you have access to a high pressure fuel gauge and some fitting, you also need to check both the upper and lower chamber pressures and the main pressue -- if there is no difference between upper and lower chambers, the FD is shot and it won't run until you replace it.

It is also possible that the pressure control valve (frequency valve) is stuck -- this will cause severe mixture control problems, either way lean or way rich (way lean in your case, I believe), but the Techron treatment should free it up. Otherwise, replace if it doesn't buzz in operation.

Peter