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Old 09-27-2015, 12:06 PM
Registered User
Join Date: Jul 2015
Posts: 138
w111 start issues

Hi all,

A quick intro…I have been very lucky to find a '71 w111 coupe with the 3.5 motor in a barn near me. It has not moved for 26 years. It was laid up after the shop rebuilding it went bust. I have spent the summer getting it ready to start, having flushed the tank, replaced the fuel pump, all fuel lines, all fuel filters, freed the injectors, reseated them, changed the oil/filter, rebuilt the AAV, rebuilt the throttle pot and tps. New plugs and ht leads. Rebuilt the dizzy and cleaned the injection points in there. I have cleaned every electrical connection I could find, etc etc. Last week I got the car to happily turn over on the starter, after wrestling with the gear selector lock out on the gearbox (column shift auto).

Today I went to start it properly. This was a big deal for the car, and me. At first I had no power to the fuel pump, which I traced to no earth. I connected the various earth cables in the boot/trunk, and had power to the pump. In ignition switch position two it hummed happily away. The thing is, it did not stop. I was under the impression that the pump initially pumps to get the pressure up, then turns off, starting up again when the car is running. I tried to start the car, and had no luck. Drained the battery trying. Had good oil pressure though. I have not checked for spark, but my question is, what would cause the pump to keep pumping in ignition switch position two? ECU? Main ignition relay? FPR? I had fuel in the lines, as there was a small leak from the fpr line.

I am new to these cars, although not new to old motors. I have a 1959 Bristol also, with a 5.9l mopar (simple as a stick), and two Volvo amazons…a 1965 and a 1970. Simple too. The complexity of the Merc has me scratching my head often, but I'm loving bringing it back to life. If anyone has any tips about the fuel pump situation I'd love to guys and girls really know your onions.


Dave's a pic:
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w111 start issues-merc.jpg  
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Old 09-27-2015, 07:00 PM
Registered User
Join Date: Feb 2011
Location: Modesto CA
Posts: 2,816
Re: No start
Two electrical events are required that you have not mentioned as being present; pulses to the injection valves, and ignition sparks.

Begin by checking for spark by pulling off a spark plug lead, and inserting a spark plug. Lay the plug on the cylinder head/exhaust manifold so that the plug shell is grounded. When the engine is cranked, you should see a spark between the plug electrodes.

The injection valves should audibly click when the engine is cranked. Some sort of stethoscope device pressed against the valve will help in hearing it. The injection valve circuits are grounded (-) at the cylinder head, and the power (+) pulse is delivered by the ECU. As a consequence, the pulses can be seen with an analog voltmeter (galvanometer type). The meter needle will deflect slightly with each pulse.

The timer for fuel pump shutoff is in the ECU.
If there are no injection pulses, and no pump shutoff, it is possible that there is a fault in the ECU.
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Old 09-28-2015, 12:37 PM
Registered User
Join Date: Jul 2015
Posts: 138
Thanks for the reply Frank.

When I got the car the ecu was unplugged for some reason. Maybe there was a problem before, I don't know. Suffice to say I will now turn my attentions to that box (briefcase) of tricks. I didn't look/listen for the injectors pulsing, I was preoccupied with the pump.

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