W111 Dies when hot.

For about the last 6 months I’ve been trying to solve a problem with my 1966 W111 250se Coupe.

This has the 6-cylinder engine with mechanical fuel injection.

I’ve owned the car for about 15 years and have done regular maintenance and it was running well until about 1 year ago.

When it starts it idles well and reeves fine and has good power for about the first 8-10 minutes’. It idles at about 900rpm.

As it warms up between 180 deg Fahrenheit. (82deg. Calcius) To 190 deg Fahrenheit (88deg. Calcius) it starts to idle rough and needs more revs to run smoothly. So at 185 deg. Far,. It runs fine at 2000rpm but dies below this. At 190 deg far. It runs fine at 2200rpm but dies below this. At this temperature it wont normally start again until it cools down.

At this stage I have removed the fuel pipe into the injection pump and the fuel flow is good (maybe 1 Litre a minute and good pressure).

I checked the ignition points and they look fine. I had a local mechanic check the Spark plugs and they are set fine. I’ve installed a new coil and distributor condenser.

The fuel system has been flushed including draining the tank and new filters.

The Air filter has been replaced and should be fine. I have run it with the fuel cap removed in case there is a problem with the fuel tank equalization, but no difference.

I’ve been told before that for a good engine to run it needs 3 things: Air, Fuel and a Spark, So it’s a matter of elimination.

I don’t believe its an Air issue as the engine runs fine when cold and still runs well at high revs when its hot.

I don’t believe it’s the spark as it runs fine at all engine speeds when its cold. I don’t believe anything changes the spark as the engine warms up?

So that probably aims at fuel delivery; so the question for me is what changes happen with fuel delivery after the engine has been going for more than 8 minutes and reaches 180 deg, Fahrenheit.

Could it be the Injection pump or any relay?

I live in the South of the South Island of New Zealand and would be really happy to pay for professional help but the nearest mechanic who knows these vehicles would be in Sydney Australia more than 2000km away. Most local mechanics know about classic Carburetor engines but nothing about mechanical fuel injection.

Any suggestions would be appreciated.

 

You gave this well defined. One detail is getting marginal.

Seems heat is interacting with fuel delivery.

-- Is your thermostat opening up well? Do you feel the hot radiator hose warming up suddenly when Tstat opens up?

-- Is your ignition a set of points switching a coil connected to rotary distributor with good plug /wires... ? Should be no problem here.

-- You do not have any O2 sensors, right? Is the injection leaning out mixture based on engine temp sensor?

• When that happens it reveals a marginal FUEL PRESSURE issue!!
• How many pumps is this...2x?
• tank + injection pump ?
• Does this injection use a "Return REGULATOR"... See if gasoline is found in vacuum line side
• else monitor commun pressure as heat rises

Don't overheat engine under these conditions.

 

Your symptoms seem related to the cold start system. In the photo, refer to the vertical cylinder on the far right with the two cooling system connections. That contains a wax bulb, similar to that found on the cooling system thermostat. As the coolant warms the expanding wax extends a pin into the governor section of the injection pump where it acts to adjust the fuel mixture. On a cold start the pin is withdrawn and the governor delivers a richer mixture. As the coolant warms the pin extends and the governor leans the mixture. Normally failure of the bulb would lead to a very rich mixture and poor fuel consumption when warm, so I doubt that is what is happening. But understanding the function of this device in the cold start system is important. I suspect there is wear or damage in the governor section of the pump that is allowing the mixture to go too lean as the coolant warms and the pin extends. I would seek an experienced specialist to look at the injection pump. Do you know if the pump has ever been serviced? One thing most folks don't know or choose to ignore - there is a dipstick on the governor section. The governor takes a small amount of engine oil to reduce wear of the governor internals.

 

that's a clever way to manage mixture without lambda feedback.

OP reaches leaner stage so that system is switching as expected between Rich <--> Lean.

Issue likely fuel pressure: pump or regulator.

 

I sold all my old factory manuals years ago, but a search found this on the SL Pagoda Group website:

The MB "Technical Data Passenger Cars" has the following for the fuel pump

Feed pressure (after fuel fine filter) - 0.8 -1.1 kg/cm2 (perhaps they meant the fuel line filter?)
Feed final pressure (after overflow valve of injection pump) at least 1.3 kg/cm2
Feed capacity at least 3 liters/min

It is absolutely good advice for him to confirm the above specs. However, from what the OP states, the engine runs well at higher RPM when fuel demand is greater, leading me to think the problem is elsewhere. The mechanical injection pump is just that - mechanical. Pressure to open the injectors is created by the plungers in the injection pump. The governor section controls fuel delivery quantity and "inputs" (such as the cold start thermostat) to the governor are mechanical. While the thermostat itself appears to be functioning correctly it is acting on a mechanical device subject to wear. With a pump of this age (circa 1965/67) there is a definite possibility that the thermostat is causing the governor to "over correct" the position of the fuel rack, thus leaning the mixture too much. Or - those are pretty rugged pumps, but the plungers could be worn, the rack worn, the three dimensional cam in the governor has worn, etc. If the OP finds the fuel delivery within spec, next stop should be a specialist - which may be difficult.

I would also suggest the he join the Pagoda SL Group. Mechanically the W113 is virtually identical to the W111/112 cars. Lots of good stuff on there. The are probably the best source for this cars .https://www.sl113.org/