1966 250SE Stalling Issue

[_Dirty_]

Hey Guys,

I recent purchased a 1966 250SE with the M129 engine.

A buddy of mine is a mechanic and has been helping me bring the girl back to it's original state mechanically.

So far we have replaced:

Spark plugs with NKG plugs
Spark plug wires
Distro cap and rotor
Fuel filter
Intake filter

Performed an oil change

When the car is cold it runs and idles like a champ, however once the car warms up it sputters and stalls out.

I took the car for a drive and after about 10-15mins of driving I lost all power from the engine, she stalled and didn't want to turn over.

Any ideas what could be causing the problem?
Areas to look into next and possibly replace/clean?

Any tips would be appreciated


Thanks,

G

[meltedpanda]

have you checked you compression? Valves been set ? ( set them cold)

[JPMOSE]

I had that problem when I purchased my 250SL in 2006. It turned out the return fuel line was blocked. If the car seems to be running too lean upon brisk acceleration....that's a sign of it! The car builds up a lot of pressure in the fuel system when there's a blockage and it takes a few minutes to occur (seems to run fine up to that point). You can disconnect the return fuel line (near the fuel tank) and place it in a bucket (or something that can collect some fuel). Have an assistant start the engine and see if fuel flows freely. Of course, there are more sophisticated ways of checking fuel pressure, but the method above is how I determined it.

In my situation, the prior owner's mechanic had removed the fuel injection pump and sent it out to be rebuilt. The mechanic placed a small steel ball in the return line at the injection pump side (I gather to prevent debris from getting in the line) and forgot to take it out when reinstalling the rebuilt pump!

Of course, there could be other problems that are causing this situation. Just be mindful of replacing a lot of parts as "trial and error" because they are quite costly! Good luck!!

[_Dirty_]

thank you gents! I'll check both.

[JPMOSE]

Please look at the following from the W113 site:

Pagoda SL Group Technical Manual :: Fuel / Start

Under Fuel tank (and other spots) you'll note references to checking fuel lines for blockages, even within the tank (your 250SE has the "flower pot" design in it's fuel tank as well).

[Idle]

I have seen this in one form. When the car is started it runs ok but after it is driven it will die when you let off the gas or stop. It is then difficult to start but it will start.

Every time it has been low compression. Once the valves were just worn out and once it was a blown head gasket.

There can also be a problem with the carb linkage, but that is likely not the case if the carbs are set up right. You could have a choke lever that has fallen off or is binding so take a look at all the little choke rods. Make sure they are all still attached.

[Frank Reiner]

Originally Posted by Idle

I have seen this in one form. When the car is started it runs ok but after it is driven it will die when you let off the gas or stop. It is then difficult to start but it will start.

Every time it has been low compression. Once the valves were just worn out and once it was a blown head gasket.

There can also be a problem with the carb linkage, but that is likely not the case if the carbs are set up right. You could have a choke lever that has fallen off or is binding so take a look at all the little choke rods. Make sure they are all still attached.

"250 SE" = injected = no carbs!

[_Dirty_]

Idle I appreciate the help, however Frank is right. My car is injected

[Idle]

I missed the E.

Check the back of your injection pump. If it is a US car there should be two solenoids on the rear of it. The bottom one is the deceleration valve.

These close the rack on the pump to full lean upon deceleration to help with emissions. Then they move the rack again to allow fuel to flow once you touch the gas pedal.

But they stick when they get older, or they fail to kick in when they should.

Some owners disconnected them since they do nothing but help with emissions. So you might try just pulling a wire off of the thing and see if the problem stops. If so you can then start shopping for a new one. If not I am afraid you might have low compression which can hopefully be corrected by a valve adjustment.

Are you getting a lot of smoke on start up? If so you might have worn valve guides which could be causing your compression problem. The good news is if you have someone that knows what they are doing these can be changed without removing the head. This is a not a job for the average mechanic but it can be done by those with the training and equipment for far less than a total head rebuild.

[Frank Reiner]

Originally Posted by Idle

Are you getting a lot of smoke on start up? If so you might have worn valve guides which could be causing your compression problem. The good news is if you have someone that knows what they are doing these can be changed without removing the head. This is a not a job for the average mechanic but it can be done by those with the training and equipment for far less than a total head rebuild.

An illustrated write-up of the indicated process would certainly make fascinating reading.

[ScooterABC]

Originally Posted by _Dirty_

Hey Guys,

I recent purchased a 1966 250SE with the M129 engine.

Spark plugs with NKG plugs
Spark plug wires

When the car is cold it runs and idles like a champ, however once the car warms up it sputters and stalls out.

First of all, what specific spark plugs and spark plug wires did you use? This car should have non-resister plugs and wires.

My first thought on your stall-out is totally different from what others are saying about compression. I'm thinking that the thermostat on the injection pump is stuck and it's causing the engine to run as if you had a choke on the car when it is warm.

[Idle]

Originally Posted by Frank Reiner

An illustrated write-up of the indicated process would certainly make fascinating reading.

The service manual has all the details.

Anyone who has used liquid nitrogen would be able to do this with little trouble.

[Frank Reiner]

Originally Posted by Idle

The service manual has all the details.

Anyone who has used liquid nitrogen would be able to do this with little trouble.

A link to the manual pages and/or scans of the relevant pages in your possession will go a long way in establishing the credibility of the above claim.

[_Dirty_]

Hey Guys,

Spark plugs are Bosch plugs which were recommended by factory, spark wires were actually not touched, return fuel line is not plugged, no smoke upon starting, power is going to electric fuel pump. However we have no way to check pressure after the fuel pump.

Not sure what else to check

[meltedpanda]

Originally Posted by meltedpanda

have you checked you compression? Valves been set ? ( set them cold)

have you done this?