vapor lock problem 1970 280 SE W108 2.8L MFI M130

[bracurrie]

Car runs very well with smooth idle even when stuck in traffic and temperatures go up. If restarted withing a few minutes of stopping engine starts immediately and idles smooth. If allowed to sit ten minutes or longer but not all the way to cold then starting is a problem. Engine hits a lick when first cranked but then stops firing even if cranking for 15 to 20 seconds. Wait two minutes or so, start cranking and in about 5 seconds the engine will begin hitting on several cylinders and after 10 to 15 seconds will begin to run but not well or not on all six. Another 5 seconds idle smooths out and engine runs normally.
Institutional experience with this problem is lacking with the techs I use so I am looking for a checklist of troubleshooting steps.
Recent work to the car includes all fuel line hoses, fuel pump and injector servicing.

[Sugar Bear]

Did the starting problem occur before the listed repairs or afterwards? It sounds like it may be losing pressure that the pump compensates for when running but not on starting.

Absent gauges installed to watch the pressure stability, powering the fuel pump before attempting to start may help with diagnosis. If running the pump makes it start better then it's losing pressure while sitting.

Later models and IIRC this car also have pressure check valves at the pump and filter. If I had to guess without tests I'd go for replacing the check valve, inexpensive and easy to do.

Yes, it is getting difficult to find experts on these cars

Good Luck!!!

[bracurrie]

Originally Posted by Sugar Bear

Did the starting problem occur before the listed repairs or afterwards? It sounds like it may be losing pressure that the pump compensates for when running but not on starting.

Absent gauges installed to watch the pressure stability, powering the fuel pump before attempting to start may help with diagnosis. If running the pump makes it start better then it's losing pressure while sitting.

Later models and IIRC this car also have pressure check valves at the pump and filter. If I had to guess without tests I'd go for replacing the check valve, inexpensive and easy to do.

Yes, it is getting difficult to find experts on these cars

Good Luck!!!

Thank you Sugar Bear! Lack of pressure that allows for fuel to vaporize in the lines is a logical conclusion.
What is IIRC?
Brad

[Idle]

One thing to check would be your cold start system. This would consist of two major parts: The thermotime switch and the cold start valve.

The thermotime switch is located on the left side of the block just behind the injection pump. It looks like a sensor with a wire going in one side and a wire coming out the other. You would have to pull it to check this, but one side will be marked G and the other marked W in very small letters.

Testing this is not hard but it requires a volt meter and a 12v power source. It will test out in a range of 25 to 62 ohms. If it reads zero ohms it is shot and will need to be replaced or rebuilt. Replacing it easy, rebuilding this is an art.

These are not cheap but they do rarely fail. These were once about $1,000 each when you could get them. Today good used ones sell for about $300 and up. If you need one then get a money back offer before you buy. Most people don't know how to test these correctly and many are offered for sale as 'untested'.

The other thing to check is the cold start valve. It is located on the right side of the intake manifold. It will be a large round thing with wires running to it and a fuel line. To test this is easy. Just hit the terminal with 12v and you will hear a slight click. If so it is working. To test it for real remove it from the manifold and place it in a large container. Hit the terminal with 12v and fuel should spray out. If the system is holding pressure it will spray out at 42psi or so.

On each start attempt you are shooting in a bit of gas but not enough to overcome the cold condition. Hence the need for a cold start system. Once you have built up enough gas vapor the engine will start and run.

This is an overlooked part of the MFI engine when these type of troubles begin.

[Sugar Bear]

Brad IIRC = If I recall correctly. Pull the dipstick, check to see if the oil smells like gasoline.

Idle, Do you think the injection pump thermostat under the two rubber coolant hoses at the back of the injection pump would cause this?

[t walgamuth]

Hard warm starts are a common problem with gas engines of that era. In my 84 500sec I fought it, spending some money trying different fixes with no love.

I finally stopped shutting it off if I was not going to be able to let it cool down before restarting. After that, no problem.

[Idle]

Originally Posted by Sugar Bear

Brad IIRC = If I recall correctly. Pull the dipstick, check to see if the oil smells like gasoline.

Idle, Do you think the injection pump thermostat under the two rubber coolant hoses at the back of the injection pump would cause this?

Yup. This thermostat is always varying the amount of fuel into the engine based on what Mercedes calls 'running condition' but the only time it ever comes into play is if the engine is hot or cold.

If the engine is hot the thermotime switch will not activate the cold start valve because the extra shot of gas is not needed. If the engine is cold the MFI thermostat will increase the amount of fuel injected because the engine will need an extra amount to fire and keep running. But it will not increase the fuel to the point the cold start valve is not needed. It will kick in after the engine is running to keep it running.

The cold start valve would flood the engine if it stayed open. There would be a lot of black smoke.

The worst case is there could be several problems. The cold start valve is easy to check so I always check it out first. If it is working then the thermotime switch is working. If it is working and the engine still will not start when cold it is time to move on to the thermostat of the MFI pump.

And the black smoke thing is important. IF the cold start valve is staying open too long it would, as I said, flood the engine. But the engine could still start. It would just be blowing a lot of black smoke out the tailpipe.

I would still check the cold start valve function first. If all is well there move on to the MFI thermostat.

And thanks for bringing this up. It has been about three years since I last worked on one of these and it turned out to be a bad thermotime switch. So I didn't have to go any deeper.

[PreferVintage]

This is an interesting thread...I had a similar problem with my 280SE, but it appeared to be due to a restriction in the fuel line. On the drivers side, underneath the car, the fuel line was almost crushed shut due to some whack having tried in the past to lift the car right on the fuel and brake lines...after I replaced the line, no more hot start problems...maybe some kind of vapor lock??

[Idle]

I once had a 123 240d that had a fuel restriction. I remember cleaning every filter I could find and then still having the problem. But I read a thread about cleaning out the steel fuel line from the tank to the engine using a .38 cleaning brush.

I disconnected both ends of the steel line that runs under the car. Then took a long piece of steel cable and ran it though the line. To the end of the cable I attached the cleaning brush and then pulled it through several times. It would be like cleaning the barrel of a .38 that was several feet long.

And it worked. A lot of gunk came out. How the line became gunken up while the tank screen was clean is a mystery. I guess years and years of use will do this.

But I have never had this trouble on a gasoline car. I have used the same trick if I was that deep into the car that I could easily do this and the lines always were clean.

[Diesel911]

Generic. On the old carbureted engines you could tell if you had a vapor lock if you poured water on the Fuel Lines (to condense the fuel) going into it from top to as far as you could get at. Keeping the Water away from the Ignition components.

Back in the 1960's I have I have seen people wrap aluminum foil around the fuel inlet lines and hoses to deter heat absorption from the Engine.