Simulating a cold engine

[bluewater]

For my 85 380SL, I have a hard time to start the engine when it is warm. But it starts fine when cold. I have read about the causes for this symptom. Without doing the real fix, I am thinking about tricking the system by grounding the temperature switch briefly when starting a warm engine. Is this going to work? Will it cause other problems?

Can someone confirm if the switch (yellow arrow pointed) in the center of the attached picture is the temperature switch? It is on the side of the thermostat housing.

Thanks in advance.

[ctaylor738]

I believe that is the switch for the auxiliary fan. You may be thinking of the oil temp switch on the side of the oil filter housing. You can try disconnecting it and grounding the lead to see if it makes a difference on the start. I doubt that it will help, though.

I can't believe I'm suggesting this, but what about a push button to fire the cold start valve?

[lkchris]

You're not likely having hard hot starting problems because of sensors.

Instead your engine's likely flooded and you can test by holding throttle partially open while cranking.

Flooding is result of leaking injectors and the fix is new ones.

Flooding has drained away and otherwise isn't a problem with cold engine.

Messing with sensors obviously doesn't "unflood" the engine, and in fact will cause an even richer mixture making the problem worse.

[bluewater]

Originally Posted by ctaylor738

I believe that is the switch for the auxiliary fan. You may be thinking of the oil temp switch on the side of the oil filter housing. You can try disconnecting it and grounding the lead to see if it makes a difference on the start. I doubt that it will help, though.

I can't believe I'm suggesting this, but what about a push button to fire the cold start valve?

Interesting suggestion. Where is the cold start valve? Is it the one I circled in the picture?

[ctaylor738]

That's it. It gets power from the fuel pump relay when cranking and ground from the thermo-time relay. You want to supply the ground, which is the brown/blue wire, with your push button arrangement. I think you can just "T" into the wire, then to switch, then to ground.

You might want to try this before making it permanent.

I strongly advise you fix this problem the right way, though. It's possibly a lean mixture, a leaky injector, or the pressure regulator/reservoir back by the fuel pump.

[abiby]

... or you could just unplug the cold start injector before starting warm to see if that helps.

As nice as that would be, my guess is either leaking injector(s) or a bad accumulator and until you determine the root cause, you're just treading water.

[bluewater]

Originally Posted by ctaylor738

That's it. It gets power from the fuel pump relay when cranking and ground from the thermo-time relay. You want to supply the ground, which is the brown/blue wire, with your push button arrangement. I think you can just "T" into the wire, then to switch, then to ground.

You might want to try this before making it permanent.

I strongly advise you fix this problem the right way, though. It's possibly a lean mixture, a leaky injector, or the pressure regulator/reservoir back by the fuel pump.

Thanks. I will make a proper fix for the warm start problem. This is more for my curiosity.

I have done a fuel pressure test. Both system and control pressures are fine. The only problem is that the system pressure drops to zero about one minute after engine is off. So, I am going to replace the fuel pump check valve and fuel filter first. If this is not going to fix the problem, I will replace fuel accumulator.

[abiby]

Originally Posted by bluewater

Thanks. I will make a proper fix for the warm start problem. This is more for my curiosity.

I have done a fuel pressure test. Both system and control pressures are fine. The only problem is that the system pressure drops to zero about one minute after engine is off. So, I am going to replace the fuel pump check valve and fuel filter first. If this is not going to fix the problem, I will replace fuel accumulator.

Dropping that quickly, there's almost no doubt it's the accumulator. Replacing fuel filter and check valve will be a PIA, but doing it twice would be terrible. You might consider changing the accumulator at the same time. Also, replacing all the rubber hoses in the immediate vicinity is a good idea, as disturbing these in a 25 year old car almost guarantees a leak.

[mak]

Intuitive diagnosis says when you replace the accumulator and filter your hot start problems will be rectified as mentioned above posts.
mak

[eddennis]

Are there any instructions for replacing the check valve? I have the hard start problem (only when hot) and have the valve and nut on the way from MB of Napierville, but have not seen any instruction for installing the valve.