150 efi starting problems

[bassjunkee462]

I have a 1996 Merc 150 EFI. I am having starting problems when the weather is hot and muggy and seems to get worse as the day goes on. It will act like it tries to start but won't. Here is the weird part. I take off the cowling and it will start right up and then blows quite a bit of smoke like it was flooded. Put the cowling back on and off I go. Does not seem to act up during the cooler weather. All of my cowling vents are clear and I even removed the foam lining from the inside. What the heck can it be ????

 

[galamb]

It could be a few things. When the air is warmer your engine needs more of it to get the air/fuel ratio correct.

Some of things that could cause your problem are:

bad air temp sensor (which tells the ECM the air temp - the ECM then adjusts the air/fuel mix accordingly) - when the sensor "dies" or the harness is bad or the sensor is unhooked, the ECM assumes it's 32 degrees (zero celcius)

dirty or malfunctioning throttle body (lets the air in according to what the ECM is calling for to mix with the fuel supplied by the injectors)

That's the top two things that come to mind (since more air (cover removed) seems to fix the issue) but those are just "guesses".

Take it into a Merc dealer and it plugged into a DDT (engine computer) - only way to know for sure. With computer controlled, sensor driven motors, the days of tweaking with a screwdriver are long gone...