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mercruiser 170 power loss when hot


oldtom's Avatar
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Join Date: May 2005
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06-20-05, 02:57 PM   #1  
mercruiser 170 power loss when hot

Have a 1986 Mercruiser 170 (4cyl), I/O Alpha 1, closed cooling system (antifreeze) for 5 yrs. Previous owner (original) warned us that he had an engine run problem sometimes after a hard run.

When outside temp gets hot 90+ degrees and engine runs for maybe 1/2 hr the engine will stall at low speed. Starts right up and acceleration OK in neutral, but stalls in gear. Can play with the accelerator (pump it lightly) while in gear and get it going and runs OK at high speed. Waiting about 1 hr until things cool down returns the operation to normal. This has been going on now for 5 yrs.

We have been through 3 tuneups (plugs, points,cap rotor, cap), wires, coil, filters, batteries, 1 voltage regulator, clean gas tank, carborator cleaner, out unit servicing, sea water pump. Engine temp is normal at 160 degrees.

We are suspecting a vapor lock problem since we don't seem to have the problem in cooler weather. Any suggestions on a test method and fix??

Our mercruiser service book talks about a vapor lock problem on 4 cyl engines; Mercruiser fix with a high volume fuel pump kit (#42725A5) and water cooled fuel line. They claim Mercruiser made this a standard item on engines made after Aug 1986. Ours was made in late 1985. I've tried 2 Mercruiser mechanics and they have never heard of a vapor lock kit or water cooled fuel line.

Does anyone have any experience with this?

Tom

 
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oldJoe's Avatar
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06-21-05, 07:37 AM   #2  
Vapor Lock

This was not uncommon in autos years ago

My understanding of the problem was a fuel line was routed too close to a high heat object, usually an exhaust manifold. When the engine was stopped and there was no cooling airflow , the fuel in the line quickly boiled. The vapor lock was the result.

Then it was apparent why autos after 1970 bring fuel lines into the engine compartment below and away from the engine, approaching from the front of the block directly behind the fan. (No fan?)

Perhaps the solution is to change the route of the fuel line using of course, safety approved materials

 
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06-21-05, 01:58 PM   #3  
Vapor Lock

Thanks for the reply. The carb, fuel line and fuel pump are all on 1 side opposite from the exhaust system. The fuel line is solid metal from the fuel pump to the carb. I'm hoping I can get a look at a newer Mercruiser (maybe late 1980s or early 1990s) 170/190 I/O and see if there is any significant difference.

One experiment I was thinking about is to wrap the fuel line and pump in a cloth soaked with cool water, while running on a hot day. If I keep the cloth soaked, vapor lock due to heating of this part should be eliminated. If this works, then I need to find out how to properly fix the problem (cool the line or insulate it).

Tom

 
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06-21-05, 10:02 PM   #4  
Vapor lock

The only idea occurring to me was an electric cooling fan from a small car that cuts in at a specific coolant temperature , that could be controlled by air temp in the confined space or a keyed switch.

But I have no knowledge of adequate space for the fan or airflow in that engine compartment.

Another description of vapor lock that I now recall is that the fuel temperture at the point of carbeuration is too high and the air/fuel mixture is altered (lean?) Your cooling the line and pump approach seems to address this.


Good luck


Last edited by oldJoe; 06-21-05 at 10:45 PM. Reason: Add comment
 
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