Mercruiser 5.7 MPI not running well

[HorizonblueDK]

Hi boaters

A friend of mine has a Campion Chase with a Mercruiser 5.7 MPI, both is year 2004.

Until recently, the engine ran perfectly, easy starting, steady idle, very responsive when hitting the throttle. But this has changed in an instant.

Here are the symptoms:

-Impossible to start, cold, without giving it a little gas.
-Stalls when hitting the throttle.
-Engine dies, when it is put back to idle, after hard reving up.

The engine has about 300 hours on it. The fuel filter has just been changed. Spark plugs looks ok, a little soot/carbon.

Distributor cap and rotor has been cleaned.

If one is very gentle with the throttle, it is possible to rev it up, and put it back to idle, without any stumble.

But fast movement of the throttle, causes the engine to stall/stumble or die.

Any ideas?

I'm not that familiar with MPI engines, but I suspect a fuel problem.

Good old carb engines are more my field.

 

[jtmarten]

Re: Mercruiser 5.7 MPI not running well

If it were a carb I'd say accelerator pump, don't know what the equivalent injection part would be, maybe a dirty TPS?
Has he run Techron to clean the fuel system?
Could be dirty injectors or bad fuel pressure regulator.
If its running rich, could be a dirty flame arrestor.

 

[Bondo]

Re: Mercruiser 5.7 MPI not running well

The engine has about 300 hours on it. The fuel filter has just been changed. Spark plugs looks ok, a little soot/carbon.

Distributor cap and rotor has been cleaned.

Ayuh,... I don't do the electronic motors either,...
But,...
A Real Tune Up,+ Complete Service is Still a Great place to Start....

Was there any Water in the take-off fuel Filter,..??

 

[Fun Times]

Re: Mercruiser 5.7 MPI not running well

You might consider looking into either cleaning or replacing the (IAC) Idle air control valve located at or around the throttle body.

 

[dirtyoldman]

Re: Mercruiser 5.7 MPI not running well

You might consider looking into either cleaning or replacing the (IAC) Idle air control valve located at or around the throttle body.

A cleaning will likely tll you at least whether or not it's the offending part.

Has he taken off any parts that could leave a large vacuum leak? That would cause htose symptoms also

 

[HorizonblueDK]

Re: Mercruiser 5.7 MPI not running well

Thanks for the inputs.

If it were a carb I'd say accelerator pump, don't know what the equivalent injection part would be, maybe a dirty TPS?
Has he run Techron to clean the fuel system?
Could be dirty injectors or bad fuel pressure regulator.
If its running rich, could be a dirty flame arrestor.

No, the fuel system has not been cleaned, but it was worth doing.

Ayuh,... I don't do the electronic motors either,...
But,...
A Real Tune Up,+ Complete Service is Still a Great place to Start....

Was there any Water in the take-off fuel Filter,..??

My friend changed the filter himself, but didn't mention anything about water/dirt.

A cleaning will likely tll you at least whether or not it's the offending part.

Has he taken off any parts that could leave a large vacuum leak? That would cause htose symptoms also

I don't think he has taken anything off, that could leave a leak, but I don't know for sure, worth checking.

I have a bunch of spark plug testers, that I will try (installed between spark plug and wire) just to rule out the possibility, that it could be an ignition problem. They illuminate if the plug is firing.

But I still think, and as I can feel from you guys, that it's not an ignition problem.