Mercruiser 4.3L MPI Hard Starting

[mat5751]

Hello All

I have twin Mercruiser 4.3L MPI V6, 2004 (OW065549 and OW065554) that are hard to start when hot/warm.

They will turn over but will take several minutes to fire. When they do eventually start, the exhaust smells rich for a few moments.

The workaround is to start in neutral at WOT then quickly throttle down. Both engines start straight away with this workaround.

The issue occurs in both engines, after they have been sitting for between 5 minutes to about 2 hours after use.

Both engines start straight away when cold.

The engines were recently serviced, show no fault codes when plugged into the computer and compression was checked and is good.

I have found the Service Bulletin 99-7 (http://www.boatfix.com/merc/bullet/99/99_07.PDF) regarding vapor lock. I do not believe that it is however as I have tried, all without any improvement at all:
- blowers
- idling after running for several minutes
- opening the engine hatch
- never using fuel with ethanol.
And, when I do try to start at WOT, it starts straight away.

I have replaced the IAC values on both engines but no improvement.

I suspect that it may be one of the following:
- Temperature Sensor - even though there is no fault code, the sensor may have some fault which tricks the computer to think that the engines are warm (when they are not) and to inject more fuel than necessary. I think this because the hard starting is cured with WOT starts.
- MAP Sensor - again for the reasons above, an incorrect temperature reasoning causing more fuel being injected than necessary.
- Low pressure Fuel Booster pump - I don't think so, because the engines start fine at WOT
- Fuel regulator - Maybe if the regulators are impacted by temperature......

Before I start throwing parts at it, any thoughts?

Any assistance would be great.

Thanks all in advance.

 

[Spark600]

Check fuel injectors for leaking fuel into the cylinders after the motors are shut down.
Google Gen 3 fuel module they had a major paint issue were paint got into the fuel rail and injectors that cause the fuel injectors to leak gas into the cylinders.
Worth looking at I guess, not sure if they came on the MPI

 

[Espos4]

When you move the shift lever to WOT, the ecm sees that as a “clear flood” and will not fire the fuel injectors when you crank the engine.

It does sound like one or more injectors is leaking after shut down, on both motors.

This creates a rich mixture in the intake manifold after shutdown, too rich to burn.

 

[Little Ducky]

Hello All

I have twin Mercruiser 4.3L MPI V6, 2004 (OW065549 and OW065554) that are hard to start when hot/warm.

The engines were recently serviced........

Thanks all in advance.

List what was done during your service.

If you engines started normally before the service was performed I would go back through and check everything that was done.

Spark plug gap, timing (if distributor cap was replaced)...etc.......

 

[dan313]

Good Morning,

I have had the same problem for the last few years. All the mechanics that I had check the motor could not figure out what why it wouldn't start when hot. This past spring I replaced my fuel pressure regulator. What I found it was loaded with paint. I contacted Merc Curiser and was instructed to take it to a certified Merc Cruiser mechanic for a warranty claim. The shop removed the Gen 3 Cool Fuel Module and found it full of paint.

Even though my motor is a 13 year old 350 Mag 2005. Merc Cruiser warrantied the Fuel Module as well as 8 fuel Injectors. I paid for the labor. I also had the fuel rail cleaned.

The paint issue is a known issue since 2004. This is why they replaced the pump and injectors.

I only ran the boat 3 times since all was replaced and so far no issues. Starts every time.

Good Luck,

 

[mat5751]

Thanks very much guys. Some good suggestions there. I track back though the last service documents and also investigate the paint issue with the fuel module - I haven't heard of this issue before. I am away from the boat ATM, but is there a way to determine from the serial number if my 2004 engines were impacted?

 

[Limetime67]

Hello All

I have twin Mercruiser 4.3L MPI V6, 2004 (OW065549 and OW065554) that are hard to start when hot/warm.

They will turn over but will take several minutes to fire. When they do eventually start, the exhaust smells rich for a few moments.

The workaround is to start in neutral at WOT then quickly throttle down. Both engines start straight away with this workaround.

The issue occurs in both engines, after they have been sitting for between 5 minutes to about 2 hours after use.

Both engines start straight away when cold.

The engines were recently serviced, show no fault codes when plugged into the computer and compression was checked and is good.

I have found the Service Bulletin 99-7 (http://www.boatfix.com/merc/bullet/99/99_07.PDF) regarding vapor lock. I do not believe that it is however as I have tried, all without any improvement at all:
- blowers
- idling after running for several minutes
- opening the engine hatch
- never using fuel with ethanol.
And, when I do try to start at WOT, it starts straight away.

I have replaced the IAC values on both engines but no improvement.

I suspect that it may be one of the following:
- Temperature Sensor - even though there is no fault code, the sensor may have some fault which tricks the computer to think that the engines are warm (when they are not) and to inject more fuel than necessary. I think this because the hard starting is cured with WOT starts.
- MAP Sensor - again for the reasons above, an incorrect temperature reasoning causing more fuel being injected than necessary.
- Low pressure Fuel Booster pump - I don't think so, because the engines start fine at WOT
- Fuel regulator - Maybe if the regulators are impacted by temperature......

Before I start throwing parts at it, any thoughts?

Any assistance would be great.

Thanks all in advance.

 

[Limetime67]

I ,too, have the same issue you are having. I have a 2005 Sundancer with twin 4.3's. The vapor lock issue is what the problem is on these motors. The fuel is basically pulled through a cool module and then sent to the injectors. The problem is the module is right next to the block and gets hot when you shut down the motors. I have installed additional blowers, re routed the fuel lines etc. None of that helps. The solution is to install additional fuel pumps ahead of the filters to "push" the fuel past the module when the engine is hot. There are a couple of youtube videos that guide you through that. Again , this issue seems to be specific to V6 4.3's and their design. The tech service bulletin dances around the poor design issue and suggests items that really don't help. Also , I suggest checking the coolant hoses leading into the fuel module (bottom of block) . Mine were badly kinked due to poor design. Not sure how to fix that but im sure it limits the water flow to the modules and thus limits the cooling capability