Engine Start Problems

Flytrade

Flytrade

Active Member
Feb 20, 2018
302
Bradenton, FL
Boat Info
2006 320 Sundancer
Engines
Twin 6.2L Mercruiser
When my 6.2L Mercruiser port engine is cold it starts fine. Turn on the engine, wait about 5-10 seconds for the fuel pump to pressurize the system, and then hit the start button with the throttle at idle. It starts, idles and runs fine at all rpm's.
When the engine is warm - has been shut down within the past couple of hours - it's hard to start, or may not start at all. I use the same technique as above, and the engine turns over and sounds like it's going to start, but when I let go of the start button the engine dies. I then try moving he throttle to various positions, and sometimes I can get it to start, and sometimes it wont.
I had my mechanic install all new spark plugs, wiring harness and distributor cap. I still have the same problem.
Any thoughts would be appreciated.
 
Agree with Cap, above... A good first thing to try is to advance the throttle at least 1/3. If that works, it's most likely the IAC. And, this would probably be the most common reason. You could also swap the IAC from port to stbd to see if the problem follows.
 
Start with what you don't know check and see if you have a good spark if you do find out what your fuel pressure is on the fuel rail.
 
I've been down this road as I have a pair of 6.2s myself. Should have 42psi at the rail with the fuel pump running. I would also do a compression test to rule out weak cylinders. I had low compression in two cylinders due to a blown headgasket between the cylinders. I had the same problem that if the motor didn't start right away when it was warm then it was almost impossible to start, started cold just fine.

Try this as well. Next time you take her out and shut her down, before you attempt a warm restart, advance the throttle slightly. Then return it to idle just as she fires up.
 
As per your suggestions above, I had the IAC and fuel pressure tested, and they are normal.
We could not duplicate the condition at my dock - I suspect the engines need to be run at normal speed and temp for a while.
I did take the boat our for a run, but turned on the blower for about 5 minutes before shutting down the engines. This seemed to solve the problem.
I was also told the problem could be with the fuel blend. It seems the blend is changed depending on use for summer or winter. I could have a winter blend - which is more volatile - but it could be causing a vapor lock because I live in Florida and we are having a warm winter.
In any case, I would like to thank everyone for your help.
 
I've been chasing this same problem for a few years now. This is what has been done to my motor.
The Gen3 fuel pump, injectors, numerous IAC valves, TPS sensor, Distributor cap and rotor all have been replaced. I still have the issue. It comes and goes. Sometimes the engine starts right up. Other times I have to advance the throttle. If you can figure it out. Please let me know.
Thanks
 
I have seen several boats with Mercruiser MPI engines over the last several years that have intermittent start/stall/lack of power etc. Mechanics chase these with computers and can't find anything wrong. Quite often the problem turn out to be a faulty fuel pump relay causing fluctuating or complete loss of fuel pressure. These relays are cheap and easy to swap out. Not a bad idea to keep a spare onboard.
 
hottoddie said:
I have seen several boats with Mercruiser MPI engines over the last several years that have intermittent start/stall/lack of power etc. Mechanics chase these with computers and can't find anything wrong. Quite often the problem turn out to be a faulty fuel pump relay causing fluctuating or complete loss of fuel pressure. These relays are cheap and easy to swap out. Not a bad idea to keep a spare onboard.
Click to expand...
Do you have a part # for the relay? I'm having the same issue and will try to swap one out. Thanks.
 
No I don't have a part# as my boat does not use these relays. I have 2 friends with MPI Sea Rays that have had the issue and keep them onboard but won't see them for awile.
 
Seems like its always the 6.2's with this issue. I've seen a few folks that have chased this. Seems like the 5.7's are not as prone. Anyone else seeing this.
 
Since you have twins try swapping the fuel pump relays and see if the problem follows?
If the problem continues follow the protocol of swapping one component at a time until the problem follows?
 
Hey guys, been chasing this problem since the day I bought the boat, this,that and the other thing, after 6 years I finally gave up and just start the boat at half throttle. Works every time
 
ExumaBlue said:
Do you have a part # for the relay? I'm having the same issue and will try to swap one out. Thanks.
Click to expand...
Most likely the IAC sensor (Idle Air Control) as others mentioned
 
Wanderer 290 said:
Since you have twins try swapping the fuel pump relays and see if the problem follows?
If the problem continues follow the protocol of swapping one component at a time until the problem follows?
Click to expand...
What the hell kind of protocol is that. How about doing some testing instead of wasting time throwing parts at it.
 
techmitch said:
What the hell kind of protocol is that. How about doing some testing instead of wasting time throwing parts at it.
Click to expand...
Previously mentioned testing provided no resolution.
Actually swapping parts is is a common practice in auto dealers in that a mechanic pulls parts from the shelf, installs them and if the problem is not resolved will return the part to the shelf and reinstall the original.
The swapping of parts between engines and other systems is also common in the aircraft business too. For the purpose of trouble shooting...
 
Last edited:
techmitch said:
The IAC is not a sensor. It is a PWM output controlled solenoid/actuator. Sensors are inputs.
Click to expand...
Yes, the Idle Air Controller is not a sensor. It's a controller. Here's what it does. When you have the throttles completely down, your butterflies on the intake should be completely closed. that means no air to speak of should be mixing with the fuel to create combustion except what is allowed. The idle air controller is controlled by your ECM. The ECM is looking at idle speed, and other stuff to determine whether or not your engine needs more air to keep running. If it does, then a little stepper motor moves a plunger back and forth to help maintain the correct air/fuel mixture to keep your engine running.
If you have to open your throttle to start and keep your engine running, then your IAC is probably not doing its job. When you move your throttle, you are allowing the butterflies on the intake to open, bringing more air into the mix. Your ECM senses this, and allows more fuel to keep the ratio correct.
I don't know about the 6.2 specifically, but if I have to change an IAC, that involves removing the intake plenum, flipping it up, or over, and replacing it, and the intake manifold gaskets on the way back to locking it all down.
 
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