WOT for 20min and engine bogs.

[bill18163]

I'm trying to help my neighbor figure out the problem with his boat. I will try to give as much info as I can but if I miss something just let me know and I then will find it.
MerCruiser with Thunderbolt Ignition. Engine SN M336055/2002/GM305-5.0L. Outdrive is Merc/Alpha 1.
He tells me that when he starts out with engine cold at WOT that about 20 min later the engine bogs down and he can't get to WOT again. It will idle but that is all. A lot of posts I have seen mention the fuel tank cap could have a plugged vent. I just called him because he is on the water right now and I told him to try it. Also I saw a few posts that mention a plugged fuel filter and maybe a problem with the fuel pump but it seems to me that there would not be a 20 min delay if either of these two devices were bad. Anyway we are just getting into this and any help or thoughts would be appreciated.

 

[Scott Danforth]

First thought is check for fuel and spark

If you don't have fuel spraying when you give the throttle, it's the fuel system and most likely a plugged vent

If you have fuel, but not spark. Go thru the Thunderbolt trouble shooting guide. Most likely the ignition trigger assembly if it's the old 2-wire version

 

[bill18163]

Thanks Scott, we will take a look at those items.

 

[bill18163]

I'm trying to help my neighbor figure out the problem with his boat. I will try to give as much info as I can but if I miss something just let me know and I then will find it.
MerCruiser with Thunderbolt Ignition. Engine SN M336055/2002/GM305-5.0L. Outdrive is Merc/Alpha 1.
He tells me that when he starts out with engine cold at WOT that about 20 min later the engine bogs down and he can't get to WOT again. It will idle but that is all. A lot of posts I have seen mention the fuel tank cap could have a plugged vent. I just called him because he is on the water right now and I told him to try it. Also I saw a few posts that mention a plugged fuel filter and maybe a problem with the fuel pump but it seems to me that there would not be a 20 min delay if either of these two devices were bad. Anyway we are just getting into this and any help or thoughts would be appreciated.

 

[bill18163]

I read somewhere on this forum about a DELCO EST Kit that would replace the original ignition system. What is the cost of one of these kits and are they reliable? I would like to learn more about these kits and possibly buy one. I have been looking to purchase the OEM replacement but the part is not available any more. The only ones I can find are used and even those are in short supply.

 

[Scott Danforth]

Before you spend $400 on a Delco EST, go thru the troubleshooting guide. It could be the $50 trigger assembly

 

[bill18163]

Where is the trigger assembly? What does it do?

 

[dubs283]

Where is the trigger assembly? What does it do?

Thinking SD means the ignition sensor for a thunderbolt system. Is located in the distributor, latest style oem has three wires and is fully potted.

It sends a signal to the module on/off hall effect to fire the coil. New age points in effect.....

 

[Scott06]

Here is the TB troubleshooting guide https://forums.iboats.com/threads/how-mercruiser-thunderbolt-ignition-systems-work.676476/

but as Scott outlines figure out if it is fuel or spark first, have you taken off fuel filter and dumped contents into a jar look for anything but clean gas

yes the delco EST is a good quality system was used as OEM on VP and some merc engines, but go through some diagnosis before throwing parts at it

 

[tpenfield]

So, your friend is running at WOT for 20 minutes? or is it just after 20 minutes of running the engine bogs down and won't throttle up?

 

[QBhoy]

After the full throttle thing for so long…I’d be checking the health of the engine. Check the oil for anything nasty…pull plugs and compression test too.