Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

[jdwarns]

Location: Annapolis, MD
Boat: 2006 Tahoe 215 I/O
Engine: V6 Mercruiser 4.3l TKS
Serial: OW344165

Hey guys, I've found some good info on this forum that's somewhat related to the problem I'm having, but no "ahah" moment yet.

Symptoms:
When cold, starts up like a dream (we're talking 1st/2nd crank) with no throttle needed, runs smooth for a few minutes, and then drops RPMs and stalls.
After initial start and stall it then requires throttle, and often a good bit of cranking, to start. It then requires gradually more throttle to keep from stalling as it continues to warm up.

Setup:
On land, using muffs.
For fuel, using a regular 5-gallon gas can with a fuel line running to the filter / water separator. Is this acceptable?
I don't think the boat's fuel tank is pressurized, or any other magic is happening in the tank, but it wouldn't be the first time I was wrong.

Observation:
TKS module doesn't "hiss" after startup.
Engine stays at operating temp of 160 (under throttle of course), but exhaust seems hotter than normal despite verification that manifolds and elbows are getting good water flow.
Exhaust actually feels somewhat normal while running, but heats up a lot after shutting it down.

Wondering if the TKS module is leanning out the mixture as the boat warms up? I've got the CA emissions setup, so the idle mixture screw has a cap over it. As such, I don't want to mess with that (unless all other possibilities are ruled out) because it's probably set correctly.

Thanks in advance for your help!

 

[jdwarns]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

OK, was able to find some more posts that were more directly applicable to my situation.
This guy has a 3.0l and just needed to spray carb cleaner in the idle mixture path:
3.0 Sea Ray dies at idle [Archive] - Club Sea Ray

Then I found an excellent DETAILED write-up from "Boat Tech" on how the TKS system works here:
http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/3-0-mercury-tks-hard-starting-when-sitting-over-week-328531.html

This write-up supports the "carb cleaner in the idle path" fix, since the TKS allows fuel from the float bowl on cold start-up but closes that path at operating temp. Once the TKS stops allowing fuel from the bowl, and the idle depends only on the fuel getting past the the idle mixture screw, it would make sense that this is my issue... a gummed up path through the idle screw causing my engine to get starved of fuel once warm.

I'll try the carb cleaner today and post my results.

 

[alldodge]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

Of everything Iv's read about the TKS I sure am glad I have EFI and regular carbs. All the best with your issue

 

[jdwarns]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

Think I need to reevaluate. Ran it again and noticed the intake manifold is getting extremely hot after a few minutes of running.
I shut it down, took the flame arrestor off, and noticed the fuel sitting on top of the butterfly valves was boiling.
Well I do believe that an overheated fuel/air mixture would certainly cause the engine to stall out.

I have good water flow coming out both the port and starboard transom exhaust ports. There's also water spraying out the prop normally.

The elbows maintain ~160 but the intake manifold and exhaust manifolds are way too hot. They should probably also maintain about 160 if all is well???

Guess it's time to take off the thermostat housing and check for blockages. It's a brand new intake manifold, so rust wouldn't be the cause. I DID run it for literally 5 seconds about a month ago with no water. I certainly hope that wasn't enough to burn up the impeller and send a fin into my cooling system. If it was, I deserve to learn this lesson.

Any other ideas while I go snooping?

Thanks!

 

[alldodge]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

It doesn't take long to burn up an impeller. Since it is being tested on land the muffs can be push the water through most channels. Suggest checking to see if this is the case. A note: just heard on another TKS issue that if you unplg the module it will keep the fuel rich, so if this solves the issue it is probably the carb

 

[jdwarns]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

It doesn't take long to burn up an impeller. Since it is being tested on land the muffs can be push the water through most channels. Suggest checking to see if this is the case. A note: just heard on another TKS issue that if you unplg the module it will keep the fuel rich, so if this solves the issue it is probably the carb

Thanks for the feedback, AllDodge. I'll give it a go tomorrow and post my findings.

 

[jdwarns]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

Alrighty... cooling issue fixed with a new thermostat.
Still not idling once warmed up.
Removed Carb, backed the idle mixture screw out until the feed paths were unobstructed, and sprayed carb cleaner into the seat.
The carb cleaner came blasting out each barrel's slot where the butterfly valves sit at idle. So that path seems to be clear and unobstructed. Not sure how to validate flow from the main fuel feed to the slots. Is there a way to spray into this upper portion of the path, or do I need to take the whole thing apart and soak it in carb cleaner?
I'm new to the whole carb scene, but trying desperately to catch up through research, so any input/advice is greatly appreciated.
Thanks

 

[incrediblened]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

It sounds like you're starving for fuel. Once all the fuel is used up in the carb. it stalls. Does it start back up and run a while after leaving it sit for 5 minutes?

Before you tear apart your carb I'd check...
1) Is your fuel filter dirty
2) Is your fuel pump supplying adequate pressure.
3) Is your fuel tank restricting flow?

But then, I'd take the carb apart, soak it all in carb cleaner, unscrew all jets and clean every little hole out with a wire from a wire brush, clean the float & needle, blow some air through all the ports to ensure that they're not clogged.

I also had a similar issue where the boat would idle for a while, then it would bog down like that. It was a loose fuel line that was sucking air and dripping fuel into the bilge. :|

 

[jdwarns]

Re: Mercruiser 4.3L starts smooth, runs a bit, then drops RPMs and stalls

Turns out my above mentioned setup of running off a 5 gallon can was not acceptable. I guess there is some sort of pressurization in the tank.
Put 25 gallons in the boat's tank, hooked it up, and it maintained idle.

As a side note, the reason I was running from a gas can was because I just installed a rebuilt long block and didn't want to commit to hundreds of dollars in gas until I knew it would run. I just had to throw away 40 gallons and it's painful. I also didn't want to just dump 5 gallons in the boat's tank for fear that it wouldn't be enough to dissolve any moisture I might have had awaiting at the bottom.

Grounded out the purple/white wire, let it warm up, adjusted the idle speed to 650, shot the timing gun, and noticed it was about 3-5 degrees off. Then the timing gun stopped working. Thought it was the gun at first but, as a check, put the clamp around a different wire and it started strobing.

Cap/Rotor were fine. Pulled the plug and it was very black... probably due to the timing, because I certainly wasn't running rich with my bad setup. Pulled all the plugs... same thing. Replaced all plugs, got running again, adjusted the timing. Running even smoother than before with all six cylinders firing every shot now.

Idles beautifully at 650 now. Tomorrow I hit the water to start my break-in period. I'll be refreshing myself on the break-in process, but feel free to chime in with any words of wisdom. I've got a zinc additive in the oil, so it should just be a matter of running it through the proper rpm range for 10 hrs?

Oh, and regarding my earlier replacement of the thermostat... the one I had in that was overheating was brand new. Go figure. I'm testing all thermostats in a pot of water on the stove now.

 

[Halibut214]

Location: Annapolis, MD
Boat: 2006 Tahoe 215 I/O
Engine: V6 Mercruiser 4.3l TKS
Serial: OW344165

Hey guys, I've found some good info on this forum that's somewhat related to the problem I'm having, but no "ahah" moment yet.

Symptoms:
When cold, starts up like a dream (we're talking 1st/2nd crank) with no throttle needed, runs smooth for a few minutes, and then drops RPMs and stalls.
After initial start and stall it then requires throttle, and often a good bit of cranking, to start. It then requires gradually more throttle to keep from stalling as it continues to warm up.

Setup:
On land, using muffs.
For fuel, using a regular 5-gallon gas can with a fuel line running to the filter / water separator. Is this acceptable?
I don't think the boat's fuel tank is pressurized, or any other magic is happening in the tank, but it wouldn't be the first time I was wrong.

Observation:
TKS module doesn't "hiss" after startup.
Engine stays at operating temp of 160 (under throttle of course), but exhaust seems hotter than normal despite verification that manifolds and elbows are getting good water flow.
Exhaust actually feels somewhat normal while running, but heats up a lot after shutting it down.

Wondering if the TKS module is leanning out the mixture as the boat warms up? I've got the CA emissions setup, so the idle mixture screw has a cap over it. As such, I don't want to mess with that (unless all other possibilities are ruled out) because it's probably set correctly.

Thanks in advance for your help!

Hoping to revive this thread as there is some good info here

Mod EDIT

 

[jbcurt00]

Hoping to revive this thread as there is some good info here

Mod EDIT

Start a topic about your motor. Link this topic in yours if you'd like.

Best not to post to old inactive topics