Mercruiser 4.3L TKS Won't start when warm.

[mikeincolo]

2007 Mercruiser 4.3L, 2-barrel carb with TKS fuel enhancement, in a Sea Ray 185 sport. Pulled it out of storage, went through spring checklist, put it on the lake and ran it for a couple of hours. Put it on the lake a second time and... Turns over on 1st key turn, engine purrs like a kitty, warms up & TKS "exits". Put the boat in reverse, no problem, put the boat in drive, and the engine stalls out! Then motor turns over & over but will not start. Leave it alone for a couple of hours, and it turns over, starts, warms up, put in drive & dies again.... Will not fail when at home hooked up to the hose and put it through the same exercise, only fails when in the lake.
Very frustrating!
I've replaced the distributor cap and rotor, put in new plugs, new fuel/water separator, cleaned and refurbished the Rochester 2-barrel carb, tested the oil pressure sending unit (good). I can hear the electric fuel pump engage but have not multimeter or pressure tested it yet. Thinking I need to figure out how to install and use an inline fuel pressure gauge.
Does anybody have any bright ideas?
Please help, I'm in Colorado and running out of summer!

 

[alldodge]

Check you shift interruptor switch, sounds like it needs adjusting

 

[badrano]

Sounds like this is a possible TKS module/circuit issue. When the engine is warm, the TKS module is supposed to remain energized with the ignition off to keep the rich fuel mixture port closed off so a warm engine can start. If the module is not energized, the fuel mixture is going to be too rich for a warm engine. After several hours, the engine is cold enough to need the rich mixture.
This thread has a pic of the TKS circuit. https://forums.iboats.com/threads/4-3l-tks-not-getting-voltalge.707874/
It's possible you may have a bad temp switch which is supposed to keep the module energized when the engine is warm.

 

[mikeincolo]

Thanks guys!
1. Didn't know about the shift interrupter switch, found the following post (link to post), and it sounds a helluva lot like what I have going on. This is an excerpt from the below-post

• The ONLY purpose of the shift interrupt system in the Mercruiser Alpha style outdrives is to get the drive out of gear (forward or reverse) when the engine is running and the boat is in the water with a load on the prop.
• Without the load on the prop, there is not enough resistance on the shift dogs in the lower unit to cause resistance to the shift cable and engage the shift interrupt system.

2. Thanks for the TKS circuit pic, best I have seen! I believe we have pretty thoroughly tested the TKS, it is getting voltage when engine warms up to about 130F, then settles in and idles at 650r's and 170F (160F is spec, but 170F is expected as I'm at 5500 ft altitude). Once motor is warm, temp-switch closes circuit, TKS is getting voltage, warm to the touch & its air intake stops making the "whistle", motor purrs...

Note: I am installing an in-line fuel pressure gauge between fuel pump and the carb, not expensive and good info now and in the future...

 

[mikeincolo]

fFollw up. So I replaced the shift interrupt switch and shazam, problem solved. And then, pretty amazing. I pulled away from the dock, got out of no-wake zone, throttle up... we're hootin' and hollarin' that the problem is solved! And not more than 30 seconds into the cruise we go by a boat with 2 adults and 2 kids on board. My buddy says "hey, they're waving atu, must want to get in on fun!" So I wave back and see that they're waving us over because they are dead in the water. I cruise up on them, confirm they need a tow, ask what seems to be the problem? Dude says they are new to boating, boat started just fine, they pulled away from the dock and then when he put it in drive the motor died. I asked if he has a Mercruiser and alpha outdrive, "yup". Anyway, I say "I think I might know whats wrong, permission to come aboard!", he agrees, I pop the cover and sure enough, Mercruiser 4.3, there's the shift interrupt switch, I press the button on the switch a few times to get it freed up (was sticky), told him to give it a try and low and behold she fired right up, shifted into drive, killed the motor. Repeated procedure, same result. Towed him in, gave him my number and a link to where I got my switch. Karma I guess. Being able to help this family out actually took some of the sting outta' loosing my boat for 2 weeks tracking down the switch interrupt (during which time I rebuilt the carb, replaced a diode, got thoroughly schooled on the TKS system, and diagnostics, etc).

 

[nola mike]

Nice! (For future reference, just unplug the switch if you need to make it home)

 

[achris]

Nice! (For future reference, just unplug the switch if you need to make it home)

And be aware that coming out of gear will put a hell of a strain on the cable. It's an emergency measure ONLY!

 

[Lou C]

Right you may need to shut the engine off before shifting out of gear, till it’s fixed properly. Had the same issue with my OMC Cobra when I first got it in 2002 but the cause was different.