97 Mercruiser 3.0LX will run/won't run

[Bondo]

I’m reaching here but by the off chance do you know what switch from digitech would fit my configuration? I contacted them and was pleasantly surprised they responded already but unfortunately they said they didn’t think they had what I needed.

Ayuh,..... Yer chasing a ghost,..... yer symptoms don't come anywhere near that switch being the problem,....

Quit throwin' parts at it, 'n Diagnose the problem,......

 

[dubs283]

I’m reaching here but by the off chance do you know what switch from digitech would fit my configuration?

Agree with Bondo here. You're putting the cart before the horse.

Troubleshooting is a step by step process, you can't skip steps. You'll just end up wasting time and money thus increasing your frustration

Yes, the shift interrupt switch can cause the ignition system to cut out, its part of the design but only when the system is sound. Can't recall a time when the shift interrupt switch intermittently causes a loss of spark in its static state. Most cases of the switch operating incorrectly have nothing to do with the switch itself as part of the ignition system. A worn/misadjusted lower shift cable is often the culprit.

Your posts mention no instance of dying while shifting, therefore you need to start from the top (spark/fuel) and work your way through testing.

 

[Scott06]

Well after following wires all morning I didn't find anything suspicious. I ended up at the shift interrupt and I got thinking. So I did some internet research and discovered that this switch if faulty, will cause the problems I have. I found out for the most part that over time the plunger will not completely reset causing an interruption and a tow back to shore. I then contacted a relative and asked if he knew anything about these types of switches as he's an oilfield mechanic and he said he deals with them quite often. He said that it is typical of these switches to build up gunk on the inside over time and when they get hot they gum the plunger up preventing it from returning to full position which could cause the problem I am having. He said the same thing that they don't fail but they do gum up. He suggested soaking it in contact cleaner.

There's no gunk on yours you can measure it with multi meter set to ohms for your knowledge base. Usually this is related to shifting in or out of gear, worn shift cable will drag the switch and trip it cutting ignition vs causing a quick ignition cutoff/ stumble required to get the dog clutch to pop out of gear...as outlined above...

If it happens out on the water just disconnect it and see if it runs again... you changed your controller would verify when in boat this isnt tripping this switch, otherwise look else where.

What you need to do is check for spark and fuel when it cuts out other wise you are pissing into a fan... I would take it out and do laps near the launch until it happens and be prepared to diagnose when it happens.

Look for spark - if no spark probably a loose electrical connection like key switch or man over board/kill switch, or loose cannon plug.

Take flame arrestor off and pump the throttle to see if gas comes out accelerator pump. Would bring a small container of gas to dump a little down carb, if it runs a bit then dies you know you are loosing fuel and need to verify fuel supply back tot he tank...

 

[manebeam]

Disconnect the switch for testing. Still haven't verified spark loss, right? If it doesn't restart when disconnected, that isn't your problem.

So I disconnected it like you said and it still starts up.

 

[manebeam]

Agree with Bondo here. You're putting the cart before the horse.

Troubleshooting is a step by step process, you can't skip steps. You'll just end up wasting time and money thus increasing your frustration

Yes, the shift interrupt switch can cause the ignition system to cut out, its part of the design but only when the system is sound. Can't recall a time when the shift interrupt switch intermittently causes a loss of spark in its static state. Most cases of the switch operating incorrectly have nothing to do with the switch itself as part of the ignition system. A worn/misadjusted lower shift cable is often the culprit.

Your posts mention no instance of dying while shifting, therefore you need to start from the top (spark/fuel) and work your way through testing.

I've never noticed it dying while shifting. It either won't start sometimes or when out on the water under load or cruising it just dies and won't restart. If I leave it for a day or two in the yard it will usually start up again with muffs on. I last had it out testing it (last summer) I had replaced a few parts and it worked beautiful for about an hour then out of nowhere, it just cut out and died and I was stuck and it would not start. Rather than thinking of checking for spark I spent several hours paddling back to shore with my son.

 

[manebeam]

There's no gunk on yours you can measure it with multi meter set to ohms for your knowledge base. Usually this is related to shifting in or out of gear, worn shift cable will drag the switch and trip it cutting ignition vs causing a quick ignition cutoff/ stumble required to get the dog clutch to pop out of gear...as outlined above...

If it happens out on the water just disconnect it and see if it runs again... you changed your controller would verify when in boat this isnt tripping this switch, otherwise look else where.

What you need to do is check for spark and fuel when it cuts out other wise you are pissing into a fan... I would take it out and do laps near the launch until it happens and be prepared to diagnose when it happens.

Look for spark - if no spark probably a loose electrical connection like key switch or man over board/kill switch, or loose cannon plug.

Take flame arrestor off and pump the throttle to see if gas comes out accelerator pump. Would bring a small container of gas to dump a little down carb, if it runs a bit then dies you know you are loosing fuel and need to verify fuel supply back tot he tank...

I will do this absolutely when the ice comes off the lake. I'm just trying to get ahead of it now before boating season starts and trying to go through things that might be causing it.

 

[manebeam]

You can measure ohms across the switch to see if it closes when actuated

How did u zero in on shift interrupt ?

If you installed a controller is it tripping the interrupt switch? On water would disconnect it if engine dies and see if it starts as this will kill ignition

I just replaced the controller in my garage and I didn't do this job as just because. My old classic 3000, the guts in it stripped out and a part broke. Rather than try to fix, I just put new on. After everything was hooked back up, it shifted smoothly from F to R and the switch visually rolls in and out smoothly as well.

 

[manebeam]

So you guys got me thinking. If it was a fuel issue when it quits, is it possible that when the fuel is sloshing around it stirs up sediment in the bottom of the tank and clogs the screen in the fuel pickup since its on the bottom as well then starves the fuel pump which deprives the engine? The tank is 26 years old and I've never had it cleaned? Just brain storming.

 

[markhodges78]

So you guys got me thinking. If it was a fuel issue when it quits, is it possible that when the fuel is sloshing around it stirs up sediment in the bottom of the tank and clogs the screen in the fuel pickup since its on the bottom as well then starves the fuel pump which deprives the engine? The tank is 26 years old and I've never had it cleaned? Just brain storming.

I had a clogged pickup in a tank once the engine would run all day at idle but put it under throttle and it would shutdown every time .. The pick up had some orange goo on it

 

[manebeam]

I had a clogged pickup in a tank once the engine would run all day at idle but put it under throttle and it would shutdown every time .. The pick up had some orange goo on it

Good to know thanks mate! How did you clean the bottom of the tank? I'm wondering if there's bugs and other crap in it. I live in an area that has 3 or 4 terrible fish fly hatches a season and I have noticed my gas cap open sometimes. Why, I'm not sure. I have pesky kids in the area so who knows lol.

 

[markhodges78]

If you have found you gas cap off I would check for water
Drain some gas into a water bottle shake it up then let it set for a few hours come back and with disturbing it to much look at the bottom and see if there's any water bubbles