1985 Merc 350 trim up/down stalls engine, missing/backfiring after fully warmed up

[kip2001]

Found the brushes and gave my 10 pin connector a complete cleaning, nothing looked bad to start but completely shiney now! Unfortunately that didn't fix the main problem. Tracing all my negative wires up at the helm I found the attached negative terminal block I didn't know about, yikes gross! Most of it appears to be an overshot spray of some sort, the metal actually didn't look corroded, shiney now though! And still the same problem.

I know I've checked and cleaned every connection except the bad one but boy that bad one is not presenting itself. I'm debating just wiring a bypass to the coil so I can spend more time enjoying the boat rather than on my back sticking my hands through small holes hoping to find something. Biggest problem there is I lose my gauges.

 

[alldodge]

You won't lose your gauges if you keep the key ON
You can hot wire the coil, turn key as normal to fire the motor up and go boating. Issues with doing it this way, is key won't shut the motor down, and if there is a short, there is no fuse

 

[kip2001]

Gave it another shot this morning trying to find the problem and disconnected all after market devices from the "run" terminal of the switch.

​​​​​I also sanded again all the main positive connections at the starter post and the negative post to the batteries.

The problem expectedly still existed so I did some testing trying to pinpoint where it exists, I hot wired the coil to the red/purple wire side of my new circuit breaker. Purple wire from key disconnected from coil (I also tried with it connected, same result). If I hit trim pump switch the rpm needle will spike down a second, go back to normal, and then slowly move down 100-200rpm until the pump hits a limit or I stop it. I also tried this with the hot wire directly on the battery, same result. Just judging audibly, if I have the key off and gauges not wired to positive, this engine bog doesn't occur.

So that leaves me wondering where this problem could possibly be, it seems to also do it if I have the gauges on and the tach disconnected from negative. At that point, there should be absolutely nothing at the helm able to effect the ignition system in the back I'd think??

Thanks for all your help AllDodge, I'll also add a switch and inline fuse to my bypass wiring just to make life easier/safer in the short term.

 

[alldodge]

Connect the voltmeter from the starter post to the positive battery post. Trying to see if there is a voltage drop across the cable. Do this with the motor running and activate the trim. Do the same from the engine block to the negative post on the battery

 

[kip2001]

Okay, it'll be a couple days but I'll do that for troubleshooting next time I'm at the boat!

My battery gauge normally shows around 14v engine running, pegs down to 12v with trim activated (this was consistent in all my wiring arrangements) and my Bluetooth battery meter right on the battery also drops when the trim turns on (it is more stable, drops from 14v low 13/high 12).

 

[alldodge]

Did you say our battery is new?
Has it been load tested?

 

[kip2001]

Not new but has been load tested. It's rated 550CCA and tested to 675.

The house battery hasn't been tested but I physically separated it from the wiring going to the positive starter terminal during my troubleshooting today. They still shared a common ground as well, they're both wired with a heavy gauge to the engine block.

 

[alldodge]

The 550 is IMO minimum, how about parallel both bats together on the start side for testing. Disconnect the house pos and use jumped cable to connect the pos together

 

[kip2001]

Okay, I haven't tried disconnecting the house circuitry yet (I will next time I'm there) but I have used my battery selector switch to use both batteries (as well as just the house) with the same results. Normally I've got the battery switch on the starter battery with my voltage crossover device to let the alternator feed the house battery once the starter battery hits a high voltage. They're also both left on a charger at the slip all the time so they are at whatever their max remaining capacity is when I'm doing this troubleshooting.

Just a thought, so the resetting of the depth finder only happens when the engine is running, I've left the key in the run position and it doesn't reset but the second the engine is running the problem starts to occur again (I'm really just guessing it's all related, it seems like these problems with ignition and the depth finder resetting occur at the same time, the depth sensor is powered by the house bank). The attached picture is a nose suppression device the PO installed on the orange line from the alternator to the positive starter terminal. Could that thing be there problem?

 

[alldodge]

Guess I have nothing, its not getting enough power from the bat, but have no idea how or why

 

[kip2001]

Yeah it's got me stumped too, thank you for all your help!

I'll follow-up for future users if I ever solve this thing.

 

[TSwantek]

I'm having the same issue with my 5.8 Volvo. It's eluded me for two years now. Thought I fixed it with adjusting my timing last year but it's back with a vengeance after filling up the tank .If you figure out what happened with your boat please let us know. Trim makes it pop if past 2\3 throttle. I'm guessing it will be a wire chasing game for me also.
has never been hard to start but it is now after the gas that was put in .But that's just part of the problem. I'm guessing since I haven't run it as full for awhile it's under more of a load. It doesn't explain the start issues and stalling when put in gear. Bout read to sell it if I didn't love the dang thing so much.

 

[achris]

...Adding to that, the outdrive trim is still causing rpm problems with the engine. The dash volt gauge goes from 14v to 12v (that I would assume is normal) but the engine audibly starts stumbling and the RPM drops. ...

That's not right. The alternator should pick up the current draw and the voltage should stay at 14v... Check the alternator output when it's loaded...

Now, these next 2 scare me...

after fixing a few loose inline fuses I somehow lost power to roughly half my circuits including the ignition key. Oddly, the blower was still working and that is fed by the same positive wire that goes to the key.

First (and I realise this was the PO, not you)... in-line fuses :facepalm: Get them out of there. 3AG fuses have no place being in a boat when water proof blade fuses are available. In-line fuses and their holders are nothing but trouble. They get corrosion on them and go high resistance (which is what your problem is sounding more and more like)....

Now for the super odd one, while I was working down in the bilge I had a fan blowing on me powered by the cigarette lighter/acc port on the console. I pulled out the 9 pin connector from where it connects at the engine and the fan lost about half its power. Put the connector back in, fan returns to full power. ....what in the what?? I had the starter battery disconnected at this point so I think there's some funky series wiring going on or maybe it's fed from multiple sources with one source being too thin a gauge? Unfortunately I had run out of time by that point so I wasn't able to get my volt meter on it.

AD already alluded to the fact that you have some non-engine electrics connected to what should be 'engine only' circuits. That really needs to change, and quickly. I did a post a short while ago... I'll see if I can find it...

https://forums.iboats.com/forum/engi...5#post10635215

Chris............

 

[kip2001]

I'm having the same issue with my 5.8 Volvo. It's eluded me for two years now. Thought I fixed it with adjusting my timing last year but it's back with a vengeance after filling up the tank .If you figure out what happened with your boat please let us know. Trim makes it pop if past 2\3 throttle. I'm guessing it will be a wire chasing game for me also.
has never been hard to start but it is now after the gas that was put in .But that's just part of the problem. I'm guessing since I haven't run it as full for awhile it's under more of a load. It doesn't explain the start issues and stalling when put in gear. Bout read to sell it if I didn't love the dang thing so much.

Unfortunately I haven't fully solved my problem, as Chris's suggestions allude to below I think it's just poor wiring on an old boat becomes an additive problem. I've run about 25 hours under power since my last post here, the engine has been completely fine except for one or two misfires after 2 hours of running. That said, the trim issue still exists and 24 hours of that wasn't above 1600rpm so the problem I think is just hidden. In addition to what I say below, I also plan on replacing the vent line to my fuel tank. I know it at least sort of works (can blow air through it) but on a fast filling pump it will spit gas out the vent so I think the check-valve is letting liquid gas slosh up past which could at times be preventing my tank from venting and in combination with the electric problem be causing some of my symptoms.

That's not right. The alternator should pick up the current draw and the voltage should stay at 14v... Check the alternator output when it's loaded...

Is there a way to do this on the boat or easier to just take it off and bring it to my local auto parts store for testing?

First (and I realise this was the PO, not you)... in-line fuses :facepalm: Get them out of there. 3AG fuses have no place being in a boat when water proof blade fuses are available. In-line fuses and their holders are nothing but trouble. They get corrosion on them and go high resistance (which is what your problem is sounding more and more like)....

Completely agree, I've been replacing them as I go. Unfortunately PO really loved the things so it's tedious work.

AD already alluded to the fact that you have some non-engine electrics connected to what should be 'engine only' circuits. That really needs to change, and quickly. I did a post a short while ago... I'll see if I can find it...

https://forums.iboats.com/forum/engi...5#post10635215

Chris............

This will be my next undertaking, I've separated the lines as much as I can reasonably do this summer. I've also got it rigged up so I can separate my coil to run off of the one battery with no connection to anything else. It's not a permanent solution but it is a summer fun fix.

Thank you for the help Chris!​

 

[achris]

Re: alternator.

Is there a way to do this on the boat or easier to just take it off and bring it to my local auto parts store for testing?

For me, yes. As I have a 100A load tester and a 0-100A induction ammeter. If you also have those, great, but I suspect you don't. So, probably easier to remove it and take it to the local car shop/auto-sparkie...

Chris.....

 

[alldodge]

Could check with a light load. Connect voltmeter to the starter post and ground. Run motor around 1500 rpm, and raise and lower trim (being in gear moving it the water will put more of a load)

 

[achris]

Just had some thoughts that may help you find the problem, and if not, at least rule out some areas...

All you'll need is a multimeter and a couple of clip leads...

Fire up the engine and get it warm... Hook the meter to the alternator output terminal and the alternator frame, then with it running at about 1500rpm hit the trim switch. The meter shouldn't drop.

Now the fun starts. With the meter positive still connected to the output wire at the back of the alternator, work your way back towards the battery negative post (alternator mounting bracket, engine block, negative cable post, etc. ), with the negative lead, checking until you find the drop. If that produces no result, do the same with the positive side, working your way back to the battery positive post with the meter positive.

Just a random shower idea...

Chris.....

 

[greno]

One of my old engines from the 70's had similar timing issues where the timing kept jumping around a little. Finally tore it down and the timing chain was so loose that you could rotate the cam about 10 deg. back and forth. Put on a new set of gears and a new chain and it ran great again. Not that this is related to your electrical issues but it may be a contributing factor to your running issues.

 

[Bdaviskar]

When you cleaned your 10 pin connecter did you notice if any pins where pushed together at slots in the center of the pins.

For my 10 pin harness ( not sure if it is the same for all) the pins have a slot in the center. I was told to make sure there was space to make sure it made a good connection to the walls of the female plug. I used a small screwdriver to pry the opening a little bigger. I was careful not to pry to much and break the male pin.

I was also also told I could check this by using both hands on both sides of the plug and wiggle to see if I could shut the engine down or reproduce the same problem you are having.

 

[Lou C]

Interesting, it concerned me that your timing jumps around that much, normally it should not, should be steady. If its jumping check for slop in the distributor shaft, could also be timing chain stretch as mentioned above.
Also a few more ransom thoughts...
I have heard that certain brands of tachs, can cause a stutter in the ignition, when they are starting to fail. Might want to try running with the tach disconnected for a while to see if that could be a factor..
Lastly, I wonder if your trim pump motor is drawing more current than normal that will drop your system voltage but not enough to trigger a circuit breaker or blow a fuse. As noted above, the alt should be able to pick up the load to keep the voltage stable enough that the ignition system is adequately powered.