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

[achris]

Interesting, it concerned me that your timing jumps around that much, normally it should not, should be steady. ..

Timing jumping around can also be bad grounds. My previous V6 had a bad dissy ground and it lost about 6 degrees of advance... So yes, it should be steady but, bad grounds, anything can be happening...

Chris.......

 

[kip2001]

Re: alternator.
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.....

Ha, definitely don't have one of those!

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)

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.....

I'll give these on-board tests a try! I've done similar but haven't hooked the meter right up to the alternator before.

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.

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.

Yikes! I hope it's not that. Just for due-diligence I'll do a check for slop on the distributor shaft.

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.

I have run with the tach disconnected, I'd have to read back through my posts but I think I found that the tach disconnected didn't make a difference, it seemed to be something else with the electrical system.

It has cross my mind that maybe the pump is starting to fail and is drawing too much power. That could be, but the misfiring and other electrical issues that crop up after the engine has been running awhile happen regardless of if I run the trim pump so there's at least something else also going on.

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.

Hmm, I'll have to take a closer look. My 10 pin connector has a pipe clamp that gets tightened around it so I kind of just assumed that squeezed all the metal contacts good and tight together. Idk if that's standard, maybe PO was having issues and that was his quick fix.

Thanks everyone for your insight and help! Lots to try out.

 

[achris]

...Hmm, I'll have to take a closer look. My 10 pin connector has a pipe clamp that gets tightened around it so I kind of just assumed that squeezed all the metal contacts good and tight together. Idk if that's standard, maybe PO was having issues and that was his quick fix....

Hose clamp was standard issue. But if it's over-tightened it will cause issues.

Chris.......

 

[kip2001]

Hose clamp was standard issue. But if it's over-tightened it will cause issues.

Chris.......

Good to know! What issues would over-tightening it cause?

 

[achris]

Overtightening will cause the molding to distort, which can lead to loss of contact, and pin or socket damage.

 

[stang32]

take a long jumper wire and go from the key switch accessory tab to the coil, test it that way. if the problem goes away, i woudl jump from 1 side of the 10 in connector purple wire to the other side.. if the problem is no longer present, then there is where your problem lies. it may be a bit difficul to seperate the wires but it can be done. also add another ground wire from the battery to the engine block, in another area. if the ground isnt already on the same post as the other grounds located by the port side transom then ad the ground there. gotta run now but i have more thoughts ill give later.

 

[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.

Problem solved!

It was my coil failing when it would get heated... These components when they get hot from sitting on top of the engine block create a lot of resistance especially when they begin to fail.
This is why it would start in my driveway under no load when it was cold, but wouldn't when it had been run around the lake a few times heating it past it's thermal threshold.
I replaced it with a cheapo and voila, never has it run this strong!
It has always popped here and there since I bought it back in 2015, but it has been getting progressively worse.
I've suggested it to my mechanic many times. He responded with they either work or they don't. B...S...!

I'm not sure If the OP has solved his issue, but if not replace the coil... It was a cheap fix. I couldn't believe that it caused a whole lot of weird gremlins.

Good Luck everyone.

 

[kip2001]

**UPDATE**

Problem solved!!! It ended up being the ignition coil! A word of the advice for people, don't assume that the PO had the correct ignition coil on the boat! Last year I replaced the ignition coil with the exact same part # from NAPA that was installed. The problem got better but didn't go away. Over the winter I did some research and discovered that that coil was internally resisted for a points ignition system. My engine is a thunderbolt IV HEI system so it should be used with an ignition coil without that internal resistance. I cross reference my engine's serial # to the correct sierra part number, 18-5438 and viola! Engine is humming along as happy as can be this year! We've gone on several 5+ hour journeys this year at all different engine RPMs and not a single problem.