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

[kip2001]

Spent some time last night messing with the electrical system, cleaned a whole bunch of connections in the bilge area. None looked terrible, but now they're all nice and shiny. Up behind the console I started tackling the rats nest of a situation the PO left me. I appear to definitely have a common ground problem, 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. Needless to say, I've got some more investigating to do.

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.

 

[alldodge]

Maybe, but could be it needs a better ground. Many add things and don't add a additional ground to handle the load, so all grounding goes back thru the 10 pin connector. Separate anything that is not part of the motor operation with a new ground and hot. The blower is one of the items that is not part of the motor

 

[kip2001]

The saga continues, I've confirmed I have good grounding to the terminals at the helm (dedicated wire), I also cleaned the contacts just for good measure.

All of my boat circuits are working except for the ignition key and anything powered by its purple wire. With my multimeter I found the red/purple wire attached to the key has 12v with the key in the off position. If I switch the key to run, the red/purple wire terminal and the purple wire terminal measure -0.46v. Turning to cranking position also does nothing. Anyone have any ideas what is going on?? I also tried hot wiring from the powered terminal to the purple one and nothing happens which leads me to believe it's not the switch. I tried unplugging the purple wire from a few of the gauges and also nothing. No matter what the testing, if I return the key to off, I measure 12v on the hot terminal of the ignition switch.

 

[alldodge]

No matter what the testing, if I return the key to off, I measure 12v on the hot terminal of the ignition switch.

You have a corrosion issue and is either ground or hot side. With not seeing any voltage with key ON I'm thinking the hot side. Get a long piece or wire so your meter can be used to check continuity of each wire on the 10 pin engine connector. Also look at the inline fuse on the Red/Purple wire behind the helm

 

[kip2001]

You have a corrosion issue and is either ground or hot side. With not seeing any voltage with key ON I'm thinking the hot side. Get a long piece or wire so your meter can be used to check continuity of each wire on the 10 pin engine connector. Also look at the inline fuse on the Red/Purple wire behind the helm

The in-line fuse on the red/purple is shiny new, the internal connectors on the fuse enclosure look a little corroded, best to just replace that guy? (Idk how I'd clean/sand those)

I'll check for the continuity.

Thanks again!

 

[alldodge]

internal connectors on the fuse enclosure look a little corroded

It could be right there in the corrosion, maybe internal to the wire. It looks connected but could be barely making contact inside the wire.

 

[Scott Danforth]

is the motor electronic ignition (TB IV) or is it still breaker points?

if its points, that would change your timing if the breaker shoe is worn

look at links 18-20

https://forums.iboats.com/forum/eng...ion-for-diyers-an-work-safe-search-version-of

if its the TB IV, here is some reading. however the ground connection at the dizzy is one place to look as well. https://forums.iboats.com/forum/eng...-mercruiser-thunderbolt-ignition-systems-work

 

[kip2001]

It could be right there in the corrosion, maybe internal to the wire. It looks connected but could be barely making contact inside the wire.

Okay, I'll bypass that fuse and see if the problem goes away (replace if it does).

is the motor electronic ignition (TB IV) or is it still breaker points?

if its the TB IV, here is some reading. however the ground connection at the dizzy is one place to look as well. https://forums.iboats.com/forum/eng...-mercruiser-thunderbolt-ignition-systems-work

It's TB IV, I believe that was stock for this engine (serial # 6921426). I just replaced the sensor inside the distributor, I'll double check that ground connection there, make sure I didn't miss the wire. I don't have 12V at the positive terminal on the ignition coil so that's where I'm at on the troubleshooting guide.

Thanks all for your help!

 

[kip2001]

Eureka! Did some troubleshooting this afternoon and I believe my problem is a faulty circuit breaker! Let me know if you think there's any fault in my conclusion, here's what I did:
Measured voltage at red/purple wire terminal - 12v
Hot wired red/purple wire terminal to positive terminal of ignition coil - 0v (voltage disappears as soon as wire touches coil)
Hot wired red terminal to coil - 12v and gauges are working!
Hot wired red terminal to red/purple terminal thus bypassing the breaker and all functions of my ignition switch work! To do this I simply put an alligator clip on both terminals of the circuit breaker, all wires to the circuit breaker are shiny metal. Seems pretty conclusive that the breaker is the problem, I measured its resistance and it fluctuated a lot from 1ohm all the way to 500ohms (it was constantly fluttering).

Okay, now for the new question, the existing circuit breaker is 50amps. Based on my engine serial number I should be using merc part # 88-93582 which is a 60 amp breaker. My gut is telling me go with what the engine serial # (6921426) wants but I just want to verify that is needed.

 

[alldodge]

Think you got your issue, the 50 amp breaker, and I would replace i with another 50 amp

 

[kip2001]

Think you got your issue, the 50 amp breaker, and I would replace i with another 50 amp

Thanks AllDodge! New 50A breaker ordered, I'll check my timing and all that once it's in and report back if that indeed *fingers crossed* solves all of my problems.

 

[kip2001]

New circuit breaker is in! Can someone take a look at this video and tell me if this is normal for the vacuum on an engine of this vintage (1985): https://photos.app.goo.gl/6bETHqqGzu51zvJf7

The timing was dead on at 8 although it does still jump to 4deg before when at idle. I revved the engine up to 3800rpm and the timing was steady at 31. Any strange idle issues like the fluctuating vacuum all go away at higher rpms. Am I still chasing an ignition issue?

 

[alldodge]

Looks ok to me, run it and see how she does

 

[kip2001]

Looks ok to me, run it and see how she does

She ran great for the first 2 hours! Not so much the last 30 minutes. Seems I keep fixing small problems that contributed to the larger issue but that problem is still elusive.

Here's what happens, pretty much same symptoms as before, after an hour or so up river we anchored at a spot we like to swim, stayed there for a couple hours and then returned home. We cruise around 7mph so RPMs were basically at 1500rpm the whole way. On the return, after about 20 minutes, the engine rpms starting spiking every few seconds, the gauge would jump 200rpms (and then return) and it felt like the engine was missing. Then the rpms became erratic, going up/down a few hundred. I then tried more gas and the erraticness went away but the miss was still happening every few seconds around 25000rpm. I took the throttle down and put the boat in neutral, rpm gauge stayed even and engine sounded fine. Put the boat back in gear and we went another 20 minutes at 1400rpm with no problems. Then increased to 1600 and the occasional miss came back, at this point I wanted to see what was happening at hire rpms and we had a stretch with no docks so I opened it up and at planing rpms (around 3300) the boat gets up there but the rpm needle will spike w/a miss and then run fine and then completely started missing for a few seconds with a reduction in power and then return right back to running fine. I did that maybe 15 seconds and then returned the throttle back, on the deacceleration it missed/rpm spiked a couple times and then all symptoms gone lower than 1500rpm.

To add, most of the time when I trim, the engine the rpm needle will quickly drop 50rpm and then return to its previous state. If I try trimming the engine at lower rpms right after one of these missing fits the rpm gauge will swing erratically up and down with the engine clearly sounding of all sorts of missing and dislikes. Stop the trimming and everything returns to normal.

Am I right to think this is still electrical? I'm wondering what I can isolate to try and figure out where the problem is. The next thing I want to check is the wiring the PO did to remove the ballast resistor going to the coil. I measured 12v on the positive terminal so I know they removed it (the coil I have requires no external resistor) but I haven't checked their splice.

 

[alldodge]

Agree its still electrical and is a corroded connection, either a connector or splice

Don't need a resistor feeding the coil. I would suggest jump from the bat or starter post to the coil. This jumper will keep the coil hot, and if the splice inside the harness is good, should stop the rpm issues.

 

[kip2001]

Okay, I'll hook up the positive post of the coil directly to the battery and see if that makes my woes go away. Thanks again for all your help!

 

[kip2001]

Alright, had some time last night to do some troubleshooting while putting the engine under load (accelerating to a plane).

I had three ways of knowing the electrical problem is present, the depth finder will reset and beep when it does (it's not powered by the ignition switch but may share a local ground), trimming the engine effects spark/rpm, and at higher rpm the engine will miss and sometimes backfire.

Disconnecting shift interrupt switch: Depth finder would beep and engine missed.

I reconnected the shift interrupt, disconnected the tach sender from the negative coil terminal: Engine would miss and even backfired, I recorded this audio during my acceleration: https://vocaroo.com/i/s0vbkWO6S6Dd

Disconnected tach sender & purple wire from key to positive side of coil, coil then wired directly to battery. After using key to start engine, I put it in the off position: Engine ran perfectly! No beep, trim worked at all rpm. I repeated the accelerating on to a plane four times and it never missed or backfired.

So based on that and the electrical diagrams, I believe I've isolated my problem to the helm. Now here's a question, could this be a faulty gauge? Right now I'm looking for a bad connection to a gauge, a PO wire improperly done, and just random loose wires (see picture, I think that wire is a ground that popped off the speedometer but I still have verify where it should be). Just want to make sure I'm not missing something that I should be looking into.

Thanks again!

 

[alldodge]

I don't think its a gauge, an my guess is its in the 10 pin connector.

 

[kip2001]

Hmm, how do I go about thoroughly cleaning all the female and male pins?

 

[alldodge]

Use something like these. Can be found at most hardware stores

https://www.ebay.com/itm/5XRound-Pi...re-Brush-Set-Stainless-Steelevs-/113090701096