1979 90hp inline 6 - spark issue

[slyem2002]

Hello. I researched postings "Post Subject: 90 hp Merc dies out Posted: 232553:: 4 July 2006" and "Post Subject: 90 won't fire Posted: 215400:: 10 Jun 2006"

but I have slightly different symptoms and would like to hear opinions on where I should start or what I electrical part(s) I should replace.

I recently bought a 1961 Debutante 19' (SabreCraft) with a 1979 90hp inline v6 Mercury outboard, serial#5160567. She is moored on the lake.

Before I bought it, the previous owner took me out in it, started and ran great, at idle speed and WOT. (although the Alum. prop is beat up and needs replacing).

I came back a day later to go out with the family (nice sunny 80 degree day). We'd been running her all day with no idle, cruising or starting issues.(We stopped and restarted several times throughout the day) In the late afternoon while heading back at WOT the engine suddenly died. It didn't sputter - it just died suddenly. I thought gas since my first tank was near empty. (even though I know running out of gas usually happens gradually) But figuring eliminate the easy stuff first, I connected the full tank and tried to start - no luck. So, next easiest is checking spark. I pulled a plug - no spark. Towed her back to the dock and decided to come back a couple days later when I had more time and tools.

When I came back a couple days later it was cooler (60 degree day) and thought, what the hell, let's see if it starts. It fired right up! Took her out and ran her for about 30 minutes with no apparent issues. Thinking I must have wiggled the right spark plug wire or something when I took the cowling off. What an easy fix! Well, maybe....

Came back next day (60 degrees again) with the family and it started right up again! Took it out on the water for about 20 minutes at WOT, then stopped for a bite to eat. After dinner she started right back up and ran it WOT all the way back home, again with no apparent issues.

Came back 3 days later, (nice 80 degree sunny day) to go out again and went to start it - wouldn't start! Pulled a plug, no spark!

Now I am baffled and without electric testing gear. Being from Seattle I am always suspect of the weather However I think I may be wrong on this one. Since it is moored and I have no trailer, I fear if I have a mechanic come to service it on a cool day it will work normal. Luckily there are several sunny HOT days coming...but my wife will kill me if I have to spend a small fortune on a mobile mechanic. So I am on my own.

Are their an electrical parts that can gradually fail or intermittantly fail? or may be sensitive to ambiant heat?

Thanks for your ideas and help.
Sincerely,
A new boat owner who is hopeless in Seattle with SeaFair only a week away!

 

[Laddies]

Re: 1979 90hp inline 6

Re: 1979 90hp inline 6

No spark on a I/6 rules of thumb for ADI ign
If there is no spark on 1 to 3 cyls = Coil if 1 cyl, 1 to 3 is a switchbox
If there is no spark on 1+2 or 3+5 or 5+6 = trigger
No spark at all = stator or kill switch is shorted (disconnect Blk/yellow wires a the boxes)
Intermittant lose of spark all cyls is sometimes a bad rectifier (disconnect 2 yellow wires from the rectifier)

 

[slyem2002]

Re: 1979 90hp inline 6

Re: 1979 90hp inline 6

Ok - so I took it out of the water on a borrowed trailer and dug in. (cooler day today) All 6 cyl. have spark!

Any suggestings for testing when all 6 are sparking?

 

[Chris1956]

Re: 1979 90hp inline 6 - spark issue

Slyem, It seems that one of your ignition components may be sensitive to temperature. It would be hard to imagine it was all the coils, so it is likely both switchboxes or the stator.

You should measure the resistance of the stator high and low speed windings when the spark is working, and again when it is has failed. I would also measure the voltage at the switchboxes(motor cranking) when the spark is working and when it is not. If you find a difference, you may have found the intermittent part.

 

[Laddies]

Re: 1979 90hp inline 6 - spark issue

There are 2 things that stop spark on all 6 cyls. the kill wire and the stator

 

[ricksrster]

Re: 1979 90hp inline 6 - spark issue

To check Stator voltages, you will need a DVA.

 

[Clams Canino]

Re: 1979 90hp inline 6 - spark issue

Also look for loose / grounding wires.... I smell a fish here.

-W

 

[Nutdriver]

Re: 1979 90hp inline 6 - spark issue

If this is the same as mine there is a small ignition coil bolted to the front of the engine right under the distributor.......coils are famous for failing due to heat.

 

[slyem2002]

Re: 1979 90hp inline 6 - spark issue

Ok, I did some simple ohm testing on the rectifier and found it wasn't repsonding like it was supposed to. I have since replaced that & seem to have the problem fixed. Time shall tell if it holds up the next time the sun warms us up in Seattle up over 90 degrees!

 

[adamcomputer]

Re: 1979 90hp inline 6 - spark issue

Ok, I did some simple ohm testing on the rectifier and found it wasn't repsonding like it was supposed to. I have since replaced that & seem to have the problem fixed. Time shall tell if it holds up the next time the sun warms us up in Seattle up over 90 degrees!

HOPE it works well for you. I had the same problem on a 1973 150hp Merc, bad trigger inside the sealed distributor. Drove us nuts, although we haver had to be towed back, it always started after a 15-20 minute rest; had it replaced and bingo, no problems. Good Luck !!!

 

[slyem2002]

Re: 1979 90hp inline 6 - spark issue

So we thought the rectifier fixed it - nope. The sun came back to Seattle & it didn't like it .

I ended up finding my temperature sensitive part - in my case it was the Switch box. I knew this because if I strapped an ice pack to the signal box (on hot days) the engine started and ran like a champ. Now that I replaced the Switch box with a new CDI Switch box, no ice required

Everything works like a champ now!

Thanks everyone for your help & advice!

 

[Kevin W]

Re: 1979 90hp inline 6 - spark issue

try simulating hot conditions.
use a hair dryer or a heat gun to warm them up.
also check radio shack for freeze spray.
its ultra cold spray to cool things off.

I have worked in electonics and these are great tools for finding heat sensitive components.
if you use a heat gun use caution it does get hot enough to melt things so keep your distance from things.