Mercruiser 4.3 runs 5 minutes then dies and no spark

[JasonOnTheRez]

This is a friends boat that I am helping out with. the boat is a 2001 Stingray 200MS and this is the original Mercruiser 4.3tks. (what does tks designate?)
The problem is that after 5 minutes of running, the motor dies and won't crank back up till the next day. The motors dies almost exactly in 5 minutes, you could set your watch by it. Before the issue it cranks right up. After it dies, using a spark tester on the coil, there is one brief faint spark but that is all. After that, it simply will not start. No spark from the coil at all. We have replaced the Distributor cap, rotor, the ignition sensor under the rotor, and the coil. Using the trouble shooting flowchart for similar problem, we have 12v on + side of coil, good ground, 12v on white/red wire on module under distributor cap, and 5v on white/green wire. This is before and after the no spark issue. In other words, 12v going into the "computer box" (Ignition Amplifier Module), and 5v coming out before AND after the issue. On the Mercury trouble shooting flowchart the only part that is troubling me is the step that says to ground the coil wire from the coil and try to touch white/green to ground to check for a spark. I get no spark before OR after the cut off issue. So maybe I'm not doing that step correctly? (could someone clarify this step?) This issue started about 45 minutes after a oil change, could just be coincidence? I don't know. The new Ignition Amplifier Module costs almost $600, so I don't want to replace it unless I am sure of the problem. Other things that I have read online on similar troubles was to check the oil sensor. These other people were have a "no spark" at all issue. It is my understanding that the oil sensor kills the fuel pump, not the ignition. At least mine runs for 5 minutes before cutting off, but when it cuts off it is down for a day. come back the next day and it fires right up.

Obviously this is extremely frustrating!!!!! Please help!

 

[achris]

TKS = Turn key start.

sounds like a heat problem. Try a couple of things. First make sure you have a good ground on the distributor. If the distributor isn't grounded properly and being ali, heats up and expand faster than the cast iron manifold it's mounted in, it can lose ground. No ground, no spark.

The other thing is to get a can of 'spray freeze' from your local electronics shop. When it stops, spray the module till it's cold. Test for speak.

Chris. .....

 

[stang32]

hmm, good points, ill be savng this post

 

[alldodge]

On the Mercury trouble shooting flowchart the only part that is troubling me is the step that says to ground the coil wire from the coil and try to touch white/green to ground to check for a spark. I get no spark before OR after the cut off issue. So maybe I'm not doing that step correctly? (could someone clarify this step?)

Take the coil wire coming out of the coil and attach to spark gap tester. The White/Red wire is reconnected. Disconnect the White/Green wire and take the wire end going to the module and rapidly strike it against a ground. It needs to be to a good ground so connection is made.

A spark gap tester is best, but can do this by attaching a spark plug to the coil wire and grounding the plug

 

[achris]

hmm, good points, I'll be saving this post

Comes from my days (in the 80s) of fixing computer boards.

(I assume you were referring to the spray freeze)

Chris..........