Thank you for that info. Had one about 5 years ago come to the shop, had the same issue. I had to take a ride with him and it took an hour to show up. Didn’t figure it out at the time but it does make sense, I don’t know what he did with the rig in the long run but we never saw him again.
Quite a few years ago I actually documented (when we had the old forum software
) my issue and all the things I did and how it all played out, and the eventual fix and results of that. It'll still be in the archive somewhere, but good luck trawling through my 27,000 posts.
I did a bunch of searches, still couldn't find it, so I just did a quick rewrite....
Enjoy.
Bought a new engine for the boat, a 1994 4.3LX. I installed it and propped it with a 21” prop that allowed it to run up to around 5,000rpm (recommended is 4400-4800) pushing the boat to about 39 knots.
It wasn’t long before I noticed that on the run home (after a day out) it would only pull about 4300rpm and the speed was down to around 32 knots. Verified that a few times and then took it back to the dealer I bought it from.
John (shop owner) gave me a 20L outboard tank and asked me to test it again with that. I did, no change. Everything ran fine at the beginning of the day, but by the end of the day, 700rpm and 7 knots were gone. John also had me check the choke was fully open, which it was. He suggested it was an ignition problem, and got a new module in (module was gen II, and mounted to the side of the distributor). We changed the module, and …… no change.
At this point John threw his hands in the air and called the Merc WA state rep, Phil Donnerly. Phil and I went through all the things John did, and again, no change. Phil called the specialists in Melbourne, and the first thing they suggested was that I buy a 23” prop to get the revs down, because ‘we don’t know what’s happening inside the module when the revs go beyond recommended’. Fortunately I didn’t go that route (would have been another $1,200 I didn’t need to spend to not solve the problem).
After about 6 months of this going on, the dealer and Merc bailed on me, with ‘it’s only a few hundred revs’, I was left on my own, and very much determined to get to the bottom of the problem.
I decided that it would need to be approached differently. I got a timing light and tacho and started checking. The first thing I noticed was that when the engine lost it revs, it also lost about 8° of advance (total module advance was now only about 6°, not 14°). So, we have the first part of the puzzle solved, but why was it losing 8°? As a test I adjusted the distributor 8° advanced at idle (up to 16° initial timing) so it would have those 8° back at full advance. When I ran the engine back up to speed, those lost 8° were back. Total module advance was back to 14°… What the???? Moved the distributor back to 8° at idle, and the total module advance is back to 6°. Super confused I packed up and went home, satisfied that at least I now knew it was an advance problem I was chasing, and I could run the engine on the flushers and not have to throw it back in the water each time.
At home I decided that I would draw up a simple ‘advance chart’, with the initial advance in one column and the total ‘module added’ advance in the other. What came back was REALLY weird! There’s no way the module knows what the initial timing is set to, yet here was ‘clear evidence’ that it appeared to. If the initial timing was set to 8°, the module would only advance by 6°, but set the initial timing to 16° the module would add the full 14°. Then, suddenly all the issues went away. I couldn’t make it ‘lose advance’ again.
It took sleeping on the problem for me to realise what happened, and what the issue was all along.
The manifold is cast iron, the distributor body is aluminium. They expand at different rates on heating. When they were cold, the ground contact (electrically) was perfect, no loss. But as they heated up, the different expansion rates caused the contact between them to change, become more resistive. As electronics likes nice solid clean grounds, all I can surmise is that a resistance in the ground between the module and the engine block changed the reference voltage for the advance curve, so the module was advancing the timing based on an incorrect reference point.
I found that there was a spare ground wire in the harness (I think for an accessory on the transom plate), and I hooked that to the distributor and never had a problem again… So, why did the problem just vanish anyway? I think because as I was changing the initial timing (moving the distributor around) it rubbed a new contact, and was no longer affected by the expansion differences, and so the module reference ground was now the same as ‘block ground’ at all times…
Anyway, I passed all this on to Phil, and then about 3 months later bought the dealership from John. About 3 months after that, all the new MerCruisers coming through had a ground wire on the distributor.
Later I may tell you about how we were the first to find out that the Vortec-headed V6s would suck water (reversion) if run at idle on flushers.
I know it's a long story, but now you know how I found it....
Chris.........
