TomZ

iTrader: (2)

I was asking the other day about replacements for the TB-IV because of some problems I was having, but I'm thinking that maybe I should be looking at trying to figure out what's wrong with it first. Here's what's going on:

When I put the boat up for the winter, the engine was running great. No issues. I kept the engine room at about 80 degrees through mid March, and kept the engine all covered up. All should have been good.

Fast-forward to April...

I sent the blower out to be rebuilt, and rebuilt the carbs while everything was apart. Put it all back together nicely. No issues getting everything back together. I didn't touch any of the electronics. I go to start the engine, and I cannot get any fire. Plenty of fuel, no spark. I take the connector off my Merc module (V6-14), verify that it looks okay, plug it back in, and try again. After a lot of cranking she starts to catch... but then starts popping through the blower BADLY. She was running like maybe only firing on five or six cylinders, and back firing through the blower. I think that perhaps the module is bad so I go and grab my old V8-24 module, plug it in, and she runs. Badly at first with a dead miss, but then she starts to smooth out in a few minutes. Finally, I'm able to start adjusting the floats and idle mixture screws and she's running nicely. Granted, I knew I would need to tweak some stuff after getting a new module (I was assuming that my V6-14 module was bad), but so far so good. I could shut the engine off, hit the key a few minutes later, and she's light off with barely any effort.

Three days later... I'm right back where I was before. Dead miss, and popping through the blower (I still have the V8-24 on the engine). I'm not sure of what the issue may be now. I believe my grounds are all good. Anyone ever see this kind of thing with the TB-IV?

Sorry for the long post, but I figure that the details are vital. Thanks in advance!

TZ

Mr Maine

Do you have another distributor module you could try? Those go bad, especially the old style ones with the external connections.

TomZ

iTrader: (2)

I don't have a third module to try. I was going to buy another V6-14, but was wondering if something else was amiss.

I guess I could try a new distributor sensor. I have a brand new one in a TB-V distributor (that set-up has a bad ICM and was replaced with a Delco).

Mr Maine

That's what I meant, the pickup in the distributor. Those go bad more often than the module itself.

donzi matt

iTrader: (1)

Also, a weak coil would cause the issue you are having. Cheap and easy to toss one in and see if it runs. If your pickup in the distributor is the old style non enclosed style without the dedicated ground wire I would update that as well as it will eventually leave you paddling.

BUP

iTrader: (9)

Like Matt said above ^^^^ the newer ign sensor are fully potted and have a separate ground wire to hook up. Mercruiser has 4 SB's over the years about the older style ign sensors. If your ign sensor is older than 10 years old I would change it regardless and the same for the COIL. Both priced is not that much under 200 bucks thru Merc.

TomZ

iTrader: (2)

I'm sure my sensor is the older style, and I doubt it's ever been replaced. I have the sensor (basically brand new) out of the other distributor as well as a new coil from that set-up. I'll give it a try when I get in after work.

Thanks everyone!

TomZ

iTrader: (2)

Came home from work and tore into it:

- replaced the ignition sensor
- replaced the plugs (most were wet from fuel because of the misfire issue)
- found that the #1 plug wire wasn't connecting properly (terminal is weak/cracked, worked enough for troubleshooting. Replacing it).
- replaced the cap

I did not change out the coil (I have a brand new one here but forgot about it).


Engine fired on the first try and settled into a rolling idle. Ran it for a little to get her warmed up. She was idling between 850 and 1000 rpm. I noticed that the idle would pick up a little, and sound very smooth, then would shutoff like someone turned off the key (key not off, gauges and warning beepers still worked). Attempts to restart were met with a little struggle, then she'd fire and repeat the cycle.

I will replaced the coil tomorrow.

What would cause her to speed up idle (seemed like the timing was advancing), then just plain cutoff like someone turned off the key? Anyone else experience that symptom with Merc's ignition? All of my grounds are good. I double and triple checked all the wiring. I also made that there was no play in the distributor so it's not a mechanical issue.

Seems I've heard of something similar to this. Anyone?

TZ

Mr Maine

The thunderbolt starts adding timing around 950 to 1000 which will cause a change in idle once it reaches that point and steps the timing up. Hopefully it's the coil, not much left.

TomZ

iTrader: (2)

This was different... the engine was idling no more than 1K RPM, and then it kicked the idle higher by itself and the exhaust note changed (I was standing on the ground at the back of the boat). I had a similarly weird issue wth a MSD 6AL that was giving up the ghost. It would idle for a few minutes, and then it would start crazily advancing the timing; crazily meaning having total timing around 45-50 degrees! This sounded similar (exhaust note). Something ain't right.

I'm going to try to coil just to rule that part out, but I think something in the module is giving up the ghost.