Thunderbolt IV Problem
 

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

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

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).

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

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.

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.

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!

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

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.

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.

Sounds like a fuel/carb issue

Griff,

What's your thought on the fuel/carburetion?

This shutting off thing is a new symptom that came up. I'm showing about 8 lbs of fuel pressure, and both carburetors have plenty of fuel. If there was a fuel issue, I would expect the engine to cough and sputter. This seems like someone just turns off the key.

Had one last summer. Pontoon with a 3.0 and Holley 2bbl.

Turned out needle and seat was sticking at times. Fuel would pour out of boosters and kill the engine like a switch was hit. Would take a long time to start, clear itself out, seemingly run okay, then instant die.

I went right for the ignition...all was good. Note: I have an inline spark tester that I used....so clearly could see it working when engine died and was trying to start. Finally took off arrestor, watched top of carb while it was running fine...and then saw the boosters start to poor fuel and then engine die. It was one of the rare times I have seen a N&S stick and fuel not come out of the fuel bowl vent.

Have also had same issue with N&S but not flowing enough fuel. After cranking a bunch enough fuel would get in bowl to start and run....but volume couldn't keep up and motor would just die.

A clue?!

Interestingly, when I got the blower and carbs back on the engine, I cranked the engine (without the coil lead connected) to prime the carburetors. I only had one fuel bowl (out of four) fill and produce a pump shot. I couldn't figure it out. I was worried that I had made a mistake rebuilding the carbs because I couldn't get the pump shot. Finally, I pulled a bowl screw, and sure enough no fuel. I found that three out of the four needle and seat assemblies were slammed shut and sticking. These were brand new AED needle and seat assemblies. Finally, I got fuel to the carbs, and fired her up. That's when I started having the erratic spark issue and popping issues. Swapping out the module to my stock Mag original got the engine started, and I was able to run it on the hose. I saw nothing in the way of fuel leaks. I even have a two minute video of the engine running. No hiccups (other than having the wrong module on it). Two days later it wouldn't run at all, and yesterday after replacing the sensor, she fired and ran well for about 15 minutes. Then the trouble started again.

Here's a little bit of a difference and a finding... this go around I ran the engine with the spark arrestor on it. I died so I went to look. I pulled the arrestor off, started the engine (took a little bit), and then saw fuel pouring out of the front carb's rear secondary booster. I cracked open the float adjustment, and got it to calm down, but still, it had done it (this was one of the reasons why I rebuilt the carbs in the first place). Maybe it was doing it again? Not sure. It was dumping fuel though. And thinking about it, it was pig-rich after trying to restart a couple of times. Another thing too... I do have the trailer rather high on the bow. Maybe it isn't even a float or needle and seat issue. Hell, I don't know.

I think I have a few things going on here.

Just to restate... there was definitely a problem with the ignition. Replacing the cap and the sensor got her running almost immediately.

It sounds like its surging at idle. Does it stall on the downward surge?????

Also, what's the timing set at???? With the V6-14, it should probably be set to about 20-24* at idle.
With the V8-24 it would only be 8* so it prob won't idle well there.

Before the issue of cutting out, it's had a surging idle. Always has. I've tried to tune it out, but honestly, I haven't had it together long enough to really mess with it, off the dyno. There were no problems running it on the dyno. Since then, the blower has been off for repair, and I did a rebuild of the carbs. I didn't change anything else.

It doesn't stall on the downward leg of the surge. It's totally random. Even etting it run at a high idle (solid 1000-1100 rpm, no surge), she'll quit. No coughing. No sputtering. Just off.

One thing that I haven't tried yet... making sure that the harness connector isn't loose. That's gotten me in the past with other boats. And I was leaning on things in that area while changing the plugs.

I was fighting this issue on a Stingray Maxim 222ss with an older 454 w/ Thunderbolt last week but haven't nailed it down yet. We finally got her to fire up and run with what sounded like a dead miss which cleared on its own briefly, then it would idle for a little bit and shut off. It would do it at High Idle (~1000 RPM) and normal idle. I was at the helm while my friend was in the engine bay and he thought I was turning the key off. Initial inspection showed the bowls were not filling but we didn't get much further than that. The boat was in bad shape when we snagged it last week in Delaware so we decided to do a full tune up anyway (Plugs, wires, cap, rotor, coil, belts, manifolds, risers, water pump, carb rebuild). We pulled the carb last week and rebuilt it over the weekend. I am supposed to go over there tonight to throw it back on with the new manifolds that are arriving today. I will let you know what I find unless you find your solution soon. Have you been able to check the timing while it is running?

I would start with the coil change. Quick and easy.

I would put a timing light on it when you get it started so you can see exactly what the timing is doing.

Another thought is the possibility of dist gear being bad and timing jumping around.

does that use the delco EST dist ? if so, the shaft can go bad and create running issues that will drive you up a wall...ask me how I know !!!! local chevy dealer here stocks several shafts !

The TB-IV uses a Merc-spec distributor; it's not a Delco EST. I checked everything... no play in the distributor. The gear is new (and properly pinned).That was one of the first things I went after,

Josh, let us know how it goes!

I plan to do all that tonight after work... or maybe this afternoon. The gear is definitely good.

TZ

Success!

I made sure the harness plug was solid and tight, replaced the coil, and changed the plug wires (had a bad connector in one so I swapped them all). I got her fired up, no real issues. She was a little rough at first, but smoothed out well once she got warned up. So ether the harness plug was giving us a bad connection, or the coil was breaking down once warned up. I'll take it!

I still have some issues with surging idle, but that's for another thread.

Thanks for all the help and suggestions!