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need help diagnosing Mercruiser 7.4 with Thunderbolt IV ignition

J

JohnGinsberg

New member
Need help. My starboard engine is a 1988 Mercruiser 7.4 (=a chevy 454) with a Thunderbolt IV ignition system. Last time it ran well was last October 31. On that voyage, it started running rough. We have done a lot of work on it (see below) and it starts very well, but the engine will not rev up at all when I throttle up, and the timing will not advance.

We have:
- run a compression test on all cylinders: OK
- replaced the fuel filter: no H2O in fuel
- checked fuel pressure: OK.
- replaced distributor cap and rotor and ignition sensor
- replaced plugs and plug wires
- replaced the ignition coil
- rebuilt the carb and then switched good carb from other engine to double check that there is no carb issue.
- tested the whole ignition system, including the ignition control module, using (1) the diagnostic flow chart in the factory manual (copied here: http://www.marinemechanic.com/site/page251.html); and (2) this test from iboats forum (see post #2, by dubs283: http://forums.iboats.com/mercruiser...ives/apha-one-v6-thunderbolt-iv-444003.html): all OK, except timing issue noted above (no advance).

I have spark at every post on the distributor, at the end of each plug wire, and in each plug. I am stuck. Boating season is passing me by. This is my first full summer as a boat owner and liveaboard. Very, very frustrated.

John Ginsberg
Gangplank Marina
Washington, DC
 
tested the whole ignition system, including the ignition control module
Click to expand...
Swap modules between engines to see if it cures it and the problem follows the module. If that proves the module is the problem it will be cheaper to abandon the TB IV ignition and install a single wire dist. w/self contained ignition.
 
Thanks very much, guyjg. I will do as you suggest and report back. It appears Mallory and MSD both have distributors like you describe, and saving money would be excellent. Have been worried this boat is going to break my bank.
 
Maybe I am wrong, but it looks like I cannot switch the ignition control module over from the port engine. It's the later version of the Thunderbird IV (triangular and has a wiring harness), whereas I have the earlier version of Thunderbolt IV on the starboard (a box with 4 wires). Looked at the single-wire distributors and see that Mallory and MSD have them. Any danger installing something like either the Mallory HEI-75 (part no. 7548201c) or the MSD 8635 on a boat engine? (Obviously, anything I do on my boat is entirely at my own risk, and I release anyone who responds from any liability whatsoever. But if you see any danger, it'd be great to know.) Thank yo very, very much.
 
Any danger installing something like either the Mallory HEI-75 (part no. 7548201c) or the MSD 8635 on a boat engine?
Click to expand...
There are marine versions to use.
all OK, except timing issue noted above (no advance).
Click to expand...
So you are saying there is no timing advance or it failed the advance test?
 
At the time the thunderbolt ign. was state of the art.

Now that being said i don't fool with them anymore.

After 2 weeks messing with one on a friends boat i finally dropped in a all in one piece design dist. with the coil.

All new wires, was a kit.

One wire in one wire out and one wire for the tach. 15 minutes the boat is still running like a raped ape, Sunday my friend went fishing had to call me to tell me how nice the boat runs.

But he didn't catch any fish serves him right didn't invite me. I couldn't go anyway the mother-in-law came over ain't i lucky.
 
Please forgive my delayed response. I was caught up working at my job rather than on boat.

MM: Thanks for relaying your experience with integrated distributor/coil. A family friend also reported a positive experience replacing TBIV ignition with a 1-wire. With guygig, that makes three for three.

guyjig: Thanks for the follow up. You wrote:

guyjg said:
So you are saying there is no timing advance or it failed the advance test?
Click to expand...

The timing has not advanced when tested with a timing light. I have not been able to run a complete timing test, because the engine has not revved up from idle.

My plan at this point--and being a novice, this may be wrong (but I do have a marine mechanic of long experience helping and advising)--is to replace the plugs again, on the off chance that any are bad. The new plugs got fouled in the course of efforts. Cleaned them with sandpaper and carb spray, and all sparked when tested out of their cylinders, so this is probably a long shot, but want to fully rule out the cheap and easy first. If, with new plugs, engine still will not rev up (and timing will not advance), will try to replace ignition system with 1-wire. The thing that makes me think this is a spark issue rather than a timing-not-advancing issue is that, if the latter, I would expect engine to rev up, then sputter out or backfire. Am I right about that?
 
Here's some advice I got on the iboats forum. Reposting here to elicit any insights and to keep all the advice I am getting in one place:

Bond-o said:
Ayuh,.... Welcome Aboard,.... The advance is controlled by the module, try swappin' 'em 'tween the motors,...
Click to expand...

Thanks very much. That seems like solid advice. The troubled starboard engine has a 1st generation Thunderbird IV ignition module (looks like a vented box). The good port engine has a 2nd generation, with a harness (triangular baseplate). You can see the two generations in this service bulletin: http://www.marinemechanic.com/merc/distributors/mercurymarine/outboard/New%20Folder/thnbolt-IV-module-replacements_EN_14.pdf. The wires look the same, at first glance. I suppose I can hard-wire.
 
You have close to a 30 year ignition system there. That's another reason to change it out.

Hey what do i know ?

I wish you well honestly and hope you find the least expensive solution you can find.
 
Also check your pump. You probably have a mechanical diaphragm pump. The diaphragm eats away over time and the slightest hole will create much confusion. If you decide to go there, there's a method to the install to avoid a lot of 4 letter words.
 
Thanks much, FD. Fuel pressure tested and good, so pump ruled out as source of problem. Going to dig back into this problem this July 4 weekend and (if I don't solve it this weekend) the next. My stepfather, who raced cars and knows engines better than I, is going to help me dig in. My beloved mechanic fell in the water drunk and having taken a valium, was helped out by a neighbor, and fell back in, right in front of the dockmaster and marina manager. He went under the dock, almost drowned and is now banned from my marina for the foreseeable future. Cannot afford to hire another mechanic. Going to solve this.

MSD has no single-wire marine distributor, but they have a three-wire. Have to check out Mallory, DUI, Prestolite. If anyone has a recommendation for a brand, let me know. Found a Thunderbird IV V8-24 ignition module for cheap, but it is the later version, which requires a harness, and Merc does not sell the harness without the module anymore ($450 for both is best available price). That's frustrating. I do not have the harness, because the ignition module on my starboard engine is first generation Thunderbird IV and is hardwired.
 
The msd 3 wires go where ? One in, one out to the coil and one to the tach maybe ?

Manual advance dist ?

Where did your stepdad race cars, maybe i know him. I'm out of Englishtown.
 
All- Just wanted to close out my part in this thread with thanks to all counsel and with a note on my solution: All problems were resolved with the second replacement of the sensor inside the distributor. Before second replacement, there was no signal telling the TB IV ignition amp to speed up its firing of the coil, so no timing advance. After replacement, timing advances beautifully and the engine throttles up. So happy to hear it roar. Master mechanic: You had asked "Where did your stepdad race cars, maybe i know him. I'm out of Englishtown." My excellent setpdad, Rich Whitehill, who helped me a ton with the boat, raced stock a Flemington and other tracks around Eastern PA. Best, John
 
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