Thunderbolt iv ignition question

[rocstar]

I just bought a 1984 Sea Ray 260 with the 260hp 350 engine. I had the engine checked out by a Mercruiser tech. He said that the way the timing was advancing, there was a chance that the ignition module may be bad. I don't know a lot about engine timing, but he said that at idle (600 rpm) it was at 8 deg. but it would jump up to 1000 RPM and advance to 14 deg.
He said that he wasn't sure about the ignition module so he didn't replace it.
I had it on the water today, and it did fine for about an hour, but on the return trip, it started to sputter when running at 4000 rpm. I dropped it down to 2500 rpm to slowly get back to into port.
My question is, is there a way to verify if the ignition module is bad?
What is the best place to buy a new one? Mercruiser or aftermarket?
Can I install it myself, or will I have to have the tech do it? I'm pretty handy, but I've never adjusted the timing on an engine.
I know it is the old style square TB IV that is not mounted on the distributor. Stem to Stern want $350 for it.

 

[Mjust]

Re: Thunderbolt iv ignition question

Try removing the ignition module and putting some thermal grease on the back of it and then reinstall and see how it performs. Sounds like the ICM is overheating and failing which can damage the ICM. Every time you remove the module you should apply some thermal grease on the back to help keep heat of the module.

 

[Reel Poor]

Re: Thunderbolt iv ignition question

There is no dielectric grease behind this module. Those are VERY reliable ignition systems. When was the last time it had a complete tune up, plugs, cap, rotor, wires, fuel filter(s)? Does the tach work like it should, or does it act up when the engine does?

 

[rocstar]

Re: Thunderbolt iv ignition question

It just had a tune up but I don't know if the mechanic took the ICM off, and checked the grease. I'll check that.
The tach seems to up and down okay. It maxed out at about 4100 RPM, but we were in 3-4 foot waves, so I don't know if it would go any higher in calm water. The mechanic did say something about the tach not being accurate with his timing strobe.
Thanks for the feedback so far.
If the ICM does turn out to be the problem, where should I purchase it? And will I need to re-time the engine?

 

[rocstar]

Re: Thunderbolt iv ignition question

Just checked my ICM. It is mounted on the exhaust riser on the starboard side, and has 3 large isolator pads to keep it from heating up from the riser, so I don't think it is a grease issue.

 

[John_S]

Re: Thunderbolt iv ignition question

T4 does not have adaptive timing, so it should not be jumping around. Check your ICM module for part number or timing advance curve id. From that, we will be able to point you to a rpm/advance curve graph. A sloppy distributer or other problems can be causing your jumping.

 

[rocstar]

Re: Thunderbolt iv ignition question

The numbers on the module are:
4C24
7520
390-7804-1

Any help on where to buy one, or the equivalent module without having to change the entire ignition system or re-timing?

 

[milkyway]

Re: Thunderbolt iv ignition question

Some are suggesting that you first check other things rather than replace your icm right away. Check your fuel, fuel lines, pick up tube, rotor and sensor, antisiphon valve, fuel vent tube for obstruction, fuel filter in t he carburetor inlet, is fire arrestor clean and clear, try spraying a can of carburetor cleaner in the carb throat, do you have fresh fuel in the tank since you just bought the boat. Did the mechanic put the engine in base timing before starting to time it ? if not, the timing will be advancing like you described it. Did you also replace all wires and caps?

My dash tach is not accurate so I no longer rely on it other than my handheld digital rpm tester. Even if the tach moves up and down by 1000 difference, it does not bother me since I can hear the engine having a steady rpm and my digital tach tester has proven this.

You need to have a manual and do your own checking. If not, you will spend substantial amount of money getting a mechanic when a lot of things can be done for a few dollars. Your time and this forum will help you. I must have saved more than $5k and a lot of downtime over 4 years by following this forum. If you need to winterize, you need to learn that too.

If you insist on replacing the ICMm try this site - ebasicpower.com for ICM Module Conversion Kit Fits MCM 5.7L/LX Alpha w/ Thunderbolt V ignition. Call them and discuss your specifications.

But the problem you have can come from a variety of causes which arise even with an engine that "does not" utilize an ICM.

 

[John_S]

Re: Thunderbolt iv ignition question

I wasn't able to match your part number to an ICM timing curve. I did look at the common T4 V8 advance curves, and at 1000rpm, all have no more than 2 degrees advance. I still wouldn't jump on the ICM until other things were eliminated. If that is the original distributer, there could certainly be play in it, and/or run out. I'd also map ICM timing curve through 3500rpms at 500rpm increments.

 

[180shabah]

Re: Thunderbolt iv ignition question

Did the mechanic put the engine in base timing before starting to time it – if not, the timing will be advancing like you described it.

This is a TB4, no base timing mode.

 

[ohioriverrat]

Re: Thunderbolt iv ignition question

i am having the same issue with a 4.3 thunderbolt. the shop has done everything except change the ICM. a thousand dollars later it still does the same thing. here is a list of things that the shop has done that had no effect..
new fuel pump
drained fuel filled with fresh
removed cleaned ground wires
removed and cleaned and readjusted carb to specs
removed and cleaned water /fuel filter
unhooked stereo
new batteries

this is very hard for someone to trouble shoot because it doesnt happen right away. it happens after you drive under load, anywhere from 5 to 20 minutes.

every time the shop does something and gives it back to me saying its fixed, but i take it out and after 5 to 20 minutes im lucky to get it back to the dock.

it will spit and sputter and not get on plane at all. it usually starts when i am running on plane and i bump the trim button or something - then the motor starts cutting out.
once it starts it will continue until the engine cools down completely

the ICM is a costly part. 300 to $400 and you might as well replace the coil while you are back in there - $40. but to be honest it is as good of a place to start as any. as for me, money is not the issue, i get very little time off from work. i cannot spend 2 or 3 days trouble shooting this and checking that. so i took it to a shop, and the shop has done everything that people here have suggested, over the course of a month to the tune of around 1100 dollars. so trust me, it isnt any cheaper. just because you want the fix to be cheap and easy does not mean it will be. id love it if it was nothing more than a stuck float, or dirt in the carb, but just because i want it to to be that easy does not mean it will be.

i suggested it be a sensor or something electronic when i took it in the first time, but they dismissed that telling me the ethanol in the gas is surely the culprit. they are idiots as most in shop mechanics are...

 

[180shabah]

Re: Thunderbolt iv ignition question

ohio - you need to start a new thread.

but

the issue only happens after it runs for 20+ minutes and NOBODY has tested the coil????

I would take a meter out to it right now and compare to the figures in your manual, then go run it till it starts to stumble, pull it off while still hot and measure it again.

Has anyone looked at your fuel vent for obstruction???