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Ignition control module

805HD

Regular Contributor
what’s up guys!

‘87 cobalt
alpha1 drive
454 BBC

Boat runs great till I stop and hang out floating or beached for a while, then it has a misfire condition around 1500 rpm.

Now here’s the interesting part. If I keep the engine hatch open while hanging out (floating or beached) the misfire doesn’t happen.

Been through the whole ignition system (grounds, coil, timing, distributor ect) only thing left is the ignition control module.

I’ve read that the ICM can be sensitive to heat when it’s on its way out.

thoughts?
 
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'87 TB IV

Typically the ignition control module only controls timing. So if you were not to achieve say, greater than 2400 rpm that would point to the module.

By any chance would you have the ability to get ICM from another boat to try. It can be from any V8 TB IV or V (V with NO Knock sensor connection)

The only difference between V8 modules SBC and BBC is the advance curve so you could use for testing purposes only.
I know the difference is a few degrees total advance and when the advance starts and how aggressive it is.

As long as you dont abuse the rpm it should work for testing.

When you say you have been through all the ign parts, What do you mean? Any replaced? Have you tried disconnecting the Tach lead at coil?

Speaking of coil, coils are very susceptible to heat
 
What’s up Jack, hope all is well with you.

I have replaced the coil, and I went through all the wiring to clean things up at the contacts.

When it does miss, if I play around with the throttle I can usual get the boat up and going on plane. Almost like it gets past that “dead zone” where it misses.

I’ve read about the tach causing problems, haven’t tested with it disconnected though. Can you elaborate on that a little more for me please.

Its interesting to me that it’s heat related. I’ve definitely narrowed it down to that.
 
Because the tach is connected to the coil if there is an issue with tach it can interfere with the coil output as the tach connects to coil -

The tach gets its pulses from the coil when it is triggered to fire.

WHat I will add based on your added description of just moving throttle a bit it goes away, Sounds more like carb related.

When was the last time you were in the carb?
 
You may be experiencing a lean condition when the throttle blades are just in the right position.

WHat is your base timing?

If it is set to 6-8 BTDC try bumping it up to 10, reset idle to ~700 and see if it goes away....

If you advance throttle and you have a lag then your accelerator pump is most likely worn, the extra gas shot is not happening when it should (as throttle moves so should gas squirt out of Accelerator nozzles exactly at the same time)

One other thought, Have you looked down at the carb when this issue occurs? Try looking at the carb throotle body and see if you may have any kind of gas drip out of the accelerator nozzles...

Just some ideas
 
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Okay that makes some sense, I have noticed the tach will jump around time to time while under way but haven’t noticed it happen during the miss. I will have to try it disconnected.

I didn’t set the timing so I’m not entirely sure but I can find out. It should be correct, the guy that did it is pretty damn knowledgeable.

I rebuilt the carb when we rebuilt the engine, the accl pump is new as well as the fuel pump. Carb is clean and looks to be functioning properly. It could probably be dialed in a bit better but when this problem occurs it is most DEFINITELY a miss. Not carb lag.

I wish I could get my hands on a loner ICM but I can’t. Perhaps I could isolate the issue by extending the ground and relocating it to a cooler area of the bilge (not mounted to the exhaust elbow).
 
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There should be non metallic spacers that keep the ICM off the elbow.

You could space it out more by increasing bolts and spacers? Use wood as spacers (dowel stock)

Try removing from elbow and just let it hang behind engine or hang it on a wire just to separate it from elbow?
 
Yea it goes spacers off the elbow, metal plate then the ICM.


I’ve been meaning to try and relocate it off the elbow to try and isolate the problem. But you know it’s been “winter” here in commiefornia so I haven’t been on the water.

just trying to get to the bottom of this before summer. It’s a drag.
 
you could also consider using electronic circuit 'freeze' spray...also known as 'circuit chiller'...to help in isolating thermal issues.

As far as the TB-IV modules goes, there is no SBC or BBC specific parts - the applicable ones all say V8-XX, where xx is the code that ID's the advance in the module.
 
Well it worked! I ran a separate (longer) ground off the block to the module and relocated it away from the engine. Problem solved! Ran all day with zero issues. That is however, until I ended up with what I’m assuming is a spun rod bearing :mad:


Looks like my summer is over before it even started. it’s always sumthin!
 
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