Verifying Tach on 1994 3.0

[markhodges78]

Ok guys im pulling my hair out here i have a 1994 3.0 my tech started acting crazy so I decided to check it with my shop engine analyzer which is a actron cp7677 I hooked it up at the tach put it on 4 cylinder ideal seamed right but half throttle it was showing 6500 rpm this is with a 14.5 23p test prop i put on so i would know it wasn't getting over revved so I put it on 8 cylinder and it brought readings back down to 3100 at WOT ok I can believe that but now its showing ideal at 360 no way is there some thing different about a marine engine thats not the same and auto any help would be great

 

[Alumarine]

What are you connecting the tach to?
Is this electronic ignition or points style?

 

[Bt Doctur]

an engine is an engine, dosent matter where. you 3.0 could never turn a 23 pitch that fast ,period. Most run 13 -3/4 x 21P and max out at the stated rpm.

 

[Scott Danforth]

my 3.0 ran a 19P and on a good day with calm waters could hit 4700 RPM

spinning a 23p, the motor wouldnt make it over 3100 RPM

 

[markhodges78]

What are you connecting the tach to?
Is this electronic ignition or points style?

Yes electronic ignintion I'm hooking test tach to ground and gray signal wire as I would an auto

 

[markhodges78]

an engine is an engine, dosent matter where. you 3.0 could never turn a 23 pitch that fast ,period. Most run 13 -3/4 x 21P and max out at the stated rpm.

Yes I know that ... the only reason I put the 23 on was I knew that I couldnt over revve motor it was just for testing the tach

 

[markhodges78]

So basically with my test tach set on 8 cylinder the 23 prop would show 3100 on test tach ok I'm happy with that and it's Beleivable but when I throttle back to idle I'm only showing around 360 rpm that I don't Beleive

 

[wrench 3]

I have seen an electronic ignition double spike before and still run not bad. It just gets a waste spark part way down the power stroke. The only way to tell for sure is to put a scope on it.
It's usually caused by a pick up on the way out but could be a module. Again the only way to tell is to check the pattern on the pick up with a scope.

 

[achris]

These ignition systems fire on both compression and exhaust stroke by design. You need to set your shop gear to think it's dealing with an 8 cylinder engine.

Chris.........

 

[markhodges78]

Ok quick update I run the boat in the yard and checked rpm with three diffrent methods at the same time. In-dash tach, the actron engine analyzer set on 4 cylinder, and a fluke meter set on hertz at first at first all three was the right inline with one another 750 RPM @ idle then the in-dash tach started climbing to about 2300 @ idle. ok so I know tach is bad but what I don't get is when I was on the water if I set the engine analyzer to 4 Cylinder then running across water half to 3/4 throttle that same setting is reading 6500 RPM ( is that even possible ) but when I change the setting on the engine analyzer to 8 cylinder it reads 3100 seams about right but at idle it only reads 360 so that cant be right I'm at a lose I don't know what to go by...... I have another tach kinda would like to know its right when i put it in the boat....

 

[wrench 3]

These ignition systems fire on both compression and exhaust stoke by design. You need to set your shop gear to think it's dealing with an 8 cylinder engine.

Chris.........

A '94 3'0L Mercruiser shouldn't be running a wast spark system.

 

[achris]

A '94 3'0L Mercruiser shouldn't be running a waste spark system.

I didn't design the system....

This is in the Merc shop manual.

 

[markhodges78]

I didn't design the system....

This is in the Merc shop manual.

I do not have a distributorless ignition I have an EST ignition and that info starts on 4B-21 and ends on 4B-25 of the manual

 

[markhodges78]

Everything I have read says two pulses per Revolution which is the same as any 4 cylinder Chevrolet engine....

 

[JustJason]

Maybe your distributor cap is bad and cross firing. Throw your tach on your car just to get a baseline. If your tach functions properly on another engine then the problem is definitely with the ignition system on your motor.

 

[achris]

Everything I have read says two pulses per Revolution which is the same as any 4 cylinder Chevrolet engine....

Yep, EST uses a distributor with cap and rotor run off the camshaft... One pulse per cylinder per 2 revolutions, just like a points system. Something sounds a bit screwy with either your test gear or something else. Where are you taking the pickup for the test gear from? I have an advance timing light (which gives revs on the screen) and it uses a pickup that hooks to one of the spark plug leads. If your system uses the same thing, try on #4 lead, close to the spark plug and away from the other leads (It may be picking up a second pulse from a lead running next to the pickup)....

Chris........

 

[markhodges78]

My Shop tach reads the signal from the tach signal wire just as a tach would and I am connecting it at the tach

 

[markhodges78]

One pulse per cylinder per 2 revolutions
Chris........

So that would be 2 pulse's per revolution just like all Chevy 4 cylinders correct ?

 

[wrench 3]

So that would be 2 pulse's per revolution just like all Chevy 4 cylinders correct ?

That's correct. The EST system is basically the standard Chevy ignition system.

 

[mickyryan]

a 3.0 cannot rev to 6500 it maxes out at around 4800 then valves start floating if I recall correctly but look that up to be sure