Logo
FULL SITE MENU

Help diagnosing intermittent power loss

N

nmuir

New member
Looking for input to a challenge



I have a 1990 20' Searay w/4.3L v6 mercruiser Alpha One 175 hp w 2BBL carb - about 700 hours.



Running I have an intermittent power loss. The boat still gets up on a plane and runs at 30+ mph, but something is not quite right. Doesn't pop a wakeboarder up the same way, tops at 35 mph verus 43, needs an extra 500 rpm+ to stay at 21 mph, and so on. So still runs but doesn't have the same power. Sometimes the power loss cycles on and off every few seconds and sometimes it just stays lower for 3-4 minutes.


I drained the fuel tank and replaced it all thinking it might be bad gas or some water. I have a racor fuel filter/water separator too. I checked the carb for debris in the float bowl and/or the float not closing but that was all ok.


I have new plugs in (the old ones didn't look too bad - no obvious clues).


The oil is a overfilled, which I will remedy regardless (changed when the boat was not level - oops). Based on the plugs I don't think it is oil fouling though.


I have not done a compression check but with the intermittent nature would not expect to start there.


As it is intermittent I cannot tell if it is heat related but it *may be* worse once warmed up (or maybe I am just running the boat more once it is warmed up and so notice it more).


So, the question is: the diagnostic path recommened and what tests I might be able to do versus rebuilding/swapping components and see when it improves - as the boat is not in the water, individual test runs take a while! Also because it is intermittent I expect some of the obvious tests like (say) 'does the coil spark' may not help.


So fuel/carb or electrical first, and what path through each?

Any recommendations? Your input is welcome!


 
When issue occurs try a timing light on each plug wire to determine which cylinder is acting up.Check timing while your at it. Has fuel pressure been looked at? Has prop hub been inspected for slippage? Is there excessive growth on hull bottom? Salt water? Is engine running warmer than usual? Electronics can be affected by heat,coil,ecm, Has cap, rotor,& wires been tested,replaced lately
 
1. to test fuel, try to acquire a small gas tank say 5-6 gallon, and hook up to water separator input fitting and test to see if there may be a fuel related issue.
Using an external tank with fresh gas can allow you to conclude if you have bad gas, water in gas, issue with inboard tank.

If this fails to fix issue then move to ignition.

It sounds like you have done this with your fresh gas and separator filter change already



There is only a few things in the ignition system. I believe yours is a thunderbolt IV. or an early V

1. spark plugs (you replaced already)and spark plug wires
2. Distributor
3. Rotor assembly, inside distributor
4. sensor pick up, inside distributor
5. distributor cap
6. Coil Typically works to much higher rpms than a stock boat needs, Coil could be an issue and there is no way to test other than replacement. (coil testers are expensive and hard to find) Typical cost ~ $30.00 +/-
7. timing module (either mounted on exhaust elbow or attached to distributor body) This typically would not allow for full throttle as timing would not advance fully. Typically only ~ 2300-2800 rpms maybe a bit higher but not able to reach 4400-4800 rpms.

You say plugs looked good..............What did they look like? was one any different then the rest? Maybe darker, Maybe whiter?

So maybe some additional history on when any of the ignition parts may have been replaced last?????

Also as suggested if you have a timing light and you can reproduce the running issue and can hook a light to each wire while it is occurring you may be able to find a wire/plug that may be not firing in essence dropping one cylinder............

After rereading your post it does sound like you have lost ONE cylinder. That would account for the lack of ~ 500 rpm or hole shot power...
 
Last edited:
When pulling plugs off wires and replacing them its easy to get a lousy connection from either a loose clip in the plug wire (carefully "mash" the clip more closed with pliers) or from an offset clip ( clip center has moved away from the center line of the plug hole in the boot).
 
Back
Top