Port side engine issue

340DA

Member
Jul 21, 2008
365
LI Sound
Boat Info
2000 Sundancer Sport Cruiser
Engines
454 Mercruiser w/Bravo III Drives
I have a 2000 340 DA with 7.4L Mercs and Bravo 3 outdrives. This weekend I was cruising at 3200 RPM and suddenly my port engine lost power from 3200RPM down to 1500 rpm. The engine struggled and tried to get back to 3200 RPM but could not. I stopped and turned the engine off and started it again and the engine ran fine for several minutes but the same problem occurred and repeated itself. This happened several times and I repeated the turn off and start up several times so I could get home.
This off/on process solved the issue only for several minutes
It is interesting to note the following:
1. the engine did not stall, it just could not maintain a 3200 rpm level
2. the engine ran fine at the lower 1000 -1200 rpm levels. At this rpm level, the problem did not occur. I was able to run the engine without issue. The engine sounds fine when I rev it in neutral
3. The engine never stalled, it just could not maintain 3200+ rpm level
4. Also during the engine slow down, the gauge for the port side outdrive trim would bounce back and forth very fast. left to right to left to right to left, etc

The marina mechanics are looking into this.... Any thoughts???
 
Do you have SmartCraft? If so, check ALL of your grounds. Also check ALL of your plug wires on both motors. Had similar issue, chased it for 2 and a half seasons. Would be running at 3400 or so and stbd motor wold drop rpms briefly (300-400) and return to the 3400. We tested fuel pumps, fuel pressure, tightened and cleaned all grounds on both motors, changed plugs (twice), had injectors cleaned (ultrasonic), and more. Still same problem. Changed plug wires and problem went away. Most seem to think had bad wire (or more) that was arcing against metal and shorting out causing this. Not saying this is your cause, just something to consider. Also - no indication on SmartCrfaft or computer when hooked up. Only random Fault Codes that didn't make any sense.
 
I agree, sounds like a ground issue because of the o/d indication bouncing and the fault is affecting the entire engine harness. Try shaking wires at high idle to duplicate the problem. Aside from that a fuel starvation condition may be present?
 
Thanks.... when you say "Ground" i am thinking you mean something electrical.....If so, I'm thinking the same thing because the outdrive gauge bounces all around when the engine drops RPMs

but then there could be a fuel starvation problem, BUT, why would that impact the o/d gauge? which brings me back to electrical
 
Fuel starvation was just a speculation assuming the o/d gauges bouncing is unrelated.
Otherwise if the ground current is interrupted all things electrical will be interrupted. Fuel pump, spark, etc.
 
are those main ground terminals located at the top of the back? bottom of the back? where on the 7.4?

Thanks for the suggestions
 
Get a set of battery cables and ONLY CONNECT THE GROND CLAMP to a good ground on the engine and ONLY CONNECT THE OTHER GROUND CLAMP to the negative battery terminal. DO NOT USE THE POSITIVE BATTERY CLAMPS and Insulate them with electrical tape to avoid unintended shorts.
If your engine runs normal then your problem is a ground interruption.
 
Just may be loose or corroded. The ecm and computer are extremely sensitive to.voltsge changes. Need to rule out the simple inexpensive stuff first. Don't just start throwing $$$ at it. Also, I changed the 90 amp fuses coming off the starter solenoid. This is where the clean power comes from that is fed to the entire system. Mine we're loose and are supposed to be tight. I replaced both as they were only $30 each. I then knew I had solid power and grounds for a minimal.price. Oh and load test batteries for short or bad cell. Not sure where grounds are on 7.4. on my 496s they are on the back.
 
Something else to consider, Merc has a problem with ignition modules from that time period. The new module comes with it’s own ground as the improvement. So the ignition module may be failing?
 
Good suggestion by wanderer, but if the terminal connections are loose, dirty and making poor connection, then re grounding the motor will not prove to help diagnose. You have a 19 year old boat, things get corroded and loose over time. Make sure everything is clean and tight. It costs you nothing but some time, and you now know it is a solid system.
 
Usually the main ground are bolted on to the studs the coupler is mounted to or where the engine mount bolts to the block.
 
If only your trim gauge is jumping around, I'd be looking in that area first. If it hooks directly to the 90 amp fuse, I would head there 2nd
 
Yes, only the trim gauge is jumping. Battery and oil pressure and temp gauges do NOT jump

BTW, the engine sounds and runs fine at low RPMs (1000, 1200). Starts up with no issue
 
I was waiting for scoflaw to chime in as he has great diagnostic suggestions. In looking at your first post, the gauge only jumps when the rpms drop and is fine otherwise. Seems to me the guage n rpm drop are related. The rpms do fine at 1500 or less and it does fine for few minutes after restart and then repeats. It supports poor and / or dirty connection as they will get hot trying to supply needed amperage. But it cannot do due to poor contact. High rpms require higher amperage. This is the case in all electrical connections.
 
i am beginning to believe that the issue is "electrical" in nature..... That being said, regarding these connections being referred to......Where are they on the 7.4L?
This may be a difficult question, but I'm just not sure where they are.
 
Follow your grounds from batteries. Will lead you to them. Will be studs on motor with several wires with eye ends connected to it. Wire brush everything and re attach. Mine had 2 on each motor.
 
The 90 amp fuse is on the starter and very common to look like a ball of rust. Look the heavy red trim pump wire over, it might of chaffed through behind your engine.
 

Forum statistics

Threads
112,946
Messages
1,422,759
Members
60,929
Latest member
Henchman
Back
Top