1996 Mercruiser 5.7 V8 LX EFI starting issue

MapisM said:
Yep, agreed.
What I meant is that if the SAME (i.e. just one) starter battery is used for both engines, the problem should not be specific of one of them.
But based on the last posts of S37, obviously he has a different setup.
Being used to consider separate starter batteries as a pointless ballast, I always forget that it's actually a rather popular arrangement... :giggle:
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Haha. Agree. A link out is useful on occasion, but totally agree.
 
Ok both Qbhoy & Mapism and everyone else

thank you so much for your input. I’m gonna go with Qbhoy as he has actually experienced the same issue along with Mapisms valued input. Hopefully I can try this in the next couple of days and revert to you all with the results......... for now,

thank you all so much

A
 
If it still doesn’t fire up check you have at least 12v on the purple wire coming from the ignition switch while cranking. If so then check it at the engine, any purple wire is an ignition on ‘running position’ supply, this will go to the Main power relay near the ECU and to the ignition module, these should have 12v with the key in the run position. If you do have voltage right down to the engine then it could be the main power relay which controls the power to the ECU or tge fuel pump relay.
 
QBhoy is correct for my money. Shouldn't be too hard to measure the ignition voltage (or even battery voltage) as the engine is cranking.
 
Anything with an ecu is very vulnerable to low voltage issues. Even apparently good batteries can suffer voltage drop on cranking, yet quickly recover when the cranking stops, which makes diagnosis difficult.
 
david_bagshaw said:
Anything with an ecu is very vulnerable to low voltage issues. Even apparently good batteries can suffer voltage drop on cranking, yet quickly recover when the cranking stops, which makes diagnosis difficult.
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Thats why the electronics on petrol injection engines and common rail diesels all work on a 5v bus. Otherwise every time you crank the engine the voltage drop would make all the sensor values invalid.Everything Mercruiser, Volvo, Mercury, Yamaha, Suzuki etc outboards, and all cars on the road since fuel systems became electronically controlled work on an industry standard 5v. So a low battery won’t affect the ECU directly, but the 12v from the ignition switch to the main power supply relay for the ECU or fuel pump can fall out under cranking if the battery is weak or the starter has a fault and draws excess current.
A Mercury Verado outboard for example has two 5v power supply busbars which then go to the various sensors and ECU, if one falls below a certain voltage you get a fault message, I know as my boss had this on his Verado 350, I eventually found the fault buried behind the main electrics box where the 5v buses are mounted.
 
spannerman said:
Thats why the electronics on petrol injection engines and common rail diesels all work on a 5v bus. Otherwise every time you crank the engine the voltage drop would make all the sensor values invalid.Everything Mercruiser, Volvo, Mercury, Yamaha, Suzuki etc outboards, and all cars on the road since fuel systems became electronically controlled work on an industry standard 5v. So a low battery won’t affect the ECU directly, but the 12v from the ignition switch to the main power supply relay for the ECU or fuel pump can fall out under cranking if the battery is weak or the starter has a fault and draws excess current.
A Mercury Verado outboard for example has two 5v power supply busbars which then go to the various sensors and ECU, if one falls below a certain voltage you get a fault message, I know as my boss had this on his Verado 350, I eventually found the fault buried behind the main electrics box where the 5v buses are mounted.
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Spot on. I know my GM Mpi V8 are all 5v sensors etc. Can cause a chain reaction of faults recorded...just from a simple thing like this. Very often leads folk down a path the wrong way as a result.
 
On some of these units there is an over rev cut out to stop the engine racing if the propeller comes out the water if that connection is faulty it could be your answer
 
Not sure what you are referring to Scottie, but I have not seen it on any marine engine I have seen and definitely not on Mercruiser. The rev limiter function is built into the ECU and limits the rpm internally.
 
I recall it on a Volvo but probably before ECU long term memory still works ok
But then we never had a very large population of big petrol engined boats in Scotland running out of fuel before you could get to a petrol pump and natural thrift may have had some effect.
 
spannerman said:
Not sure what you are referring to Scottie, but I have not seen it on any marine engine I have seen and definitely not on Mercruiser. The rev limiter function is built into the ECU and limits the rpm internally.
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Actually, I can't recall of anything meant to completely stop the engine upon overrevving also before the ECU era, at least in Merc engines.
Btw, on fast boats, while cutting off an engine upon overrevving might be good for protecting the engine itself, it would be a recipe for destroying the transmission upon re-entry, after jumping out of the water.
That's the reason why the throttleman sensibility has always been crucial.
 
Quite agree Mapism, I remember when I had to test the 2 Nortechs we had, at 120 knts you had to be very quick on the throttles if she took off, once you got some air going through the tunnel it only took a small wave to induce flight!
 
scottie said:
On some of these units there is an over rev cut out to stop the engine racing if the propeller comes out the water if that connection is faulty it could be your answer
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hi Scottie. On these things the rpm limiter is from the ecu. All the best.
 
Hi again everybody

As I have little experience with electrical systems and "lack of" Mercruiser engineers in my region of north Scotland I have called in the help of a reputable motor engineering firm Precision Engine Services to help with my issue. After discussion with the owner (whom I know back in the day) who is also an electrical engineer he has mentioned he will check just about everything you guys have suggested, so he too is on the right track, he is very experienced in V8 EFI as well. I am now sharing this thread with him to let him get the flavour of experience that has been offered on this site for which i am very greatful indeed. The only thing causing a slight delay is of course Covid-19 but he has committed to getting down to the boat in a few days time as his business is unusually quieter at present. So once again I will let you all know the outcome just as soon as I get around to it on the boat itself.

For now, thank you all once more.
 
All, whilst waiting for the electrical engineer myself and a good friend (Marine Mech Eng. ) went back to the boat with limited resources and checked the following items below, see the results for yourself.


Stbd Engine (Cold Start) 3 x linked Batteries with 12.7 Volts recorded prior to starting attempts.
  • First attempt the engine started immediately (as it always does) with a recorded voltage drop from 12.7 V to 12.4 V. while cranking
  • Engine ran for 5 minutes and produced a steady reading of 14.2 Volts which was taken from all 4 batteries onboard.

Port Engine (Cold Start) 1 x independant Battery 12.7 Volts recorded prior to starting attempts.
  • First attempt on the single battery the engine failed to run with the recorded voltage drop from 12.7 V to 11.3 V very quickly.
  • Second attempt with the battery link switch engaged with the volts recorded at 12.7 V and reduced to 11.9 V over approx 4 seconds and the engine started when releasing the key from START to the RUN position. (Below 12 V as mentioned in the thread).
  • Second scenario - Stbd Engine running and battery bank at 14.2 V, The Port engine started first time on the key, Voltage drop from 14.2 V to 12.2 V over approx 2 seconds.
  • Further tests were not successful as on the first attempt and proved there was always a substantial voltage drop upon starting the Port Engine (Generally down to and between 11.6 V & 11.9 Volts) resulting in continuous cranking times and the engine intermittently starting when switching the key back to the RUN position.
  • Connections and cable run from the isolator to the starter are in good condition and connections secure. Would this indicate a possible issue with the internals of the starter motor? Dirty brushes etc, (Personally I’m not to sure on the internal workings till I get a drawing). as mentioned earlier in the thread both coils are confirmed good as they were swapped.
Additional:

- We have not swapped the starter motors to see if the problem exits with the other engine
- We have not checked the voltage on the purple ignition wire
- We can confirm a large voltage loss on the single battery when cranking each time with or without the link switch engaged.
- Once cranking is stopped all batteries return to 12.7 v very quickly.


Until we get the electrical eng. are thinking is either a starter motor and/or solenoid issue losing voltage or the ECU itself

Any comments.......p.s. sorry we didn't get the purple ignition wire done
 
Oops.....sorry guys forgot to mention

- we allowed the starboard engine to run and with a total of 14.5v recorded we fired the port engine it started on the key at approximately 12.3v recorded, we tried to repeat this but couldn't get it to do it again ???
- we also allowed the stbd engine to recharge to 14.5v (3-4 minutes) and then tried to start port with the link switch engaged, it too started on the key with approx 12.7v recorded. we tried to repeat this but couldn't get it to do it again ??? always firing on the key return to run position again ?????..........Wow we are so close to getting this solved but just something missing and cant quite get there until we have the electrical eng. and equipment with us........thanks all so much again for getting us this far .
 
Remove clean and replace all the earth connections and starter positive connections. Examine each terminal and the wire at it for any sign of corrosion or fracture.
 
sealiner37 said:
Oops.....sorry guys forgot to mention

- we allowed the starboard engine to run and with a total of 14.5v recorded we fired the port engine it started on the key at approximately 12.3v recorded, we tried to repeat this but couldn't get it to do it again ???
- we also allowed the stbd engine to recharge to 14.5v (3-4 minutes) and then tried to start port with the link switch engaged, it too started on the key with approx 12.7v recorded. we tried to repeat this but couldn't get it to do it again ??? always firing on the key return to run position again ?????..........Wow we are so close to getting this solved but just something missing and cant quite get there until we have the electrical eng. and equipment with us........thanks all so much again for getting us this far .
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Definitely that engines battery is weak. 100% that’s your issue I’d think. The cells will be gone inside and any strong power from the other good one just gets dissipated into the other bad one. When you’re running the other engine and they are linked out..it will be just Enough to fire the port. All tells me that this battery is weak and is the issue.
 
Also worth knowing that an indication of 12v odd doesn’t mean the battery is always good. It could still be in bits inside and show 12v...but won’t give you the power to full effect. I’d try switching over battery from the other engine. Or maybe better still. Run jump leads from the other battery (with that engine running) straight into the positive battery cable and to an earth on the port engine. By pass that battery altogether. If it starts normally you have proven it’s that battery. Try that.
 
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