1996 Mercruiser 5.7 V8 LX EFI starting issue

S

sealiner37

New Member
Joined
13 Apr 2020
Messages
12
Visit site
Hi all

I am at my wits end on this one with sparce Mercruiser dial up support and I have turned to you all for help. I have a twin 1996 Mercruiser 5.7 V8 LX EFI's (petrol) and I have a starting issue on one of them. When I turn the key, the engine turns over nicely but wont start as soon as I let the key go and the key returns to the run position the engine fires and runs sweet. This actually feels like wires are crossed or something but checked against the second engine and all wiring is good (the same). has anybody had a similar issue ?? and could you advise.

Thank you all very kindly for your support in advance.
 
Try swapping the key switches round ie wire up the left engine to the right key switch and vice versa, if the problem then moves to the other engine then the issue is with the key switch.
 
I have had the boat over 12 years now but did not use it a lot as I was stationed abroad most of my working life. I recall that it would sometimes do this and other times not. Now that I’m retired and have the boat close to me I am able to both use it a lot more and service it a lot more. So the issue has been there but not as prominent as it is now, it simply refuses to start on the key only when I let it go.

I have swapped coils, swapped ignition barrels, installed new distributor caps & rotor arms, renewed spark plugs and cables. Cleaned VST’s , renewed fuel filters, checked injector sprays and swapped them over, and installed a new fuel pump as well. And still the thing won’t start on the key? struggling to find support from a Mercruiser qualified engineer
 
If it always fires and runs when you release the key I'd be tempted to just leave it as is, you could be chasing this and swapping/buying new parts for ever and never solve it.

So if it's still actually running fine, and you've tried the easy stuff (ie swapping the ignition switches round) I'd just put it down as a bit of character.
 
As Julians post it sounds very much like the key switch but I guess if it was faulty then it would not crank. Suspect a break break in the wire from the start terminal of the switch to the ignition side. Needs checking voltage at the switch terminal and then keep going through the circuit

Thinking a bit more, sounds like lack of 12+v to the coil on crank. Fairly easy to check that.
 
Last edited:
Why are you folks insisting on the key switch (which would have been my first thought either, fwiw), after the OP confirmed that he already tried swapping the ignition barrels?

Since the engine starts and runs well as soon as the key is back in run position, I would think that no current reaches the spark coil whenever the starting motor is running, but I can't think of any logical reason for that.
One attempt that could be made easily enough is to spin directly the starting motor with a jump cable (leaving the key in run position, of course), and see if the engine starts right away.

A weird behavior indeed, anyway.
In fact, what julians suggested in post #6 has some merits. Either that, or an exorcist! ?
 
MapisM said:
Why are you folks insisting on the key switch (which would have been my first thought either, fwiw), after the OP confirmed that he already tried swapping the ignition barrels?

Since the engine starts and runs well as soon as the key is back in run position, I would think that no current reaches the spark coil whenever the starting motor is running, but I can't think of any logical reason for that.
One attempt that could be made easily enough is to spin directly the starting motor with a jump cable (leaving the key in run position, of course), and see if the engine starts right away.

A weird behavior indeed, anyway.
In fact, what julians suggested in post #6 has some merits. Either that, or an exorcist! ?
Click to expand...
He has not changed the key switch , he has checked the wiring .
 
Hi
I know exactly what this is. It boggled me for a while when I had the same issue.
so these efi and also the mpi engines need nothing other than a superbly conditioned battery state to fire them.
despite the engine or the other engine seemingly turning over fairly healthily, what’s happening is that the vast majority of the demand is taken away from the ecu and ignition system whilst you crank it over. All the demand is given to the starter motor and away from the ecu and ignition system that fires it. When you let go of the key...for a brief second, the load demand from the starter is released and given back to the ecu and ignition system...and enough to fire the engine. Or at least give a kick. It’s a tell tale sign of a weak battery (or weak battery route to that particular engine if twin set up) on most modern type fuel injected engines. They take so much demand to fire these things that something such as cranking can make the ecu think less of firing the engine.
honestly, it threw me for a while...especially since the engine turns over well enough. But it’s certainly a fact these days and becoming more common an issue as time goes on.
 
All, sorry to have you doubtful of each other and it’s likley my fault on the wording anyway, but for clarity....... I checked both the wiring and swapped the ignition barrels at the helm position which is the same as your car ignition i.e where the key goes. I have found that by having twin engines it is a god send for swapping and fault finding which saves you money on expensive parts. The big disappointment for me is lack of Mercruiser support ? thank you all for trying to help and please do continue I find this thread soooooo helpful, but please don’t fall out with each other over it ......?
 
seastoke said:
Well isn't the barel the part the key goes in and not the switching part ,I could be wrong
Click to expand...
Yes you are correct, I think the OP just used the incorrect terminology. Looking at post 4 none of the components changed would effect a cure to his ACTUAL problem of not starting on cranking but catching/starting when the key is let go to the run position.

As I said, I would check power to the coil on crank. I know its an EFI engine etc but was a common problem on old ballast ignition systems where power was missing on crank but ok on run.
 
Hi
Have a read at what I posted above. I’m almost certain this is your issue. It’s a common thing with these GM TBI and MPI engines. I’d almost bet money on it.
 
QBhoy, what you suggested is a thought that crossed my mind, but I didn't mention it because the OP said that only one motor is affected, and I assumed that they are both driven by the same battery bank.
But if that wouldn't be the case, yes, definitely something worth checking, and very easily: it's sufficient to swap the starting batteries.
 
MapisM said:
QBhoy, what you suggested is a thought that crossed my mind, but I didn't mention it because the OP said that only one motor is affected, and I assumed that they are both driven by the same battery bank.
But if that wouldn't be the case, yes, definitely something worth checking, and very easily: it's sufficient to swap the starting batteries.
Click to expand...
Mapism...initially you’d think that a two motor set up and battery system would have such an issue, but as you will definitely also know, if that engines battery is even a little less than it should be, health wise...even if the two batteries are linked out...the supply to that engine could be compromised for sure. It’s now a known thing...despite common sense telling you thay if it’s cranking, the battery is fine. It’s just not the case though, with these merc and Vp GM efi and mpi, gxi etc engines. It’s now a known issue.
 
Qbhoy

I think your onto something that hasn’t been checked. So if I hook up a fully healthy battery direct to the engine by-passing the other engine and battery bank I may solve this issue ???... also I was advised when I bought the boat to start the starboard engine first as it helps to boost the independent port engine. There is also what’s called a “link switch” which applauds full power from the main battery bank to the port side. Final question: would it make a difference if I had non leisure marine batteries as opposed to normal heavy duty engine start batteries ?
 
I had written a much more
sealiner37 said:
Qbhoy

I think your onto something that hasn’t been checked. So if I hook up a fully healthy battery direct to the engine by-passing the other engine and battery bank I may solve this issue ???... also I was advised when I bought the boat to start the starboard engine first as it helps to boost the independent port engine. There is also what’s called a “link switch” which applauds full power from the main battery bank to the port side. Final question: would it make a difference if I had non leisure marine batteries as opposed to normal heavy duty engine start batteries ?
Click to expand...
yeah...so it makes even more sense now. if That engine side has a weak battery, even being linked out to the other good one, the supply and amps is compromised for sure. Honestly, they need nothing other than a perfect battery to crank them and fire at the same time. It totally threw me when it happened to me. Couldn’t understand why it would try and start when I let the key go. Had the ignition barrel out and everything. Then it happened to another guy at same time with similar engine. Hadn’t charged the 7 year old battery since mid winter and it was now spring start up time. The other guy hadn’t charged his for ages. Put two and two together and tried a known good battery on both boats. Started then instantly. Put other battery back in straight after. Same thing. Slight kick or only just starting when let key go. Hopefully you are the same.
 
QBhoy said:
even if the two batteries are linked out...
Click to expand...
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:
 
You must log in or register to reply here.

Share this page

Top