5.0 MPI cranks but no start

[Siphon]

Serial No 0m340696

Have a crank but no start condition on my 1999 Sea Ray 5.0 MPI/Alpha One. Boat had been running relatively well prior except wouldn’t fully rev at WOT, only ran about 4K RPM. Had recently installed a lift pump to alleviate a vapor lock problem I had on a trip to Lake Powell this spring, added the pre-lift pump canister style filter when I got home and then encountered the issue with not revving.

Boat’s been sitting on the buoy for a week with a fair amount of rain and now won’t start. There was a couple inches of water in the bilge but nothing extreme. Thought maybe moisture distributor cap so I swapped that out and no change. Checked voltage at the coil and got 12v. Voltage at the coil wire to the distributor was 13.5v. Checked the ignition fuse and it’s fine; jiggled main fuse wires and looked around for loose connections. Screw-on fuel filter is new, replaced it when I put the pump on. Checked it and it’s full of fuel. Haven’t put a gauge on the fuel rail yet but I poked the bleeder valve and got a good spray.

Ideas?

 

[Siphon]

Forgot to say, tried throttle only advance and can get it to sort of cough and almost start, but not quite. After trying that a few times I got a puff of fuel-smelling smoke from the spark arrestor.

 

[alldodge]

When your cranking the motor, do you see at least 300 rpm on the tach?

 

[QBhoy]

Hi
the mpi will crank apparently quite healthily but not quite fire...and sometimes take a cough as you let the key go. This happens when the battery is anything less than absolutely tip top condition. Happens to mine and many others...usually at the start of the season or after a long time away. I’d be pretty sure that’s the issue. Totally threw me at first. Just because it would turn over fine. Just that these things demand such power, that if you’re battery is weak...it won’t try to fire until the load is reduced when you stop cranking. Then it for a split second thinks it has enough to fire..but it’s too late.
Ocasionally tou can catch it on the throttle whilst it takes that final cough. If you’re lucky. Charge or replace the battery. Pretty sure that’s it.

 

[Siphon]

AllDodge, didn’t look for that, I’ll check.

QBhoy, I wondered that too but I don’t think so. I have dual batteries wired to separate the starting and charging functions. The switch I have enables me to turn them both on for starting iso I tried it, but no dice.

 

[QBhoy]

AllDodge, didn’t look for that, I’ll check.

QBhoy, I wondered that too but I don’t think so. I have dual batteries wired to separate the starting and charging functions. The switch I have enables me to turn them both on for starting iso I tried it, but no dice.

I see what you are saying but if one of the batteries is weak and left linked to the other healthy battery...the weaker one will always have a detrimental effect on the other too. So they could still both be bad.
It may be something else...but I’d definitely be making sure you’ve ruled it out. Honestly now, it’s threw me and others a few times before figuring out this was a real issue with these mpi

other than that, fuel delivery and diz cap would be high on the list...but you say you’ve covered that so...yeah definitely have a serious effort with a known good battery friend. All the best.

 

[Siphon]

Understood. They’re AGMs, about one season on them and had a good trickle charge over the winter. I’ll bring them in tonight and give them some juice, see if that makes a difference.

 

[QBhoy]

Understood. They’re AGMs, about one season on them and had a good trickle charge over the winter. I’ll bring them in tonight and give them some juice, see if that makes a difference.

Worth a shot. Please let me know how you get on.

 

[aerobat]

when its about to go with throttle only look at the IAC .

 

[Siphon]

AllDodge, I get 300RPM for the first couple seconds and then it goes down.

The starting battery is on the charger and says it’s only at 50%. But I’ve been cranking on it a lot now... who knows, we’ll see later tonight when it’s charged up.

Aerobat, the IAC was new last season but this boat has been going through them. I put one on the season before, too. Maybe I have a voltage spike somewhere or the saltwater is hard on them.

 

[alldodge]

I get 300RPM for the first couple seconds and then it goes down.

Unless the tach stays up, then the ECM is not seeing its firing and will not allow the injectors to fire.
Need to see if there are any codes, crank sensor is a possibility

charger and says it’s only at 50%

Taking a bat down to 50% can damage it, being a AGM it does have a bot more spring back then a lead acid.

 

[Siphon]

No change in condition on a full battery. Still not seeing sustained 300RPM. I guess I’ll be calling a tow to get it to the marina.

 

[Siphon]

Hey where would I find the crank sensor? Might as well try that before I let it sit on a trailer for a couple weeks waiting to get fixed. The engine compartment on this boat isn’t a real fun place to work but if I can get to it I may as well give it a shot.

 

[alldodge]

Crank sensor (item 9) is on the timing chain cover

https://www.mercruiserparts.com/bam/subassembly/31747/7585/110

 

[Siphon]

Crank sensor didn’t resolve the issue. Turned out to be fouled plugs from water in the cylinders — leaky exhaust manifolds. Not sure how it didn’t hydro lock, but somehow that didn’t happen and compression looks good except one cylinder low by a couple PSI. These manifolds and risers only have two seasons on them so I didn’t really suspect anything like that. I did check the oil for milky look but found none. Guess the saltwater caught up with me sooner than I thought it would.

Thanks for the help, just thought I would post an update.

 

[QBhoy]

Mmm. Unless the baskets have failed you wouldn’t think they would fail that early.