Mercruise 5.7 will only stay running 1 second Does oil pressure shut off ignition ?

[Brenan]

Hi All!

It's been a couple years since we had a boat, but we got back in and bought one, and ran into some issues. Currently the boat will only start for exactly 1 second and dies immediately. It has spark, and acts like a fuel injected motor with no oil pressure. If I pump the throttle a couple times it may run for 1.5 seconds (although it might be in my head!). It's been interesting. I've been cleaning and doing things one at a time to help rule out and allow me to save old parts for spares. When it was running good it would start up with no pumping of the throttle and idle perfectly, running smoothly.

I've diagnosed all my other issues with boats in the past, just by searching but this one has me a little stumped.....

Engine serial: 0M358751 Out of a 1990 bayliner Trophy 24'
TKS 2 barrel Carb (pulled it open and things looked ok, blew out the jets, cleaned new accelerator pump, etc The powervalve was not coming out without possible damage so I left it as things looked ok. I didn't test the TKS as I didn't really understand it at the time. It OHMS at 37 and was warm and appeared to turn off when we were diagnosing the 1st go around.
All fuel screens and filters pristine ) (which is actually odd)
Fuel vent works fine.
Seller replaced battery, gets about 14.3 volts off alternator
Plugs looked ok, little sooty but not bad. THey are short however (the parts guy sold me long ones, so I am looking into that)
Thunderbolt V ignition. one at a time, I replaced cap, rotor, coil, ignition sensor inside of distributor (it was all pretty rough looking)
Engine off, I have 12.5 volt at coil and dist wire, but only 11.5 on the way back to dist via White w/green wire (per the merc test protocol of thunderbold 5). Not sure if that is ok?
I jumpered the coil + to the battery and no change. I DID NOT disconnect the other wire at + coil, that is my next step
I did not jump the oil pressure switch as fuel was in the carb but I think maybe it cuts out the ignition?
Currently have it plumbed to a 5 gallon Tank to rule out the boat.
Motor has electronic fuel pump. Fuel pressure good at Carb 6.5 . I hooked up a clear line and thought I had air bubbles for just a second then solid fuel
Compression (172-181)
As I was diagnosing the starter gear got ground up and I had to pull the manifold to get to it. I looked at it and ordered a new exhaust manifold set today which will be here next week. They were kind of past their useful life anyhow
One of my checks I threw the timing light on it and didn't notice any advance (However I couldn't get it to run prob high enough to do anything)
The gauges and electronics seem to work fine.

Boat ran great on test drive 30 minutes, we shut it on/off a few times, went through the gears. Then we left it for a couple hours, and I drove it to the dock (approx. 30 min). Next day went to move it, started find, warmed up fine, then died. Ran like crap so we got a external fuel tank and it did the same crap. Got it to the dock did the usual checking and it was running ok. It wouldn't accelerate super well (seemed like the accelerator pump which is why I pulled the carb, but once it got going it took off great for about 10 minutes. Then died, had to kicker back to dock, now it will only start for 1 second and die. I replaced all the electronics and pulled the carb after it was doing this 1 second run bs. It's weird like each day it just got worse?

Now it is down to starting for 1 second and stopping. (My hunch it has something to do with a sensor killing ignition, or the TKS module. I think I had a couple other problems. With the manifolds out, I replaced the plugs, cleaned up a bunch of nasty connections at the starter.

Plan is to
Jumper coil to bat and remove the current wire to rule it out
Jumper oil pressure switch
Replace Ignition amplifer/thingy
Tinker with the TKS
Cruise around the marina with the kicker to get my water fix...lol
Verify plugs in boat are proper (Based on serial they gave me long head, but there was short head in there before (which is what I reinstalled new with after comp test))

Thanks guys!!!!! Any ideas it seems easy....but....?

 

[Watermann]

Wow, you've covered a ton of ground trouble shooting so far. Yes the oil sensor will kill the motor if oil pressure is too low or the sensor is bad.

 

[Brenan]

So I am stumped now. To.no avail Jumped oil pressure switch, played with tks. It runs for a second or two and has to be something easy. Is it safe to jump other senders? The sensor on the cooling jacket only has 4.9 volts going through it. It grabs so smooth every time I think its almost there..... Argh. . Any help will let me preserve what little hair I have left....lol

 

[Scott Danforth]

May have a failed ignition module. Mine died in water and had similar symptoms.

 

[Fun Times]

Since you have a TKS carburetor that didn't come standard on the engine serial number you posted, you probably have a failed TKS diode assembly, Merc part number 83-865913A01.

TKS (Turn Key Start) carburetors. When the engine is warm ...

Unless the cylinder heads aren't original to the engine, you'd need ACDelco MR43LTS, NGK BPR6EFS or equivalent spark plugs.

 

[muc]

Wow, you've covered a ton of ground trouble shooting so far. Yes the oil sensor will kill the motor if oil pressure is too low or the sensor is bad.

The oil pressure switch will kill the fuel pump but the motor will still run on the gas that's in the float bowl. No stock MerCruiser ever had a oil sensor that would kill the ignition.

Agree you should test the diode.

Also make sure you don't have a problem with your emergency stop tether cord/switch

 

[tlewis1]

I would try to spray a little fuel down the carb before it dies, it may help rule out a ignition issue or a fuel issue. Hook up a remote switch to the starter or have someone else hit the key while you spray fuel down the carb.

 

[achris]

I would try to spray a little fuel down the carb before it dies, it may help rule out a ignition issue or a fuel issue. Hook up a remote switch to the starter or have someone else hit the key while you spray fuel down the carb.

If you do this ^^^, do it very carefully. Easy to get a backfire through the carb and singe off the eyebrows, or worse.

 

[achris]

Unlike a points system where there is a connection between the starter and the coil (bypassing resistance wire etc) the Thunderbolt V module has no such bypass. There also a connection between oil pressure (and gear lube level switch) and the ignition system, but that appears to only be for the audio alarm system. But without knowing exactly how the circuitry in the module is laid out I can't say for certain. I do know that Merc had a decision with Guardian to not create any condition that would kill the engine, only reduce power. That was done for vessel safety reasons, and I can't see them changing that in this engine. So, I suspect you have a module problem. One final check I would do is to remove and insulate the tan/blue wires off the oil pressure switch and the gear oil monitor bottle, and the yellow off the temperature sensor switch (this is not the gauge sensor).... I would also unplug the knock sensor module.

If you still get this stalling after the key is released, check that the key isn't actually powering down the engine (watch the instruments), then replace the module...

Chris........

 

[Brenan]

Thanks guys!!! Much appreciated! We did a little more tinkering just now and going to keep cleaning stuff. I will see if we can make any head way tomorrow and see about the tks. .I like to understand my boats anyways so the time is well spent.. And yes I am ultra careful with fuel and boats. Its got a good spray of fuel shooting in, if anything it seems to be a hair on the rich side, but that's just a hunch. I

Fun times, that is my fear is they are the wrong plugs. My Intel says they should be the long ones. I am going to see of I can figure out. My initial assessment is its the original block with new intake man, carb, etc

I will keep you guys posted...

 

[Brenan]

OK success! We got a new ICM and it didn't change anything. Retested with the thunderbolt v and it turns out the new ignition sensor i had purchased on my first diagnosis was bad!!!! Next time I will run diagnostics on new parts! I put the original in and it fired right up!!!, so not really sure I solved the root problem other than it is super likely I cleaned all the connections and grounds. The brand new sensor ( in dist) was for sure bad tho. Off to get another one. I noted it had not been installed with any greese so I will confirm that when I reinstall the nand I am not confident that the original was 100%
I would have packed a spare ICM anyways. We found a used one .... Now that i know how everything works future issues swill be waaay easier to figure out..... And the kicker is a champion ,


Thanks for all the help. I have a little more tinkering to do and I think we will be good....

 

[Scott Danforth]

BTDT :lol: