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8.1 won't stay running.

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sammi

Regular Contributor
Ok, back to this problem. Serial number om314427. 8.1 mercruiser. We put new regulator in and have a pressure of 43 it idle. Key on engine off 50, then when pump shuts off it holds at 34. What our problem is now is it is hard starting and once it does start it only runs for 4-5 seconds then shuts off. Fuel pressure holds at 43 while running. Swapped IAC from side to side, no change. When it's trying to start it reminds you of a carbed engine with a stuck choke. Like it's catching on most cylinders and winding up but just won't start. Then it finally does. We removed and cleaned fuel rail, replaced injectors, new filters and fresh gas.
 
Unbolt the PCM and lift it away from the coils. Leave it plugged in and run it. See if the problem goes away. Leaking plug wires can scramble the PCM/

Your fuel pressure is off. but will not cause your issue.

Lose of power on the 5volt system will also shut down the engine. I would unplug the water pressure sensor and take a look at the plug and pins in the sensor. The sensor can leak water if older and short the 5volt system
 
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Ok, so last night my buddy put new spark plugs in. He checked water pressure sensor plug and it looked ok. He left it unplugged and started engine. He said it ran great for 10 seconds then it shut down almost like you turned the key off. Then it would not restart. One question. This boat has the fuel shut-off solenoids on top of tank. If that shuts off, would it shut off quickly like that or would it continue to suck fuel out of water separator etc? As well, I could have sworn fuel pressure was good when it shut down before last night. If it is a bad oil pressure switch, does that **** down the fuel pump? What I'm getting at is wouldn't fuel pressure drop if it was a bad oil pressure switch? As well, if the 5 volt system is shorted out, what is causing the engine to shut down, does it stop the fuel pump, which would show a decreased fuel pressure as it shuts down?
 
As Chris mentions, your fuel delivery is probably OK. Look for horses, not unicorns. Did you try relocating the ECU? Spark plugs swap unnecessary for troubleshooting purposes since it fired and ran. Suspect the ignition system and in particular the spark event chain.
 
He didn't do that mostly because if it didn't fix the problem was it because that "was not the problem" or did we not relocate it far enough. We figured we would just change spark plugs wires then we would know for sure that wasn't the problem but we can still give that a try. As far as spark plugs the engine ran but very poorly. After spark plugs change it ran great for those 10 seconds. We think plugs were fouled from previous high fuel pressure problem which appears fixed.
 
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Ok, my buddy finally got around to lifting PCM up and out if the way. No change. Engine starts and runs for 4 seconds and shuts off.
 
sammi said:
Ok, my buddy finally got around to lifting PCM up and out if the way. No change. Engine starts and runs for 4 seconds and shuts off.
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I am hesitant to tell you to switch the Crank Position sensor from engine to engine. It is in the back of the block. If you choose to do this be careful.

When you have good fuel pressure and the ignition coils are not zapping the pcm the CPS is the most likely thing that causes a no start. Never had one cause start/stall, maybe now we have.
 
Swapping the CPS is the last test you can do without a CDS to check the 5volt reference system. Depending on your build (your serial number listed above is invalid) you may have only one 5volt reference system (Power 1) and if that is taken down by a bad 3 wire sensor the engine will shut down.

What is this engine in? I/O or I/B?
 
It's an inboard in a 34 cruisers inc. unfortunately it had to go to storage yesterday so my buddy can't do any more this year. I've heard some people talk about a bad oil pressure sender that shuts down the engine. Do you think that could be it?
 
Ok, boat is out of storage. Borrowed a rinda scanner and got 3 codes.191(fuel level circuit high). 118(steering POS circuit high) and 94(pitot pressure (boat speed sensor) low. This is an inboard engine. No other codes. It did say guardian 1 unspecified. To rehash, engine starts right away, runs for 3-5 seconds and shuts off. Fuel pressure stays normal when it dies. Engine serial number om 314427
 
Ok, maybe this is a clue. Rinda gauge says we are getting 5 volts at the senders. Here's the goofy part. When we hook up a fuel pressure gauge on the fuel rail we are getting 43 psi(or close to that) the rinda scan tool is only reading 4.0 or 4.5 I don't remember. Where does the scantool get it's reading from that would be so different? When the motor cuts out it just sounds like it's starving for fuel.
 
sammi said:
Ok, maybe this is a clue. Rinda gauge says we are getting 5 volts at the senders. Here's the goofy part. When we hook up a fuel pressure gauge on the fuel rail we are getting 43 psi(or close to that) the rinda scan tool is only reading 4.0 or 4.5 I don't remember. Where does the scantool get it's reading from that would be so different? When the motor cuts out it just sounds like it's starving for fuel.
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The scan tool does not read fuel pressure.
You issue is fuel related

I would pull the fuel injectors and have them flowed. Hook your fuel pressure gauge up to the rail and purge the fuel system into a glass container and look for water. I would also pull the gen2 fuel cooler and clean out the screen under the regulator(PN 805406) along with checking the cooler for holes.
Your Fuel pressure is elevated.
 
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We did pull the water separator looking for water. It was good and fuel is fresh. My buddy had a mechanic replace the screen last fall thinking that was the problem. What should fuel pressure be? We did put all new injectors in last year and cleaned out the rail. We didn't bleed the rail. We just figured if pressure was good it must self bleed. As well we wondered about an antisyphon valve but fuel pressure stayed high even when it dies so we kind of ruled that out. What do you mean by having the injectors flowed? I'm not sure if this is a clue or not but after letting it sit for a day when we try to start it it runs the longest. About 5-7 seconds. And time after that it runs about 3-4 seconds and quits. Sounds exactly the same as a carbuerated engine running out of fuel.
 
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The scan tool just reads the data that the ECU has sampled and collected.....

The ECU doesn't 'read' everything essential to the engine making full power...only enough to do the job it was asked to do.....
 
sammi said:
We did pull the water separator looking for water. It was good and fuel is fresh. My buddy had a mechanic replace the screen last fall thinking that was the problem. What should fuel pressure be? We did put all new injectors in last year and cleaned out the rail. We didn't bleed the rail. We just figured if pressure was good it must self bleed. As well we wondered about an antisyphon valve but fuel pressure stayed high even when it dies so we kind of ruled that out. What do you mean by having the injectors flowed? I'm not sure if this is a clue or not but after letting it sit for a day when we try to start it it runs the longest. About 5-7 seconds. And time after that it runs about 3-4 seconds and quits. Sounds exactly the same as a carbuerated engine running out of fuel.
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Where are you located?
 
Erie, pa. It sure sounds like a fuel delivery problem. But what I don't understand is fuel pressure is 42 even when it dies. Can the low-pressure fuel pump be bad but there's still enough to give 43 PSI?
 
And one other question. I read somewhere that a guy had the same problem and it turned out it was a cracked fuel pickup that was sucking air. We plan on trying to start it on a remote tank tonight but our thoughts are if we have good fuel pressure at the rail it's probably not antisyphon or clogged pickup tube. What are your thoughts?
 
You have two pumps a lift pump and a pressure pump. Lift pump pulls fuel out of the tank and pumps it to the pressure pump which delivers it to injectors. Somewhere between tank and injector is where your problem is happening. Are you working from an actual shop manual or are you spitballing?
 
Spittballing I guess but there is a method to our madness. We know there is 43 psi at the fuel rail, injectors are new. We hear fuel pump turning on(assuming that's the high pressure pump since we have high pressure). Switched PCM s side for side no change, new oil pressure cutoff switch, switched IAC side for side no change. Rinda scanner says no pertinent codes. We have 5 volts in that 5 volt system. Checked for water in fuel, none found. Next step is to do remote gas can but I don't see that that can be the problem if we have 43 psi at the fuel rail.
 
So tonight we tried to run on remote tank and got the same thing. Ran for 4 seconds and died. Then we started it again and when it started to quit we gave it a shot of ether. We were able to keep it running so that rules out electrical issues, right? Now we need to figure out why with 43 psi at the fuel rail, is it not running?
 
Pretty good indicator....

The Rinda has access to many parameters that may be worth monitoring when the issue occurs - it may provide insight to help pinpoint the problem. If there's no restriction and the pressure is adequate, the injector not firing will cause "no fuel" - the question is why? It could be as simple as a bad ground connection to either (or both) injector banks....
 
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