454 Mag MPI Warm/Hot start issue

[JW in Texas]

'00 454 Mag MPI, Serial# 0M021074

This started the night I brought the boat home in 8/18, 5 tows later & numerous attempts by multiple "certified" mechanics including the dealer & we are still at square one. Frustrating is a vast understatement.

I purchased the boat in 8/18 from Lake Cumberland with approx. 395 hours. Always a marina boat on a lift, first owner put 125 hours on it in 16 years, second put 270 on in 2.5. Brought the boat back to DFW area.

Boat starts perfectly every time when cold, never an issue. Left us stranded on it's maiden voyage. Started right up, went out, stopped for a couple hours, went to leave & it just cranked, no catch. Get towed in, next morning, fires right up. We were below a 1/4 tank so I thought maybe contaminated fuel. It did live in a marina it's entire life. Pull the separator & sure enough, water. Drained the tank, Seafoam'd, new fuel pump, plugs, batteries & oil change. Took the boat out a couple weeks later, same issue. Start looking at spark, seems weak & coil is original so new cap, rotor, coil. Try again, same issue. In desperation & to eliminate basic ignition, I do the ignition module, typical GM style under the cap. Use the boat several times including a long weekend with dozens of starts, no issue...Yay! Not quite, late last season, same issue. This time, it's taken to the dealer. They spend a week with it, bow to stern, find no issues & cannot duplicate problem.. Pull codes, none. We decide I will wait until this season when it warms up & try to duplicate it.
I took it out last Monday, run it for over an hour, back to dock, kill it, let sit 20 minutes & presto, won't start. They bring down Mercruiser Computer, all looks good. Check spark, very solid. Fuel pressure is at 40-45 psi cranking, but we do have aeration & it bleeds pressure after sitting for a few minutes. Pull plugs and they are damp, possibly leaking injector{s**. Take it back to shop right then, new plugs, fires right up. Sent the injectors out for cleaning & balancing. Took boat out yesterday, ran great right up until we were ready to leave out in the middle of the lake. Had been sitting a couple hours, crank, crank,crank, no love. Get a tow back, 45 minutes into the tow, it starts. I had been running the bilge blower the entire time going back, but I don't feel like this is vapor lock. I have tried starting fluid every time this has happened & it doesn't even bump, ether has zero effect while cranking. I have had lots of experience with old cars & boats, ether always does something. At the end of my rope. Thanks for any guidance,
J

 

[QBhoy]

Definitely changed the battery for one good enough ?

 

[JW in Texas]

Yep, two brand new proper size marine spec. Also rebuilt & upgraded the alternator to 120A. Charging was a little suspect at idle & it has upgraded stereo/amps. Figured better safe then sorry & I have a great local starter/alternator guy.

I really want to believe vapor lock & plan to try non-ethanol fuel & 10 minute idle with the hatch up to cool down before I shut off. Plan on going out & burning off the half tank I have tomorrow & refilling with non-ethanol. I just don't get why ether wouldn't at least make it do something when cranking if it was a fuel/vapor lock issue. As I have researched, I have seen cranking with the throttle wide open takes the injectors out of the mix so i might try that when it does it again. Driving me bonkers...

 

[alldodge]

Lake Cumberland

Howdy, my neck of the woods

Sorry about the issue but it sure sounds like vapor lock (VL) to me.
Your getting good spark and even with either there is no hit at all.

I've had VL and it always happens after a run and it sits. If your fuel pressure is within spec, Im thinking VL

I fixed mine but returning to tank and not to filter

 

[JW in Texas]

That would be the next step. Tank fill is on the starboard side near the separator so wouldn't be too difficult.

 

[tpenfield]

All that stuff that you replaced so far . . . those things were not the problem. I'd sure like to have the computer attached to the engine right away when it does not start. Not sure if the engine had cooled down enough, by the time the computer got attached, as you described. Also, would like to see if it is throwing a spark when it does not start.

The tougher problems to solve are the ones when the engine does not throw a 'code'.

 

[JW in Texas]

Ted,
Yep, I admit I got frustrated & threw the kitchen sink at it. Mostly inexpensive, but it has added up & we still have a boat that we love that my wife is leery of going out on.

When I was able to duplicate the problem last week, I ran it around for over an hour, brought it back to the dock, let it sit for 20 minutes so prime heat sink zone & it didn't restart. We hooked up the DIACOM & started reading, no codes. Now that I have read more of your posts, I will look at individual sensor reading volts next time around. When we checked the fuel pressure, it was zero. When cranking, it was 40-45 psi with some aeration that gradually cleared up. We checked spark & it was very strong, not surprising with all new components. That is when I started wondering what the PCM used to control start/no-start, spark or cutting fuel. My guess was spark as that is the traditional method in automotive & since we had that, I ruled that out. We pulled a couple plugs and they were fuel fouled leading us to think leaky injector(s). Put a new set of plugs in within 30 minutes so still hot & we verified it wouldn't start just prior to putting the new ones in. With new plugs, fired right up. The injectors hadn't been out & the boat is 20 years old so that looked like a likely culprit. After more research, I wonder if the temp sensor is telling the PCM that the motor is cold & giving it more fuel thus fouling the plugs while cranking. I assume the temp gauge is on that same sensor though & it reads accurate so why would the gauge show the correct temp & yet the PCM gets an incorrect reading.

It still sounds like VL, but I don't understand why ether in the spark arrester has no effect whatsoever. Windy here today, so going to go run the remaining E10 out tomorrow, fill up with non-ethanol & try cool down/hatch lifted for 10+ minutes before shut down. Thinking if that works, I will re-route the Cool Fuel return line from the Separator to the fuel fill so I get cooler fuel instead of hot, recirculated. Open to any ideas & thanks again for the help!

 

[alldodge]

Last time it wouldn't start, you mentioned aeration, where was this seen?

 

[JW in Texas]

Clear line from the fuel pressure gauge back to tank. We ran the exit line back into the fuel fill.

 

[alldodge]

OK you use a clear line for temp setup. There would be no issue with seeing air in a return, then motor would use some of it but not all

Was the temp line connected during last no start?

If so, when key turned ON and FP runs for 2 seconds, does pressure bleed right back off or does it hold around 40 psi?

 

[JW in Texas]

Temp line has always been connected. After FP pressurizes, it holds around 40.

 

[JW in Texas]

One more thing I have noticed, the single warning beep you usually get when you key on is intermittent when this is happening. When I start from cold, if memory serves it always beeps once.

The boat always starts right off the next morning after it has left us stranded the night before. This last time after new injectors, it started after a 45 minute tow back with the blower on most of the way. That would have been 4 hours or so after we had shut it off to float. The other 4 times we got stuck, that was not the case. We had to walk the boat back on the trailer & it wouldn't start until the next day. Makes me wonder if I have/had two issues; leaky injectors & VL. With leaky injectors, it needed overnight for the plugs to dry so it would fire. With fresh injectors, it did start after 4 hours or so, I went back out & cruised around for 30 minutes & it ran great then we put it on the trailer. Previously, no start until well over 12 hours after shutting off.

 

[alldodge]

So when it won't start the key is turned to ON and there is no beep?
When it does start there is a beep?
Does the FP run when it doesn't start?

 

[JW in Texas]

The beep is intermittent that is why I don't think it is telling us anything. When it won't start, sometimes it will beep, other times it won't. Either way, it still doesn't start.

 

[alldodge]

Your sure it has spark, and good white color spark as seen on spark gap ?

 

[JW in Texas]

Oh Yeah, very strong spark @ both the wire & plug. Bright white, knock you on your ass strong

 

[alldodge]

Sorry threads are blurring together

Strong spark and cannot get it to start even with either.
Plugs are wet with fuel

 

[JW in Texas]

Sorry threads are blurring together

Strong spark and cannot get it to start even with either.
Plugs are wet with fuel

Ether has never helped. Only time we could duplicate this on demand, we tried ether & no help. We then pulled plugs & they were wet. Immediately put in new plugs, fired right up. This last Sunday after fresh injectors, again ether didn't help. Never pulled plugs & it started on the way back under tow,

 

[alldodge]

Need three things to run, fuel air spark

Good spark, plugs wet, Ether doesn't help, fuel pressure doesn't leak down, new injectors
We have all three but it won't start.

Injectors are not leaking loosing pressure but plugs are wet.
The wet plugs is it fuel or water (smell)?
Pull a plug and take a match to it, does it burn?

 

[JW in Texas]

Yep, that is what I have always said & it was rote when we were dealing with carbs, no sensors & points/HEI.

3//28-When plugs were wet, it was prior to doing injectors. That is why we thought leaky injector. Fuel Pressure had bled to zero in 30 minutes and the plugs were wet, therefore looked like leaky injector. 40-45 psi Fuel Pressure during cranking. Installed new plugs, fired up so sent out injectors for refresh.

4/3- First time in the water since new injectors & plugs. As usual, fires right up at launch. Park & hang out for a couple hours, won't start. No idea if plugs were wet as this was supposed to be a pleasure trip & I didn't bring tools. I know, stupid! Ether again doesn't help. Boat started on return tow back to ramp, approx 4 hours after it had been shut off.