Random hard start, bogging, backfiring, 1998 232 454 mag mpi

[Mwmx54]

I’ve been running into a bit of an issue for the last 12 months, the boat usually runs fine, but lately more so than before, the boat will start up just fine, but bog or die when I try to take off, and barely run accompanied by some back firing, usually about 10-20 seconds of revving it up and down will get it back to normal operation and it will run fine until I either slow down or shut it off. Sometimes as I throttle down the boat will stall also. After some research I tried changing the iac, which didn’t help. Does anyone here have experience with this issue? There is a few posts about the same problem, but not really any that gave solution. Boat has about 500 hours and well maintained.
Thanks

[AsLan7]

.
So glad you found us M54...

Those MPI motors are pretty reliable. Could be a timing issue. Bad fuel. etc. Easy to go down rabbit holes on the net. Do you have a marine tech close by?

And welcome aboard mate..
.

[jhnmdahl]

Could be plugs or wires, water in the fuel, or a dozen other things. Backfiring sounds like timing (as do the rest of your symptoms), but it could be slow ignition due to plugs, bad fuel, fuel filter or water separator, or other things too.

[Mwmx54]

Thanks for the welcome, I have been here several times looking through the posts over the years, always loved cobalts ever since my parents bought a 1999 232 back in 2000, so when this near identical 1998 came up for sale I hopped on it, and it gave use about 7 years of trouble free fun. My background being an auto mechanic, which my career changed from that about 10 years ago, I have always thought I was pretty knowelegable in that arena, but this has me stumped, I never realized how little I actually knew when the engine doesn’t have an obd port. But, I did change the fuel filter water separator, and found no water, changed the fuel lines thinking maybe ethanol, that I very rarely use unless I have to made them swell, also checked fuel pressure while it was acting up, and actually found that it had quite a bit of air come out, but even after bleeding the air out, it still acted the same, I changed the cap, rotor, plugs and wires, checked and double checked the gaps, cap and rotor still look perfect, no corrosion checked all electrical connections, and vacuum lines, it’s had several tanks of fuel through since this first started about a year ago, intermittent problems are hard to track down. It’s most common when the engine is warmed all the way up. If I had to guess, it’s starving for fuel, just getting enough to idle. It usually idles smoothly, although at a very low rpm, my understanding is it has a mechanical fuel pump, which is known to be very reliable, and would most likely not cause an intermetint issue.
Do these engines have some sort of variable timing? That would be something to look into, but it runs great 90% of the time.

[dustinm]

If it acted up more when cold I would say it is suffering from a lean miss ie the IAC valve, but only when warm I would lean toward ignition side. What I would do - spray the inside of the cap and rotor with WD-40. I would also consider trying a proven coil.

I guess you could have a cooling system temp sensor acting up when it gets warm.

[jhnmdahl]

No variable timing on a boat that old. It sounds like you guys are on the same paths I'd be looking at. Idle air control valve is cheap, and I might fiddle with that before the coil.

[dustinm]

No variable timing on a boat that old. It sounds like you guys are on the same paths I'd be looking at. Idle air control valve is cheap, and I might fiddle with that before the coil.

He has tried an IAC

Its hard to accurately diag over the internet... so many variables

[Mwmx54]

Yeah at this point I’m thinking it could definitely be a heat related issue, the temp gauge reads correctly when the issue is happening, I also tried unplugging the coolant temp sensor while it was happening(thinking maybe it had a default it would go to when unplugged), and it made no difference. So I’m thinking some sort of vapor lock, unlikely as that’s more of a carb issue I believe, or possibly the coil like dustinm said. I don’t know of anywhere I could borrow one from, so I may have to bite the bullet and hope, but at $200 for the merc oem one I’d rather try testing it first, but that’s hard to do when it only acts up at full temp, does anyone have experience with the sierra coil? It’s $100 but if it ends up being a dud that would make this diag even more difficult. Or is there a way to test the coil? I’ll also add, a couple times it has died as we pulled up to camp, and the next morning it fired right up just fine, so cooling down has made the issue go away.

[dustinm]

The reason I say proven coil is that I had bought a brand new quality coil for my chevelle and after a few weeks it wouldn't start while warm. I could tell by the tach that I wasn't getting any spark but as soon as I let off the start position the tach would jump up... I suspected cables, then battery..ect... until I threw on an old coil from the 60's...

You should be able to test this with a cheap automotive coil, if you cant find one to borrow for a weekend. Then if it's the problem replace with a Merc part as they're marine rated

[bruceb58]

I would be putting a fuel pressure gauge on here to verify pump pressure. Very easy test.

In addition, make sure you have a good/new distributor cap. Don't just trust looking at it...buy a new one.

If you suspect bad injectors, use an IR temp gun under medium engine load pointed at each exhaust manifold port from each cylinder. A cool one means nothing is happening on that injector.

[bruceb58]

my understanding is it has a mechanical fuel pump, which is known to be very reliable, and would most likely not cause an intermetint issue

in 1991 the Gen V block came out...no mechanical fuel pump from then on.

This is what you have:

https://www.crowleymarine.com/mercruise ... uel-cooler

[Mwmx54]

Very good advice, I did notice some erratic tachometer movement, but just figured the gauge was acting up, definitely need to look more into the coil, it was reading about 4K sometimes when idling at a more like 500-700. And that’s good to know about the fuel pump, I thought that was just a cooler down there, I think I’ll find a coil to try out next, and take a fuel pressure gauge with me next time I go out. Any idea if this or something similar would work for testing? https://www.summitracing.com/parts/sum- ... /chevrolet
Maybe not this one in particular, but will any coil made for a 454 of that era work?

[Jclark]

Could it be vapor locking? Mine has done very similar, only on hot days, running around and then turning the engine off for 30-60 min. It always starts normally after sitting but then wants to sputter when getting up on plan. It’ll sputter 5-10 seconds and the take off like usual. Every time it does it I think it’s going to stall out and quiet but it never has. As of 2 yrs ago I have a new plugs, wires, rotor. I think they also put in a new coil.
I put in a second hatch blower and run them frequently while underway. I also started using Marvel Mystery oil additive in the fuel. I believe these steps have helped as I only remember it happening once this summer. My mech told me to also open the hatch when floating but that’s hard to do with peeps on board.

[bruceb58]

Very good advice, I did notice some erratic tachometer movement,

As a test, disconnect your tach from the back of the tach. Bad tachs have been know to affect the coil since that is where it gets it's signal.

[bruceb58]

I also started using Marvel Mystery oil additive in the fuel.

If you have cats, that is probably something you shouldn't be doing.