2005 Mercruiser MPI 5.0L misfiring(?) when in gear, idles fine

Hello,
I have a 2005 Mercruiser MPI 5.0L (running a bravo 3, if that matters). Seems to idle fine, but as soon as it goes into gear, it starts misfiring. This motor is pretty new to me so I don't know the full history, but it did just get a new distributor cap and rotor and new plugs. I am not even sure where to start looking but I'll take any advice!

I've been reading through other Mercruiser misfiring-type of threads here and one thing that keeps coming up is the switch from 2 wire to (apparently much better) three-wire ignition trigger. What is this, and how can I check which one I have?

Here is a video of the general sound:


Any other advice or starting point would be appreciated, thank you!

Going under load requires increase fuel supply. I would start with checking fuel pressure 43 psi

alldodge said:

Going under load requires increase fuel supply. I would start with checking fuel pressure 43 psi

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Finally got my friend with a fuel pressure gauge to stop by. Seems to be reading around 45PSI so I doubt fuel pressure is the issue? Really good thought though, appreciate it

What is your serial number?

searay250 said:

Hello,
I have a 2005 Mercruiser MPI 5.0L (running a bravo 3, if that matters). Seems to idle fine, but as soon as it goes into gear, it starts misfiring. This motor is pretty new to me so I don't know the full history, but it did just get a new distributor cap and rotor and new plugs. I am not even sure where to start looking but I'll take any advice!

I've been reading through other Mercruiser misfiring-type of threads here and one thing that keeps coming up is the switch from 2 wire to (apparently much better) three-wire ignition trigger. What is this, and how can I check which one I have?

Here is a video of the general sound:

Any other advice or starting point would be appreciated, thank you!

Click to expand...

Mpi uses a different distributor and ignition than the thunderbolt system that has the sensor you speak of. Mpi fires off crank sensor .

the crab style distributor caps on mpis are known to cause misfires has your been changed

Scott06 said:

Mpi uses a different distributor and ignition than the thunderbolt system that has the sensor you speak of. Mpi fires off crank sensor .

the crab style distributor caps on mpis are known to cause misfires has your been changed

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Ah yeah, I might not have the right terminology but I definitely have the crab cap, I had a new one put on like 2 weeks ago

alldodge said:

What is your serial number?

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0W61xxxx

copper terminals also https://www.amazon.com/Performance-...44-98c0-8490dc2d2da6&pd_rd_i=B0845M26MW&psc=1

searay250 said:

Ah yeah, I might not have the right terminology but I definitely have the crab cap, I had a new one put on like 2 weeks ago


0W61xxxx

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I missed the dizzy cap in first post.

You are right in your research that the 3 wires sensor on TB ignitions is better, just your MPI engine uses a different system.

Hopefully AD can come up with some ideas for you

Bt Doctur said:

copper terminals also https://www.amazon.com/Performance-...44-98c0-8490dc2d2da6&pd_rd_i=B0845M26MW&psc=1

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Darn.... is this cap compatible? I spent like $180 on a mercruiser brand cap (or maybe it was sierra or something, either way it was well over $30). Oh well, live and learn I guess. Good to know for next time

searay250 said:

Darn.... is this cap compatible? I spent like $180 on a mercruiser brand cap (or maybe it was sierra or something, either way it was well over $30). Oh well, live and learn I guess. Good to know for next time

Click to expand...

I think it is a standard GM cap used on their trucks I dont think the cap is marine rated , just the dizzy

searay250 said:

0W61xxxx

Click to expand...

Good luck

Well, I have new information. I put a little seafoam through (gas and a little in the oil) just to clean it out, give it the best chance of running well. Ran it for a few mins the other day just to get the seafoam through the system and it ran like normal, minor misfires but generally okay.

Went to start it up today and it ran for a minute and died out. I thought, okay, that happens. Started it with a little extra throttle and let it warm up for a minute, and then brought it back down to idle.

It started idling low, and would almost die, and then would rev back up for 5-10 seconds, and then almost die for 5-10 seconds, and just go like that until it died (after about another minute or two). Started it up a few times and it was just doing this weird cycle of almost dying and coming back. Those cycles got shorter and shorter until it would start up, get down to a low idle, come back once, and then die out.

Here's what it was doing:

So basically it was running worse and worse and able to stay running for shorter and shorter amounts of time until by the time I left it, it could only stay idling if the throttle was up so it was at like 2k rpms. When I would drop it down to idle it would immediately stall out and die.

What could cause this? It feels very sudden, like it was misfiring a little but generally running strong and now all of a sudden... this.

Thanks

alldodge said:

Good luck

Click to expand...

Also @alldodge what do you mean by this? I'm not sure I understand

searay250 said:

Also @alldodge what do you mean by this? I'm not sure I understand

Click to expand...

You don't want to provide the full serial number, so I'm not able to look up your motor.

alldodge said:

You don't want to provide the full serial number, so I'm not able to look up your motor.

Click to expand...

Oh! I don't think it's that I don't want to provide it, it's just that usually folks want the serial number range and that's good enough Just a simple misunderstanding, glad I asked. My full serial number is 0W611224

Thanks
Really need to scan the motor for codes, but until then

Do you have mechanical or digital controls, I'm assuming mechanical?

The running worst as time goes on is showing that as the motor heats up the ECM starts reducing fuel supply like it should. The issue is the motor needs more fuel due to an unknown reason

Could be low compression, vacuum leak, injectors clogging, timing is off, sensor error or bad gas. Seafoam isn't going to fix anything IMO and definitely not in a few minutes

Check the HVS timing


Measure the 5V reference voltage on the TPS, TMAP or Water pressure sensor to see if it's exactly 5V on Gray wire. The black wire on each sensor is ground and the other wire is sensor readings

Measure TMAP and TPS voltages at idle and try to see how they change as motor has trouble idling. TPS should be around 0.6V

You don't have a 2 or 3 wire module which is used to control timing with the Thunderbolt ignition. Your timing is controlled by the ECM and the Crank Position Sensor

@alldodge Yes, I have mechanical controls. When you say scan the motor for codes, do you mean like with an OBD-type of scanner? Is there an OBD port on these, or is it some specific mercruiser thing?

Thanks for all the info. I will check the easy stuff first.... new gas from an external tank, spray for vacuum leaks, check compression, new plugs, fuel injector cleaner, etc. and report back.

No OBD port, boats have unique connectors based on which ECM and Rinda makes the scanners. Can get a TechmatePro or Diacom will need connector 94006. The Techmate is hand held and self contained, the Diacom uses a laptor

The lower cost kits in the link have the single connector. The higher ones have more and different connectors

Rinda Kits

Did you check ALL the sparkplugs for looking brand new ? Or are they cruddy looking ?

Pretty annoying update... now the motor won't start at all. The only thing I can think of is that for a few days I've been running it off of an auxiliary tank (one of those 6 gallon red outboard tanks). I can't imagine this would make it not start. A hose comes out the built in tank and goes right into the fuel pump/filer. I took the hose off of the input side of that pump/filter setup and plumbed the external tank right into the pump/hose. It has a primer bulb, I've tried with the primer bulb but I've also removed the bulb entirely and tried just with an open hose going into the external tank. The vent is open, when I prime using the bulb, the bulb gets hard, although it softens up as the motor is running, which seems weird. But like I said, I've also tried removing the bulb and just putting the hose right to the bottom of the external tank, but still, no dice.

It turns over great and sometimes pops like it wants to start but it doesn't fire. Almost feels like a fuel issue, given what was happening the other day (with the cycling between low idle and normal idle) so it's almost like it has been getting less and less fuel over a couple of days and now just won't start.

I think I need pretty basic help, like what should I be actually checking? I would like to figure out what's happening here, it must be fuel, air, spark, timing, or compression, right? What seems like the most likely given that it was misfiring a little on the main tank and then started to slowly die out as I moved to an auxiliary tank?

I would really, really appreciate any basic help. Nothing is too dumb for me to check right now, and it would be impossible to overexplain things to me apparently, so feel free to talk to me like I'm a moron given the circumstances

Thanks

Spark gap tester to see if your missing spark. If spark is there than we look for fuel. Place on coil feed and if good there, move to a few plugs