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

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searay250

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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!
 
alldodge

alldodge

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Going under load requires increase fuel supply. I would start with checking fuel pressure 43 psi
 
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searay250

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alldodge said:
Going under load requires increase fuel supply. I would start with checking fuel pressure 43 psi
Click to expand...

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
 
Scott06

Scott06

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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
 
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searay250

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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
Click to expand...
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?
Click to expand...
0W61xxxx
 
Scott06

Scott06

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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
Click to expand...
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
 
Scott06

Scott06

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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
 
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searay250

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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 :)
 
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searay250

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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
 
alldodge

alldodge

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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
HVS Distributor Timing.jpg

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
 
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