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

[searay250]

I am not positive on your engine, but the coil may get power in two different ways:

• One while the starter is engaged (key in the start position)
• Two when the engine is running (key in the run position)

Need to find a wiring diagram for your engine, please search around on the forum at the Don S MUST HAVE technical info in the electrical section. Also just search this forum and perhaps someone will post it. Troubleshoot it to ensure you have coil voltage in both these key positions. Report on this or ask questions as needed.
...
Otherwise, no oil or wetness so that is good, and you can see the little mark in the middle of the rotor tang showing it is making contract, also good. Regardless install another set. And check that firing order in the process!

Now once verified coil voltage at both key positions and with a new cap / rotor and proper firing order (heck should replace the wires while at it as well) give it a try and post.

Thank you! This distributor actually has a repair plate already, it just wasn't on for this picture as I had to remove all of that to get the cap off. I think I misspoke... I didn't mean to make it sound like the cap was moving. By "very little play" i mean more like I felt like I could maybe twist it hard and get some of the plastic to bend momentarily, but it's not like it was moving freely or anything

But, it sounds like the consensus is to get a new cap anyways. This is the one that was suggested to me earlier in this thread, is it a good one? https://www.amazon.com/Performance-...-CAP1010-DF/dp/B0845M26MW/140-4275339-0125666

I will also check the firing order.

Re: the voltage, here's my thought process, but please tell me if I'm wrong... if the question is whether the coil gets power with the key in both positions... I think the answer is yes, and here's why:

The coil definitely gets voltage while the key is in "start" position. I have verified this several times. So the only question is whether it gets voltage when it's in "run". And i think it does, because *when* my motor kind of accidentally and randomly catches, it keeps running when I put the key back into "run." That's when I get the strongest spark, according to the test light. So I don't actually have any reason to believe that I have weak or no spark even at the plugs when the key is on "run", the only thing I know right now is that I don't have spark at the plugs when the key is on "start" although I do know that I'm getting spark at the coil, at least. And my guess is that, since I don't even have spark at the plugs when it's in "start", my issues are so much more likely to be caused by something with the dizzy or wires than something more obscure like randomly not having spark when the key is on "run" as that not only doesn't explain, but also actively contradicts, my observations.

But, happy to be told that I'm wrong here! It's so easy for me to miss obvious stuff when, as you note @ESGWheel, anxiety is... higher than is should be haha

 

[ESGWheel]

I have not used that particular cap and rotor but its on my list to do so (I too have a 5.0 MPI but VP, same engine base). I see no issues with it and it was recommend.

Recall you are our ears, eyes and hands. If you are 100% confident have voltage at both key positions, I will avoid the Click and Clack tactic and not treat you as a “hostile witness” But to be clear > this is VERY odd what is going on. You should be getting spark at the plug wires, but you do not. This is why we are urging the new cap and rotor and if the wires are more then 5 years old (or unknown) replace those as well. We are a little into the parts cannon space, but the spark at the coil and not at plugs is driving us there.

Are you sure you have spark at the coil? As in snap, snap, snap, rapid fire like a machine gun and not snap….. snap…... snap… Please confirm.

Thanks, and keep it up, this is a head scratcher for sure. But my confidence is not diminished > you have a solid engine as mentioned before based on the compression nos. and you will have great stories to tell back on the water when all said and fixed

 

[Scott06]

I'll try bending the little tang up just to get really good contact and verify that the rotor is spinning. How would I know if it's leaking spark to the ground?

Look for signs of arching in the cap, also if you get spark into the cap and not out of cap to plugs or wires. Just the gap tester you have- should jump consistently 3/8" 9nice blue spark not orange) or so. Testo it off the coil wire , then you should get the same size gap jump off the plug wires. if you have less spark strength at the wires you are loosing it somewhere in cap/rotor/wires.

Thank you! This distributor actually has a repair plate already, it just wasn't on for this picture as I had to remove all of that to get the cap off. I think I misspoke... I didn't mean to make it sound like the cap was moving. By "very little play" i mean more like I felt like I could maybe twist it hard and get some of the plastic to bend momentarily, but it's not like it was moving freely or anything

But, it sounds like the consensus is to get a new cap anyways. This is the one that was suggested to me earlier in this thread, is it a good one? https://www.amazon.com/Performance-...-CAP1010-DF/dp/B0845M26MW/140-4275339-0125666

Stay away from the cheap amazon chit... summit racing has a sierra cap for like $78 , quicksilver is ,about $125

If the dizzy already has the repair ring on it could be as simple as the cap on fitting right.

 

[Scott06]

Please don't do that

yeah why use the spark gap tester he already owns when you can become the path to ground.

 

[searay250]

I have not used that particular cap and rotor but its on my list to do so (I too have a 5.0 MPI but VP, same engine base). I see no issues with it and it was recommend.

Recall you are our ears, eyes and hands. If you are 100% confident have voltage at both key positions, I will avoid the Click and Clack tactic and not treat you as a “hostile witness” But to be clear > this is VERY odd what is going on. You should be getting spark at the plug wires, but you do not. This is why we are urging the new cap and rotor and if the wires are more then 5 years old (or unknown) replace those as well. We are a little into the parts cannon space, but the spark at the coil and not at plugs is driving us there.

Are you sure you have spark at the coil? As in snap, snap, snap, rapid fire like a machine gun and not snap….. snap…... snap… Please confirm.

Thanks, and keep it up, this is a head scratcher for sure. But my confidence is not diminished > you have a solid engine as mentioned before based on the compression nos. and you will have great stories to tell back on the water when all said and fixed

First off, I might be a hostile witness.... I would consider it more like an "accidentally ignorant" witness but I think the end result is the same

I feel like I have very fast consistent spark off the coil but I will take a video to confirm, so other can see the timing and double check my intuitions.

Look for signs of arching in the cap, also if you get spark into the cap and not out of cap to plugs or wires. Just the gap tester you have- should jump consistently 3/8" 9nice blue spark not orange) or so. Testo it off the coil wire , then you should get the same size gap jump off the plug wires. if you have less spark strength at the wires you are loosing it somewhere in cap/rotor/wires.

Thanks. Yeah when I sometimes DO get spark off the coil spark plug (using a tester in the boot), it's not very often and it's definitely orange. I'm pretty sure it's blue off the coil but I will double check that as well.


Thanks!

 

[ESGWheel]

1) I feel like I have very fast consistent spark off the coil....

2) Yeah when I sometimes DO get spark off the coil, it's not very often and it's definitely orange.

These sentences appear to be conflicting. Recall you need the snap, snap, snap rapid fire off the coil, less so off the spark plug wire.

• Is sentence no. 2 for the plug wire and not the coil?

 

[searay250]

These sentences appear to be conflicting. Recall you need the snap, snap, snap rapid fire off the coil, less so off the spark plug wire.

• Is sentence no. 2 for the plug wire and not the coil?

Ooops! Yes, good catch... I meant:

1) I feel like I have very fast consistent spark off the coil....

2) Yeah when I sometimes DO get spark off the plug, it's not very often and it's definitely orange.


I'll edit my original post now to prevent future confusion but I appreciate the catch, that was definitely my typo

 

[ESGWheel]

i forgot to mention that these types of dizzy caps usually have the cylinder no. stamped on it for each wire...

 

[searay250]

Hi all,

First, I want to thank everyone who has helped with this so far. I completely redid everything "downstream" of the coil and the engine was able to fire up. I am not convinced that I would have been able to get this going without help, and I want to express my gratitude to you all.

I'm pretty sure that the combo of a new cap and rotor was the difference-maker, it's the only thing that I wasn't able to kind of independently test. I would not have ever bought a new set, thinking, "these are new! Nothing can be wrong with them!"

So, thanks!

Unfortunately, there still seems to be an issue, although I'm not sure whether this is related in some way or not. When it fired up, it was idling a little high, so I figured I'd adjust the idle down just a smidge. At some point, I bumped the throttle, and it made a really, really weird almost screeching noise. After some fiddling, I found out that as soon as it throttled up even a little, the noise would start and then slowly die down. I captured the noise on video here, with me just nudging the throttle a tiny bit:

Eventually, after maybe 2 minutes of additional runtime (with some engine shutoffs in between) the noise started to just be there all the time, even at idle:

I shut the engine off as I didn't know what was going on. It kind of seems to be coming from the throttle body/intake area, although I'm not confident enough about the location really pinpoint it for sure.

Has anyone... heard anything like this before? It sounds really, concerning, whatever it is.... I would appreciate any thoughts on what this noise could possibly be, and/or if it's a sign of my engine about to blow up or something!

Thanks again, everyone

 

[cyclops222]

It sure sounds like bare metal rubbing together. Or a super odd air leak.

 

[searay250]

Yeah it's so weird. The strangest part is that this noise was NOT there last time the engine ran not even a week ago. I am trying to think through what I actually touched on this motor recently.... spark plugs, plug wires, spark arrestor, distributor cap.... I really think that's it. It's so hard to remember, I guess this is why @ESGWheel (rightfully) notes that the owner of the boat is often an accidentally hostile witness, ha!

This noise seemed at first to be related to RPM because it would show up as soon as the RPMs increased and last even after the throttle was returned to idle but die down along with the RPMs, but then it started to just make the noise all the time, which is alarming. Seems to run and shift fine despite the noise, although I have not actually tested under any kind of load beyond just putting it in gear and making sure the prop turns.

Really stumped on this one. It feels like this could run the whole spectrum from "what a weird but harmless noise" all the way to "yeah your engine is going to explode in 2 more minutes"

Thanks

 

[nola mike]

No such thing as a new noise that should be ignored