Mercruiser 350 Intermittent RPM drop while cruising

[QBhoy]

Taking in a little water could do this too ? Can’t remember but did you look at the plugs ?

 

[achris]

Taking in a little water could do this too ? Can’t remember but did you look at the plugs ?

Post #14...

 

[tank1949]

I am assuming that your ignition has knock sensor? Perhaps your "knock" sensor is causing issues. Disconnect it and see if motor "caughs" the same. Intermittent failure will drive you "nuts." Also check for plug wire broken and leaking fire, which is going to ground. Run it a night with muffs on and start wiggling wire. You will easily see or worse, feel it. Been there!

 

[siriussr]

I was able to find out more about the engine. Original engine serial # is 0F598644, and the reman engine fits the same category. It has 12 intake bolts, so its a non-vortec engine, so the plugs are correct. The ignition pickup sensor was replaced 2 years ago with a new style Sierra one. Note that all my replacements are with Quicksilver parts. I have not done a compression test, but the boat will run 39mph at 4500 rpm WOT w/ 4 people and loaded, so I don't think compression is an issue.

I am hoping to get out on the Puget Sound this weekend- Ill be able to tell if it is still doing it or not. If it continues, here is my list (in no particular order):

Pickup sensor
Shift Interrupt Switch (supposedly new in 2017)
knock sensor (not sure if I have one since i believe it is Tbolt IV, but ill search)
Carb Rebuild (it hasnt been rebuilt since 2008)
High Tension Coil/coil pack
Ignition Module (this is last resort)

PS: the 2400 SCR was a new model in 1994, and was indeed the replacement for the 2300 SCR

 

[alldodge]

I don't see anything on your list that would cause your symptoms, those items either fail or they don't.

Either a connection or fuel issue

 

[siriussr]

More fun discoveries- I found the starboard side core plug under the engine mount is dripping coolant... time the replace that and pressure test the system.

the mechanic who did the pre purchase inspection didn’t do all that great of a job...

 

[tank1949]

More fun discoveries- I found the starboard side core plug under the engine mount is dripping coolant... time the replace that and pressure test the system.

the mechanic who did the pre purchase inspection didn’t do all that great of a job...

Surveyors are just as bad!

 

[siriussr]

A good update- I took it out this weekend, and after two hours of running, it only lost RPM once. I filled the tank all the way before this too, so I think it could be either a fuel pump or anti-siphon related problem if it persists. Thank you all for your help! This weekend made us fall in love with our 'new' boat.

 

[Lou C]

I've never gone more than about 6 seasons without needing to rebuild the carb, at least with my Quadrajet and I check the gas that is in the filter every season and its never had water in it as long as I've had this boat (17+ years). That might be worth a look. Have you dumped out the gas from the fuel filter and inspected it? I also had an issue with a sticky anti siphon valve but that was much more obvious and was fixed by my mechanic when I first got this boat (2002). Electric fuel pump or mechanical? If electric how are all the connections/terminals?
Can you trigger the interrupt switch when its idling to see if the 'miss' feels the same? I had a sticky interrupt switch on my OMC a few years back and it was not a miss but very obvious.

 

[QBhoy]

Glad you got it sorted

 

[siriussr]

Fun news... this is back. It have since replaced the anti-siphon valve and fuel line. Will be doing the separator, fuel pump, carb rebuild and tank pickup next... anyone got any ideas otherwise?

 

[siriussr]

I should also note- thsis engine has an Edelbrock 1409 4 barrel marine carb, which seems to be different than what it came with from the factory- could a bad fuel pump be the culprit?

 

[siriussr]

Carb rebuild did not fix the issue. (I am posting this info so it happens to anyone in the future, they can use this as a reference)

fuel pickup is next, followed by round 2 electrical cleaning.

 

[alldodge]

Hope you can find the issue, gremlins are hard to find

 

[Rick Stephens]

The 1409 is about the easiest carburetor ever made to do a rebuild on. Or to calibrate. It does take a little study to figure out the calibration and it has definite parts of the carb to consider for the different throttle settings. If you find that a given throttle setting seems to have the issues, then might be needing to do calibrations for that range of carburetor function. I just mention this in case you see a pattern that might indicate a bad calibration for a given throttle setting as opposed to something that is restricting fuel flow only after running a while. I have some experience with that carb and can help should you need to look there. Not suggesting that is the problem, just pointing out the possibility for your thinking.

I hate gremlins.

Rick

 

[siriussr]

I rebuilt the carb, but I am having trouble resetting the idle/low speed settings. over 2200 rpm it runs fine. under that, there is some small surging and is slightly rough running but doesnt die. I replaced the shift interrupt switch and checked out the fuel pickup, but I am still having the problem, but it is happening significantly less that before.

 

[Rick Stephens]

Might try hooking up a vacuum gauge and seeing if you have a major intake leak somewhere. I had valve guides totally thrashed on my last motor and it really effected idle smoothness and how much RPM I had to maintain to keep it running. The vacuum gauge needle would bounce like a rock screener at anything under 1800 RPM. And you should do a compression check, just to establish a baseline. Too easy to do not to weigh that as well.

 

[alldodge]

Think you need to get back into the carb, something isn't right. Your first post said the idle is perfect but anything above 1500 was the issue. Now its better above and won't idle.

While you may have a vacuum leak as Rick said, and do check that first

 

[siriussr]

I rebuilt the carb again. It idles perfectly now! BUT, now I am having trouble with rpm just off idle. When I give it a little gas, rpm moves as expected to 1500 rpm, then drops down to 1100 and runs rough. I slowly give it more gas and it stumbles along until 2200 rpm where it will suddenly surge up to about 3k. I can back it off and the rpms match throttle position until about 2k rpm, where it then starts to run rough again and drops to 1100 rpm. I have tried all 3 settings of the accelerator pump and spent a lot of time dialing in the idle screws, but no matter what it does the low rpm stumbling. Any other tips?

 

[siriussr]

I may also add that this is indeed a thunderbolt v ignition system. I found the knock sensor and capped purple/white wire.