2001 Mercruiser 7.4 rpm limited to 3600- need advice

[ScubaDave16]

Hello. I have a 2001 Maxum SCR 2900 with a mercruiser 7.4 MPI engine. Last year it ran just fine (if not a bit underpowered for that size boat) and I could get to 34 mph or so at over 4400 rpm. This year it started out running OK, but quickly the rpm dropped to 3600 rpm and since then only rarely does it get above that and not long enough to get on plane before again dropping to 3600 rpm. at 3600 rpm my speed is only about 16mph. I am consistently getting 38-40psi at the fuel rail through all rpm range. The engine will rev to over 4400 rpm in neutral. the engine runs fine at 2000 rpm and 8mph. smooth and stable. even when struggling at 3600 rpm it is smooth with no hiccups, backfires, or missing.

other info- i have added new gas in May, I put STABIL in the gas before storing last fall, I have changed the fuel filter, put fuel injector cleaner in the gas, I have cleaned the flame arrestor. There is also a very loud hiss from the engine upon deceleration, but it has done that since ive owned it. I pulled the distributor cap and cleaned it and the rotor. I have not been able to replace the plugs yet.

Someone suggested the fuel pump, but the cool fuel module is extremely expensive and very hard to replace and with consistent 38psi I kinda doubt this is the issue. others have suggested the fuel injectors, but it seems if one or more was clogged or dirty, it would run rough.

my boat has no trailer, so taking it to a mechanic is essentially impossible unless one can come out to me which so far none are willing to do.

any help is appreciated. Thanks

[mmwjr]

First these engines are not designed to operate at 4400 rpm for more than short bursts. Not sure how you have been operating but you mentioned running at this rpm.

How many hours are on the engine? These engines have a safe mode that will limit rpm if knocking is detected. I would perform a compression test to assess the health as a start. A complete tune up; cap, rotor, plugs and wires are probably past due.

[ScubaDave16]

Engine has 718 hours. I didnt run often at high rpm. But it used to be in the low 4000s rpm range. Unfortunately all serial numbers are wiped from the engine sticker so i dont know the exact cap and rotor to get. When i pulled the existing one it seemed ok. No carbon or excessive corrosion I have so far been unable to remove any spark plugs with the tools i have available

[mmwjr]

The engine is a GM 7.4(454) so you just need the distributor cap and rotor for that, Mercruiser, Sierra, … Are the spark plugs rusty? I’ve worked on these engines before and if the exhaust is leaking raw water will drip onto the plugs. It’s difficult to spray them with any penetration stuff with the exhaust manifolds in place, you may need to remove them first. Been there done that.

[shrew]

Max RPM range is 4400 - 4800. My 1999 2800 (Effectively the same boat) used to cruise at 21-24kts at 4000 depending on conditions and tide. Below 18 kts it would wallow and start to drop off of plane.

Can you reach 4400 in neutral (briefly)?

[ScubaDave16]

So to follow up. since the last post I had a mechanic come look at the boat. He ran a scan tool which showed no saved codes at all. I had him change plugs, wires, distributor cap and rotor. the plugs were unremarkable. With all that done there was no change. He swapped out the timing advance? module. no change. I even dove down and manually checked the stern drive and propellers. they spin freely. compression on cylinders ranged from 135-150. fuel pressure stable at 38-40 psi. flame arrestor cleaned. we checked and throttle cable is not binding and when WOT, the throttle is wide open at the TB. we had it full throttle and it just sits at 3600 RPM and mushes at 15mph. doesnt cough, backfire, hiccup or anything, just constant 3600 rpm. it does rev a bit higher in neutral.
He is out of ideas as am I. anything else to check?

[mmwjr]

It only takes minor marine growth on the bottom and drive to create drag that can prevent a boat from planing and reaching high rpm. Have you checked this?

[ScubaDave16]

Yes the bottom was recently painted and no significant marine growth is present. I am still leaning toward fuel supply issue because of the sudden drop in RPM that occured in May. the engine has been pretty much limited to 3600 rpm since then. if it were a clogged or faulty fuel injector, wouldnt there be some performance issues at lower RPM?

[mmwjr]

Being that this is a MPI engine a bad injector would only affect one cylinder and I don’t think the engine power would be as low as you are seeing. Was the fuel pressure measured at 3600 rpm while under way? If not this should be done to see if the pressure is holding up at load. Did the mechanic look at the knock module/sensor?

[shrew]

The Cool Fuel modules in the 8.1L (496) used to have an issue because they painted the inside of the module. Gas (possible ethanol?) would turn the paint into a sludge that would begin to clog the injectors.

You might want to have someone pull and inspect the injectors to see if that is the issue you're dealing with. I'm not sure if that issue impacted the cool fuel module on the 7.4's but there were not a lot of 454 MPI in production between the carb 454 and the release of the MPI 496.

[ScubaDave16]

well an update on my engine. the guy working on my boat checked knock sensor and TPI sensor and both were found to be OK. He also verified that the fuel pressure is OK. He is supposed to change out the fuel pump next weekend, but even he doubts that that is really the issue. someone else suggested that maybe an exhaust flapper had come off and was restricting the exhaust, but that requires pulling the lower unit and that unfortunately cannot be done until the boat is out of the water and then i am not able to test it until May! I am completely out of ideas. still limited to 3600 RPM no matter what I do. any direction at this point would be welcome.

[mmwjr]

To see if a flapper broke off all you need to do is remove the exhaust elbow as the flapper is just below it on top of the y pipe. Now any broken pieces will be at the bottom of the y pipe or in the exhaust passage of the transom assembly.

[HavreDeGrace2400]

A friend of mine has a MPI engine that has burned out some fuel relays resulting in poor fuel delivery. I think in his case its been caused by running long periods on idle with low voltage from a crappy alternator voltage regulator. Anyway, whenever he replaces these little $10 boxes his performance improves till he does another extended slow idle and burns it out again.

[mmwjr]

Originally Posted by HavreDeGrace2400

A friend of mine has a MPI engine that has burned out some fuel relays resulting in poor fuel delivery. I think in his case its been caused by running long periods on idle with low voltage from a crappy alternator voltage regulator. Anyway, whenever he replaces these little $10 boxes his performance improves till he does another extended slow idle and burns it out again.

Interesting. Has he considered temporarily connecting a digital voltmeter to see if the low voltage theory is correct?

[HavreDeGrace2400]

There are two of these little cube fuel pressure relays atop the engine. I have two also. They're standard GM parts my friend has gotten from an auto parts store for $10 each.

We've only gone by his dashboard voltmeter. What is happening on his boat, is that unless his engine is over 1000rpm, his alternator is not charging and hsi dashboard voltemeter reads 12 and if over long enough time...even lower.

On my boart, and what i assume is normal... is that my voltmeter also doesnt go above 12v untill i eventually RPM over 1000. However unlike his.... once my alternaor kicks in it is charging at 13.5ish even when i got back to idle rpm.

So this guy... he idles from one side of still pond to the other and back... and after like 30 minutes of low voltage these little fuel pressure relays overheat. chatgpt says low voltage overheats them, and sure enough this guy runs long periods on low voltage. They eventually stall him out. They become too hot to touch for even a split second.

And when he tries to go on plane he barely can. When he replaces the overheated relays, the boat goes back to fast on plane.

Side note, he has a promariner battery charger with 2 banks. One charge bank isn't working he says... but it is still connected to battery #2. My theory is that this might be causing the alternator behavior. Certainly, it is causing battery#2 to always be low.

So... to the original poster.. pop two $10 boxes in tehre and see if anything changes.

[mmwjr]

Your alternator is working correctly but your friends isn’t. Hopefully the OP try’s what you are suggesting as I’m curious as to if this solves his issue.

[shrew]

Originally Posted by HavreDeGrace2400

What is happening on his boat, is that unless his engine is over 1000rpm, his alternator is not charging

On my boat, and what i assume is normal... is that my voltmeter also doesn't go above 12v until i eventually RPM over 1000. However unlike his.... once my alternator kicks in it is charging at 13.5ish even when i got back to idle rpm.

This is expected behavior by ever alternator I've ever troubleshot. In order to charge, the excitor wire needs to be connected to the battery, and typically the engine needs to be above 1K for the alternator to begin sending a charge.