Mercruiser 383 Mag Stroker MPFI Bogs Over 3000 rpm

[even-rude]

New member. I could use some direction. I bought a 1999 Cobalt 233 with a 2019 Merc 383 Stroker Engine and have been working through deferred maintenance issues. The last one is the engine bogs down over 3000 rpm. I checked the tank vent (OK), the tanks is clean, the screen intake is clean, i changed the check valve that was stiff and I was convinced was the problem (it wasn't), new fuel filter (WIX metal body), new fuel/water separator (Sierra), low pressure pump runs fine. I can't get to the high pressure pump in the cool fuel unit because the stanchion of the boat doesn't allow clearance to get the plastic box open. However, I get 37 psi fuel pressure on the fuel rail when ignition is on but haven't started the engine and when I turn the ignition off, the pressure holds for many minutes before it starts to subside. I pulled the pressure regulator and it appears serviceable. Diaphragm is intact and there is no fuel in any vacuum line. I ran the engine at the dock in neutral and the rail pressure remains the same within 1 psi or so at 37 up to 3000 rpm. Not sure what it's supposed to be but the engine starts easily, runs smoothly and when under load coming out of the water to plane does just fine. It cruises effortlessly up to 2900 rpm that gives me almost 30 mph but put the beans to it and it "fits and starts" and refuses to move any faster. I ordered a high pressure pump but I'm pretty convinced that's not the problem. I'm thinking that maybe the ECU is somehow getting a signal from something that won't allow it past 3000 rpm and that makes it seem like a fuel issue. I don't have a way of reading any stored codes. Can someone give me some direction of suggestions on what do to next?

 

[kenny nunez]

There is a switch on the shift linkage plate that prevents the engine from going over a certain speed if the shift is in neutral, sometimes that is the problem. It is there to make sure that the drive is fully engaged.
I am sure someone on the site can post a picture of the switch and possibly the Mercruiser troubleshoot process.
I had a similar problem with my Donzi that I purchased that had been in storage for a few years, but it was the spark plugs, it would start cutting out around 2800 rpm and would not go any further. I also performed all the fuel system checks and even had the fuel injectors tested. I went to NAPA and asked for the most expensive spark plugs they had. Instant 5000 RPM @ 70.

 

[even-rude]

Wow. Thanks. I have replaced both the cables and adjusted them for shifting properly. I think my project is next to work on looking for the switch and seeing what I can find. Thanks again. I'm pulling my hair out.

 

[even-rude]

Well, the neutral safety switch is apparently not my problem. I unplugged the switch, put a multimeter on the leads and it measured closed (as the label on the switch said it was NC). Then I went to the helm and put the throttle full over. Immediately as FWD is engaged, the switch shows open -- as it should.

 

[dubs283]

37 psi is close to spec but should be more around 42 psi steady at all rpms/load. Based on the fuel supply troubleshooting it's possible the high pressure pump is faulty. Only other test I can think of is a fuel supply vacuum test or running on a remote fuel tank.

A faulty in gear/neutral switch will cause an audio alarm/set a fault above roughly 3000 rpm. Seems like you ruled out that possibility with the continuity test.

You haven't mentioned checking/replacing any tune up parts. The flat/crab style distributor cap is a common culprit in high rpm/load issues. Best to start fresh with the correct spark plugs, oem cap/rotor, plug wires if they're more than 5 years old and fuel filter(s).

Wouldn't hurt to verify proper timing advance as well however aside from properly clocking the distributor there is no timing adjustment for the engine

 

[even-rude]

Thank you for your input. After this neutral safety switch test this afternoon, I pulled the rear two spark plugs on the port side based on kenny nunez's gracious response about his bad spark plugs. The two I pulled looked fine for wear (though I plan to replace them anyway) but they look like they have been running in a "rich" mixture which may be a result of the ECM misinterpreting sensor inputs. I looked at the wires and they look good but it's a cheap investment for this annoying problem. I will put the "distributor" cap on the list of things to replace as well. {Thanks for your suggestion.] Regarding the vacuum test for the gas tank, I got a new check valve and installed it backwards then blew through the fuel line from the low pressure pump and could easily push air into the gas tank so that's not a problem (then, of course, i changed the check valve for the proper direction). My friend, (with a similar boat and more money and less time than me who uses a mechanic -- mechanic said 1) the water pressure sensor, if blocked, will limit the engine RPM and there's some ignition sensor up top (that I'm not yet clear on with my friend's description) will do something similar. So tomorrow, it's: spark plugs, maybe wires, a water pressure sensor and looking for another sensor up top.

 

[dubs283]

Those spark plugs are toast, fouled with excess deposits.

Reviewing the pics posted I'd guess this is a saltwater boat with raw water cooling? Based on the patina and other aspects I'd guess the distributor cap needs replaced at a minimum. Doesn't appear that the risers/manifolds have ever been removed for inspection either

Is this a bored over/stroked 5.7 or an oem 6.2 mpi?

 

[even-rude]

This is now and has always been a saltwater boat. The original owner repowered it in 2019 with a new OEM: MCM MX 6.2 Stroker replacing the 7.4 that was the original engine. I have already ordered a new distributor cap and plugs. I have also installed a fresh water flush system using all the drain locations on the engine and risers with a quick disconnect to a garden hose that bypasses the thermostat.

 

[even-rude]

So now I have replaced the distributor cap which turned out to be a pain. The "ear" closest to the manifold on the lower portion of the distributor that holds the cap on cracked when I was unscrewing the cap. Had to improvise with JB Weld to make a new screw hole. Will probably have to invest in an aluminum distributor body. BOAT = Break Out Another Thousand. Replaced all the wires and the rotor. Took the boat out yesterday. Still have the 3000 RPM ceiling. Spark plugs are arriving Tuesday as is the water pressure sender. Thinking to self: I wish there were an easy way to read stored codes in the ECU.

 

[alldodge]

You have a HSV distributor and cannot be replaced with another type without many other changes

Dorman makes ear repair. This may not be yours, its just FYI
https://www.ebay.com/itm/2944377836...6VAh4f6cg%2FU%2B%2Bs|ampid:PL_CLK|clp:2047675

You need a scanner and replacing parts without knowing what the issue will cause more problems

 

[even-rude]

So many times people don't follow up on how they actually overcame their problem. I gave up randomly changing parts. I had many suggestions -- some good and some no so good -- but I followed and did most of what I could. Finally I decided that without some help from a real mechanic familiar with my engine and some tools and knowledge to apply, I was going nowhere (not so) fast. I found a local mechanic and delivered my boat. He told me that the codes he found were: 94 pitot pressure low signal; 75 fuel level sensor 1 high signal; 130 trim adjustment sensor overvoltage. He told me that none of these should cause my condition. I told him that when I got the boat, the first time I put it in the water, It overheated and I limped back to the dock. (I changed the impeller, the thermostat, the temp sender.) He suggested checking compression to cross that off the list of afflictions which I did -- taking out my brand new plugs and then measuring compression in each cylinder -- 155+/- psi across the board. He related a story to me that he installed a brand new 383 Stoker from Mercury in 2021 and he had a similar problem with limited RPM. When he was unable to find the issue, he insisted they send him a new engine (under warranty) which they did the problem disappeared. Mercury told him that the returned engine had a short in the wiring harness which fooled the ECU into limiting the RPM and this might be my problem too but first -- there were a list of sensors he suggested changing including MAP, IAC, TPS and a couple others though he said that these would be more symptomatic of an engine that had problems starting which mine did not. A new wiring harness, he informed be could be over $1000. Yikes. This morning, I put the boat in the water and idled out of the cove to the open water and "gave the engine the beans" She came out of the water as she always does, effortlessly and continued to accelerate past 3000, 4000,, 5000 WOT without complaint or hesitation. I finally have a boat! The only thing my mechanic actually did was to clear those codes from the ECU. So there you have it. Thanks to everyone who responded with suggestions. It is greatly appreciated when you're frustrated and looking for some direction from those with knowledge or experience.

 

[alldodge]

Thanks even-rude for closing the issue