1983 mercruiser 260 350cid Low RPM issue

My engine is a little newer (1995 model) Indmar with EFI and Electronic ignition. The long block rebuild is basically the same. One thing that happened to me a number of years ago that may help you if your engine has an Electronic Control Module (??). I had a minor leak in the raw water pump, and the impeller began to self distruct as well. The ECM would not allow the engine to overheat and reduced the RPM. The engine Temp was always normal, seemed to run fine at the reduced RPM. The RPM would rise to about 32-3400 RPM at about 3/4 throttle, and then more forward throttle movement produced nothing. Had the raw water pump and impeller replaced...that was it.
Not sure about my current problem though...I will let you know.

 

The easiest thing to do is install a borrowed prop with 2" LESS pitch, then check the RPMs.

 

Suggesting something like wrong camshaft for boat engine perhaps. this comment from skibus might be on to something. cam timing may be out as well.

 

At least that behavior is exactly correct and expected. Only 300 RPM drop?..they can't be that fat.

Putting the correct prop on it will bring the RPM up but you still need to account for the variation of loading and what that will do as well.

 

Thanks Gents for your input...
The engine rebuilder offered no information at all, and simply stated it was a remanufactured Vortec 350 engine with all marine specs and that he does over 500 engines a year.

Compression test was good, and some kind of a new black magic "Sensor" was installed.

The marina took it out a few more times and it began to achieve higher RPMs (whether it was a sensor or not, I am not sure. ) I think they were a little stumped as well. They began thinking it was the old ECM. (1995 boat was one of the first with the Electonic Fuel Injection!)

I now have the boat back and it seems to be running well. I am slowly breaking the engine in with varying power and such. I've had the boat since new, and the top speed in the owners manual states "43 mph" I have almost always been able to achieve this (clean hull, new plugs, wires etc...) with max RPMs about 42-4400 with stock prop. Manual also states that WOT max rpm is 5000.

Its been a great boat over the years, but this is sadly the 3rd engine. (1150 hours) I use it basically only for barefoot water skiing, so I wonder if we are stressing it a bit much (hard pull out of the hole and 38 MPH constantly)

I guess I am rambling on a bit, but the bottom line....personally I think it took an hour or so of running for the new engine to seat and ECM to sort itself out???

Thanks again for your help.

 

The 1995 and 1996 ECM are very different. 1996 was the first year for OBD II, while 1995 was the last year for OBD I, so you may just have a ECM issue.

It's also pretty important that you check the cam spec's as there are 3 (that I know of) Vortec 350's for 1996. The plane Jane is about 190 HP with lots of torque (~300 ft. lbs.), while other versions produce more HP, with various torque ratings.

 

Thanks PAR...this rebuilt Vortac seems to be breaking in nicely now...but will keep in mind your notes regarding the ECM...
If I get into it in the future, do you know if the ECMs can be changed, or do I need to replace it with the original OBD I?
Cheers

 

If you have an OBD I engine, it'll have the appropriate wiring and sensors. The OBD II version is the Vortec, with the two part intake and sequential EFI, not the throttle body of the OBD I (some early OBD II's came with TBI), so it would be difficult to screw this up as the sensors and fuel delivery hookups would be different. On the other hand you probably could take a Vortec long block and force it to be a OBD I with some modifications.

Check the number of intake bolts you have. If you have 12, it's a regular first generation small block, but if you have 8, it's a Vortec and has different heads too. In remanufactured or marine applications, the Vortec is often call the RamJet 350, but it's still the Vortec 5300 L31.

Any reasonably skilled gear head should be able to sort this out for you. Marina mechanics aren't the brightest bulbs in the chandelier. You probably just have a bad sensor or two, but any good mechanic can find these and replace it.

 

That's great PAR...I am learning slowly...thanks for the lessons!
Checked the old owners manual from Mastercraft (1995) It stated clearly that the "Corvette LT1 engine" had multi port fuel injection and the "Standard Performance" engine had "Electronic Fuel Injection, Throttle Body Type"
The latter is my engine for sure however the intake manifold has 12 bolts (six a side)...the engine rebuilder asked specifically about this prior to putting together the engine for me, as well as the serial numbers on the block, and on top of the head between the rockers. Each rocker panel cover fits on with 4 center bolts. I suspect OBD I based on the year. (95) Engine rebuilder assures me it is a Vortec 350 built to marine specs.
Had it running again today...all is good so far...5 hours...with a short power 'test' up around the 4,000 rpm range...

I'll give it a few more hours and change the oil.
Thanks Again!

 

Are you sure it's TBI? I mention this because the first Vortecs also came with central sequential fuel injection (CFI). The piece on the intake looks much like a TBI unit, but in addition to this are injectors at the ports. This is why I asked about the intake manifold bolt count. The CFI units uses a two piece composite manifold, while the TBI a regular single piece iron manifold. If you have 12 bolts, it's a TBI. This is correct for a 1995 OBD I. The obvious check is the TBI unit, which should have two injectors mounted on the driverside of the throttle body, squirting directly down into the two bores.

 

Yup...this is it...So now I get to say for sure...TBI (12 bolts) and the manifold was one big heavy piece of iron (some discussion took place between me and the marina as to whether this was to be replaced by the engine builder as part of the long block replacement...it was not and I ended up carrying it around between the rebuilder and the marina.) and indeed "two injectors mounted on the driverside of the throttle body, squirting directly down into the two bores" This is it...although the injectors and bores are mounted laterally across the engine. (must be the Indmar mod of the intake (?))
And I have to think about this a moment as well...the driver side is on the right on the boat, but I know what you mean...this had me confused for a while last year when I was trying to figure out the correct numbering of the cylinders, and the most comprehensive explanation I got was always refering to the "driver side" 1,3,5,7!!
Thanks for this PAR, you have been a great resource. 1995 OBD I, early Vortec...I can see more on the internet now I understand what it is I have. (So far so good on the re-build...same amount of top end revs, providing the same speed as the original boat.) I only hope this engine lasts longer than the other two!


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The drivers side is an automotive reference . . . my bad.

Looking at the front of the engine, with the crank pulley facing you, the right side of this engine is the drivers side (in the USA and all other places that drive on the proper side of the road ). These cylinders (as you correctly noted) are 1, 3, 5, 7.

The CFI setup on later OBD II versions of the Vortec have a composite, two piece manifold.