1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

[Darin Jordan]

Good Day.

First, let me preface this post by saying that I'm a good mechanic and have quite a bit of experience with motors of all kinds... so I'm not completely clueless about mechanical issues on motors. I just don't have a lot of experience with marine exhaust systems or issues that may arrise.

Here is what I'm dealing with.

After pulling my 1990 Maxum 2400SCR w/ Mercruiser 7.4L and Bravo 1 out of storage, I found that the antifreeze in the closed system was quite low. The boat was put away after flushing the fresh water side, and was stored with a heater in the engine compartment to keep the moisture down and keep it warm in colder weather. I live in Washington State, and we did have a colder winter, but nothing super severe.

Upon further inspection, I found that antifreeze in cylinders 3, 5, 6, and 8. 8 had enough in to prevent the starter from turning the motor over.

With the plugs out, the motor turns over fine with the starter. There is NO oil in the cooling system, and no water in the oil.

I've owned the boat for about 5-years, when I purchased it from my Mom after my Father died. He had owned it since it was new.

Since I've owned it, the anti-freeze has needed to be topped up after sitting for longer periods, and when running the boat, there would be some of the white anti-freeze fog coming out of the exhaust when you'd first fire it up when warm, but the plugs have always been just golden brown, with no sign of the motor actually burning the coolant...

According to the owners manual, the heat riser gaskets are suppose to be replaced every season. To my knowledge, they've NEVER been replaced...

I suspect that the prolonged storage this time just exaserbated an issue with leaking heat risers allowing antifreeze to get into the exhaust manifolds, which then ran down the manifolds and into the cylinders.

My current plan is to:

1) Spray SeaFoam into all the cylinders and down the intake to prevent any further possible damage from corrosion... Luckily it's almost pure antifreeze, so there shouldn't be much if any...

2) Change the motor oil to verify that no water is in the oilpan.

3) Do a compression test. Also watch for any signs that compression is getting into the oil system or cooling system (leak down test if necessary)

4) Reinstall the plugs and see how the motor runs.

If the motor is running smoothly and there are no signs that the heads or block is cracked, then I'm going to proceed by:

5) Removing the Heat Risers and if necessary the Exhaust manifolds and replace whatever appears to need replacing.

6) Change the oil again.

7) Put in new plugs and run the motor.

So, to all you experienced boat mechanics out there... does this sound like a reasonable approach? I know it could be something much worse (cracked/warped heads, etc.), but since the motor has been running fine, has NEVER been overheated or abused, and has been well cared for, I find it hard to believe that these worse-case scenarios would just happen while it's sitting... But, I'm open to advice...

Thanks!

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

Ummm, no. The exhaust manifolds and risers are cooled by raw water, not the closed cooling system. (Anti-Freeze)

If your getting anti freeze in the cyliners when it sits I would strongly suspect a leak in the intake manifold or the heads. (And or gaskets.)

People have told me that, but are there not two closed system? One that is "half-closed", and the other "full-closed".

I'm certain that I have anti-freeze lines going to my heat risers... but I'll double check just to make sure...

Also, if antifreeze were going through the intake or heads and into the cylinders, would I not have indications of it begin burned on the plugs or in the exhaust? I just don't see those typical signs...

I'll check it all out, just to be sure...

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

As to the plugs appearance, you said you have seen anti-freeze in the cyls. right? It is what it is, its in there, and its not showing on the plugs.

My point with this was that in every other motor that has a cracked head or leaking intake, etc... the antifreeze gets into the cylinders and is burned by the combustion process, leaving a white residue on the plugs and causing a steady white stream from the exhaust. Or it just cleans the plugs completely off... This doesn't appear to be happening here.... they are all golden-brown, just like a perfect combustion should be...

I suppose it might be possible that the plate is leaking and anti-freeze is getting into the exhaust manifold and running back into a cyl. with the exhaust valve open but it seems a stretch.

Like I mentioned... the coolant has been having to be topped off for a couple of years of running the boat, but only after it sits for long periods of time. NEVER after running the boat... even after burning three 70-gallon tanks through it out on the Snake River over a week long trip... And in all that time, there have never been any running issues or indications of anything wrong with the motor itself.

Would also seem rather odd to have warped/cracked/or otherwise engine components on BOTH sides of the motor at the same time. I suppose it's possible... but I've never seen it on a motor that was otherwise running perfectly.... usually only after the motor has been severely overheated, etc...

Other than wishful thinking... I think these are some of the reasons why I suspect that the riser gaskets have failed... this and the fact that the owners manual states that they should be replaced EVERY season... which seems excessive to me, but like I said, I'm not a boat exhaust expert...

I'll take all this under consideration as I check things out. I'll post updates as I find what I find. Maybe it'll help someone else in my position.

Thanks for the info! If anyone else wants to chime in, feel free... Would love to hear some more POSITIVE experiences if they happen to exist!

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

That really suggests a slow leak at the engine intake. Probably not enough to even notice when running but accumulates when sitting for long periods.

That would be a "preferred" over the more toublesome alternatives! Intakes are easy for me to do... I may change it just to be safe... and to give the heads a good look if everything else checks out.

Thanks for the conversation... I'll dig into this deeply and report back what I find... Hopefully the base motor and heads are fine and it's just one of the alternatives... I can deal with those a lot easier than having to pull the heads or motor.

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

OK... I cleared all the anitfreeze out of the cylinders and lubed them with SeaFoam. I then changed the oil. Sure enough, there was about a quart of antifreeze in the oilpan. I'm figuring that bled down through the rings while on top of the pistons... Added back some straight 30wt, which will be changed again immediately after this ordeal os completed to get the remainder of any contamination out.

Once I refilled the oil, I cleaned up the plugs (they were only about 4 hours old) and put them back in. Motor started up on the first crank and I ran it for about 25minutes... temps were around 170 and it seemed to be running just fine. No signs of compression getting into the cooling system. No oil in the cooling system. No odd noises or rough running. There was a little white smoke at first, but that was to be expected and it cleared up after a few minutes.

I then ran some Marvel Mystery Oil into the carb to fog the motor. Ran it for a few more minutes after this, then shut it down.

After it cooled a little, I started draining the coolant and taking things apart.

I have things all torn apart now... Removed the exhaust and intake manifolds and separated the Exhaust Elbows (which I've mistakenly been calling the "Heat Risers")...

While I can't find any obvious signs of water leaking, I did find some cooling system issues. A lot of blockage in the exhaust manifold fittings leading to/from the Heat Exchanger. Also a LOT of crud in the Exhaust Elbows near the separator gaskets. I'm going to spend some time today inspecting everything to get a better picture of what it all is.

When I removed the intake, there was moisture inside the lifter galley and up under the splash shield... foamy white oil/water... typical of water in the oil. I haven't looked real closely yet, but it looks like water may have been leaking in from the intake gasket. Still doesn't explain the antifreeze in the cylinders... but I'll know more once I inspect it more fully and get things cleaned up.

I think my next steps are going to be to cleanup the intake manifold (damn thing is heavy... would LOVE to put in an aftermarket aluminum manifold and FI system...) and to leak-test the exhaust pieces.

I'm going to post a couple of pictures that I have questions about. I'll do that in a followup post or two...

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

Sure sounds like a leaking intake manifold/gasket. (Anti-freeze is pumped through it.)

As to testing the manifolds, risers/elbows I have a sticky in the Adults Only post at the top of this forum on how to do it without pressure testing. Should be of help.

I sure hope you are right. That's what I think as well. I'm going to examine it more today.

I have ready your sticky and was planning on doing that this week. THANKS!

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

Here are some pics of the Exhaust Risers as I was taking things apart.

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

A few more shots... Lot's of "gook" in there...

My main question on this one is are the Risers suppose to look like that? It looks like, in the ring around the actual exhaust port, that they are filled in on the lower half to 2/3rds... is that normal?

Threw in a shot of the exhaust manifold as well...

I guess another question would be ARE these parts reusable, or should they be replaced, barring any obvious leaks, of course... They are about 20-years old now.. though the motor/boat only has 286 hours on it... some in salt water...

I'm heading out to the garage. I'll get some more pics to show a little later today.

 

[rodbolt]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

you got some serious nasty issues going on inside the block.
appears either air has been in the system,the engine coolent doesnt get changed every 3 years or both causing severe block corrosion.
may also wish to flush the heat exchanger freshwater side.
see all that nasty inside the riser. thats what causes the hot spots that lead to riser gasket failure.
its also why they want the risers off for flushing periodically.
it has been burning coolent by your own admission,thats what the white smoke and distinct sweet smell has been.
looks like the gaskets are the original merc versions that incorperate the block off plate in the gasket.
carefully clean the mating surfaces and check for flatness with a .004" feeler guage and a straight edge. any more than .004" vmeans surfacing time.
check your risers with a magnent, some were cast stainless. if cast stainless clean up and reuse if not I would opt for new.
use an automotive type flush on the freshwater side then flush according to the instructions then flush once more to be sure.

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

Once I get the intake installed again, I will run a full cooling system flush on the motor... I removed the heat exchanger and opened it up and it's very clean inside the fresh-water side... no blockage at all.

Here are some shots of the intake manifold and motor side. Clearly it's been leaking in places.

 

[Darin Jordan]

Re: 1990 Mercruiser 7.4L in Maxum 2400 SCR - Heat Riser issues?

By the looks of the starboard side riser, I think I'm just going to opt to replace these on both sides. I'll have the exhaust tanked and I'll leak check them. If they checkout, I'll likely reuse. Sure would like to find something MUCH lighter... can't believe how heavy this stuff is... but I know it's a boat and it has different requirements... the car racer in me wants some stainless headers and an aluminum intake!!

Also took the fresh-water pump out and it's all still there. I'll replace the impeller, obviously, but at least I know it didn't fail... just worn.


OK, so my next question... Are there preferred brands for these risers? I'd like it to be as direct a replacement as possible. Any recommendation for sources would be appreciated... I'm assuming that iBoats.com might be a good place to start??