Mercury Overheating?

I have a 1982 50hp "M50", in-line 4 cylinder. New water pump (impeller and entire pump housing replaced), rebuilt carbs. After I did ignition repairs (new trigger), I took the motor to a local (well-trusted) expert to set the timing and idle mixture.

The motor is running great, and peeing strong, but after about 15 minutes from initially starting up (cold) and then some hard running (pulling a tube) it will begin to lose power. Throttling back to 4k rpms and just running along it does fine. I noticed that it is "peeing" steam when warmed up and running at 4k. Returning to lower rpm it pees warm water. Working it hard after warm-up, the power loss occurs much more quickly, just a few minutes.

The tube from the cooling water port on the block to the pee-pee nipple is clear. Could there be an obstruction in the cooling passages?

The mechanic suggested replacing the (cheap) fuel line, so I got a new bulb and bought the "real metal" fitting for the engine end of the fuel line. The engine usually needs a few squeezes to get fuel for starting up, even if warm. Is there a way to test the fuel line? The tank end fitting is from Wal-Mart, could that be an issue?

Any thoughts on what to check next would be appreciated. Thanx in advance.

Re: Mercury Overheating?

Rebuild your popit valve.

Re: Mercury Overheating?

Is there a way to verify that the poppet valve is the issue? It's not clear to me from my (Seloc) manual what has to come off to get to the poppet valve or thermostat. Thanx

Re: Mercury Overheating?

What's your serial #? I don't believe an '84 50 hp 4-cyl has thermostat or poppet valve.

It's possible you may have obstructions in the water jacket which would require removal of the cover over the back of the block, or the exhaust manifold covers, for inspection. Another though is a slight exhaust leak which manifests itself at higher speeds/loads and is pressurizing the block such that it's trying to blow out all the cooling water.

The cyl block water jacket cover at the back of the block would be the least-intrusive cover to remove. I've seen 'em have silt/debris buildup such that it's halfway up the block. This definitely impedes water circulation! You can scrape/blow out the debris and then flush water up the water supply tube (with the lower unit removed).

You'll probably have to pull the powerhead to get at the exhaust manifold lower bolts. This is also a good time to check the water tube upper grommet, which can only be accessed with the powerhead off. If this grommet is squeezed partially shut, you'll get marginal flow and poor cooling.

But I'd pull the block cover first. Watch for any bolts that don't loosen right up, as they're probably corroded in place. You'll have to use a combination of heat (from propane or MAPP torch) and penetrant. PB Blaster is Good Stuff!

Just an afterthought, with the lower unit down, check the impeller for any "freak" failures. Also check the intake side of the gearcase for obstructions to water flow. Before tearing down the powerhead, connect a supply of water to the copper supply tube and let 'er rip. Any circulation issues should be evident. Water should flow very freely thru the powerhead and dump out the middle of the center section.

Your motor shouldn't ever get that warm even if it's run hard. I definitely wouldn't run it any more until you get to the bottom of the problem.

HTH & keep us posted on what you find........ed

Re: Mercury Overheating?

S/N is: 6223380

Compression checks at about 100 to 110 on all cylinders.

There is a cover on the (left)side of the block where the throttle linkage mounts. Is this the cover that I need to remove? It does look like the cowling is in the way of removing the lower bolts.

Is there any chance that this (complete tear-down) can be avoided by backflushing thru the cooling water outlet on the top of the block? Or from the intake side, by removing the lower unit?

Re: Mercury Overheating?

10 PSI diff in compression is getting on the marginal side for one of these. Are the lowest readings in the upper cylinders? Hopefully heat damage hasn't already occurred. A Seafoam decarbonizing treatment would be a good idea once you get things sorted out.

You might be able to gain something by backflushing. It sure can't hurt. Although if you do have a lot of sediment/corrosion buildup inside, it's probably firmly adhered to the metal. So don't expect miracles. It's worth a try, though.

Yes the exhaust manifold is the cover on the same side as the throttle arm. You'll have to unbolt the throttle arm and lay it out of the way to gain access to the cover bolts. Also the shifter arm will be in the way. If you remove the lower wrap-around and undo all the cowling mounting bolts, you may be able to pull the lower cowling down far enough to gain access to the lower bolts. But it's gonna be tight. More than likely you'll need to pull the powerhead.

If you get no results from backflushing (note this should be done with the lower unit removed so you don't flush a bunch of garbage into the pump), I'd pull the cyl block cover (the one over the spark plug holes). With the lower cowling dropped down a bit, you should have "easy" access to the lower bolts.

It's far more likely that if you've got buildup of crud in the engine, you'll find it in there (rather than exhaust manifold).

Anyway, have fun and Be Gentle with the bolts!...........ed

Re: Mercury Overheating?

Pulled the lower unit and hooked up a hose to the nipple on the block (pee hole outlet from the block). I didn't see anything come out the bottom other than water. I noticed that some water came thru the tube (where the pump output inserts), even more came down around the exhaust tube.

Running water the other way (up the tube) there was some backpressure (I was just holding the hose to it), water came from the pee outlet as well as down around the exhaust tube.

Could the motor be going lean enough to cause it to overheat? It needed a pump or two on the bulb to re-start after running, which makes me suspect a fuel issue. It seems like water is able to flow thru OK, and compression that is decent seems to negate a leak from exhaust into the water jacket. (or does it?)

More Info: The motor is on an old (1957) aluminum boat (Crestliner Voyager 18'). Prop is 10.3" diameter x 13 pitch. Lightly loaded and trimmed up a bit, it runs 5500 rpm and about 26-27 mph. When pulling a tube, I get about 5000 rpm. This is when I've seen the problem with losing power and peeing water + steam.

Thanx much for your advice and assistance. I'll keep posting as I learn more...

Re: Mercury Overheating?

It does sound like you're getting circulation thru the motor. One question, other than running full tilt, did the block and exhaust manifold stay relatively cool (or even lukewarm) or were they getting warmer than that. Especially at idle in the water; the block and manifold, in a cold body of water, should be almost stone cold. If the motor is ever getting so warm that you can't lay your hand on the block that's way too hot.

Since you have this on an older hull, I'm wondering how high the motor is mounted. Specifically, where is the anti-ventilation plate on the lower unit, in relation to the keel.

If the motor were mounted too high, you could start to run out of water and cause a lot of problems at higher speeds.

You know, a water pressure gage would sure tell a lot, wouldn't it? Might be a worthwhile addition. I'm thinking your motor probably has a plugged hole on the back cover for a fitting to run hose to a gage.

If you are having a water starvation problem due to motor height or hull design, I don't know if there's any other way you're gonna be able to detect that unless you have a pressure gage.

Just a thought. Diagnosing this type of unusual problem long-distance is so much different than being there! Lotsa possibilities but it's all going to boil down to one cause, ain't it!!!

HTH......ed

Re: Mercury Overheating?

The anti-ventilation plate is in line with the bottom of the hull, when trimmed up it can clear the water by a bit, but the issue has occurred when trimmed down and pulling a tube. At idle or even pulling along at 2k rpms, there is a nice strong stream of warm/hot water (you can hold your hand in it). We're in South Carolina, so the incoming water isn't cold to begin with, but the motor doesn't seem hot at idle. (Black motor in full sun is pretty hot without even running it tho !)

I don't know how likely it is that there is an exhaust leak into the water jacket, but this should show up immediately, so my next test will be to start up, go asap to open water and open up the throttle while someone watches the stream. And yes, the lower compresssion readings are on the upper cylinders.

Other stuff to try: Needing to pump the bulb to restart makes me think the fuel is having trouble flowing well. I have an in-line filter that I can pull out or replace with something larger. Pulling a tibe, it runs at 5k, so I'll drop two inches of pitch and try that as well. (But one thing at a time).

Re: Mercury Overheating?

Update: Pulled off the baffle plate cover (throttle side of engine). Gave each bolt a whack or two or three using a screwdriver and hammer to hit the head sideways, then a hit in-line with the bolt. They all came out OK, tho they were covered with chalky corrosion.

The baffle plate is sound and looks to be in good condition, but a big (thumb sized) ball of carbon fell out. I couldn't tell exactly where it came from but I think it was blocking or at least part of the top cylinder's exhaust, maybe the top two ports. Could this cause overheating?

Not sure if I've found the issue or need to keep snooping. I didn't find any major blockages on the "water side" of the baffle.

Re: Mercury Overheating?

If you think the carbon was enough to even partially plug the area, maybe. But a restricted exhaust would also cause the motor to be down on power.

Since you've torn into it this far, might as well pull the back cover too! Then you can run water up the supply tube and see if it enters the block with sufficient volume/force. I don't see any way of knowing for sure unless you do pull the cyl block cover.

It's a good sign that you were able to get the bolts off, though. Just be careful on the back bolts, they're 1/4-20 and easily busted.

Re: Mercury Overheating?

I snaked a wire (bent like a corkscrew on the business end) up the copper tube of the lower unit - got nothing. On a hunch, I grabbed the tube and yanked it out (it's just hanging in a rubber grommet). Sighting down it, I could see a blockage, which turned out to be a chunk of old impeller just big enough to hang in there.

BTW: The mercury man (dealer/service guy who's been working on them since forever) insisted that I wouldn't find anything of interest under the rear cover (what I would call the head). So that's why I didn't look there.

I also rebuilt the fuel pump of the top carb, I suspect it has been running lean. The top two cylinders had some scoring, but the rings were free and compression is good. I also re-plumbed the fuel line so that the top carb gets "favored".

Motor is running strong and peeing good at full throttle! Stream is VERY hot after a run, but not scalding, reverts to quite warm at idle. All seems to be well.

Thanx