Overheating help needed

Destroyer · #1 07-27-2015, 04:42 PM

Hi
About a year ago, I bought a used engine for a very good price to eventually use as a kicker on my boat. The engine serial number is A901218, it has a brown stripe cowl and as far as I can tell it's probably a 1986 vintage. It's an electric start, long shaft Mercury 9.8 HP engine and I just mounted it on my boat the other day, replacing my 7.5 hp engine. I started it for the first time just to check it out and make sure it was running ok. It started easily and runs great. Nice smooth, no backfires, etc.

BUT

The telltale stream is almost non-existent. At high rpm I get a nice stream, but as soon as I throttle back into what I would consider trolling range the telltale goes away and is quickly replaced by steam. Also, when I have the engine running at high rpm, the telltale that does come out is very very hot to the touch.

Thinking that it was probably a bad impeller, I took off the lower unit and looked at the impeller. It's in good shape, nice and flexible, not really worn. I then checked the intake for any blockages. The screen and pipe is crystal clean. No sign of corrosion or anything. So next I pulled the copper pipe feeding up to the block. Same thing, extra clean. Then I thought, ok, maybe it's a t-stat problem, so went looking for it and there is none. A t-stat was offered an an option for this engine for running in very cold waters, but normally there is none and looking at the place where it shows to be mounted, nothing is there.

So now I'm faced with the block or....? I'll replace the impeller tomorrow just to be sure, but like I said, it looks to be in really good shape.

Is there a poppet valve in this engine and if so where would it be? Also, is it ok to shoot high compressed air (120 PSIG) backwards through the block from where the telltale hose comes out in order to make sure that the block is clear?

I really don't want to pull the block and take it apart if I can avoid it, so any insight or help would be greatly appreciated.

phatdaddy · #2 07-27-2015, 07:21 PM

brighter bulbs will be along,d. but i'd change impellor & housing first. and use merc parts, not "will fits"

spareparts · #3 07-27-2015, 08:11 PM

Quote:

Originally Posted by phatdaddy

brighter bulbs will be along,d. but i'd change impellor & housing first. and use merc parts, not "will fits"

this

sunny717 · #4 07-27-2015, 08:23 PM

My impeller on my sailboat motor (9.9)rude looked good too, but it was spinning on it's bronze bearing. Make sure your motor is deep enough in the water to pick up water in the pump. I lube my impeller with Dawn liquid dishwashing detert. when I install.

Destroyer · #5 07-27-2015, 11:10 PM

My engine is replacing a short shaft 7.5, and it's a long shaft, so there,s no problem with not getting water. The EZ-IN bracket will lower it well below the anti-cavatation plate. Not worried about it since I'm not looking for speed.

The impeller that's in it right now and the housing are original mercury parts. Like I said, the impeller looks healthy, but I'll replace it just to be sure.

Spare.. I don't understand your post?

smokeonthewater · #6 07-28-2015, 06:20 AM

Spare's post is telling you be agrees with phat's post...

I do too... Replace the impeller and housing before we go any further.

IF you still have a problem then we'll dig in.

Destroyer · #7 07-28-2015, 02:41 PM

Quote:

Originally Posted by smokeonthewater

Spare's post is telling you be agrees with phat's post...

I do too... Replace the impeller and housing before we go any further.

IF you still have a problem then we'll dig in.

Thanks Smoke. That's what I thought he meant, but wasn't sure. A new genuine Mercury impeller kit (Impeller, key, o-ring and seals) is $17 off eBay (I bought 2) and they are expected to arrive on Friday. A new housing kit is $90 - $110 off eBay. There doesn't look to be anything wrong with the one in my engine. No cracks or leaks, no visible wear, seals all good. WHY are you guys saying replace the housing also?

And as to the rest of my post. Is it okay to blow compressed air backwards thru the telltale hose into the engine just to make sure everything's clear?. There's no t-stat or poppet valve in these engines, so really I don't think there's anything to impede the airflow (except a blockage if there is one). For that matter, is it okay to hook a garden hose up to the telltale hose and run low pressure water backwards thru the engine?

THEFERMANATOR · #8 07-28-2015, 03:25 PM

Pull the plugs out of the engine, hook a water hose up to the tube going up to the engine, and run water through it. See if wtaer gets into the cylinders, this wil ltell you right away if you have a corrosion or gasket issue. One of the 1st things I always do when a pump is in question is to hook a water hose up to it with the lower off, and run it that way.

phatdaddy · #9 07-28-2015, 04:21 PM

also noticed in the parts breakdown there are 2 different impellors & housing. you said impellor looks new, po might have tried to solve overheat issue with new impellor and got wrong size.

these are the prices i'm finding

89981 impellor 12.91
89980 impellor 20.34

70941a housing 59.19
92970a housing 31.35

these were on boats.net

Destroyer · #10 07-29-2015, 04:20 PM

Phat, looks like you win the overheat derby. I used my micrometer on the driveshaft and then my veneer calipers on the ID of the old Impeller. there's almost .020 slop. Seems Mercury used 2 different driveshafts on these engines and as you correctly called it that results in 2 different impellers and housings.
The PO obviously installed the wrong one. It's just enough that the pumping action just isn't there.There shouldn't be any slop between the shaft & impeller. There are two impellers that can fit that outboard. The 47-89981 has a .456 ID the 47-89980 has a .438 ID. The .456 is what I have, the .438 is what should be there.