1991 200 Merc 2.5L Carb - Overheat at mid-range

I have been fighting this issue for 4 years with limited success.

If cruising pre-plane, and anywhere up to 3000rpm on plane, will overheat in 1-2 minutes. I can idle all day long, run 3500-4000-5000 rpm all day long without issue (can run harder all the way to the gas station, LOL). As soon as I try to take my little girl tubing at 25mph, BEEEEEEEEEEEEPPPP.

I believe I'm propped well, running 21" standard 3 blade, getting almost exactly 1mph per every 100rpm, tops out at 57mph at 5400ish rpm.

I've replaced impellor, t-stats, have checked that pickup tube fits securely into housing grommet, screens are clean, cavity in divider plate and below is clean, replaced poppet assembly last year (this actually helped a lot, but did not solve the issue). No visible cracks in housing. Gaskets are new with the impellor.

Have read quite a few threads indicating possible blockages below the power head. Is this my next step? Seems logical if it is...there's nothing left to replace!

No...I don't have a pressure gauge on cooling system...sorry. But heat gauge is calibrated correctly, motor runs mid-gauge at 4000rpm, no issue. As soon as revs drop below 3000, even while still on plane, temp creeps up and eventually will prompt alarm when gauge tops out. Idling brings temp back down to mid-gauge.

I bought this damn thing cause I've wanted a Black Max since I was a kid and our cottage neighbour put a 1983 225 Black Max on his 17ft family runabout...I loved hearing that thing roar past us.

Help me restore the faith. I have a buddy who's a former Merc tech who's willing to help...he already did a big job on it when I had to replace the steering pivot assembly when I first bought it. He's scratching his head too.

Cheers,

Brian

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

Brian, I would recommend you inspect the waterpump cover for scratches and the inpeller for wear. If you find any replace it. If not, sometimes the waterpump bases warp a bit, and need to be replaced.

Other than that, maybe the prop is a bit tall. Can you try a 19" pitch prop and see if it improves?

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

Wow- I could have written this myself. My 1993 Merc 150 XRi was doing the EXACT same thing. Fine at idle, fine at high speeds but anywhere in mid-range and POOF- overheat. I had done all the stuff as well. However, I never had a pressure gauge until two weeks ago. I installed the gauge and took the boat out. 2 psi at idle, 12 psi at full blast up and running. But it was all over the place anywhere in between. Sometimes good, most times not. I went back home, put a new water pump kit in it (full kit, upper and lower housing, etc.) and tried it again. Success! Water pressure was exactly where it was supposed to be and the temp gauge never went over 160. My water pump was 2 years old with maybe 25 hours of runtime on it, but the vanes had taken a set as usual. It didn't look to be excessive, the sleeve was perfect, the vanes were not visibly worn. But it wasn't doing its job. I can now say that my engine is running as it should. I had a defective t-stat that I found a month ago but that wasn't it. It turned out it was the pump even though it was only 2 years old.
Spend $50, get a pressure gauge and even if you only use it as a diagnostic tool it will help pinpoint the issue. I never had one before but now am a firm believer in their importance. There is a great chart with the pressure situations you will encounter and they are invaluable for finding your issue.

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

On the 2.0 - 2.5 liter V6 mercs the water temp varies quite a bit within normal operation, but the water pressure is pretty predictable compared to RPMS. It also varies, but in a predictable pattern. If you know the pattern, one pass while watching the pressure gauge and you can probably diagnose any cooling system fault.

So never mind the condescending "sorry", go get the recommended water pressure gauge.

BTW, I'm betting on a combination of scored base plate and too tall a prop. If it won't turn over 6000 without pulling a skier, the prop pitch is too high for pulling.

On my XR4, I change props when I'm pulling on a rope. Just common sense to properly load the engine.

hope it helps
John

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

j_martin;3470352 I'm betting on a combination of scored base plate and too tall a prop. If it won't turn over 6000 without pulling a skier said:

Isn't it mean to take his money on such a sure bet?

He might need it to buy that second prop!

Click to expand...

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

John, have to agree with you on the prop suggestion. If I pulled the kids with my 19 Tempest I ran into issues faster than if I changed to my 13 Mirage. Granted, the 13 is waaaaay low but it came with the boat and is a pulling beast! But the overheat came on much slower with the Mirage as it wasn't loading the engine nearly as much as the 19 did.
For what it's worth, there was zero evidence of wear in my pump assembly other than the set in the vanes. I've been told by several people who "know" Mercs and they have all told me that the 150 were particularly temperamental where water temps are concerned. Don't know if that is true or one of those old wive's tales but mine sure was persnickety. My old 50hp would run for days with so much as a hiccup despite the conditions but my XRi has been much more attention-intensive. Great engine and it runs like a scalded dog but she defintely needs more love. LOL

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

What type of boat is this on??

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

We're chasing ghosts till you get a water pressure gauge installed and read it to us.

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

Ignition timing arm must be spot on, no sticking or binding anywhere in its travel, some motors like just a wee bit more air gap in the throttle pick-up to the carbs, it gives a couple degrees timing retard until near full throttle, but be careful, too much air gap retarding timing too much and it will also cause mid-range heating.

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

Hi guys, thanks for all of the replies to the issue. Great bunch of fellas you are.

So...water press gauge...how to install

Boat is a 1990 Malibu 175 Closed deck runabout. Surprisingly well-matched with the big V6, though 150hp would be lots.

I looked at the impellor wear plate...not grooved or anything, just kinda polished where the impellor turns on it. I have a hard time believing that it has anything to do with the impellor housing, but, without pressure gauge...i really couldnt tell ya. i know the pee stream isnt a garden-hose of pressured water coming out...i think its pretty weak for an engine this size with that huge vane pump in it.

i can get the revs up to about 5800 if i really let er dangle down a straightaway but the hulls centerline is very round at the back and she really starts to chine walk at about 57mph so i dont hold it there long. thats with the plain jane 21 inch aluminum prop. this boat cruises beautifully at 3500-4000rpm, i dont really want to jump down to 19 inch if i dont have to

so...water pressure gauge first...if pressure no good, do i replace water pump housing complete, then if thats not it, tear off the powerhead and see if i have a blockage or debris or a bad gasket at the divider

cheers guys...sorry for the terrible grammar...crapppy new laptop and i cant figure out the keyboard for some characters

PS i want to deal this boat. looking at 1993 Baja islander with 4.3 205hp. love it...but it needs interior done so trying to hammer the price down.

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

First. All the water that goes out the tell-tale goes through the thermostats first. It's theoretically possible to have a dripping tell-tale and a perfectly running cooling system. (real cold water, light load on engine) What the tell-tale is useful for is to let you know when your cold started engine is warm enough to go full bore without sticking with cold seize.

What you look at after you install the pressure gauge depends on what the gauge says at different engine speeds. Pressure could be high or low at different times.

If it chine walks at 57, your steering is defective (loose) or deficient (wrong steering, like single cable.) 150 ponies and speeds over 50 require rock solid steering, either dual rack properly adjusted, or hydraulic with no air in it (slop). Engine mounts have some bearing on it also. Running some boats on pad requires finesse steering, which requires real tight steering and a bunch of seat time. Usually these are 80 mph or faster boats. I run a 150 and Morse dual rack NFB steering on a hull quite similar to your Malibu, though it's a bass boat. In smooth water or a light chop I can let go of the steering completely at 57.

hope it helps
John

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

So...here I am, several months later. I gave up in the fall despite all of your tips...too busy. So this month I replaced the whole water pump and housing assembly...it was only 50 bucks so it was a no-brainer to me. When I fired up the engine and let it get up to temp in my driveway at mid-idle, I went over every inch of the motor very slowly and carefully. I found a trickle of water coming from the ports where the steering yoke clamps enter the top of the mid-section of the motor. This tells my feeble mind that I have cooling water escaping somewhere below the powerhead. I have gone through the trouble of tearing the engine apart and have the powerhead off. There are two plates between the powerhead and mid-section assembly, separated by gaskets. The gasket between the powerhead and the exhaust manifold plate came off in several pieces when I pulled the powerhead with the hoist. The gasket between the exhaust manifold plate and the mid-section plate is completely intact with no signs of damage. This is the area that I would have suspected I have a problem, if the gasket was not intact. What I find strange is how the pickup-tube seal in the lower plate mates with the casting in the exhaust manifold plate...should there not be some kind of sealing surface here to prevent pressure loss and water escaping?

As for all the replies from the fall re: water pressure gauge...I need some help...where do I get it, what type, and where on the engine does it get installed? I would have hooked it up to the telltale hose but maybe that's not the place?

Re: 1991 200 Merc 2.5L Carb - Overheat at mid-range

The exhaust adaptor plate gaskets under the motor are very important in cooling as a leak here will allow coolling water to by-pass the motor completely. As will a loose fitting water transfer tube grommet into the adaptor plate.