150 Merc xr2 overheats at high rpms

[CU2NITE]

My 1985 150 merc xr2 overheats at high rpms. It runs at normal temp. when idleing and normal until about 3500 rpms then it shoots up to 195 at full throttle. I have changed the impeller and houseing, also replaced stats, and head gasket, and the compression is good 125 in all. The only thing I saw that concerned me was I changed all plugs and after I ran the boat for an hour I checked them and they all looked the same except #3 had a lot of oil build up on it, the others were fairly clear. The boat runs great, idles at 800 rpms and full 6000rpms at 55 mph. Could this be a problem with my carb like a plugged high speed jet? ALso I get good water pressure out of pisser. If I throttle up in nuetral the temp goes down. Any reply would be greatly apreciated.<br />I love boating!!!!!

 

[grandx]

Re: 150 Merc xr2 overheats at high rpms

I run a pair of carbed mercs, and mine and others run warm. Mine generally when trolling at 2500 rpm with a single engine. 6000 rpm at WOT, I believe is too high, if I'm not mistaken your engine should be 5000-5600 rpm at WOT, check into this, you may need to re prop to achieve the proper rpm.. I think you should drop about 200 rpm with another 1" of pitch.. if running a 19 pitch prop try a 21 etc, this should drop your max RPM and probably get a few more MPH. The oily plug does not sound to me like a jet problem, looks like its getting fuel, just not burning it all. A little oil on the plugs is normal. You may be running a little lean on one or more of the other 5 cylinders that show no oil on the plugs (look for a jet problem on those)... A lean condition would cause more heat. No alarm at 195 f, right. Look for more responses, they'll come.<br /><br />Just my 2 cents, I'm going to borrow a line from Schematic "take it with a grain of salt, heck i've never even seen your engine"

 

[CU2NITE]

Re: 150 Merc xr2 overheats at high rpms

Thankyou for your response. I checked my clymer manual and wot should be 5500-6000 so I think I'm ok there. I don't get the horn and I did check to se if it was working. How hot does your mercs run? How hot is to hot? 195 scares me! Do carbs get out of ajustment on their own? I did change the power pack recently would this have anything to with the carbS? Also how long does it take for new plugs to get stained with oil? I certainly don't want to run lean and cause myself big trouble. The old plugs were all stained pretty good, some worse than others. <br />Wow what a great site!!!

 

[jim dozier]

Re: 150 Merc xr2 overheats at high rpms

You and I are having a similar problem, although with different engines. I copied the following from another post of mine responding to a guy with a similar problem with the same engine as mine (1987 Suzuki DT85).<br /><br />Fish4970 I am having and have had problems with overheating at WOT on this engine. I had replaced the oil hoses while I had the powerhead off to replace a leaking engine holder gasket (which is beneath the power head gasket). This engine runs at about 185 degrees F. at 1/2 to 7/8 throttle but at WOT it creeps up to 200+ degrees F. and will set off the alarm (which by the way has been replaced) after 3 or 4 minutes. The engine runs pretty good otherwise. In an effort to fix this I have:<br />1. Replaced water pump impeller (twice). It overheated quickly when I bought it and replacing the water pump the first time really improved it, but eventually I noticed the WOT problem.<br />2. Replaced thermostat. Even tried it for a while without thermostat.<br />3. After consulting a Suzuki mechanic I replaced the head gasket with a newer OEM version and cleaned out the clogs in the head cooling passages.<br />4. Installed temp gauge.<br />5. Pulled the powerhead and replaced the exhaust cover gaskets and cleaned exhaust cooling passages.<br />6. Replaced temp sensor. This seemed to work for a while but the problem either returned or was still there and I just couldn't tell right away. <br />7. I should have replaced the engine holder gasket the first time I had the powerhead off but one mechanic said I wouldn't have to. He was wrong.<br />8. My other mechanic asked if I had properly installed the water bypass valve when I had the head off. I did but I am wondering if maybe the spring on the bypass valve is old and the valve is leaking too much at high speed. It is the only thing left in the cooling system that hasn't been replaced.<br /><br />This engine pees like a fire hose and I have I think eliminated every other possibility. I think I will remove the head one more time, have a machine shop plane it flat, and replace the bypass valve and spring and hope for the best. <br /><br />I may (emhasis on maybe) have finally fixed the problem. I backed off (retarded) the max ignition advance by about 1/16 inch on the mark. I don't know how many degrees that is, I have not put the timing light on it yet. I had not done this previosly because the max advance was visually right on the mark. Anyway the engine temp does not creep up past 185-195 degrees F. anymore and I was able to blast around at WOT all day. I don't have a tach or speedometer but I didn't notice any difference in top end speed or rpm. Hope this was it, time will tell.

 

[grandx]

Re: 150 Merc xr2 overheats at high rpms

CU,<br />My mercs are 96 models with 5-56K RPM, thats where I got that from. From everything I've heard from many other carbed V6 merc guys is that "they run hot". I'm same as you, in certain conditions I watch my temp gage rise to 195 F, mine cools down at idle or WOT 5300 rpm just gets warm at 2500 or so when (1) engine is pushing the boat for trolling (7mph). For my 96 models "alarm to sound at 240F and stop at 210F... I've kinda learned to live with it... if alarm doesn't sound till 240 must not be serious at 195, right? I have a clymer and a Merc manual for my pair. Clymer says engine should never exceed 20 F over thermo (165 or so). Merc manual says nothing except when horn sounds... At high rpm's the poppet valve controls water flow more than the thermo, you may have a look into the poppet if your engine is equiped with one... I suspect you'll find it's ok also. Let me know if you find anything interesting, sorry can't be of any more help. <br />Just a small note on the previous post about the suzi, if Jim fixed his problem you can't really argue, but generally speaking.. if you "retard your ignition timing (from in spec), you get some less power and more heat..." at least that the way I understand it, but if it worked what can you say, ignition timing can be a bit complicated maybe he found the "sweet spot for his engine"?

 

[Kenny Bush]

Re: 150 Merc xr2 overheats at high rpms

You may want to retard Max advance timing about 3 degrees. This will knock the heat off a little. You may lose 100 or 200 RPMs but at least you won't scour the piston/cylinder. Retarding the timing will not increase combustion temperature.

 

[jdkzrt]

Re: 150 Merc xr2 overheats at high rpms

My opinion (and I also have an XR2 that used to run hot)---<br />Install a transom mounted water pick up. You have no worries at high trim angles. The faster you go, the more pressure you have. Flushing and running on the trailer is a breeze. <br />It worked for me.

 

[grandx]

Re: 150 Merc xr2 overheats at high rpms

I'm not going to get into great detail, as I'll probably stick my foot in my mouth. <br />I agree that retarding timing won't increase combustion temp (2200 F), just don't know how much it'll reduce it, but the engine temp is only 200 F. <br />I think it's a balancing act of power/heat/and heat removal from the engine... You wouldn't want to retard the timing so much that your burning fuel that is not contributing to pushing the piston down and just heating exhaust ports. There's got to be a perfect timing for any given condition, and I'll admit that I thought that was probably the manufacturers max advance +0 -1 degree (as long as there was no pinging). I'll retard mine a bit to see if it helps. I do think these Mercs have a problem with removing the heat, hence JDK's additional water pick up that solved his problems. I've asked my Merc rep, and he said if there's no alarm sounding I'd run em... I run em, but it still bothers me and I take a peek at every hot Merc post looking for an answer.

 

[jim dozier]

Re: 150 Merc xr2 overheats at high rpms

I ended up retarding my Suzuki 3 degrees, from 21 down to 18 degress BTDC. I did not notice any drop in performance, but I don't have a tach and it probably dropped a 100 rpm and I just can't tell. In general, retarding ignition will reduce temp somewhat (Old timers may recall that the number 3 cylinder on VW air cooled engines was retarded a couple of degrees relative to the other 3 cylinders because the oil cooler was in the way of the air flow and it tended to run hotter). It is a balance, too much and performance will suffer. I view this as a temporary fix. I am still wondering why my Suzuki overheats. I have been through the entire system and the only thing left is to replace the spring loaded bypass valve in the head on the outside chance that the spring is old and because of multiple overheating episodes has lost some of its spunk.

 

[CU2NITE]

Re: 150 Merc xr2 overheats at high rpms

JDK whats a transom mount water pik? It sounds like I need one of those. How expensive are they, and where can I get one? Thankyou all for your responses! GRANDX I will check out my poppet.

 

[grandx]

Re: 150 Merc xr2 overheats at high rpms

CU, When you fix it, don't be shy, let me know what did it.

 

[CU2NITE]

Re: 150 Merc xr2 overheats at high rpms

Grandx I'll post my results as soon as I find something. Thanks for all your information. I hope I get a reply with information on that transom mount water pick up. That sounds like a great deal. <br />Can't wait to get back on the water!!

 

[jdkzrt]

Re: 150 Merc xr2 overheats at high rpms

CU - my boat is at my cabin, so I can't post a pic for a while, but the pick up is an aluminum bracket bolted to the transom. This bracket holds an adjustable (slides up and down) water pick-up tube. This ube is basically a piece of 1" x 1" square aluminun tubing, slash cut at the bottom, that scoops in water that runs along the bottom of the hull (it protruded below the bottom of the hull at most 1/2").<br /> The top of the pick-up has a fitting to attach a piece of hose (I use 5/8" garden type hose). This hose is looped up to near the steering cables, then back down to the water pick up fitting in the gearcase.<br /> This hose must be long enough and tied off to not interfere (or become kinked) when steering or tilting the engine.<br /> I bought the pick up at a performance shop (since closed) about 12 years ago. Worked fine ever since.<br />

 

[moderator1]

Re: 150 Merc xr2 overheats at high rpms

Moving to Mercury questions.

 

[CU2NITE]

Re: 150 Merc xr2 overheats at high rpms

Just an update on my overheat problem. I still have it. I changed my poppet valve and even cut off a bit off the spring. (heard of that in another post) but no luck, still shoots up to 195 at full throttle. It runs great at half speed. I think it might just be the hot weather and the warm water. I think I'll look further into the water pick up attachment. Thank you all for the info and advice. This is a great site!!<br />CU

 

[CU2NITE]

Re: 150 Merc xr2 overheats at high rpms

Another update on this overheating problem. I took the water jacket off the head to check for obstruction and found a small rock blocking one of the water passages right next to the temp sender. Ran it on ears and noticed stronger tell tale. I have a feeling I might have found the culprit. As soon as I run it on the lake I'll post my results.<br />CU