Re: Merc 140 - Problem 1 - Engine Overheats at High RPM
The saga continues, and I still need your respected advice...<br /><br />So, my 1974 Merc 140 overheats. It was holding 135-145 degF up to 1500 rpm, overheating slowly below 2500 rpm or so, and quickly above 3000 rpm or so, but on Labor Day it started to overheat slowly even at idle. I am trying to diagnose and fix all overheating problems. Here is my description of the raw water cooling system, and what I have done so far:<br /><br />The stern drive inlets are clear. I replaced the stern drive water pump (impeller, upper housing, base, gaskets) 3 weeks ago. I was careful in mating the copper tube into the plastic guide tube.<br /><br />I have inspected the raw water delivery hose that is visible underneath the port side of the upper drive housing (tilted up all the way). It is in good shape, and all clamps are tight.<br /><br />The hose that goes from the transom over the top of the engine to the t-stat housing is in good shape. I pulled it from the t-stat housing, and with earmuffs got strong flow at idle, and up to 3400 rpm (momentarily). In the lake, at idle, I don't get quite as much flow (no garden hose pressure), but it was still good flow at idle. This does not prove that I have good raw water flow at high rpm with hot exhaust going through the stern drive--more on this later.<br /><br />The t-stat is the correct model and I have tested it on the stove. I have run the boat in the lake without the t-stat. Other than running about 5 degF cooler at each rpm, the engine still overheats in the same manner. The problem is not in the t-stat.<br /><br />Today, I pulled the upper and lower t-stat housings. Their inner surfaces are like ALL inner surfaces I found throughout the system--rust colored, but no real scale or rust buildup. The passages were wide open.<br /><br />I looked into the passage in the head, and stuck my fingers in as far as I could. It looks very good (rebuilt head was put on about 4 years ago).<br /><br />Then I pulled the engine circulating water pump. It looks very good. The impeller vanes are rust colored, but they are still thick (not corroded away), and the pump turns freely with no wobble. The seals appear to be good (I've never had any leakage).<br /><br />I looked into the passage in the block, and stuck my fingers in as far as I could. It looks very good, rust colored but wide open.<br /><br />*** There is no way I can tell if there is a blockage within the block or head. With the front housings off, does anyone ever seal a garden hose to the block opening (duct tape or something), and run it full blast to see if there is flow out the head opening? Or how else could I test it, as long as it's open now? ***<br /><br />Note: I took my alternator upper bracket bolt out, but the darn thing won't pivot at all. Suggestions? I sprayed the pivot bolt as best I could with WD-40. Not much space in that part of the bilge.<br /><br />I then pulled the manifold/riser/boot/carb. I eventually got the riser off the manifold. They both look very good. I ran the garden hose full blast through each of the riser passages, and got good flow to the elbow side of the riser. I ran the garden hose full blast through each of the manifold passages (that go to the riser--I ran the water backwards to check each passage), and got good flow out the riser top feed hole. I don't think these are plugged at all.<br /><br />The rubber boot looks very good. No melting, no blockage.<br /><br />The rubber exhaust flapper is gone. Surprise (1974). I cannot see it looking down the exhaust elbow with a flashlight. A mechanic told me Merc flappers generally break in half and blow through (unlike OMC flappers). I ran the garden hose full blast into the elbow, and the water streams out the prop center, and some out the two bypass holes at the base of the gimbal housing.<br /><br />The large rubber exhaust bellows that go from the transom to the stern drive are in good shape.<br /><br />That is the entire cooling system. I have found nothing wrong. Here are my thoughts of what could be causing the overheating:<br /><br />1. Blockage within the engine block or head. UNLIKELY, I think. How do I check this withough pulling the head?<br /><br />2. Exhaust flapper clogging lower part of elbow. UNLIKELY, I think. How do I check this without pulling the stern drive upper (drive shaft) housing?<br /><br />3. Stern drive water pocket cover warped, could be leaking water/steam at high rpm when hot exhaust blows on it. LIKELY, I think. A mechanic mentioned this to me about a month ago. Also, as posted above by akriverrat, see the discussion at:
http://www.marinemechanic.com/site/page150.html <br /><br />I have a little time before continuing on this, so your advice would be GREATLY appreciated. I have to go out of town on business most of this week; when I get back, my gasket kits should be here, so I can start putting everything back together again.<br /><br />I have a minor question about reassembly. Threads that are involved in a water seal, like the water pocket screws, get coated with Quicksilver Perfect Seal (gasket compound). I've been told I can put Quicksilver Special Lube 101 (lube with Teflon) on the threads that mount onto the engine (manifold studs and bolts, circ pump bolts, t-stat housing bolts, etc.). Is this a good lube for that, i.e. does it act as a good anti-seize in a hot environment?<br /><br />Thanks in advance for your ideas and suggestions.<br /><br />Steve
