Engine Overheating


(Original message from Gordon Ellis gdellisjr@newyorklife.com requesting overheating info was not captured, but several responses are below.)


From: BrendanG@aol.com
Sent: Tuesday, July 03, 2001 6:41 AM
Subject: Re: Overheat Problem, '77 Islander 36 With Perkins 4.108

Gordon, If you have a water inlet strainer, check that. Another susceptible area is the inlet side of the cooling tubes for your heat exchanger. Remove the plate and clean the tubes. It is a 10 minute job. Are you getting good water flow out of your exhaust. How long before the motor overheats? Is it OK if you run it slow?

Larry Gotch

From: SCHUGO@aol.com [mailto:SCHUGO@aol.com]
Sent: Tuesday, July 03, 2001 10:06 AM
Subject: Re: Overheat Problem, '77 Islander 36 With Perkins 4.108

During a conversion of from Perkins to Yanmar, I discovered that stainless steel
wet exhaust muffler was constricting water flow either by backpressuring system or by reduced lift ability in exhaust so that Yanmar was overheating although the Perkins did not. (Yanmar has less gas exhaust but larger engine exhaust stack.)

In any case replacement of muffler with a fiberglass one dropped temp on Yanmar 20 degrees C, eliminated much of steam in exhaust, and greatly increased visible water flow without any other adjustments. I don't consider this technically logical, but I can now be less technical and cooler on that subject anyway.

Don Schumacher



From: John Isakson [mailto:jisakson@whidbey.net]
Sent: Tuesday, July 03, 2001 6:55 PM
Subject: Re: Overheat Problem, '77 Islander 36 With Perkins 4.108

On my 72 I36, I put in an Atomic Diesel a number of years ago. The year before last, I had a very similar overheating problem. I found that on the inlet end of the heat exchanger, pieces of the zincs would plug some of the tubes.

Overheating was a bit of a crap shoot in that various numbers of tubes would be plugged at each start. I removed the heat exchanger and removed the zinc pieces through the hole that the zinc is screwed into. Much to my surprise and further education, I found that
zincs are not eaten away from the end back, but from the outside inward until the only metallic zinc left is pencil thin that then it breaks off and into pieces. These pieces fit nicely into the tubes without really entering the tube to flow through. The result is a blocked tube. After I got all of the pieces out through the hole, things were back to normal and still are.

One thing I learned from all of this is that I replace my zinc after a shorter interval and before the metallic zinc gets too small in diameter.

John Isakson (jisakson@whidbey.net)


I had/have a similar problem, but was going to wait a few more weeks to provide a full report to the group. Anyway, given the similarity of our problems, here's my status as of today (my fix is 90% complete). This will be a fairly longgggg email, so those that aren't interested or hate long verse are forewarned.

Before getting too long winded, though, I would first emphasize a careful reinspection of the raw and fresh water cooling systems similar to the suggestions you're receiving from other members to ensure there is no blockage or restriction in the raw or fresh water systems. Then, depending on your answers to some of the following questions, I would wonder if your prop size is overloading the engine at the higher 1800 RPM. Although you should normally be able to achieve 1800 plus RPM without overloading the engine, you omitted some info that might help diagnose your problem.

1. Have you always experienced this RPM limitation since owning the boat?

2. If not, have you changed props or have any significant bottom growth? Marine animals in your intake thru hull?

3. Have you removed the raw water hose where it enters the exhaust system to check for restriction/scale build up in the metal exhaust fitting at this point?

4. How long does it take to overheat at 1800?

5. Do you get any black exhaust smoke at 1800? at 1500? I have come to realize from my mechanic this also means "faint" black smoke which can be very hard to see (my mechanic sees it; I have a hard time seeing it). Black smoke can indicate engine overloading. Another way to look for black smoke is to see if there is any black discoloration of the hull around your exhaust outlet.

6. You don't mention what speed you get at various RPMs, or the maximum RPM your engine achieves under load. For max RPM under load, you don't want to keep it there, but just slowly "floor it" UNDER LOAD (in gear) for no more than 10-15 seconds and see what RPM you can attain--being very sure NOT to exceed allowed (redline) RPM. I would perform this under way, and not at the dock because you have higher loads exerted at the dock. In any event, if you're max RPM is significantly less than rated intermittent max RPM, this might indicate prop size is a problem.

Now, a description of my original problem, although worse than yours, very similar:

I have an I-36 with a 1971 Perkins 4-107 and a fixed 2 bladed prop size 14x8 LH. My transmission is a Borg Warner Velvet Drive, with 1-to-1 engine-to-prop ratio. My maximum RPM under load is only 2100 RPM. My mechanic (Tom List in Sausalito--who I'd highly recommend) and the engine manual say maximum intermittent RPM (redline) should be approx. 3000 under load. After running the engine for 30 minutes under load at any rpm between 1200-1800, my engine starts to overheat. If I drop the RPMs down to 800-1000, the engine cools down. Obviously I go nowhere fast at these lower RPMs (3 knots with a following wind) and am not doing my engine any good.

I replaced my heat exchanger six months ago when the old one's cooling tubes corroded through. Although a necessary repair, this didn't solve my overheating. In doing some cheap "hit and miss" diagnosis, I started to replace the fresh water thermostat, and discovered my engine had no thermostat installed! Fearing real problems I gave up and ran to my mechanic. Although there are a few engines that will actually overheat
without a thermostat (i.e., Detroit), the Perkins is not supposed to be one of them.

My mechanic went through the entire cooling and exhaust systems, and also checked for a leaking cylinder head gasket (bubbles in the water header tank, water in the oil, or oil in the water header tank). Thankfully, he was sensitive to the effect on the check book and didn't just start replacing parts. In the end, he flushed the fresh water system, which removed some debris, and replaced the raw water impeller AND cam (which had a chip in it), and installed a 160 degree thermostat. This brought my water temp down to 160 to 170 degrees at 1800 RPM.

My mechanic was somewhat perplexed, though, because he didn't think any of the items he did were sufficient to cause the overheating I was experiencing. Upon reflection, though, I think the real key in my case was flushing out the fresh water side. My guess is there was really more crap flushed out than my mechanic realized, particularly since I had recently replaced my old heat exchanger that blew out at least one corroded tube. I do not think the new raw water impeller and cam appreciably changed the amount of raw water flow out my exhaust.

My current engine status and next steps follow. At 1800 RPM, I have faint black exhaust smoke due to overloading from my prop according to my mechanic, and achieve 6-6.5 knots in flat seas. I have a very hard time seeing it, but my mechanic doesn't. If I go above 1800 RPM, my engine temp will start to slowly climb, which my mechanic believes is the prop size. Also, I don't really get any higher speed with any higher RPM. Recall my max RPM under load is only 2100 RPM, and I have no problems seeing the telltale black exhaust smoke at 2100! Next steps--I will be hauling out next week to
repitch my prop, among other things. According to my mechanic's prop source, the correct prop for my engine/transmission configuration is a 12x6 if 3 blade, or a 12x7 or 8 if 2 blade. I'll probably go with the 12x8.

Unfortunately, given my engine is 30 years old, and probably vexed, I'll have to seriously consider repowering before I do any major repairs to this engine. Hopefully, that's still a few more years.

In a few more weeks I hope I send a very short follow up email stating my new max RPM, cruising RPM (targeting 2000-2200 RPM), and boat speed.

Best of luck solving your overheating problem.

Ralph Greenwood


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