Wednesday, April 17, 2013

Onan Generator Continued

Drift Away's little 7 1/2 KW generator runs like a top, but has an overheating issue.  You regular readers will recall that I tried the easy stuff, replacing the radiator cap and testing the thermostat.  On to the next step.

I read the Onan Master Service Manual, and could find nothing about how to flush out the generator's cooling system.  I was on my own, and that's never good.  I put on my work clothes (Pam would be proud) and shone a big light on the metal beast.  Inspecting the cooling system, it seemed to me like the most likely culprit would be the heat exchanger, as reader John of Augusta commented yesterday.   The heat exchanger is a jacket containing a bundle of narrow tubes that seawater flows through, cooling the "fresh water" that surrounds it, as the image below shows.

Dare I take a wrench to it?

This is always a monumental decision for me.  You know how it goes.  You have a dozen bolts to loosen.  They all come off with effort but one.  That one either won't come off, ever, as if it was welded on, or it snaps off with the gentlest of turning. Or maybe you drop a little part way down into the deepest inaccessible parts of the bilge.

We're headed out in a couple of weeks and want to anchor at Cumberland, so we might need this generator.

I decided to go for it.

In the photo above, the heat exchanger is the tube shaped gizmo with hoses connected to it.  It's purpose is to transfer heat from the anti-freeze (called "fresh water" for some reason) to salt water pumped into the exchanger.  

First, I removed the end cap to see what was inside.  It actually came off easily.  This is what was inside.

The bottom half is plugged solid with... something.  Salt?  Calcium?  Gunkola?  Whatever it is, I think this is the problem.  Not too much heat exchanging going on here.

I removed the hoses and unbolted the exchanger from the generator.  Not knowing what to do next, I put it in a tub of vinegar.  I let it soak for six hours or so.

While waiting, I searched the internet for other home remedies.  If vinegar doesn't do it, my next move might be a radiator shop.   Being a stubborn do-it-yourselfer sometimes, especially when I've come this far, I decided to press on.  I came up with Coke (removes rust) and toilet bowl cleaner (acidic).  I had both.  I decided to pour some toilet bowl cleaner into the mix since Coke is sugary.  It foamed a little.  Then I let it sit for a couple of more hours.

At 5 PM, I pulled the exchanger out of my cleaning solution.  The crud was soft.  I took a screwdriver and scraped at it and got most of it off.   Then I used a coat hanger to clean out the tubes.  This is what it looked like then.  Go back up a bit and look at what I started with.

Big difference!  Since Pam was working and I had to babysit the dogs (which actually means yelling at them every ten or fifteen minutes to stop barking at whomever happens to be walking by) I let it sit overnight.  Today, after showing the boat to a prospective buyer, I'll put it all back together and see what happens.


  1. I will bet you a victory beer that your problem is solved.That looked about 50% blocked off.Good job cleaning it up. Now if you can put it back together without any parts left over! Good Luck with the prospective buyer.

  2. Apparently, you missed my comment, yesterday that solved it for you...

    1. Sorry Tim, but I never saw your comment. I even looked in the blog's "awaiting moderation" file and there's nothing there. You suspected the heat exchanger too?

  3. No, no, I posted,

    "It is obvious to me, that your flibber-thwacky is clogged with your flacker-doodle.

    No worries. Just send me a check, whatever you can afford."

    I am apparently unable to post to google from my laptop. I have no idea, why. It shows me as logged in, etc. It even gave me the message that my post would be available after moderation.

    but hey, you go ahead and do an acid rinse on your heat exchanger. On mine I couldn't get to my flacker-doodle and the acid (white vinegar) soak/rinse worked well enough to compensate. It got me through another five or six years until I just bought a new one.


  4. Could you imagine if you had to pay a boat yard to fix the Overheating problem!!
    I'll bet that's the problem-- Only 50% cooling won't do.....

    Nice Blog Post-- Can't wait till the Bleaker Mountain Blog gets go'n!! I'm sure it will be a Hoot!!

    You'll figure it all out....

  5. A lot of that blockage sludge could be generations of dissolved zinc off the pencil zinc. Gray, crumbly stuff. Be sure to fit a new pencil zinc. Good luck. J