OIL CHANGE FOR AN UNDERMOUNTED, CONSTRICTED ONAN 2500 LP GENERATOR

There are plenty of short informative vids and write-ups on how to change an Onan generator's oil, but almost all of them entail side-mounted units which are easily accessed and hassle-free.  The Wendlands (Roadtreking) published this post on how to check the oil on an undermounted unit, but stopped short of showing how to change that oil, unless I missed another post.

Our particular 2007 Airstream Interstate has an unusually tight lay-out for two reasons.  Number one, it's not built on an extended ("EXT") Sprinter model, so it's shorter than many Class Bs, including most newer Interstates.  And number two, it has an under-mounted spare tire that impinges on the generator space.  For these two reasons, I thought I'd throw out some oil change tips, while not repeating the entire set of details that have been published by others.  You might want to see Tito's YouTube as a general primer for this task before continuing to read the account given below.

Note that our unit is a 10-year-old Onan 2500 propane (not gasoline) generator.  Apparently the modern-day 2500 is either gasoline or propane-powered, but ours only accepts propane.

It's also very dirty in this pic.  No sense washing it until after the oil change is done. 

First of all, let me say that this job is approximately one hundred times easier if you can ramp your vehicle.  It's extremely difficult to work under the chassis with the Interstate being so low to the ground.

These are the Interstate-specific ramps I built using wood I that picked from trash piles as our subdivision perimeter fence was being replaced by a contractor.  Blog post with instructions here. 

It's also kinda neat if you can have a friend keep you company as you do this work.

Just as I got very comfortable working underneath our Interstate, so too did our dog. 

Because I wanted the genset to be at the low end of the vehicle (which I thought might help with used oil drainage), I had to do what I call a "backwards foursie" - all four wheels up on ramps, mounting them in reverse (the configuration of our engine exhaust pipe does not permit rear wheel ramp drive-on).  The Sprinter does not like that maneuver because it tends to confuse the anti-lock braking system (ABS).  But it can be done if one is careful (see DISCLAIMER).

The oil drain plug is easy to locate (see Tito's video linked above).  There's no way around the need for a 9/16ths socket for this job, as Tito says.  No other tool will substitute because the plug is recessed.

I've got the socket around the plug, but here's an important point below. 

Whether you are draining the oil out of your genset or draining the oil out of your Sprinter engine, don't let 'er rip, as they say.  Use the socket to loosen the bolt, until you begin to see this.

It's now loose enough so that oil begins to leak past the threads.  

From that point, you should be able to unthread it the rest of the way using your fingers.  This prevents your socket wrench from getting drowned in oil when it starts cascading out.

Bolt removed, let drain 5 or 10 minutes into the oil pan below.  It will take a while.  

Inspect the plug for wear or damage.

Looks good to me. 

You'll notice that the oil above looks very clear, like it's brand new.  That's because we haven't used our generator in the almost 3 years we've owned our rig.  But we recently switched out our old propane refrigerator for a Vitrifrigo 2-way electric model, which means that the pressure is off our tiny propane tank.  No longer needing to conserve propane for the refrigerator while boondocking, we decided that we'd go ahead and try using the genset in certain circumstances - we might as well, because it's already there, it works, and in fact it's almost in new condition despite its age, because it was never really run.  So, if we are now planning to use it, that means that we need to do the required maintenance tasks.  Three-year-old oil should be changed regardless of how many "miles" (or hours) are on it.

Now here comes the tricky part.

This (yellow funnel) is not necessarily the best way to get new oil back into the genset.

After you've let everything drain and have replaced the drain plug, it's time to put the new oil in.  But both the spare tire and the chassis block good access to the fill port.  Even if I had dropped the spare tire, the frame for the tire mount would still have been in the way.  I had my husband hold this funnel while I tried to maneuver the quart of oil around to get a good fill, but I still managed to spill a bit of it down his arm.

They do make a small pump (something akin to a jumbo hypodermic needle with a flexible tip) that can be used for this sort of thing - I didn't know that until after we finished the job.  A simpler method might be to pour successive fractions of the quart of oil into a smaller, shorter container, say about a cup or two in size, and then lift that up to the funnel successively until the entire quart is transferred.  It's bound to be a bit messy, so make sure you have a tarp under your vehicle.

And speaking of new oil, what kind should be used?

Our Onan users manual says very little about the grade, other than it should be SAE 30.  But we decided to use this.

This formulation is de rigueur in my old near-north stomping grounds where freezing temperatures prevail for much of the year, but it's so rare where I live now that Rotella was the only brand I found that was even sold in a quart (consumer) size, given that it almost never freezes here in the Deep South. 

We now have a lithium battery system, and lithium batteries cannot be recharged in below-freezing temperatures.

From Google. 

Therefore, my husband wanted to cover a "just in case" boondocking scenario in which we couldn't charge the batteries, but still needed a source of power.  Obviously, then, the genset should contain a type of oil that is suitable for use below freezing.  Hence the 15W-40.

Image from Onan's (Cummins') generator handbook (PDF link).  

If you research this oil issue on the internet, you might find references to the use of "non-detergent" oils in generators.  My husband looked into that issue, and basically concluded that, as long as the Onan reservoir was not over-filled, foaming should not be a problem and a regular oil could be used.

Speaking of filling, I also read (as in, RTFM) that the dipstick should not be screwed in when reading the oil level.

The dipstick is also the reservoir cap.  Sorry for the blurry pic.  It's challenging to take pics one-handed under the chassis. 

In other words, for measuring purposes, you're just supposed to plunge it in, and read what's on the stick, without tightening it, which lowers the stick more deeply into the reservoir.  This Onan 2500 takes exactly one quart of oil, and after we got done re-filling, the stick read slightly higher than a quart, even given the amount that I lost by spilling it down my husband's forearm.  However, the tail of our vehicle was dipping backwards by a couple of degrees due to being "foursie ramped" in our sloping driveway, so the stick was probably reading higher than actual.  According to internet reports, as long as you don't add more than one quart to this generator model, you should be OK.  If there is too much oil, the generator will shut down and may start "foaming".

After you get the dipstick replaced, it's time to clean the outside of the generator thoroughly.  That way if oil starts coming out of any given orifice, you'll be able to see it.  Go back six or eight hours later and examine the drain plug for evidence of slow leaks.

See, now the underside is shiny because I cleaned off all the dirt.  That's the drain plug in the lower center-ish part of the photo.  No drips are visible. 

And there you have it - one more money-saving DIY task.  And not a very difficult one.

That way you can spend it on some other aspect of your Interstate, hahahah.