RV Doctor – Oil Won’t Drain When Changing Oil in Generator

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March 1, 2009

Dear RV Doctor,

We just purchased an RV with an Onan Marquis generator. We went to change the oil for the first time and after warming up the generator we could not get the oil to drain any faster than a drop at a time. We opened up the drain valve and unscrewed the filler cap. Other than that we’re not sure what to do.

Kandie Nield, (Hagerstown, MD)


Kandie, I hate to be the one to ask this, but are you absolutely sure the crankcase is not simply empty or very low on oil now? I’d suggest pouring in about a half a quart, leaving the drain valve open and see if the new oil immediately drains. If it does so, that generator was quite low on oil when you went to change it. If the new oil, however, does not drain normally, then you apparently have a blockage inside the crankcase or in the drain valve itself.

You can try using a flashlight and mirror to see if any blockage is visible. If not, it may necessitate having the generator removed from the coach and the oil pan removed for a thorough cleaning.

But first pour a quart of cleaning solvent or very light oil into the crankcase and stir it around with a screwdriver reaching through the dipstick/cap cover. Perhaps that will dislodge or dilute any sludge blocking the drain opening. You may even try to snake a flexible wire up through the drain valve in case the blockage is only located there.

Be sure to flush it thoroughly and to fully drain everything in the crankcase before adding fresh oil. Don’t forget to replace the oil filter as well!

(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)

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  1. gmas

    Wow… its not that bad… as I doubt you would scrape anything enough to plug a port. But, I don’t think I would use air pressure in the crankcase as it would or could quite possiably blow the seals out… Most engines can’t stand crankcase pressures much more than 5 to 9 lbs… so if you pressurize the crank case… you would need more pressure than what it could stand to blow anything out. Using air to blow the other way… into the valve… would only then put the defect into the crankcase…

    No I think I would leave the air hose alone on that one.

    Indeed contacting the manufacture is a good idea after you check all the external issues that could not be a part of his product. Indeed good idea to have a certified service center do the work if its still new and under warrentee… that should be the first stop in the chain of events too…

    I also recommend that you seek out some of the oils which have the anti-galling extream pressure/temps.

    Here we find that the aircraft oil.. works for air cooled enigne… from phillips XC seem to work the best… the 20w-50 XC is what we have used and now have gotten over 2000 hrs out of one onan genset… and it never has been apart yet… but, the key to long life is clean oil.. with the filter on the engine.. we change our oil at the 50 hour interval.. along with the filter… or about once a month… that way if the oil gets low at the time.. we don’t add.. we just drain and refill… works for us…

    After 100 hrs our genset then quit burning oil… (got down to where you can’t see a loss) and we considered it broke in… we keep tabs on the oil useage as it is a good indicator as to the engines health. We also split open the filters and examine them for any material… sending out a sample to the local oil analysis every 100-200 hrs…is also a good deal to determine its health and longitivity… all of which are done for aircraft engines also… if you have lots of time on your hands and really up on knowing everything about the engine on the genset.

    happy trails.. the adventure contenues

  2. James Jackson

    I don’t like the idea of putting anything in the generator to prob around with. As it could damage or leave a scrap that could plug a port and or cause further problems. I would rather use air presure, you may have to go to Lowe’s or Home Depot and Napa to find parts to modify so you can controling air lose. But before doing anything You should contact Onan Service, be prepaired with model and SN number. Determine if your generator is under warranty. Having anyone work onn it other than a authorized service center could void your remaining warranty.

  3. gMAs

    Good answer… low or no oil… new engines need breaking in.. and some of the time they use a light mineral based oil… it gets somewhat consumed by the engine when it runs the first couple of times…

    We have seen where attendents did not look to make sure the little plastic gasket on the oil bottle was still in the cap.. thus it makes its way into the oil pan… when they dump it in… that then could also head over and block the outlet of the drain valve…

    We have also seen where the valve itself is a problem… and right out of the box needs to be replaced…

    Either way.. adding some oil to the pan.. and then checking the drainage is a cheap first try…

    If not.. next comes taking the drain valve off and seeing if you get flow… if the plastic piece is in their you need to use the magic fingers (extractor) or force it back to where the fill opening is and recover it that way…

    If its the valve itself… take it back and have it replaced… and don’t use teflon tape to put it back in with… as the teflon could get into the engine oil gallery and really mess up the bearings when its running…

    Good luck… let us all know what you find… as some post a question and then never come back and tell what the problem resolution was…

    happy Trails.. the adventure conteues..

  4. Thomas Becher

    If it were so low, it wouldn’t run as Onan has a low oil switch. I don’t know what valve is on the Marquis but the Emerald has a crappy valve. I’d pull the valve and check it out. It’s right next to the fill opening, so you may be able to look in there to see and maybe fish a snake thru the valve.

  5. Mike Hammer

    Did you check the oil level before trying to drain it? Good rule of thumb, check oil before you use generator or any engine for that matter. Generators run very hot and if you do not use good oil you could find that it is burning it. Onan does not have a oil but Kohler and B & S do and my understanding is that at least with the Kohler oil the main difference is that it has a higher flash point than regular motor oil to run in hotter engines (non-water cooled). I would try putting something in drain hole to see if clogged if it has oil in it.

  6. PAT

    Is the coach on a level surface? Is it possible that the coach is leaning the opposite direction of the oil outlet? A little out of level can make a big difference.