My 88 Minicruiser - good and bad

[zero]

My 88 Minicruiser with a 22RE has used oil since I got it around 5000 miles ago.  About 1/2 a quart every 400-500 miles.  I put new valve seals in guides and valves. It made no difference.  Runs fine. No smoke. No discoloration on the spark plugs.  So I assume just general long-term wear.  I've been using Shell Rotella T 15W-40 since I owned it.  As an experiment, I just switched to Mobil One, 10W-30 "synthetic" High Mileage.  Just drove 800 miles and did not use a drop of oil.  I'm kind of surprised. I did not change because of oil usage. I did it to see if I could experience any change in gas-mileage.  So the good is I used NO oil at all.  The bad is my gas mileage. I am kind of perplexed over this.   This RV got a pretty consistent 14 MPG when I first got it. I DID make it heavier as time went on. But I also removed the roof-top AC and it should have a little less wind resistance?  This last trip - I removed some weight and changed the oil to Mobil 1.  I removed the extra gas tank, microwave, helper springs, and the water-heater tank.   Then took a very steady trip never going over 60 MPH.  We got 12.2 MPG for the trip.  There are no error codes in the computer. That being said, I put in brand new injectors and an oxygen sensor.   Tailpipe will always show  black soot inside if I rub my finger in it that makes me think it runs rich.   I have no way to test the air-fuel ratio as it comes out the tailpipe  - but the ECM is supposed to take care of that. At this point - there is nothing I can "adjust."   Maybe it is just a case of  a tired engine.  It still runs perfect so I can live with the 12 MPG for now.  It's possible the last trip was extra windy and that caused the low reading.  I am still a bit disgusted.  I've considered rebuilding the engine and converting to a 5 speed manual trans.  Also thought about putting in a 5 cylinder Mercedes turbo-diesel.  Just not so sure I want to go through all that  work and then find out it was not worth the bother.  I've love to come across someone else's conversion first that worked well.  Next year we might camp all summer with the Chinook instead and maybe then I'll lay this thing up for a while and do some work on it. I DO have an extra 22RE engine in by barn.  What I don't like is having no control with the EFI system as far as adjusting the fuel-air ratio.  I don't know if it is possible for a computer to get a little "dingy" as it ages and maybe compute the wrong air-fuel ratio yet not trigger any error codes?

[Wade]

Just throwing this out there.  Is it possible that the thermostat is off?  Perhaps the synth oil causes it to run cooler and the thermostat isn't adjusting correctly.
 Running cool could give you those conditions.  FWIW, have you ever read through this?

http://www.22reperformance.com/22RE_Performance_-_Installation_Parts.html

Kind of an interesting general read of what to verify on our older 22re's.

I just got my toy and was planning on putting Shell Rotella T 10W-30 in it when I change the oil. Probably dino or T5 but might go T6 if they have it in the 10W-30.

Wade