Several comments come to mind, relating to various aspects of your problem.

First off, and nobody else has mentioned it, could the problem be not in the engine at all, but in the odometer??? I mention this because mine has become intermittent in the colder weather, leading to an apparent mileage drop from 29-32 to about 21-23. In my case it's easy to spot because the speedo is affected too, but since mine is a 740 the instrument itself is different, and perhaps the odo can go in and out on a 240 without affecting the speedo.

Second, compression testers vary quite a bit. Your 140 lbs may well be someone else's 180 lbs. Unless you have a really high quality professional grade gauge, the readings are unlikely to reflect the actual PSI generated. You can still gain useful information from a cheap gauge, as they are usually consistent with themselves---that is, if all your cylinders read the same, you are probably in good shape.

Things to keep in mind while doing a compression test. All plugs should be removed to get good cranking speed. Coil wire should be connected to a spark plug with its body grounded so there is a gap for the sparks to jump (to avoid burning out anything in the ignition system) Put the gauge somewhere you can see it an note not just the maximum it pumps up to but the reading after eack kick of the needle. For example, 80 lbs on the first, up to 120 on the second, 150 on the 3rd, 160 on the 4th, 165 on the 5th. Again, what you are seeking is consistency. If the first and second increments and the final reading on all cylinders are within 10% of each other, your engine is healthy, the actual numbers are not that important.

You mentioned having done a valve job recently, and that the engine started using oil afterwards. It is unlikely that there is anything wrong with the work that was done. As I understand it, what happens is that since the valves seal so much better than before, air (and oil) is now being sucked past the rings upon deceleration instead of the valves. What weight oil are you using? Maybe you should try 15W40, which is what the owners manual recommends for temps above 20F. 10W30 is recommended for temps below 80F, and is a good choice for winter, unless you experience Arctic conditions, in which case 5W30 might be better.

Gas additives shouldn't cause that much of a drop. The maximum amount of ethanol allowed in gasoline is 10%. I have heard that mileage will drop 20-25% when using E85 (85% ethanol) instead of gas, so that would only account for a drop of about 2-3% when using 10% ethanol--maybe a difference of 1 MPG.

I have an 88 745 with 120K miles. I converted it from automatic and 4.10 rear to M46 and 3.31 rear. I am getting 29-32 MPG, in mostly rural backroad and highway driving. I never use anything but 87 octane. I tried a tank of 93 and found no difference whatsoever, either in mileage or in the way the car ran. I use Shell Rotella 15W40 in the three warmer seasons (northern VA, not too different from your climate) I have a couple of minor oil leaks, yet use only about 1/2 quart between oil changes at (factory recommended) 5K miles. When it comes out, the old oil is the color of Coca Cola. I do tend to drive gently---it is rare for me to exceed 3000 RPM. That doesn't mean I don't like to go fast, I just take my time getting up to speed.

Consider this as not sage advice, just my own ecperience.

Pulled the accordion air hose on my 1988 244GL. Lo and behold, there were two places on the bottom side where the little peaks had holes. New hose = better gas mileage.

The holes were invisible until I had the hose off. Turned out to be a low-cost fix, my favorite kind.

Un-metered air makes for a lean burn, which the 02 sensor picks up. ECU gets the message which takes steps to add a little fuel to the mixture. You might do a hands-on feel of all the vacuum hoses, especially those on the nice hot rubber-hardening intake manifold.

Good Luck,

Bob

:>)

what a beeeuuutiful baby - i wish mine looked like this under the hood. seems you love this lady to bits, and still she wont play ball. ahhh.
--
Flame Trap UK

SPH:

Pull your plugs and take a look at them. are the electrodes and insulators kinda tan and perfect, or are they black with soot? I'm assuming you are not burning oil. If the electrodes are tan and you have no CEL lit, all is good and write it off to crappy fuel and colder weather.

jorrell
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89 244 171K miles, 92 245 241K miles, 06 XC70, 00 Eclipse custom Turbo setup

I've been in the same boat too with my MPG, being in the 21 MPG range with appx 80% highway (65-80 MPH) and 20% city (WOT up to 35-40 MPH).

I recently bought a used AMM on E-Bay for $20 (including shipping) and when I installed it, the car ran bad and illuminated the "Check Engine" lamp, and threw off a 1-2-1 code. Put the old one back in, reset the idiot light and car ran normally again. I'm still waiting for a refund from the guy who sold it to me...

I thought my AMM might have been getting old (247K) and I thought this might be the reason for MY poor MPG, but I'll have to wait on this one.

I also changed out my original fan clutch, but it had no effect, and might have actually caused the MPG to go down...

Have you checked the thermostat in the air cleaner housing? That being broken can easily cause the drop in MPG you quoted. Is the throttle plate clean? I don't think it would cause a huge drop, but it might have a small effect.
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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 245K looking forward to 300K badge. >>You haven't really worked on a car until you draw blood<< :-}

I thought the new gas formulation phased out MBTE, and replaced it with ethanol possibly up to 10%.

If that's the case, the addition of the dry gas may be hurting the mileage.

I would fill up with a fresh tank, pump the tires back up to 40 psi and see what kind of numbers you get.

The onset of colder weather has some effect also.

Keep us posted.

--
Bruce S. near D.C.

Coming from me it will not be sage advice, but just a couple of working points to throw out for the heck out it.

Have you done a proper compression test recently? Checked the resistance values hot and cold for the ECT, tested at the ECU plug? Also, check the running temp of the engine with a known good thermometer, ir guns are nice and quick. Maybe the 'new' thermostat is fubar, running cold, and the gauge is not picking it up.

The best diy test( no scope) for the O2 sensor, imho, is with the propane torch and dvom. Check out the bottom of this page, http://mymiata.paladinmicro.com/Miata4-WireO2.htm
--
Gary Gilliam Sumerduck VA,' 94 940 Regina, '86 240GL

Just remember that with the colder weather comes more activation of the Choke and as such a richer mixture ... and resulting lower mileage .... I somehow doubt that the additives have much to do with the mileage loss ....

Brett in the Great White North ;-)
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Brett Sutherland & the 1.5 million mile 122 CANADIAN --- WINDSOR, Nova Scotia the birthplace of HOCKEY www.ecvintagevolvo.com

Okay, no one else has pitched in yet, and this is a long shot, but....

[preface: I don't know where you live, but maybe if you're in an area where they mandate winter formulations of oxigenating additives in gasoline, and this IS the time of year when they would start doing so....]

I'd suggest that maybe they've started adding oxygenating additives, or ethanol/MBTE/etc. This is the time of year when, in some areas of the country, this would start. Is it mere coincidence that your mileage dropped at this time?

I remember, when they put in MBTE in the winter months, my cars' mileages dropped.