Elmer the 86 subaru 4x4 wagon , my question why is my mpg only 20mpg ??

[NoahDL88]

Your 20W-50 isn't helping your economy either, unless its burning a great deal of oil, or leaking like the Exxon Valdez you really should be running a thinner oil, more in tune with what it was designed to run with.

 

I'd expect you to pick up 2-3 mpg by switching back to 10W-30

[WoodsWagon]

Running thicker oils in older engines is a bad idea, and a self-fulfilling prophecy. The thick oil cant flow into the bearings properly, so the bearings starve and wear quicker, then the clearances open up and you need the thick oil to build any pressure. 10-30 should work fine, and improve the mileage a fair bit, especially on short trips around town. Oh, and don't trust the oil pressure gauge on the dash, the senders go bad quite often and give false readings.

 

Now, an 86 carbed 4x4 shouldn't have a feedback carb, computer, or 02 sensor if it's a federal emissions car. It may even say it's a 1986 "light truck" on the emissions sticker under the hood. Check under the steering column above the pedals and behind the lower dash trim to see if there's a big silver box bolted to it. Most 86's have the bracket with studs sticking out of it on the column, but no computer.

 

If your exhaust y-pipe rusted out, someone could have swapped one from a SPFI car onto yours, and just left the cut off O2 sensor in as a plug.

 

Don't shift into 1st while moving, the synchro is undersized for the job that's being asked of it, and it's 25 years old. The same 5mt design was used up into the 2000's, and most of them balk at shifting into first while moving. You can jam it in, but that's worse on the synchro. Just wait till your stopped and put it in.

 

If your choke isn't hooked up properly or moving smoothly, and you have to pump the gas to get it started, that could mess with the MPG's. Make sure it's fully closed when stone cold, and fully open when warmed up.

 

Taking the carb apart, cleaning and resealing it, and replacing all the vacuum hoses could do great things for it. But you need to be careful to make sure all the parts go back where they came from. The hitachi non-feedback isn't a bad carb.

[WoodsWagon]

Do tell,MR. Expert

 

What do YOU think the synchro is for?

 

Nobody else seems to know.

 

The synchro is meant to spin the input shaft and clutch disk up or slow it down to match the speed of the selected gear being engaged. It has a friction cone clutch, usually brass, sometimes carbon coated, that engages on the flank of the gear and is splined to the shaft the gear freewheels on. As you push it into gear, the shift collar (more of a sleeve really) pushes the cone clutch into the side of the gear. The drag of friction matches the shaft speed to the gear speed, and the cone clutch shifts sideways a 1/2 tooth while this is happening to block the shift collar from engaging the dog teeth on the gear. When the speeds are matched, the rotating force on the cone clutch drops to the point that it no longer blocks the collar. The dog teeth line up, and the collar slips onto the gear, locking it to the shaft.

 

Now, that's all well and good, but if there's a large speed difference between the shaft and the gear, the synchro has to work harder and longer to bring the input shaft and clutch disk up or down to speed. This is what's happening while your pushing on the shifter, but it wont go into gear. If you push steadily but longer, it will go in properly, but you're essentially smoking the clutch surface in the synchro. Once it's worn out, it won't provide the friction that offsets it a half tooth and blocks the collar from going into the dogs. You will now "grind the gears" which is actually the shift collar grinding on the dog teeth on the side of the gear. These are much less substantial than the gear teeth themselves, and once you grind them enough, you will get it popping back out of gear under load. Wear the dog teeth down further, and you will have to forcibly hold the shifter in gear to get it to work.

 

So.... there's a couple things to do. You can double clutch, or no clutch, up and down through the gears. You're using the throttle to match the engine, clutch disk, and input shaft speeds to the expected speed of the gear your going into. You're doing the job the synchro was intended to do, so if you do it right, you reduce the wear on the synchros. Do it wrong with no clutch, instead of double clutch, and the synchro is now fighting the power and inertia of the engine instead of just the input shaft and clutch disk. That will destroy them in a hurry. People that think they're cool by not using the clutch, but aren't good enough to accurately speed match the engine, do this by briefly unloading the drivetrain, popping it into neutral, and pushing on the shifter and letting the engine speed come down until it pops into gear. This is hell on the synchros and why the transmission in my Tacoma is in such bad shape. I caught the previous owner doing it on another car of his and that explained why my truck was so balky.

 

Second thing you can do is sequentially down shift, so rather than jumping from 4th down to 1st as you come to a stop, keep the clutch pushed in and shift through 3rd and 2nd on the way down to 1st. That will spread the load of bringing the input shaft and clutch disk up to speed through 3 synchros instead of putting it all on one.

 

Third thing is just wait until it's stopped, and put it into gear. That's the simplest, and the least wear on an already old gearbox.

 

There are Subaru 5mt's out there that will shift into first while moving. They came from cars that were only shifted into first while stopped. I've had one, so I know they exist. However, with "normal" use, the synchro will quickly wear to the point that it takes a while to work, so the shifter is blocked for longer than you expect. That's indicative of an undersized synchro for the job demanded of it. It's strong enough to get first to go in reliably without crunching when bringing the shaft speed down from 1000rpm to 0 when first engaging it, but not enough to bring it from 1000 up to 3000rpm when shifting down into if from a higher gear while rolling.

 

3rd gear synchro has the same problem as Miles pointed out, it seems to wear out prematurely. However, it's unrelated to 1st gear.

 

This has nothing to do with low MPG's, and shouldn't descend into name-calling, sarcastic, and caustic comments. Really people.