quest for better mileage

[xirdneh]

Off and on I make an attempt to figure out why my three tercels differ in gas mileage.
average driving daily is 34 miles on country roads and 5 miles city
car mileage compression timing noise
Yellow tercel 27-29 mpg 145-150 psi 15 deg no pinging
Blue tercel 29-31 mpg 150-155 psi 16 deg no pinging
White tercel 36-38 mpg 120-125 psi 15 deg pinging

Compresssion was on warm (not hot) engine
Timing at idle with vac adv hoses attached
Pinging during lug period while up shifting

Also held engine at medium high rpm and watched timing mark
White tercel mark hovered top center (90 deg off horizontal)
Blue tercel mark hoverd at abt 110 deg

Now i just need to know if this means anything?

[Petros]

Wow! The while one is excellent MPG, we got to find out what is different with the white one! Tell us more about about it.

From my university engine studies I know that the higher the compression and the more advanced the spark (it results in higher combustion pressures) the more efficient the engine. And the most efficiency occurs with spark timing that gives MILD ping. heavy ping is not just destructive to the engine, but also inefficient and produces a lot pollution.

So with that understanding I would say you are getting exactly that with the yellow and blue Tercel, but that is all thrown out with the white one.

Perhaps the vac advance is working on the white one, and the others are not? The vac advance improves the part load efficiency (as in a part throttle cruise mode). You can not tell by driving if the vac advance is working or not since under acceleration or full throttle and idle it plays no role, only at part throttle steady state driving.

Are the cars the same years? Are they the same in terms of 4wd, trans, and weight? Is any of the emission controls disabled in the white one, if so which? Is the engine equipped the same (carb, A/C, air filter, etc.)? What are the total miles on each, and each engine if different?

Tell us, tell us, please. We need to find out what is different. I wait with bated breath!

[neonsport]

One big variable with these old Tercels is the carburetor. My 86 2WD was down to 28-30 highway MPG with it's original carb (the idle was lousy as well), but it jumped to 35 when I swapped on a reman. Unless they're all fresh, the carburetors on your Tercels are likely operating at varying levels of efficiency.

[takza]

Yep...variables....

* 2 or 4WD?

* timing at idle with the vac hose on.......tests your vac advance... and partially whether the key holding your front pulley fits snug

* pinging I'm thinking has to do with mal-adjusted emissions stuff and maybe carbon buildup..and basic timing

* blue terc has it's basic timing set 10* less advanced than the white one...or it's centrifugal advance doesn't work?

I'd say the white one is 2wd? Also uses some oil...basic timing more advanced?

[xirdneh]

Yep...variables....



I'd say the white one is 2wd? Also uses some oil...basic timing more advanced?

Yellow (4x4 with '87 tranny and diff.) is an '83 with re-ringed engine that has abt 60,000 miles on it, standard carb and who knows what is or is not working in the vac hose/emissions catagory. the idle is low, steady and quiet.

Blue (4x4) is an '87 with an original engine and carb out of an '83 and has abt 220,000 miles on engine. the idle is medium high and somewhat erattic.

White (4x4) is has a re-ringed engine with about 35,000 miles on it. the engine had about 220,000 when re-ringed. carb is standard and it idles low and quiet.

all three cars have relativly new vac adv diaphrams and all three have had wd-40 sprayed into distributer counterwieghts area (about 1.5 years ago)

[takza]

White (4x4) is has a re-ringed engine with about 35,000 miles on it. the engine had about 220,000 when re-ringed. carb is standard and it idles low and quiet.

all three cars have relativly new vac adv diaphrams and all three have had wd-40 sprayed into distributer counterwieghts area (about 1.5 years ago)

You should blueprint this one? Lower compression? I'd like to get that mileage.

I've got the high rough idle.

[Petros]

What year is the White one?

Though it appears likely it is the vac advance that is working better than the others, perhaps the EGR is plugged/malfunctioning too. Perhaps you can check the timing for us at idle with vac line plugged, and then then plugged in, and the same at 3000 rpms (vac line both unplugged and connected), on all three cars. THat will tell us what is different with the timing and vac advances.

It is odd that it has the this car has the lowest compression but has had the most recent ring job. Perhaps it needs a valve job too, or just poor fitting rings?

[xirdneh]

What year is the White one?

Though it appears likely it is the vac advance that is working better than the others, perhaps the EGR is plugged/malfunctioning too. Perhaps you can check the timing for us at idle with vac line plugged, and then then plugged in, and the same at 3000 rpms (vac line both unplugged and connected), on all three cars. THat will tell us what is different with the timing and vac advances.

It is odd that it has the this car has the lowest compression but has had the most recent ring job. Perhaps it needs a valve job too, or just poor fitting rings?

not sure what year the engine is. it was taken from a 4x4 tercel that had a larger flywheel and slightly different transmissin. i never asked what year it was. the guy told me why but i do not remember the details. something about the car coming from japan differently than the made for america ones. the body and running gear are '83.

does the vac advance really make that much difference if it is slightly malfuntioning?
does the EGR really make that much difference? i have cleaned them on all three cars but did not see any difference working or not.
i have rebuilt all three carbs
the yellow car has a new cam but stiill has the lowest mpg
not sure about vac hose conn being 100% correct due to the differences in years of moters, carbs and electronic brains.
i wonder why the white car pings so much? when the timing is less advanced than the others.
i am leaning towards the carb playing the most major role is the good mileage. i'm not sure why though. i rebuild them the same way.
one thing i wonder about is the second barrel of the carb and when it kicks in. i wonder if that is the main factor?

[Chris]

In regards to your white car pinging....this might give you something to ponder.

One of my Tercels, that I bought new, has over 350,000 miles on it. At about 60,000 miles I had noticed it would ping on hot days with a load on it ( A/C on ). I started using mid-grade gas. That solved the problem for awhile. More miles, more pinging. So, I went to premium grade gas...OK for a while, then more pinging. All this time the ignition timing was set dead on specs and the EGR system was maintained and working properly.

Last winter I pulled the head for some maintenance and decarboned the combustion chambers and piston tops. The results are...no pinging, more ignition advance than ever and it's back to running on 85 octane.

[takza]

This might crap out your O2 sensor? Or maybe not.

viewtopic.php?t=152

[xirdneh]

update

the White car is still getting fantastik mileage
just replaced both front axel hubs due to bearings howling (did not seem to affect milege)with salvaged hubs from hatchback

the Blue car is down to 26 mpg (during winter months) tried different carb (same result)
just put another carb on last weekend (we shall see)

Yellow car , removed engine due to failed gasket between rear main mount plate and block
(after market gasket only had abt 60,000 on it) bought new one at toyota dealer (its teflon coated metal)
re-installed different engine (128,000 orig miles out of '86 tercel hatchback) in car and only got 26 mpg. on first tank Bummer

My work buddy has a Grey 4x4 tercel. he has been getting abt 26 mpg since aquiring the car 6 months ago
i changed his carb and reset timing. still getting 26 mpg.

one thing dawned on me this weekend
the White car is the only one that does not have power steering.

[sdoan]

Some ideas...

The 1983 and '84 had slightly higher gearing. Depending on how you drive it this may favor your mileage.

Also the weight of the gear oil may make a difference. Driving 25 miles each way I get barely 30 mpg on the freeway, but on long trips, even with a fully loaded car over mountain passes it will get 34-36 mpg. My best guess is the weight of the gear oil in the trans. - as the gear oil warms and thins the car gets better mileage. It also shifts more easily when warm.

Check that ping - my 1983 used to ping and I now consider it a symptom of a vacuum leak. The '83 would ping, particularly when hot at mid throttle (60 mph on gentle freeway hill). Not a bad death rattle, just ping. Timing set 5 degrees BTC with one vacuum line plugged. It got good mpg for a while, but slowly got worse - it dropped from 30-32 to 26-28 (over a couple years). Then it burned a valve. I found that one of the distributor diaphragms was leaking, but it could have been a leak anywhere in that vacuum advance circuit. The combination of lean mix and advanced timing probably made the combustion temps hot enough to burn the exhaust valve.

[Snax]

one thing dawned on me this weekend
the White car is the only one that does not have power steering.

For anybody wondering, removing the power steering belt still leaves these cars very driveable. It's not the typical heave-ho like most cars with power steering. I'd put it back on for any serious off-roading however. It's there for a reason.

[Petros]

I found that one of the distributor diaphragms was leaking, but it could have been a leak anywhere in that vacuum advance circuit. The combination of lean mix and advanced timing probably made the combustion temps hot enough to burn the exhaust valve.

a leaky dist diaphragm would mean less advance at part throttle condition (like at freeway speeds) and less fuel economy. Likely all the difference in economy on all cars (presuming they run good otherwise) is caused by how good this dist. vac advance is working.

Leaner would make the valves hotter because the fuel burns slower and transfers more heat to head and valves. Advanced timing cools valves as compared to retarded timing, more of the fuel is burned before the exhaust valve opens. I burned two exhaust valves running at 5 deg BTDC, none after running at 10 deg BTDC.

[takza]

Found that the advance diaphrams went on my dist...so have the lines plugged now. Have a couple other dists I'll be checking out soon.

What helps pinging and ring/valve stem seal is upper cyl carbon cleaning and a kerosene flush when changing oil.

I'd say a pint of water thru the carb and a pint of kero for 20 mins above idle right before an oil/filter change should be standard. Plus using a PCV jar.