Topic: Poor MPG

[woodash]

Hello. I own a 1980 Toyota 4x4 Longbed. It has the original 20r motor, an RV Cam,  a L52hd tranny, G102 rear axle, and 31'' tires. It has 262,000 miles on the odometer and I recently replaced all the top end seals and gaskets because of an overheating issue. This seems to have solved the overheating problem, but now I have a new problem--10 MPG. I have always got at least 15 MPG, whether climbing hills, or hauling a load. I replaced all the fuel lines from the filter to the pump and to the carb. I have run about 50 gallons of fuel and still poor fuel economy. No visible external fuel leaks. The idle speed and timing are right on. The valves were adjusted. Air filter good. Throttle plate operating as it should. Any suggestions on what the problem might be would be appreciated. Thanks.

[MiniSimp]

Here is the answer http://www.youtube.com/watch?v=tpQWaI1lspc

[79coyotefrg]

it sounds like the choke  is not opening all the way,  the 20R has a water heated choke

[79coyotefrg]

btw, G102  is the code  for  4.37 gears  to Marlin

[KDXSR5]

This is going to sound really dumb, but here it goes... are your hubs unlocked? Without hubs unlocked I get 17-19 in town, but with them locked (not necessarily in 4wd) it drops down to 11-12 mpg. It was just an idea. Good luck!

[dt]

needs a new shortblock,thats what I think

[CrazyYota]

I only get around 15mpg in my 83    with a rebuilt engine, I call it the Phat b!t$h 

[woodash [OP]]

Actually, I drive fairly conservitively. Albeit mainly city driving. Hubs are unlocked. The choke appears to be open all the way once it reaches normal temp. to allow more air in. Yes, a new motor would be ideal, but I am on a slim budget. I do have an oil leak from either my inner or outer seal of the rear axle on the passenger side. This could be causing the drum brake to lock up or drag some??? Perhaps this is the problem. Have not had time to drop the axle and investigate. Is their a dye that can be added to the fuel to search out fuel leaks? Also, my evaporative fuel canister is dead. I have not been able to locate a good used one, but it has never been a problem till now. This truck is an original CA model with all the smog equipment still intact. Any more ideas on what the problem and or answer could be to my poor fuel economy?

[Not A Toy]

IM pretty sure if your brakes were locking up enough to down your mileage you would know I.E smoking or just plain no more brakes or alot of noise. and if a seal was leaking I dont know why that would cause your brakes to lock up anyhow? I dont recomend dumping anything in your fuel tank,but i might be wrong. trace the lines and see if you can see fuel leaking because if it were leaking at enough of a rate to cut your mileage in half youd see it (ecspecially where you said you just replaced them you may have left something loose). and you dont really need to drop the axle to see if your seals are leaking??? is the truck running like always?  maybe replace your sparkplug wires. maybe your carb needs a good cleaning/tuning. you need to provide as much info as you can for us to help you on here.

[CrazyYota]

I do have an oil leak from either my inner or outer seal of the rear axle on the passenger side. This could be causing the drum brake to lock up or drag some???  and or answer could be to my poor fuel economy?

I dont think that would be the problem. my rear axle was leaking into the drums, the only problem was the brakes because they would not work.

Actually, I drive fairly conservitively. 

Me too

[woodash [OP]]

I assumed that if oil is saturating the drum and pad it would cause the brakes to fail and possible lock up if they swelled enough. I know one or both of the passenger seals are bad in the rear axle. If it is the inner seal the bearing will have to be pressed by a machine shop. That is what I meant by having to drop the axle to repair. However, the brakes have been lousy as usual. No difference in their performance or noise. Brake drag was just an idea I had. As for the engine, it purrs nicely. It smokes white in the morning when cold, but it drives the same. The carb is a Holley re-manufactured I got a little over a year ago to pass smog and is in like new condition. I clean it often and set its idle speed to about 850 rpm @ 8 degrees BTC. I have not replaced the spark plug wires yet. I did about a year ago. I will try this as a remedy as well as look for leaks at the tank and leading to the fuel filter. All fuel lines that were removed when the head was removed were replaced and are currently leak free.

[Not A Toy]

man sounds like your on top of things good luck let us know what happens.

[93tonkatoy]

When was the last time you changed the distributor cap and rotor? If enough corrosion builds up on the contacts in the cap, it will effectively retard your timing and cause a weak spark. I have the EFI, but I normally change mine every 6 months, or my mileage suffers.

[woodash [OP]]

I changed the cap, rotor and plugs about a month ago. Every 6 months or so the truck won't start because of bad plugs from oil saturation. I believe it is from worn piston rings. This is the original motor after all. The spark gets to weak, so I changed it all out a month ago, when this occurred. One thing I forget to mention, is that I did manage to brake a part on the engine that I believe is called the BVSP emission component. It has two lines that plug into a sensing unit that is mounted on the bottom of the thermostat housing. I epoxied the plastic part that broke, but who knows? I guess when the truck reaches a certain temp, a vacuum advance occurs and changes from one hose to the other, but I am not positive on all the emission operations.

[woodash [OP]]

I found out that the broken part is actually called  a BVSV (bi-metal vacuum switching valve). Some say it can be bypassed and is not really that important. However, my manual says it is connected to the choke opener diaphragm. A broken BVSV would not stop the choke operation even after the truck reached operating temp. and the extra fuel was no longer needed.  I was able to track down the part at NAPA. It was an expensive part at $50. At the same time, I put in new Denso spark plugs, new Bosch wires, a new Fram air filter, and the new BVSV. Hopefully, these replacements should address the problem of poor MPG.

[kneedownnate]

Bear in mind, you'll likely lose a little mileage due to running winter gas.  Not every engine seems to show a difference, but some do.  Also, if your engine was running hotter before it was burning gas more efficiently.  I suggest running a 195* thermostat.  You may also try running a test.  Run 5 tanks of cheap gas and take down your mileage, then run 5 tanks of quality gas (chevron, etc.) and see if there's a difference.  My 91 2wd got slightly better mileage when I switched over to quality gas, though it took atleast a couple tanks to really start to notice the difference.  And no, I didn't drive any different, I never really do.

[woodash [OP]]

No success yet. The poor fuel economy started after the head was taken off and the seals and gaskets were replaced. I ran winter gas in the truck before this job and the truck always acheived 15 mpg. I did shell out the money and replaced the BVSV. Does anyone have an opinion on this part and its importance or lack there of??? Also, my  evaporative fuel vapor charcoal canister is dead. Can this be bypassed??? If so, how? I get various reading of gas in my tank based upon how much pressure is built up in the fuel system. I forgot to mention that I have a 22.5 gallon fuel cell in place of the stock tank. This pressure can be released by opening the gas cap or pulling the feed hose from the evaporative canister. On hot summer days, if the tank is completely filled, fuel will actually leak out the overflow line at the bottom of the canister. Is this canister my problem? I have yet to find a replacement, so if bypass is an option, someone let me know.

[woodash [OP]]

Anyone have any feedback???

[93tonkatoy]

It sounds like the fuel cell needs a vent. when the engine is off, pressure builds as the fuel evaporates, pushing fuel up into the charcoal cannister. You need to connect a line from the top (highest point) of the fuel cell to the vapor inlet of the cannister. this will allow the vapors to bleed off, and the charcoal will absorb them. The cannister should also be connected to draw air through it while the engine is running, to pull the vapors back out. If the charcoal has been SOAKED with fuel, I would replace it. But, this SHOULD not be causing a large drop in mileage.

I would do a compression check and suspect burnt valves. An engine with burnt/ improperly seating valves will still run, but the power per fuel used will suffer - you will have to press the accelerator farther down to go the same speed. Just a possibility.

[woodash [OP]]

Where do you suggest I look for a replacement charcoal canister? Toyota no longer carries my canister. I have not had any luck at pick n' pull places as my truck is a CA model and the canister is specific to my year and model. Also, the compression is at the low end of the acceptable range at approx. 120-125 psi. All cylinders are even at this range. I believe this to be from worn piston rings as adding a drop of oil in the spark plug hole raises the pressure slightly. The valves were adjusted to an rv cam setting of .008 intake, .012 exhaust. The cam did have an excessive amount of play. I believe this was called the cam trust clearance. It probably should be replaced, but it worked at giving me 15 mpg before. What do you mean by burnt valves? Thanks.

[Plekto]

Well, exactly that.  Too much unburnt fuel and heat literally melts the edges off of the valves so that they don't really seat correctly(mind you we're still talking thousandths of an inch most likely, but that's enough). 

The typical situation is that it then dumps some of the fuel out the exhaust or pulls extra air into the cylinder, depending which valve(s) are affected. 

If the head was off, though, it's very likely that the valves were checked for this, though - it's standard to check things like this when it's apart.(rings too, usually).   

Since you have a carb, though, my gut tells me that that's the culprit.  The jet(s) on it tend to erode over time and gunk tends to build up in the strangest places.(plus it may be set too rich or malfunctioning)

[dt]

you said the head was replaced,so I assumed your valves are not burned,but when you put a rebuilt head on an old motor the rings suffer from the tight seal at the valves Ive had a lot of 20r's and when the mileage go down severely and quickly the rings are worn out,Ive never had a 20r BLOW UP but rather die off slowly,they start using oil and getting mileage thats not acceptable,thats when I swap out my block

[woodash [OP]]

The 20r head was not replaced. The head gasket was replaced along with the other seals/gaskets for the top end that came in a kit. This was done to combat an overheating issue. With this I was successful. The head was checked for flatness and the valves and springs checked too. All appeared well and the same head, valves and springs were installed. The valves were adjusted after install. The piston rings have been slowly dying for some time. I have low, but equal, compression. I suspect that this is the problem as well. I just cannot believe how suddenly the mileage dropped from the tight seal. I will no more when I calculate my mileage from my latest tankful with all the previous adjustments made.

[Plekto]

My guess, then, is what dt said is happening.  Is it burning a lot more oil than it used to?  It's probably at very low compression but the oil that's getting into it is keeping the pressure up just enough so that it still runs.   

Yeah - they are like old tractor engines. They just run worse and worse but won't actually die.

[woodash [OP]]

No, it is not burning more oil than normal. The truck actually runs very good. I just spend more time at the pumps than on the road recently with an average mpg of 10.

[RN37DD]

i had a 20r carburetor rebuilt/ remanufactured by ?? said holley on the box from kragen.. got bad mpg but passed smog. then changed carburetor and got better mileage. also whats the condition of your exhaust system? a clogged cat well make for bad mpg also.

[Plekto]

The only thing I can think of that's left is the carb has bad jets or similar and is dumping large amounts of fuel into the engine.

[woodash [OP]]

The catalytic converter and carb are new within the last two years. I replaced them both, along with the air pump to pass smog and register the vehicle in CA. Yes, the carb was a Holley remain from Auto Zone or Napa. It was nearly $500 with the particular water choke and smog ports. Perhaps, I need to have it inspected. I clean it regularly and performed a 'lean drop method' to get the idle mixture and idle speed just right.  I currently have the truck at 8 degrees BTC at approx. 800-850 rpm at normal operating temp. I am no carb expert though, but it seems like the most logical problem source. My mechanic is pretty good with carbs.  I will see if he can look at it.

[Plekto]

It could be as simple as something wasn't connected right with the water injection system or carb or whatnot when the engine was put back together.  Or something just broke on the carb.  That happens.

Shoot, almost anything seems to happen with us.  Lol.