Hello All, I have a 1991 Toyota pickup Deluxe, 2WD, 2.4L, automatic (4-speed), standard cab with 78,000 miles.
I am only getting 12.5 to 14 mpg on the highway. It has new plugs (at 60,000) and a new air filter (at 75,000 miles).
It has oem size tires. Any ideas on why the MPG is so low on the highway, I was expecting 20+ mpg.
I did check the brakes and they were not dragging/hot, and I cannot see / smell any fuel leaks.

Thanks....

 

Is that an efi engine or carb? First i would check the timing.Also the vacuum advance if it has one.

 

EFI engine

 

Low highway MPG can be frustrating. Let's explore some potential causes:
1. Aerodynamics and Tire Pressure: Ensure the tire pressure is at the recommended level, as specified in your owner's manual. Proper tire pressure can improve MPG. Also, consider removing any external accessories that might be affecting aerodynamics, such as roof racks or bug deflectors.
2. Air Filter and Intake: Although you mentioned a new air filter, double-check that it's properly installed and not dirty. Inspect the intake manifold and throttle body for any signs of dirt, grime, or carbon buildup. Clean or replace them if necessary.
3. Fuel Injector and Fuel System: Dirty or clogged fuel injectors can decrease MPG. Consider having them cleaned or replaced. Also, inspect the fuel filter and replace it if it's old or dirty.
4. Ignition System: Although you mentioned new plugs, check the spark plug wires, boots, and ignition coil for any signs of wear or damage. Worn-out ignition components can decrease MPG. 5. Transmission and Torque Converter: As an automatic transmission, the torque converter might be slipping, causing the engine to work harder and decrease MPG. Consider having the transmission serviced or checked by a professional.
6. Oxygen Sensor and Engine Management: A faulty oxygen sensor can cause the engine to run rich, decreasing MPG. Check the oxygen sensor and consider replacing it if it's old or faulty. Also, ensure the engine management system is functioning correctly.
7. Idle Mixture and Idle Speed: If the idle mixture is too rich or the idle speed is too high, it can decrease MPG. Check the idle mixture and idle speed, and adjust them if necessary.
8. Accessory Belts and Tensioners: Worn-out or loose accessory belts can cause the engine to work harder, decreasing MPG. Inspect the belts and tensioners, and replace them if necessary.
9. Catalytic Converter: A clogged or failing catalytic converter can decrease MPG. If you suspect a problem, have it inspected or replaced by a professional.

 

Incredible!

 

By investigating these potential causes and gathering more information, you should be able to identify and address the issue, hopefully improving your highway MPG.

 

Little ai copy and paste huh? When pasting, paste as plain text, then it will show the correct text color for the page.

 

I can’t read replies from copy and paste it’s just black, also copy and pasting other replies from other users in the same thread……I fear the AI will eventually run rampant ruining human responses that have actually had the issue. Yay.

 

Check timing. Checking TPS wouldn't hurt. I posted an easy way to set TPS in the stick section
Plugs ideally are basic resistor ND or NGK. Any other brand I'd replace.
Pull the plugs, keeping the order straight , and post a picture. Curious what the electrodes look like.

 

Thanks all for all the suggestions.
One thing I forgot to mention, the past owner deleted the fuel filter, he actually replaced the fuel filter with a piece of hard fuel line with like connectors (no clue why). So I think my first thing I need to do is get a fuel filter in, and then remove/clean the fuel injectors. Then next is to replace the O2 sensor and check the timing, then try other items on the list. Currently, she starts immediately (even in -10F), idles nicely and appears to have good acceleration.

 

Try to get an OE fuel filter, aftermarket’s are super cheap. I got an aftermarket one and when tightening it the metal was so thin it tore a crack in the filter that was so small it took me a month to figure out where the gas smell was coming from. The Toyota filter is the best you can get.

If you do get aftermarket just be careful when tightening the bracket and fuel lines

 

1. The tire pressure reccommended by Toyota is on a sticker inside the back side of the driver's door. Very easy to find!
2. Also check that there are no "air leaks" in the tubing between the filter and TB. Especially the two big corner flex pieces. They are prone to cracking, thus, an air leak. Just pull them off and look into them while flexing them. If there's any doubt, there's no doubt. Replace them. They're cheap and readily available.
3. When you install the fuel filter, make sure it's coorect oriented. It will have a flange at one end. That goes to the rear, not the front. Essentially, it will appear bass-ackwards. Also, get a number of extra crush washers from the dealership for the filter. You can only use any washer ONCE. If you put one on and tighten onto it, however slightly, use a new one the next time, or you'll have a leak. Be aware, those crush washers have a malicious habit of leaping out of your hand and vanishing, Won't be on the ground, won't be on the frame, nowhere. As soon as you put a new one on, it will magically appear again. They're worse than 10 mm wrenches. Be warned.
4. Also, a dab of silicone dielectric grease in every connection will be a big plus. It heps keep the crud that floats around in an engine compartment out. Helps make for high quality connections. Keeps the silicone of the boots soft and flexible as well.
5. Eh, I got nothing, myself.
6. The oxygen sensors are slated to cbe changed in The Book every 60,000 miles. Much more than that, replace them!
7. The best way to ensure the mixture is correct is with a good fuel filter, injectors, air filter, O2 sensors, and so on. Clean, high quality parts will make the difference. The idle speed is adjustable on the throttle body. Screw on the top, left side of the entrance to the TB, underneath a large cover screw. Check that it's rubber o-ring is in good shape. It's small, and readily availble. The idle speed should be 750-800 RPM. Ensure the Idle Air valve is functioning correctly.
8. Very good advice. Heck, if in doubt, again, there's no doubt. Replace them. They're cheap, and readily available. Keep the ones you remove in a corner of the truck someplace. Even the best new belts can fail unexpectedly. If you have a spare belt available, you can put the old one back on until you get a new one. Can get you home on a dark and stormy night. Same with things like the distributor cap and rotor, plug wires, and so forth.
9. I've found that taking a rubber mallet to the cat nce a year or so, and beating it up can break loose the crud and corrption in them. I still have the factory cat on my Pickup, and it still works great.
10. be prepared to replace the oil in the transmission, and differential. Lube the diveshaft up real good. It's important to do that at least once a year. Check the differential breather. Pull it off the diff and make sure it's clean, and functioning correctly.
11. If you haven't yet, flush the brake system out until it's flowing clean, new fluid from all 5 bleeder valves. Yes, 5. Don't forget the LSPV!

Ok, ok, shutting up now...
Pat☺

 

I’d like to add, read the plugs, put a vacuum gauge on it, perform a compression test. All general well-being indicators.

 

Odd they previous owner went to the trouble to delete the fuel filter. Makes you wonder why.

 

 

Shame.

 

Sorry about the delay, just got around to some of the updates. Got a fuel filter in, and pulled and cleaned the fuel injectors (using a can of carb cleaner and a 12 volt wire to open up the jet, the jets flowed nicely, but did replace the rubber seals on them). Use a half can of carb cleaner to clean the inside of the intake manifold. Replaced
the PVC valve and its rubber seal. Replaced the O2 sensor with a DENSO brand. See picture for the one I removed, it was pretty carboned up. Going to do a mpg test on it Saturday to see if any improvement on mpg. All the vacuum and rubber lines were in good condition, as well as the air intake plastics.
[img alt="O2 sensor
"]https://cimg2.ibsrv.net/gimg/www.yotatech.com-vbulletin/1080x486/o21_1622540e1f709f89cc54d1f6aa99e145cf36fecd.jpg[/img]
O2 sensor

 

wow, that o2 sensor is a disaster.

running rich destroys catalytic converters, might want to check that as well.