90 Toyota Corrola...bad gas mileage

Hello All,

I have a problem with my 1990 Toyota Corolla. It seems I am only getting 20 mpg while mostly driving highway. This seems awfully low to me. A little history on the car...I had to replace the head gasket and have the head machined about 8 months ago. Once I got everything back together the car seemed to run a bit rough. Over the past months I have done everything I can think of to rectify the poor mpg. I have cleaned the fule injecters manuly and with fuel injector cleaner, I have replaced the fule filter, I have inspected all the fule lines, have timed the engine more than I can count, replace the plugs and wires, and it still has poor mpg and it idles rough when the transmission is engauged.

A few months ago the driver side CV joint went out and I had to replace it. This helped the driveability of the car, but my mpg was still low. I am going to replace the 02 sensor this weekend, but would that cause such poor mpg? I have also thought of checking the exhaust and muffler for cloggs.

Another tid bit of info that might help is when driving the car, everything goes great until I hit about 55mph. After this point the car shifts into 4th gear(OD) and the whole car shakes really bad. If I floor it, it drops to 3rd gear and accelerates fine and shifts into 4th gear at around 70mph and drives better but still has the shakes somewhat. Now if I ease on the gas and slowly accelerate, I can keep it from shaking and it rides much better at higher speeds. And also accelerates better when I apply more gas past 70mph.

I am gonna go ahead and give her a once over this weekend, but if anyone has any info to add, I am all ears.

Thanks for you time,
Luckybob Luckybob wrote:Hello All,

I have a problem with my 1990 Toyota Corolla. It seems I am only getting 20 mpg while mostly driving highway. This seems awfully low to me. A little history on the car...I had to replace the head gasket and have the head machined about 8 months ago. Once I got everything back together the car seemed to run a bit rough. Over the past months I have done everything I can think of to rectify the poor mpg. I have cleaned the fule injecters manuly and with fuel injector cleaner, I have replaced the fule filter, I have inspected all the fule lines, have timed the engine more than I can count, replace the plugs and wires, and it still has poor mpg and it idles rough when the transmission is engauged.


With all that this might sound stupid, but have you check the air filter?
A clogged air filter can rob Gas.

Also you might want to remove your throtlebody and look for excessive carbon build up. This 2 can cause gas mileage to drop.

Another thing which is highly missed.
THERMOSTATS! If your thermostat is sticking open and your car is running way cool. It will also rob your gas mileage.

Just a few sugestions. I have already replaced the thermostat. Did that when I replaced the radiator and replaced the head gasket. I have not replaced the air filter, although it looks fine, I can spare to put a new one in. Might have problems I can't see.


I will try to clean the throttle body and all fuel line this weekend.

Does anyone have any suggestions as to the shaking question?

Thanks for your replies,
Luckybob Hi,
I don’t have any answers but I have a bunch of questions.

When you say you’ve timed the engine more than you count, are you referring to cam timing or ignition timing or both? Have you had to repeatedly reset the timing or are you just rechecking? Exactly what procedure are you using to set the ignition timing?
Have you checked for vacuum leaks and proper vacuum hose routing?

As far as the shaking is concerned, do you notice the shaking in the seat or in the steering wheel?
If I understand you correctly the shaking is most noticeable when the engine is under cruising or very light load conditions, is this correct? Do you feel the shaking is an engine or other driveline problem? If you feel the shake is related to an engine problem try disconnecting the vacuum hose to the EGR valve and see if any improvement is felt.

Unfortunately a lot of things can come into play with poor mileage and shaking problems. I’m just trying to narrow down the possibilities a little. I have checked the timing on the engine a few times because I have done it myself, had a friend do it, and also took it into a shop to have it checked. I did the ignition timing, which isn't much because the distributer only has a small turning radius. I had the cam timed by the shop, and had my friend check my timing.

I have not check for vacum leaks, but this was mentioned to me before. I am not sure about how to do this. I would suspect I need to get some pressure gauges to check it. Mabye Autozone loans them?

TO tell you the truth the shaking seems to be coming from the driver side drive terrain. The only thing I could think that could be causing this is I replaced the CV joint not to long ago and perhaps I messed something up? But I don't htink that is the case because the car was shaking around 55mph before the CV joint was replaced.

The engine starts shaking really bad when I am accelerating and the car shift to 4th gear and I hit 55mph. If I floor it and the car goes past 55mph in 3rd gear I am home free. But as I said if I baby it and give very little gas it accelerates very slow in 4th gear, but it passes 55mph without shaking.

The car also has a slight shake when the car is stopped, but I have it in gear. Meaning I switch it to drive. If I switch it back to neutral, the car is smooth and quiet.

I do appreciate your help, and tommorow I am going to change the air filter, change my 02 sensor and check the drive terrain on the drivers side. I will also look into getting some gauges to check for a vacuum leak.

Thanks for your help,
LuckybobCar Repair Talk's forum.