What is the cause of my decreased mileage?

1989 Camry 4 cylinder automatic driven in California, mostly city driving and pretty much the same type of driving since I've had it. No snow.

Purchased in December 2014 with passing smog test. Idles smooth at about 700 rpm. Shifts okay. No water or smoke exiting tailpipe. Starts okay.

In the beginning of January 2015, I: changed and gapped spark plugs (copper NGKs that Autozone's website said are proper), spark plug wires/distributor cap, distributor/ignition rotor, alternator and PS belts, synthetic oil, long-life coolant, air filter. My MPGs went DOWN after doing these interventions

I checked the hoses and didn't see any cracks. I confirmed spark plug gaps as correct and that they are the correct ones. There is no discoloration or debris on the spark plugs

Date Mileage (averaged over the entire month)
Dec 2014 25.7
Jan 2015 19.0
Feb 2015 16.8
Mar 2015 19.1
Apr 2015 16.9
May 2015 17.0

Thanks!

 

Two other questions:

Can I use platinum spark plugs to improve mileage, or are they improper to use in this 1989?

I tested the spark plugs by reinstalling the old Nippondensos after confirming the proper gap. The car immediately idled roughly, idle speed at rest varied from 400-700 rpm, and tail pipe made a putt, putt, putt sound. These resolved immediately upon reinstalling the newer spark plugs. Obviously, something's wrong with one or more spark plugs. What would that likely be? Thanks

 

No discoloration on the old plug electrodes? The engine runs fine with the new plugs?

You can use whatever plug you want but mileage is not going to change, the plug will just last longer.

The decreased MPG started after your list of repairs was done?

The engine has a cold start injector. A fuel injector that add more fuel when the engine coolant is cold. The injector is on a timer. Make sure the injector is off after a few minutes and is not leaking fuel when it supposed to be off.

Sometimes the O2 sensor starts to fail and the result is a richer mixture and decreased MPG.

Suggest if not having one, getting a Haynes manual or other service info.

 

Thanks.

I first noticed the decreased mileage immediately AFTER I had changed the spark plugs, ignition rotor, and cap/wires. My notes said that the car idled smother after doing this. I double checked the wire position and connection.

I have a Haynes manual, but didn't find a list of ideas for this problem. Haynes describes testing the cold start injector via resistance in the terminals, but not testing for leaks. How would I do that?

The check engine light works and goes off after starting the car, as do the other dash lights, so I don't think that the oxygen sensor is faulty.

Thanks

 

An oxygen sensor can start to degrade over time and not be bad enough to cause a check engine light. It could just give the ECU a false reading on the amount of O2 available to the engine.

 

Thanks.

I suspect the oxygen sensor is the original one from the factory.
How would I test the oxygen sensor?
If I took the car in for a smog test, would I likely diagnose the problem with low mpg?

My mileage fluctuates widely from fill up to fill up. I thought that the in-tank fuel pump might be bad, as my gas gauge seemed to move slowly toward 1/2 way, then rapidly down, but repeated fills after driving only 50-100 miles ruled this out.

 

Hope this helps.
How to Test the Toyota Oxygen Sensor | eHow

 

As a point of reference, I was driving back from the beach one year in our Caravan. We had been doing about 22 mpg, fully loaded, for most of the trip. Out of the blue it dropped to 16 mpg and stayed there the rest of the way. Turns out, the Ox sensor had locked in at a low voltage(lean) and the fuel control was pouring in the fuel to try and bring it back up. New sensor, back to 22 mpg.