Reader Question: Mr. Davis,
I have a question that I have not been able to find an answer to on other forums and I hope you can help me out!
I have a 1998 Nissan Altima that I was getting 26–28 MPG with it. When I changed the oil, I ran a bottle of STP complete fuel system cleaner through it (I usually use valvoline, but they were out at my local store), and got less than 20 MPG.
I figured it was just the blend with the cleaner, so I made sure I ran it almost to the bone before I fueled again. This is now the third tank of gas since I used the fuel system cleaner and I am still “barely” getting 20 MPG when I used to get 26-28! Are there certain additives in some of these mixtures that can foul fuel injectors?
The motor runs the same as it did before, with the exception of the poor mileage, no noticeable missing, and smoking, loss of compression or power. What about the possibility of ruining the O2 sensor with any of these fuel system-cleaning products? Loosing 80 miles of driving per tank seems ridiculous from an additive!
I do not think the additive did anything to your injectors or any other internal engine part. The additive COULD have dirtied the oxygen sensor, which in turn adjusts fuel mixture and plays a vital role in fuel economy.
I would expect there to be a change in engine performance and possibly a check engine light to be on if there really was a problem with the sensor. I have however seen a lazy oxygen sensor that was slow to make fuel adjustments NOT turn on a check engine light and not hamper running conditions.
So, if you just want to take a guess, replace the oxygen sensor. It definitely will not hurt anything. If you have a State Inspection Emissions Test in your state, you might want to pay the fee to get your tailpipe emissions tested. If you are really running as rich as you say you are I would expect to see higher than normal emissions numbers.
Other things that would affect fuel mileage:
(1) The need for a tune up – spark plugs, wires, fuel and air filter (dirty air filter can greatly affect fuel economy).
(2) Using the wrong weight engine oil – thicker oil is harder to move and requires the engine to work harder.
(3) Under inflated tire pressure – probably one of the biggest factor in fuel economy.
(4) Ignition timing is improperly set – unless you have a distributor, the computer is controlling the ignition timing.
(5) A vacuum leak under the hood somewhere – look and listen for a hissing noise under the hood with the engine running, could you have knocked a wire connector or vacuum hose connector loose when you changed the oil?