My driver is a 98 Honda Civic and recently the check engine light came on.

The codes I read were PO170-Fuel Trim Malfuntion
and PO171-System Too Lean.

My car when warm will sometimes idle extremely low, say 200 rpms. Something is going on and I'm looking for a starting point. Any of you guys come across these codes before?

Dave

System too lean means that the PCM is trying to ADD fuel but has run out of adjustment and it still needs more fuel. I would suspect a vaccum leak. Don't know Hondas real well but know Chrysler extremely well and the OBD II systems pretty much all work the same. Look for a reason the car would be running lean-low fuel pressure, vaccum leak, possible pcv problem.

In addition to what's already been offered might be the Mass Air flow sensor if it's a hot wire type and its dirty. Thats what I always found on Mazda's for the P0171 lean code. You may be able to "clean" the wire with carb cleaner.

Update. I filled up my tank of gas and mileage rate has dropped by over 10 mpg! This may be an O2 sensor. Is there a sure way of testing it before spending that money for a replacement?

I looked at the mass flow sensor and it is just a bulb that pokes into the air cleaner. I didn't see the wire type. Could this still be it?

My idle continues to be extremely low. I pulled a couple spark plugs and the rim of the plugs is carbon black. The tip itself is clean.

Dave

The bulb that you see in the air cleaner is probably an Intake Air Temp or IAT sensor. Follow the tube from the throttle body down, the MAf will be between the throttle body and the air filter itself. I've seen really dirty air filter elements take out the MAF. You have the symptoms of a bad MAF to me.

I had basically the same thing on my '98 Lumina recently, turned out to be a plugged fuel injector ... Fuel trim (lean) code.

Turns out my Honda does not have a mass airflow sensor. I tried reading the current from the O2 sensor but my meter isn't sensitive enough.

Dealer said the MAP sensor acts in place of the MAF sensor. I'm not sure how manifold pressure is the same as airflow?

SSuperdave, you were right, the sensor I pulled was the intake air temp.

Dave

You have what is referred to as a "speed density" sytem. Instead of measuring air speed with a MAF sensor the MAP sensor comnbined with the TPS input tells the PCM how much air "should" be going into the engine, this is actually a more effeint way of running as MAP sensors rarely fail where MAF sensors fail quite often in comparison.

Just to give an update, I had to replace my battery. Very low starting amps and was not charging. I did this on Sunday and so far, I have not had any low idling problems. Perhaps the battery terminal was too corroded for a good connection or something, I dunno.

Gas mileage still appears off but I discovered the exhaust manifold is cracked so I have to deal with that now...

Dave

Originally posted by ScoobyDoo:

Gas mileage still appears off but I discovered the exhaust manifold is cracked so I have to deal with that now...

Dave You just found your lean code...