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Discussion Starter #1
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

[ 06-19-2004, 01:25 PM: Message edited by: Scooby Doo ]
 

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A good place to start is to check the fuel pressure. See that it is within spec and increases as manifold vacuum drops. One thing you can do is hook up the pressure guage and energize the fuel pump by shorting out the pump relay contacts for a second or two. Then shut off the fuel pump and see that the pressure holds. If it does not, clamp off the return line with a pair of vice-grips (presuming a rubber return line - not rigid). If the pressure holds after clamping, the pressure regulator is leaking.

The fuel trim code simply means that the computer has maximized the injector pulse width and still can't get the mixture (as read by the O2 sensor) set to the stoichiometric ratio of 14.7:1. The O2 sensor probably isn't bad, since a lean mixture gives maximum output on the O2 sensor. Exhaust leaks and a faulty air injection system can cause the computer to see an abnormally lean mixture though.

Read the plugs and check the fuel pressure for starters.
 

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I had the same code on by 96 Ford deathstar, it was a vacuum leak from a rotted rubber hose right on top of the throttle body. I don't know what kind of rubber car manufacturers are using, but it sucks. Turns to a really soft gelatenous mass and the outer surface rubs of on your hands if you touch it.
 

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Discussion Starter #4
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 be it?

Also, if the fuel pump was going south, shouldn't I notice a problem when I'm accelerating at high rpms? So far, there has been no hesitation in that condition.

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
 

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Discussion Starter #5
I'm still troubleshooting my Honda. I thought it was the mass airflow sensor and it turns out the Honda doesn't have one.

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

I tried reading the voltage from the O2 sensor to test it but my meter isn't sensitive enough.

BTW, a lean signal by the O2 sensor give minimum output.

Dave
 

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MAP = Manifold Absolute Pressure a device converting intake manifold pressure into a voltage for ECM input with the addition of other inputs for look up of timing and fuel requirements. Used mainly in Speed Density systems. ECM "collects" all these inputs, 'looks up' on a table for timing & fuel requirements and executes requirements.

MAF - Mass Air Flow a device measuring the amount of air the engine is ingesting, also used with other inputs to determine fuel and timing requirements. Used sometimes in conjunction with MAP sensor. A faster "tuning" system as a whole because ECM now knows from the MAF input the amount of air ingested, thus ECM makes its decision on timing and fuel requirements.

MAP sensor can be troubleshot using a hand vacuum pump and ohmmeter. Connect pump to MAP nipple and ohmmeter to two pins. Apply vacuum and watch ohmmeter for a smooth transistion from WOT to idle, ie., no vacuum to 29" vacuum.
 

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That car should have an OBDII compliant engine controller on it. One thing about OBDII is the codes are more dependable that the 1980-1996 controllers. Get a cheap hanes manual or even chilton. They should have a sensor diagnosis tree in there that will allow you to check the MAP, O2 sensor, and anything else you suspect might be contributing.

Based on the codes it sounds like the ECM fattened the mixture enough that the engine should have burned rich, but the O2 sensor told it was lean. By the way, lean means high O2 sensor voltage. Try grouding the O2 sensor input to see if it has any affect. It's highly probable that, fuel pressure, injector issue, MAP sensor problems, or an exhaust leak could cause an abnormally lean reading.

What do the plugs say? If they are black with soot, then the computer don't know WTF is going on.
 

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Discussion Starter #8
Per my Haynes Honda book:

"The oxygen content in the exhaust reacts with the oxygen sensor to produce a voltage output which varies from 0.1 volt(high oxygen, lean mixture) to 0.9 volts(low oxygen, rich mixture).

Plugs do have soot on them so I agree, the computer is confused. I do continue to idle around 200 rpm's when the engine is hot. And I continue to have poor gas mileage.

I did check the voltage on my O2 sensor yesterday. I read a steady 450 mV. From what I understand, I'm supposed to end up with a reading that bounces from lean to rich very regularly as the computer is making adjustments. I unplugged a vacuum hose to induce a lean condition and the O2 sensor was unresponsive so at this point, its the prime suspect.

Dave
 

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My bad, you are correct - lean means low voltage.

O2 sensors will become slow to respond. I suspect it is either caused by soot on the bulb or the fresh air side is plugged with road goo.
 

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Discussion Starter #10
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
 

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Thanks for the update scoob. Seems like symptoms are caused by at least two or three things. Let us know how it does after a couple of drive cycles.
 
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