Q: Rich idle only, negative STFT

asked by on

I've been having an issue lately with rich idle. Additive trim (stft) dips into the negatives very quickly, long term trims are right around 1. CEL for P0172 keeps coming up. The car is only running rich at idle, leading to the ECU to pull fuel which is affecting my trims and causing the code. It can correct enough to where the idle AFR is 14.7, but engine load at idle is around 30%. Compression is good, 160-170psi across all 4 cylinders. No exhaust manifold leaks, just checked that. If I induce a vacuum leak at the intake manifold it will help with the fuel trims. O2 sensor reads spot on with my wideband AFR gauge, so I dont suspect the o2 sensor is going bad. Car is setup to be MAP based, not MAF. Any help would be appreciated!

My car has 160000 miles.
My car has a manual transmission.

Some common causes of too much fuel being delivered in a situation like that you describe are: a faulty coolant temperature sensor (the temperature signal is a part determinant of injector pulse width); leaking fuel injector(s) allowing too much fuel to enter the combustion chamber; a faulty fuel pressure regulator; and worn spark plugs. To resolve this, the recommended service to request is a check engine light diagnostic during which a certified mechanic from YourMechanic would test the appropriate individual vehicle components or sub-systems (often a sensor, circuit, and the like, as noted in the short initial list above) to pinpoint the fault that has caused P0172 to set. If you have further questions or concerns as you decide what to do, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Rich idle only, negative STFT

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Audi A4 makes noise In Sport Mode

Switching to sport mode only changes the transmission software. So I don't think you are actually experiencing a transmission problem. But the "sport mode" program enables the engine to reach higher speeds before shifting gears, so it could be that...

Q: Tiptronic transmission -reverse gear slide 2002 VW Jetta

Hello - your problem looks like it might be a problem with the valve body of the transmission. Wear within the valve body results in low clutch engagement pressure - forward or reverse - and slippage as you have noticed....

Q: 03 Honda Accord EX which starts, shakes, then dies, yet when restarted cranks up just fine and runs. Ideas??

My first guess is that you may have a leaking fuel injector. When an injector is leaking from the tip, there is no visible sign. When you shut off the engine, the tip of the injector will drip into the...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.