Weird Live Data P0172



I have a 2000 Malibu 3100 V6 and my live data showed something interesting with fuel trims. After it switched from Open Loop to Closed, the LTFT went to 0.0% and plummeted immediately back to -29.7%. When it was at 0.0% for 3 seconds there was no misfire it sounded normal with a steady 750RPM. The upstream O2 is working great obviously. My MAP sensor reading was 6psi but some say 5psi is what it should be at idle? I am curious if it’s a faulty cam/crank sensor since they’re 17 years old. The old injectors were replaced and that didn’t help too much. MAF sensor was 3.78g/s average. Again I thought that 3 second span of perfect 0.0% LTFT was weird. Could the PCM be bad?


The computer can’t calculate the fuel trim until the upstream o2 sensors come on-line, which takes a few minutes for them to warm up and produce accurate data. At that point the computer goes into closed loop mode and can calculate the fuel trims. The manifold vacuum at idle is usually in the 15-20 inches of mercury range. If you type “convert 16 inches of mercury to psi” into the google search bar, you’ll see what that is in psi. Around 8 psi.

Yes, but more likely there’s a problem among these three

  • o2 sensor
  • MAF sensor
  • fuel injectors


MAF is new and O2 is a Bosch 13474 and works well with voltage fluctuation. The fuel injector harnesses could have a short or bad circuit. I remember accidentally pulling wire ends out of cylinder #1 injector plug in and then I just stuck them back in but I bet that could be a big problem. I do have a new harness for that cylinder injector. The Denso O2 would be fixed rich or lean for 2 second fluctuations so that’s why I replaced it with the OE Bosch since that was what came off. Funny it runs -27.5% LTFT with Bosch O2 instead of -29.7%!


All of the injectors have to pass a specified amount of gasoline at the rated fuel rail pressure. That number is coded into the software that calculates the fuel trims. On my old Rabbit I could pop the injectors out one by one and measure the volume fuel flow, and compare one to another too. It was simple to do b/c the injectors were attached by long-ish fuel hoses to the fuel distributor (what would now be called the fuel rail). That’s not as easy with the modern kind of FI where the injectors are attached to the fuel rail. But you may have to rig up something like that to test the fuel injectors. They need to deliver the proper flow rate, and they must match each other. A shop with fuel system experience could do this for you of course. If you try it yourself, one bit of advice. Get a big fire extinguisher you know works, and do it away from buildings.


Just to add. I notice that the LTFT stays fixed -18.0% at 3000RPM. Then when I let off the pedal the LTFT would go to 0.2% and it sounded great with no missing then immediately plummets -27.5%


Negative fuel trims means when the computer uses the MAF data to calculate the fuel injection rate, the O2 sensor complains it is running too rich. And the computer then cuts down the gas by 27% to satisfy the O2 sensor. The O2 sensor rules when there is a disagreement, b/c the O2 sensor is a better measure of the tailpipe emissions. During quick transitions of the gas pedal the MAF responds more quickly than the O2 sensor, so there’s a brief period where the MAF rules b/c the O2 sensor hasn’t had time to complain. That’s a possible explanation for what you are observing there. I expect you’re looking at one of these being the problem

  • Faulty MAF
  • Faulty O2 sensor
  • Faulty injector(s) or imbalanced injectors
  • Outside air leaking into the engine, bypassing the MAF
  • Outside air leaking into the exhaust system, not coming from the combustion chambers.
  • There’s some chance one of the other fuel injection rate calculation sensors is faulty. Engine coolant temperature sensor, intake ambient temperature sensor, throttle position sensor are the other main inputs to that calculation. If you Malibu uses an MAP device in addition to the MAF, that’s another one.


MAP means manifold absolute pressure. It pokes right into connects to the intake manifold through an air-tight seal and senses the air pressure inside. At sea level the air pressure is about 15 psi. That’s what the MAP sensor should read if the engine wasn’t running and the car was at sea level. A little less if the car is at a higher altitude. At Denver the air pressure is about 12 psi. With the engine idling the air pressure is less than that, b/c the engine is creating a partial vacuum in the intake manifold with the throttle valve nearly closed. For most engines that idle vacuum is in the 15 -20 inches of mercury range. Inches of mercury is just another unit system to measure pressure. 15 inches of mercury is about 7 psi. So at idle with the engine providing 15 inches of mercury vacuum, the pressure in the intake manifold should be the ambient air pressure minus the vacuum reading, or at sea level 15 psi - 7 psi, or about 8 psi. If you’ve got a higher vacuum than 15 inches of mercury, which is the sign of a well tuned, good compression engine, then the MAP at idle would read a little less than 8 psi; so yes, 5 or 6 psi would be a good reading for a MAP at idle.


When car was warned up at idle it started to go over 195°F towards 225°F because fans suddenly stopped turning on. Then the OL and OL-Drive took off the Closed Loop control for 5 seconds then it went back. I’m thinking ECT sensor.


ECT 's rarely fail, but are easy enough to test.


Is this by chance a flexible fuel model . . . ?