I'm a diesel guy so troubleshooting these gas engines isn't my thing... help me out here!

I'm told that it was running low on gas (low fuel lamp was on), but they didn't hear the chime yet. The engine cut out at a stoplight, they were able to get it started (no throttle needed, but running rough) and drove into a service station and filled it up thinking it was out of fuel.

It wouldn't start aftewards. I drove up there and was able to get it started but had to hold my foot partway down on the accelerator pedal to get it started and to keep it running. Drove it back here (a couple miles down the road) and started looking into it this morning. No codes at first. I started it up and ran it for 5 minutes or so at about 2000-2500rpm and got it to throw a code.

I scanned the codes and got a P0172 System too rich (Bank 1). The exhaust has that hot-stinky-catalytic converter smell too it.

It won't idle, you have to keep your foot on the accelerator to keep it running. It spits, pops and sputters. I read about these engines being bad about breaking valve springs. Hopefully its not that - doesn't seem to be odd noises coming from the engine.

At about 2 grand you can still feel a slight stumble.

At 3 grand and above its running smoothly.

Where should I start? Thanks for any advice.

 

You should start cheap.
Have you stuck with a regular tune up like spark plugs?
What about cleaning your TB? makes a big difference.
Your getting a faulty o2 code. Id start by doing the simple stuff like plugs and clean your TB, if that dosent work id be willing to bet your o2 is out. Or going out. If you decide to get a O2 be sure to only get NTK,NGK. Are trucks dont like other brands. Rockauto.com has them for almost half the price. One last thing UPstream and Downstream do matter on our trucks also

 

Thanks. If this information is of any use... let me know. They were taken after running the engine for several minutes, the first at 2500 rpm and the second at 1000, the lowest I could get it to run at without stalling.

Anything look way out of wack?

 

Im not to sure about all of the correct numbers, i havent used a tool that gave me that much INFO in a while.

 

I have, but mine doesn't read out using those abbreviations.... can you tell me what the following mean?:

F/T 1x1
STFT1
LTFT1

I also notice that you are reading 0.7 and 1.0V for the O2 1x2, which would lead me to believe you are getting a very rich reading 1.0V = Less than 10 AFR, which is EXTREMELY Rich. Here is a graph for narrowband o2 sensors.



So you've got an issue somewhere with an O2 Sensor, my guess is it is one of the Upstream (Pre-Cat) O2 Sensors that is out of wack.

Can you take some screenshots at idle (without holing the throttle down?

And if possible please post up a graph of each of the 2 (or 4) O2 Sensor Readings in Voltage versus time.

It will also help to give us Throttle vs RPM and Timing vs RPM concurrently.

My only other guess to a problem is a pretty big vacuum leak, so I'd also suggest spraying some brake cleaner on the vacuum lines while the truck is idling and see if you hear any idle drops, indicating a vacuum leak. or if you have a smoke tester, get to work.

 

Pull at least one plug per cylinder and check burn condition. Did this "event" just suddenly happen out of nowhere? Was the engine running fine prior to stalling at the stop light.

Check oil for anything that doesn't look right.

Check the coolant for anything "foul"

My first instinct suggests that you may have sucked up some water in the tank, but sampling the fuel rail, and starting the pump would be my next step with that.

The fuel rails have no returns to the tank on these engines, if water gets introduced, it can and often will sit there until it gets sucked through the engine.

You might try a "red bottle" gas line antifreeze (anhydrous isopropyl alcohol). or two or three. absorbs water to allow it to be burned

Use a scanner to see if there are any additional codes other than the single rich.

Inspect the intake manifold for any possible cracks.

 

Luckily you only have 2 O2s

 

Ok, here is the update.

I pulled the throttle body off and cleaned it up. I believe this truck only has 2 O2 sensors, one in front of and one behind the cat. I went ahead and replaced the sensor that was in front of the cat. Still running like crap.

Unfortunately I can't get ya'll any numbers of the engine running at idle, because it won't run at idle at all. I have to keep it at a minimum of about 1000 RPM to stay running, and at that RPM its still running terribly.

I wasn't driving the truck when the problem occurred, but as far as I know it was running just fine before it first stalled out at the light.

I like the idea about the water contaminating the fuel in the fuel rail... it would make sense if it was sitting at the bottom of the tank as it was running low on fuel. Is there any way to bleed fuel out of the fuel rail to see if this could be the problem? Since the person that was driving it managed to get it into the service station, they filled it up. I drove it back here which took about 10 minutes, and the total time I've had it running while trying to fix the problem is probably about 10 minutes, so a total of 20 minutes or so of running.

After I replaced the O2 sensor, I cleared the codes and ran it for a little while again, and after about 3 minutes the P0172 code showed up again.

 

F/T 1x1 - Fuel Trim Bank 1 Sensor 1
STFT1 - Short Term Fuel Trim B1S1
LTFT1 - Long Term Fuel Trim B1S1

I'll have to try again tomorrow and see if I can get some graphs... the program I'm using doesn't do them very well, and I don't believe I can graph multiple readouts at the same time.

Your theory about it running very rich seems good as well, again, I'm not a gas engine guy, but it would make sense that it would run really poorly at low RPMs because the engine is just being bombarded by fuel compared to the amount of air it receives, but at the higher RPMs where it consumes more fuel it smooths out because it is consuming more fuel, right?

 

The truck won't even idle, yet only throws one code? That seems suspect. What are you using to read the codes? Does it read stored codes?

Also, I'm no mechanic, but the timing looks really advanced in your 2k+ readings. Is the truck tuned at all (Superchips or the like)?

I don't think the O2 sensors are necessarily bad, I think they are complaining about how rich with fuel the exhaust is. You said you replaced the first sensor? Try removing the second one from the pipe, but leaving it plugged in. I'll bet nothing changes, meaning the sensors are good.