03 Neon sxt 2.0 sohc automatic p0171

My car has the p0171 lean fuel system. My pressure checks out good to my knowledge. I'm getting about 50 psi, the needle on my guage fluctuates a little i think from the vibration of my engine. I did a snap throttle test and the needle goes down maybe 1 psi or 2 at the most of a second then returns to normal. I did a volume test and it checks out as well.

Fuel trims: With the car in park the fuel trims are normal. When I did the snap throttle a few times really fast I noticed the positive fuel trim maxed on maybe 1 out of 5 consistent snaps for a brief second.
In drive when I am going down the road the trims almost immediately max out around 32 or so and stay there.

I've never heard of trims looking mostly normal on snap throttle but consistently maxed out while driving. A garage I took it to said fuel pump but I have been betting against that.

This is real good video that can help you determine if you are loosing fuel or if the MAF is misreporting by watching the upstream O2 sensor during a wide open throttle pull.

I didn't think dodge used maf sensors.

That is correct for neons. They do not have maf sensors.

But the video may still be helpful. I was wondering before how to tell if i'm running rich or lean at wot (because I know the pcm ignores 02 readings at this point ) but I obviously never thought too hard about just watching my 02s at wot.

Part of the problem for me also is that the neons use a reference voltage on the signal wires and i'm not familiar with what range of numbers I should be in for stoichiometry.

cheryl hartkorn wrote: I didn't think dodge used maf sensors.

Good point!
I kind of remembered that as I was typing out another post, lol. Thanks for pointing that out.

Does it idle normal? I ask because a MAP engine with a vacuum leak would show normal trim numbers at idle. It would interpret the increased manifold pressure as a load condition and increase injector pulse width to keep up with the perceived demand. The result would be higher than normal idle.

The idle is never too high. Right at start up it idles around 1k to 1100 but that's in open loop I believe.

When the car warms up the idle sits around 744 and usually sinks to 500 at red lights but I think that's another issue all together.

350boatracing wrote: But the video may still be helpful. I was wondering before how to tell if i'm running rich or lean at wot (because I know the pcm ignores 02 readings at this point ) but I obviously never thought too hard about just watching my 02s at wot.

Part of the problem for me also is that the neons use a reference voltage on the signal wires and i'm not familiar with what range of numbers I should be in for stoichiometry.

During your WOT run, watch for the upstream o2's to stay above 800mv right up to the shift into second gear.
If you're not comfortable with the o2 data, you could try the same thing with a fuel pressure gauge taped to the windshield.
I've done that more than few times.. :whistle:

I did run the car the other day with the guage hooked up and I didn't notice any significant drops. I have a returnless style fuel system should it drop at all while doing a snap? I also feel that since it says lean the whole time while driving that pressure would stay dropped and not just drop a pound for a second and then read normal. Unless my fuel specs are just low altogether. I read somewhere it says 43 to 53 psi for those cars but I'm not sure if it is for that exact year.

The spec I have is 58psi +/-5
So at 50, you're already out of range, then dropping another 1 or 2 psi on snap...
Looks like a probable cause of the p0171.

Thanks for the specs. I was at 53 before at initial (koeo) pressure. I'll have to verify whether its 50 or 53 at idle and snap. I will get back with the results in a week or two hopefully. I will also try to get the scan tool again to look at my trims under wot.

I didn't get a chance to check the fuel pressure again but I have pictures of the graphs for WOT and a snap throttle. They are a little blurry but most are still readable. I'm new to reading and understanding how the signals work together from all the sensors so if anyone would mind taking a look and seeing if there is anything obvious that is wrong I would appreciate it. One thing I noticed is that the iat temp was like 74 degrees fahrenheit and it was about 14 degrees fahrenheit so I'm thinking it's because of the engine heat.

now for WOT

WOT contd...

one last WOT pic

In the second set of pictures, am I seeing 23% throttle at WOT? Maybe nothing or maybe my eyesight. Just looks like it to me. Seemed odd.

Yeah, I see the TPS and MAP low, not sure if the scanner caught the WOT snap or not. The data bus rate may be too slow to show the kind of detail you're looking for.

I also saw the long term trims go to 32% in the first set of pictures, BUT that was during open loop operation. I do like that the O2 signal isn't pegged lean, at least while you were testing it. Could be going dead at other times

If you're willing, maybe try taking another capture? Narrow down your PIDs to engine speed, MAP voltage, both O2's, TPS voltage, and loop status. Wind out first and second gears at WOT. Highway on-ramps are great for this.

Then, save the file on your Modis. That way, you can use ShopStream Connect to easily review the data later, OR send it to us! Most members around here that have a Snap-On scanner also use ShopStream to manage data captures like this.

If I can figure out how to get signals from the scope maybe I'll just manually tap my signal wires so the data capture should be a lot faster. Being its a dodge I was hoping to get my readings from park with a snap throttle to spare the transmission but if I need to I'll go abuse it a little in 1st and 2nd when I get time.

You don't need to attempt to kill it. Just a full throttle drive. Don't need huge revs though. A problem may still show up on the spot but the fuelling strategy is totally different from neutral to driving because of the load difference.

Can anyone verify that the shopstream connect link is safe to download?