2008 Cryslet 300 fuel trims

I've got a 3.5l Chrysler 300. For the most part it runs and drives good. The main issue is that on medium throttle, it surges a little. It'll take off like a rocket for 1-2 seconds then drop power for a
Second or two then takeoff again. It'll do this until you get to speed or let off. It was really bad after the engine swap but hasgotten way better..
Fuel trims are -3% stft & +20% ltft at iidle.Revved up, fuel trims go to 0% stft & 15%-20% left. No codes, 23-24 mpg hiway @ 75-80 mph.

Surprised there's no codes with a long term value that high. You need to figure out why it's trying to dump so much extra fuel in.

That's why I'm here. What would make Short Term read lean or near zero but cause Long Term to read high no matter the condition.

Ideally, you want to see your short terms fluctuate between +5 and -5% and your long terms near 0%. If you're not seeing that, read on....

Lean condition: If the oxygen (or air/fuel) sensors report they're seeing a lean condition- that is, there's more air than fuel detected in the combusted mixture- the computer will attempt to increase the amount of fuel going in slightly then wait to see what the sensors report back (this increase will be reported on the STFT PID, by the way). If the sensors still report the mix is lean despite the increase, the PCM will try to shoot even more fuel in and, again, wait to see the sensors' response. (STFT value will rise even more). If, at this point, if the sensors say the mix is good (stoic), the PCM will keep fuel delivery at this higher level. If the car continues to be driven and ignition isn't shut off, the STFT values will fall (back to ~ the +5 and -5% range), and the LTFT values will increase by the same amount. This doesn't mean that amount of fuel commanded in has changed, rather the PCM has now "learned" and committed to memory the higher level of fuel that is now required to make everybody happy. Thusly, when the ignition (eventually) gets turned off, the computer doesn't need to go through the whole rigor moral of the above again to know what things should be at. Once the engine warms up, it knows where to go to.


There are times where an O2 will report a lean condition regardless of how much fuel a PCM tries to throw in. In that situation, you will see both the short and long term values pegged high and stay that way. How high they go depends on the car.

Anyway, keep in mind that at no point in the first scenario has the PCM "fixed" what was causing the lean condition. It just compensated for it thereby making the vehicle less polluting than it would be otherwise. The problem is still there. The CEL may come on (depending on how out of whack the trims go/the threshold the manufacturer has set) alerting the driver something's up, which may prevent the car from passing inspection the next go-around.

Sooo, long story short here is the high LTFT levels and low STFT levels you're seeing are normal.... given your situation. Your sensors think they're seeing a lean condition, the PCM commanded more fuel to go in until sensors reported stoic. Car continued to be driven this way, STFT went back down as LTFT levels went up/system learned what the new fuel delivery rate should be once car enters closed loop in the future.

The causes of a lean air/fuel mix vary and Google could probably give you more of the whole laundry list than I could. Off the top of my head, low fuel pressure, poor flowing injectors, vacuum leaks are some popular ones. Your O2 sensors may be faulty also, which may be telling the PCM things are lean when they actually aren't. That one's always fun.

i think that it has to do with your throttle try cleaning it or adjust it because air is going in maybe beacuse is dirty and gets stuck that why your lean issue i think

Thanks for the excellent explanation. Kind of read the same thing online. So last night driving home on the interstate at 75mph I plugged the scanner in and watched it at 2500 rpm. Stft was bouncing between -3 & +1 for both banks equally. Ltft bank 2 was holding at +5 & Bank 1 was +9-10. Let of the throttle and they really jumped up with equal separation. No matter the rpm, Bank 1 was always 4-5% higher than bank Bank 2. I read that ideally, stft + ltft should be no more than 10%. I'm thinking I must have an air leak on Bank 1 or a weak injector. I already had one fail on that bank. No codes until it finally gave out. Idled a little rough before that but never knew why. Fixed it and now it runs smooth. Just trying to get it running the best I can. Still has the hesitation thing going on. Itd be a beast if it wasn't for that. Fun car to drive with decent gas mileage.

qwerty.qwerty wrote:

What would make Short Term read lean or near zero but cause Long Term to read high no matter the condition.

Short story is, the job of the LTFT is to keep the STFT at zero. High LTFT at idle that decreases as RPM rises points to a vacuum leak. Depending on the severity of the leak, the RPM's might have to be quite high to overcome the added unmetered air. With Chrysler a code will not set until LTFT hits 35.

As with most DIY'er your testing equipment is probably limited. Using your scan tool set your PIDs for the STFT and LTFT only. This way you'll get a bit faster readings. Note the trims at idle then increase RPM to 1500. Note readings again then increase RPM to 2500 then 3000 noting the results at each interval. If the LTFT moves to or towards 0 you have a vacuum leak most likely.

Get your baseline data before doing any repairs or adjustments. Cleaning the throttle body and MAF will always help, but do one at a time then recheck trims after each. Vacuum leaks can be challenging so be patient. Resist throwing parts at the problem. You can use water to test for vacuum leaks. If you have propane that is a safe method. Brake or carb cleaner work well but are flamible.

+or- 10 trims are considered acceptable.

Let us know your results and testing equipment you have so we can better help you.

Fuels trim shown above.
2500 rpm:
Bank 1: stft(-3 to +1) Ltft (+9 to +10)
Bank 2: stft(-3 to +1) ltft (+5 to +6)
Idle:
Bank 1: stft(-1 to +1) ltft (+15 to +17)
Bank2: stft(-1 to +1) ltft (+8 to +10)

Dirty MAF and or throttle body would show even trim values both banks. Bank 1 has higher LTFT than bank 2 but both get better with higher RPM. Suggests leak is on bank 1. Check the intake runners and injector seats on bank 1.

As scannerjohn mentioned, the problem seems more pronounced on bank 1. To expand on some things he said, set your scanner to display only the LT and ST fuel trims, locate which side of the engine is on bank 1 and grab a propane torch and shoot the (unlit) gas in and around that area slowly moving it around while the engine is running. Look at the scanner PIDs while doing and see if anything changes with those values. Also listen for RPM changes. Where it changes is where your vacuum leak may be.

Doing this in a garage or on a non-windy day with the rad fan off helps keep the propane where you want it to go. Making sure the compressor doesn't turn on (if equipped) or turning off anything that might change the load or RPM while you're doing the test is also a good idea. Add a short section of hose to the propane torch if you're having trouble getting the gas close enough to where you want it.

Try and keep the propane away from the intake, when you're doing this part of the test. If you see trim values on your scan tool go up on both sides simultaneously, you may have inadvertently shot some propane in there.

You can apparently spray water from something like a spray bottle to look for leaks, too. I know Paul has done that in at least 1 of his videos. I don't remember if doing that affects RPM or fuel trim readings any, though. Perhaps you're just looking for where it bubbles.