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My 96 Dodge Neon throws a P0171 code.
When I’m on the highway I would get up to speed to about 70-75 and then cruse at that speed. Every time I’d let off the gas, the car would jolt like it had misfire of some sort. Doing that 2 or 3 times when then lead to a CEL. Above 70 the jolt is very noticeable. Going between 60-65, you can barely notice it. Below 60mph, no jolt. When ever I’m below 60mph I never get the CEL.
I would put my cheap reader on the car and it would come up as P0171. I’d clear the code, look for loose hoses, tighten things up, but I’d get on the highway again, letting off the gas at 70+mph a few times and the P0171 would come back.
I attached a fuel pressure gauge on the fuel rail and was reading about 50psi at idle. The service manual says it should be about 49psi. At idle, fuel pressure looks good. When I rev the engine, the needle my gauge may drop a psi or two when I initially hit the throttle but it returns to 50 instantly. With the car is off, the fuel pressure will drop down to 40 psi after about 5 mins.
Today I received a Actron CP9185 scan tool in the mail so I can get better look at the data behind the code. The freeze frame showed that ST and LT fuel trims were both above 20% at 63 mph. It also showed I had a another code for a high fuel level voltage, but no OBD code to go with it. Fuel gauge doesn’t come down off of full until you drive about 60 miles, and 1/4 tank seems to really be 1/8 tank.
I cleared the codes and took the car for a spin on the highway. This time the car didn’t do its misfire thing nor it did throw a code. It always has in the past, but the only difference today was I had the scanner plugged in. I started recording when I was on the on-ramp, and tried to let off the gas when I was at speed to see capture anything related to the misfire like symptom.
After downloading the data, my LT fuel trim was all over the place. It would be -9 in one frame and then +21 in the very next frame. Is this normal? Attached is my downloaded data.
I wonder if something is out of sync and some sensor is updating fast enough when I let go of the gas or there is some issue with my fuel pump.
Any ideas?
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