Tibbychick

So I got my CEL scanned and it's code P1128, Long term fuel trim error - System lean (Fuel System Lean - Part Load). Guy said it's a dealer code.

Probable causes:

1. Low Fuel pressure - fuel filter plugged (I already replaced the filter, so it can't be this)
2. Failed fuel pump (Not this either, the car runs fine)
3. Engine mechanical condition (that's pretty vague, but I've kept the car in shape)
4. Large vacuum leak (Guy at the store said leaving the car on while gassing up could cause a vacuum leak, and I do leave it on sometimes but I don't think it's this either)

I've replaced the TPS sensor, MAF sensor (with a used one though, I thought mine may be blown, who knows if either is) both O2 sensors, and just recently put in a 1.8 ECU.

Some issues I have are:

The O2 wiring was redone a few years ago. When I lowered my car, the was a period of time when it was scrapping on the ground. The wiring may be bad, but then wouldnt that throw a CEL related to the O2 sensor?

Could it be the IAT sensor? I had it dangling in the open for a bit, perhaps it's blown?

I've always driven with the screen of the MAF removed, would that cause this CEL? Some of you have said no, but I wanna be sure.

Any ideas?

DTN

TROUBLESHOOTING FOR DTC P1123, P1124, P1127, P1128 DTC Diagnostic Item P1123 P1124 P1127 P1128 Fuel Trim Additive Air System Too Rich Fuel Trim Additive Air System Too Lean Long Term Fuel Trim Multiplicative System Too Rich Long Term Fuel Trim Multiplicative System Too Lean
DESCRIPTION a. Intake air system. b. Exhaust system. c. Evaporative emissions control system (includes purge control solenoid valve). d. Fuel injectors. e. Fuel pressure regulator. f. Fuel pump. TROUBLESHOOTING GUIDE [FOR SHORT-TERM FUEL TRIM] a. ECM is in closed loop operation. b. Engine speed is below 1000 RPM. c. Engine coolant temperature is above 158°F (70°C). d. Canister purge system is not operating. e. Mass air flow is less than or 6.6 g/s. [FOR LONG-TERM FUEL TRIM] a. Engine load exceeds 2.0 milliseconds. b. Engine speed is below 1000 RPM. c. Engine coolant temperature is above 158°F (70°C). d. Canister purge system is not operating. e. Mass air flow is less than or 5.5 g/s. TEST PROCEDURE






















Now, keep in mind that long term fuel trim is what the ECU has "learned" Short term is quick adaptation, long term is adjustment adaptation. Make sure you reset your ECU when performing checks.

The first thing they say is to check your intake air system for vacuum leaks. A larger MAF or an unobstructed/turbulent MAF would be the same as a vacuum leak.

Bullfrog

so this is possibly from the larger MAF sensor housing?

Tibbychick

No, because it's only been on the car for two days. This is a longterm CEL, its been on there for at least a year now. So it's gotta be from the MAF screen being out.

HyundaiKitCoupe

beats me. welcome to the anti-emission club. i don't think it's your O2's. are they extended and how? how is your exhaust?

i would guess what DTN is saying, the different MAF is probably turbulent. but what do i know.

Tibbychick

Yeah the O2s are extended. The exhaust is ok, the flex pipe on the down pipe is a bit stretched out. Im having it replaced hopefully this weekend.

As I said, this code has been there for at least a year now. Probably longer. I've had the MAF screen out since the first day I had the car. Same with the black tib, always had a CEL, and always had the MAF screen out. I just never bothered to get the CELs scanned until today.

DTN

The MAF sensors are engineered to have a certain ammount of restriction, the air is supposed to flow a certain way, the tube has to be a certain length and diameter, the heater has to have a certain resitance and so on and so forth. The MAF sensor is not something to be messed with unless you're tuning. Definately don't mess with the MAF while you've got a CEL.

Loneshark

ya know, this turbulent air thing boggles me, I cut mine out (the diffuser in the MAF) and i dont have a cell ! car runs great, idles great, wideband reads satisfactory... ??

Tibbychick

Ok, just put the old MAF in with the screen, and reset the ECU. CEL is gone, I'm letting it run for 15 mins. I can't drive it anywhere till later.

One thing I found odd is that when I pulled the fuse for the ECU, it was a 60 amp fuse, while on the diagram on the fuse box cover it's pictured as a 20 amp fuse. Would that make a difference to anything, or should I not worry too much about it?

Jed118

I'll send you the diffuser, but you'll have to glue it in place... it will work as long as you epoxy the sides in place.