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Discussion Starter · #1 ·
Hi guys, this is my first post in these forums. Im having issues with my 1999 Mustang GT regarding idle and an o2 sensor. Starting with the idle, without ac on, the car will idle at 950 to 1k rpms. The moment ac is turned on, the idle immdiately goes down to 700 to 750 rpms. It can also start surging sometimes between 500 to 800 rpms while the ac is on and off rarely but at higher rpms. Sometimes it will stall. The problem with the o2 sensor is bank 2 sensor 1. On the scanner bank 1 sensor 1 reads normal voltage above 0.1v but b2s1 always reads low voltage usually around 0.02v to 0.04v. Im getting codes p0133 and p1151. I saw a small exhaust leak right before the problematic sensor and it was welded but the voltage remains low. The sensors were replaced a few months ago and still getting the codes and weird idle and running rich. Sometimes the long term fuel trims can be high above 15%, other times they are low near 0%Any suggestions?
 

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Discussion Starter · #2 ·
I got an update, i let the car idle and looked at the scan tool and manipulated the wiring for the o2 sensor from the side of the car and saw the voltage rise to normal for a few secs. Seems like its a wiring issue. The idle idk if it will get fixed with this though.
 

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The idle could be a couple of things. It could be the IAC. I have read reports that say you cannot clean them but I cleaned mine with brake cleaner getting the carbon deposits out of it and it started working fine again. That was several years ago and its running fine. If the IAC doesn't work right the motor will not go into the high idle required for the air conditioner (also for the brakes and power steering). It doesn't hurt to clean the IAC every once in a while.

It could also be the butterfly valve inside the throttle body (TB). The IAC requires a certain amount of bypass air to operate correctly. Two things can cause it to shut too tight. Putting on a new throttle body that has never been adjusted. This is what I did. Or normal wear and tear on the TB stop screw causing the original TB to close too tight.

If it is the stop screw other symptoms are a surging idle and car dying when slowing or turning.

The stop screw is not an adjustment screw for controlling idle. That is not its purpose. If at anytime when adjusting the stop screw idle changes then you have gone too far. Playing with the system I discovered it takes more than one complete turn of the stop screw clockwise to change the idle. Usually the adjustment doesn't take more than a 1/16th turn clockwise.
 

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Discussion Starter · #4 ·
Hmm so for the idle it could be a completely separate issue from the faulty connection for the o2 which makes the car run rich then? Ill get the new connector spliced in, im crossing my fingers that it helps alot, but from what your saying it could be different issues. I put a stock throttle body on it, the idle had been adjusted with jb weld i think so when it was removed to set it to normal, I think it remained how it was before. Idk tbh I gotta see after the o2 sensor fix what happens.
 

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"Sometimes the long term fuel trims can be high above 15%, other times they are low near 0%Any suggestions?"

Is your MAF clean? When does the fuel trim change? At idle is it near 0 or 15? When you cruise does it hover at 15%? What about short therm? What does short term do when long term is hi? As for 02 voltage it could be bias lean, I watch my o2 and it fluctuates hi to low but I was reading that the pcm won't give a code unless there is no change for like 30 seconds.
 

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I put a stock throttle body on it, the idle had been adjusted with jb weld i think so when it was removed to set it to normal, I think it remained how it was before.
Idle isn't set by the throttle body stop screw. Only by-pass air is set. Why the JB Weld?
 

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Discussion Starter · #7 ·
Sorry I havent checked these forums in a while, Im guessing the Mechanic had adjusted the screw but it couldnt go any higher due to the threads not being long enough so he put the JB weld on. I managed to get the O2 sensor to read above 0.1v, the connector was the issue. It now runs in closed loop if im driving it but when im idling for a long time, It will go into Open Loop (Fault). I dont know if that is due to the long tube Headers maybe or I dont know if there is another exhaust leak somewhere there. When im accelerating I can hear a weird noise it is hard to describe. With the o2 sensors both working now, I attached a picture of the graph for them in closed loop. Also while in closed loop, the LTFT1 and LTFT2 always stay at 25% at idle and will only drop down once you get the rpms up to maybe like 4 to 6k to like 14% on both banks maybe less sometimes. If the car ends up going into open loop fault, the LTFT1 and LTFT2 will be at 0% and the O2 sensors upstream will both read 0v. I checked for Vacuum leaks and sealed all the ones I could find using a vape to flush them out and everything remained the same. One new thing I did discover was that the vacuum line that runs from the back of the throttle body to the evap purge solenoid in the passenger side fender was flattened. I need to replace this line but in the meantime I plugged the part that goes into the Throttle body and it did not really change anything except for a gas smell sometimes so I reconnected it since nothing changed plugging it up. Still will replace when I get a chance. Also in addition to all that Im getting the codes p0153 and p0171 as a confirmed fault with the CEL on. The idle issue I think is not related to these probably but who knows. The idle surges sometimes but rarely with the ac on. Also when im trying to move the steering wheel with ac on at slow speeds to maybe turn out of a parking lot, the car can stall and it gets really annoying. For example today i was leaving a parking lot and I had my ac on. I was doing a turn in the lot to get out and when I pressed the clutch to disengage the transmission so I can brake, the car stalled. It seems as if it cant compensate for load.
 

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Discussion Starter · #8 ·
Also I forgot to add, the car has no Cats from what ive been told. For some reason the o2 sensor bank 1 sensor 2 runs at 0v and bank 2 sensor 2 runs high at around 0.9 volts. I know these o2 sensors arent used for the performance of the vehicle but could there be a reason why they are like that?
 
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