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Juan Crespo
Juan Crespo, Tech Trainer
Category: Ford
Satisfied Customers: 1493
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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Ford Mustang Gt: 03 Mustang Gt STFT 33% LTFT 0 at idle on both

Customer Question

03 Mustang Gt STFT 33% LTFT 0 at idle on both banks. Fuel pressure fluctuated between 43 and 52 according to scanner rough idle. Fuel pressure gauge shows less pressure than scanner reading 33 and 44 or so. Disconnect mass sensor idles perfectly.
Submitted: 5 months ago.
Category: Ford
Expert:  Juan Crespo replied 5 months ago.
Hi there.Your LTFT should be around 0 %. It being at 33% means your PCM has recognized that the engine in your car is running too lean all the time for some reason, and is trying to compensate. Have there been any modifications done to either the hardware or software?
Customer: replied 5 months ago.
The STFT is +33% and the LTFT is at 0 on both banks,on idle. There have been no modifications done. I did try replacing the Mass sensor with an Autozone part with no success in hopes of remedying the problem.
Customer: replied 5 months ago.
I was getting a code P1131 which i failed to mention on the earlier post, which i have since reset.
Expert:  Juan Crespo replied 5 months ago.
I see that I got the trims mixed up...Actually, since the PCM uses Oxygen sensor data to set fuel trim, P1131 makes sense as it indicates an Oxygen sensor that's not switching from lean to rich. This could be due to bad connections, bad wiring, bad sensor or bad PCM.Easiest way to diagnose this issue is with the aid of a NGS tester, hardest is by taking direct readings from the harness using a fast voltmeter capable of reading voltage variations occurring within 100 milliseconds. Which one do you have available?Also, have you had to add any coolant lately?
Customer: replied 5 months ago.
I have access to an Autel MS908P diagnostics scanner that I assume can read the O2 sensor voltage in live data. Does that sound like a tool I can use for what I need to find? If so what am I looking for in the voltage readings of those four O2 sensors exactly?
Expert:  Juan Crespo replied 5 months ago.
I'm sorry, but I'm not familiar with that Autel scanner (there are so many aftermarket tools it's just impossible to keep up with all of them...). What that means is I that don't know whether or not it can read voltage changes happening every 100 milliseconds or less.With the engine at idle in Closed Loop, we're looking for the two upstream Oxygen sensors to switch from Lean (0.2V) to Rich (0.8V) constantly within 100 milliseconds. Can you do that and let me know the results? If your scanner is not fast enough, can you get either a fast voltmeter or a scope so you could test directly from the harness?
Customer: replied 5 months ago.
Ok let me check that and I'll get back to you asap.
Expert:  Juan Crespo replied 5 months ago.
Customer: replied 5 months ago.
Both upstream banks no change on idle unless rpms are raised
Customer: replied 5 months ago.
Both upstream sensors read lean without change unless rpms are raised switches between lean and rich.
Expert:  Juan Crespo replied 5 months ago.
OK. What we need to know is whether or not the O2 sensors are switching from Lean to Rich within 100 milliseconds and at what engine speed. Can you get me that data?Also, will the voltage and switching be affected if a lean condition is introduced, such as disconnecting a vacuum hose? If so, how?Lastly, will voltage and switching be affected if you spray carb/choke cleaner on and around the EGR valve? If so, how? How many miles on this engine?
Expert:  Juan Crespo replied 5 months ago.
Sorry, need more info...Both sensors reading Lean at idle means the PCM should be trying to enrich the mixture by keeping the injectors opened longer. Can your scanner read injector on-time? If so, what is it?Also, this is beginning to look like a fuel delivery issue. Can you check and see if any misfires have been recorded?
Expert:  Juan Crespo replied 5 months ago.
Another reason for lean fuel trims and longer injector on-times would be unmetered air coming in, such as that from a vacuum leak. See the attached service bulletin Ford put out to that effect.Please let me know what you find.
Customer: replied 5 months ago.
Bank 1 sensor 1 reads .2 (lean) with no change on idle between 650 - 740
Bank 2 sensor 1 reads .3 (lean) with no change on idle between 650 - 740
Introduced vacuum leak through a hose, causes voltage to drop to 0 on both sensors and reading remains lean on both. Increased rpm to 1600 both sensors switched once between from lean to rich and back to lean.
Expert:  Juan Crespo replied 5 months ago.
Well, it looks like you're not going to be able to provide all the information I need in order to formulate a precise diagnosis. With just the data you did provide, I can say the issue is probably being caused by unmetered air entering the engine - I would have had more certainty had you provided injector on-times.Follow the inspection procedure indicated on the TSB I sent you and you might discover the cause to be a simple thing like a leaking PCV elbow coming out of the base of the throttle body. If that doesn't do it, then we'd have to deal with the returnless fuel system.Please let me know what you find.

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