-25% to -30% Fuel Trim at Idle, worried about CA SMOG - Ford Mustang Forum
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post #1 of 6 (permalink) Old 08-25-2017 Thread Starter
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-25% to -30% Fuel Trim at Idle, worried about CA SMOG

I am about at my wits' end...

A few weeks ago, I took all of my non-CARB compliant parts off and put my stock parts on to go get smogged here in the People's Republic of California. Well as soon as I did that, I started throwing misfire codes in cylinders 2 and 8. I swapped COPís and still had P0302 & P0308 codes. Swapped injectors, still had the same codes. I also noticed that my Long Term Fuel Trim was hitting -25% to -30% in Bank 2 during idle. Bank 1 was actually around -5%, so not too bad. Past 1200 RPMís LTFT was at ~1.00 or 0% so all good there.
Not knowing what to do, I did what any self-proclaimed YouTube Certified Mechanic would doÖ I cleaned out each fuel injector by spraying carb cleaner through it while pulsing it (connected it to the car battery) and changed the O-Rings and caps, then swapped them from one bank to the other. I swapped the COPís from one bank to the other, replaced the spark plugs, replaced the EGR valve, the IAC and cleaned out the PCV. What was interesting is the spark plugs in Bank 2 were pretty beat up. It looked like they had been through a lot of detonation. I donít remember feeling anything like that, but itís possible I just wasnít paying attention. Based on -30% fuel trim that would make sense I thinkÖ
I also found out I had an exhaust leak at the manifold to mid-pipe flange on the passenger side. Maybe this was letting O2 in and the sensors were getting false readings. So last night I crawled up there and made sure both passenger and drivers sides were mated up correctly with the gasket installed (thick edge facing towards the manifold flat side towards the mid-pipe). I also sprayed starter fluid all over my intake parts looking for a vacuum leak and couldnít find anything.

So where Iím at now:
No codesÖ YetÖ But Long Term Fuel Trim on BOTH banks is getting close to -20% at idle and Iím worried Iíll screw up my spark plugs again AND I wonít pass SOMG here in California. I also had this weird thing happen to me yesterday, where I let off the gas and put the car in neutral while coming to a red light and the car jumped to 3000 RPMís and would not drop. After about 30 seconds I turned the car off and back on, still at the red light and it reset itself. I really have no clue what to do next. Iím going to take it into the shop next week and have them take a look, unless someone here can help out with some ideas for me to try this weekend.
As always, all of the help on here is very much welcomed!

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post #2 of 6 (permalink) Old 08-25-2017
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IMHO, you're doing the right thing......needs an advanced software program (Pro Mechanics) to see what is really going on........ could even be the ECM itself....


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post #3 of 6 (permalink) Old 08-25-2017 Thread Starter
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Advanced software huh? So I guess I'm outta luck? I guess there is no sense in trying to guess by replacing more parts and just spending more money when I don't have a definite answer. Will the software be able to tell if the ECU is messed up?
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post #4 of 6 (permalink) Old 08-26-2017
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Does your car have a tune for the performance parts?
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post #5 of 6 (permalink) Old 08-26-2017
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Quote:
Originally Posted by gezus101 View Post
What was interesting is the spark plugs in Bank 2 were pretty beat up. It looked like they had been through a lot of detonation. I donít remember feeling anything like that, but itís possible I just wasnít paying attention. Based on -30% fuel trim that would make sense I thinkÖ
^^^IMO this is a very troubling symptom. It's the type of symptom to be expected IF the timing has slipped on one side.

Has the MAF been cleaned?

Here's some information on an affordable advanced ODB2 scanner. You may find the graphing capabilities very useful.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-for...elm327-usb.57/

2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.
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post #6 of 6 (permalink) Old 08-28-2017 Thread Starter
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@neuron :
I just had Bama Performance send me a stock tune with the rear gears set at 4.10. I don't know if a SMOG shop can detect when the computer has been modified, but I know they sure as hell will know if the speedometer is off.

@wmburns :
I had read online that timing might be an issue with something like this. The car is 13 years old, I'm sure a little timing chain stretching is possible right? I guess it's time for a little tune-up. Over the weekend, I cleaned the MAF with some CRC MAF Cleaner and put the old IAC valve back on. I haven't had a RPM surge since and the car is only peaking at -16% LTFT at idle but spends most of the time between -9% and -12%. Better than before! I was also able to complete the Drive Cycle so the P0100 code isn't there anymore. I'll take it to get smogged sometime this week, hopefully, it will pass. Too bad I threw out the box for the IAC valve... Piece of Junk...

But I still think -16% at idle is high. After I get this bad boy smogged, I plan on dropping a 2.6L KB in there (underdriving the crank) and everything needs to be in good working order. I'm a little afraid to put a supercharger on without the LTFT being closer to 1. What do you guys think?

Thanks for the help too!!! I wish I could actually post "answers" to this forum rather than just ask questions, but I'm still a moron....
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