We just got a new Actron CP9190 scanner at work to use when the Modis is just too bulky to carry around, and I tested it out on my car quick to see how it worked. When flipping through the datastream, I noticed that my LTFT's were sitting at a whopping 16.4 percent. I was like WOW that explains the poor gas mileage.

Odd thing is, it's NEVER tripped a SES light, and it runs great. It's got a new fuel filter and fresh tune-up (plugs, wires, ICM). STFT's are right at 0. I'm thinking vacuum leak, but every time I've been under the hood I haven't heard a thing, and I have a good ear for those kinds of things.

Any ideas where to start digging?

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Mods: Clear corners, gutted airbox, Alpine CDA-9886. Plans: 4x SensaTracs, high-flow, FM 80, 3.5 pulley, and for gods sake some good rubber.

Marty (olblueeyesbonne) and charliemax have also been fighting this issue. They haven't figured it out yet either.

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Bye Bye:

RIP sandrock

Is it possible that it's just a PCM glitch? My car has zero modifications other than a gutted airbox, and I don't see how that would effect the fuel trims. I'll wait and see what they find out.

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Mods: Clear corners, gutted airbox, Alpine CDA-9886. Plans: 4x SensaTracs, high-flow, FM 80, 3.5 pulley, and for gods sake some good rubber.

It won't throw a code (P0171) until both the LTFT & STFT get that high. Check both trims again at both idle & 2000 - 2500 rpm. Since vacuum is highest at idle if the trims are higher at idle then it's most likely a vacuum like. If the trims are higher when the rpm increses then you may have a fuel delivery problem. Post the results of this test when you get a chance.

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2006 Chevy Monte Carlo SS

Do all 3 cars share a common mod? Or any other intake mods? Maybe the MAF can not read air flow correctly?

I know Marty has the Intense FWI, Ported Exhaust manifolds, 3.6" pulley. and some other mods. Not sure what charlie has for mods.

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Bye Bye:

RIP sandrock

I'm completely stock besides a gutted airbox.

I just pulled the numbers again. At idle: 0.0 STFT 16.4 LTFT at 2500: 0.0 STFT 8.6 LTFT

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Mods: Clear corners, gutted airbox, Alpine CDA-9886. Plans: 4x SensaTracs, high-flow, FM 80, 3.5 pulley, and for gods sake some good rubber.

I can't imagine why your STFT is at exactly 0. That really doesn't seem possible. Can you try this scanner on another car or try another scanner on your car?

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2006 Chevy Monte Carlo SS

My car often runs at -10 LTFT at idle and -4 to -2 when cruising. Fuel mileage is on par for what other folks get.

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2012 Chevrolet Cruze Eco - Current car

1999 Buick LeSabre Custom - Former car

Learn from the mistakes of others, that way when you mess up you can do so in new and interesting ways.

Thats what I was thinking...

Maybe all three have a PCM failing in a "common-but-uncommon" way? Meaning, it is common for them to fail with this condition, but it only happened to (lets say?) 15% of them.

While at idle, it sits at 0 with the occasional jump to ~3.5. It goes right back to zero. At 2500 RPM, it sits at zero and occasionally dips to -5.5. 90% of the time it sits at zero.

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Mods: Clear corners, gutted airbox, Alpine CDA-9886. Plans: 4x SensaTracs, high-flow, FM 80, 3.5 pulley, and for gods sake some good rubber.

FWIW.... I have/had at least 3 things going on right now. this is the short version.

The main symptom was an elevated boost gage at idle. then I started getting P0171.

1- Checking MAF vs MAP movements, they weren't moving in synch. MAP would go up, but the MAF value wouldn't change. these were very subtle and at idle. If I threw on an accessory, MAF the same, MAP/boost gage would move up. Replaced EGR pipe and reseated TB and sensors with red gasket sealer. Boost has behaved better but not cured. MAF and MAP now move together. I had replaced all vacuum connectors and sprayed literally 15 cans of carb cleaner around every edge which could take in air. The idle stumbled once and never again. A week or so of heavy driving and no more P0171, since I did the above work.

2- Fuel pressure, and there are two symptoms. Nobody will ever catch this checking alone. They will just tell you your fuel pressure is too low. The pumps have two "fuel pressures". When I power up the pump, on the '98 it should be 50+psi. But it is 44psi. The reason is that the pump does go to 50psi, but depressurizes so fast, by the time you turn the key, and walk around to the engine bay, its over. 44psi. The unexplainable is the pressure release. There is a hissing noise in the engine bay. To me it sounds like it is coming from the connector at the main fuel line and fuel rail. But where's the fuel? A few inches behind that is the EVAP purge valve. that's the only thing which could be releasing no fuel and odorless air? I checked the FPR by detaching the vacuum, and didn't smell any fuel.

The other symptom is leaving the gage on after shutting off the car. Within an hour, the pressure goes from 44psi to practically nothing. To solve a problem which didn't exist (after diagnosis by a dealer and two mechanics) I replaced the fuel pump and the check valve is in that pump. Since there is no sign of fuel or fuel smell, the only way that fuel can back into the tank is out of the pulsator in the sending unit. It connects the pump to the fuel line. If that's the case there still has to be a pressure leak in the front of the system. It has to have something to replace the fuel as it moves back from the engine or it can't move. Otherwise, it's essentially in a (bad name) vapor lock.

I am putting this aside for now because there is another problem. But maybe my info will help. I will follow this thread with great interest and add to it should I move along. With LTFT at 16.4, there is a fuel delivery problem. Since LTFT gets so much info from idling, i have ordered a new IAC,too. BTW, I have two MAF sensors. Either they are both broken and dirty exactly the same, or they are working fine.

3- The MAP and MAF value steadied as I said. But there is a charging system problem. Finally bought a multimeter, hooked to the battery terminals and driving a round. Battery is new and strong, just short of an Optima. At ease, it reads 12.3. Turn it over and the circuit runs at 14.20v. Remember, I told you the boost gage was ALMOST corrected. Alot more testing found as I loaded on accessories, the boost gage would go up (and both MAF and MAP readings, now) at idle. Accessory demand took the circuit down below 13.v in brief moments, and also started to eat away at the 14.2v the system was trying to get back to. When I stopped and idled, the boost gage was up and the running circuit was down to 14.0. Turn the accessories off, and drive for ten minutes and it was back to 14.2 and the boost gage had leveled down near -10, where it should be.

I only bring this up because if the circuit is strained, voltage to the pump could be compromised. Tomorrow I'm putting in a new alternator and a new coupler in the S/C. Then see what the new day brings.

My only mods are a gutted box/K&N and a 180* T-stat.
The only other idiosyncrasy of the car, is that my SECURITY light stays on really long.

I don't want to jack the thread, just a quick answer, but the PCM has me interested. What's the best and cheapest way to get a new PCM, or have mine reprogrammed with a whole new stock program? For $100, I can get an Intense or ZZP, but the last thing I need is performace programming muddying the picture.

I have been workin' on this stuff for months. this really was the short version.

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I finally got the Tech 2 hooked up again after a week of running the low-mile replacement pump. I replaced the fuel filter and strainer on the pump when replacing the pump.
My LTFT's aren't at 16.4 anymore @ WOT, but aren't down enough to make me happy. The LTFT is locking in @ 14 now with STFT's @ 0.
I'm not sure if having the 00+ fuel rail with the extra pulsator is a problem or not.
I'm going to replace my plugs this week as well as the wires and ICM as I'm getting a heavy miss (14-16*KR) every so often. It doesn't build up, it will just randomly go from 16* Advance and 0* KR and then spike to 16* KR, but before I can react (let of gas) it will continue to build rpms and have 0* KR.

I'm going to take the ZZP S1 TB and ZZP TB spacer off to see if one of the gaskets isn't leaking.

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DHP Powrtuner & Tech 2
1998 GTP Coupe
1999 SSEi 70k miles
2000 Silverado 2500 Ext Cab 4x4/6.0L/4L80E
2000 Trans Am WS6 LQ3 (LQ4 w/LS3 top end) 4L60E
2001 SLE
2002 Monte Carlo SS Top-swapped FSIC Gen V TOG's, Mezeire.....
2002 GTP 40th Anniv Ed Coupe
2004 GTP Comp G 'Bottom-swapped'

Do you have the screen in the front of the throttle body installed?

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2006 Chevy Monte Carlo SS

You describe it as the fuel pressure regulator releasing fuel pressure into the return line, which it should do, but not beyond the minimum fuel pressure specification.

This is all great info. I'm still not sure what I should look for on my car though. I plan on re-doing the top end of the engine some time in the future anyways, gaskets and such, so hopefully that'll solve some of the issues. Like I said, it really doesn't bother me that much because the car runs just fine as it is, and still gets OK mileage save for the gas that's leaking out at the moment.

Honestly unless someone else figures it out, I don't plan on pursuing the problem any further.

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Mods: Clear corners, gutted airbox, Alpine CDA-9886. Plans: 4x SensaTracs, high-flow, FM 80, 3.5 pulley, and for gods sake some good rubber.

FWIW, I heard replacing the injector O-rings helps a great deal. I don't know.

I'll find out in a few weeks once my car gets LIM gaskets and all the other goodies.

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2012 Chevrolet Cruze Eco - Current car

1999 Buick LeSabre Custom - Former car

Learn from the mistakes of others, that way when you mess up you can do so in new and interesting ways.

So maybe you can imagine how thrilled I am to run at +16.4 LTFT. Doesn't exactly leave room for system adjustments for top efficiency.

Once I get this straightened out, I plan on some mods.

Hope I'm still alive to enjoy them.

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Come to think of it, I don't believe the MAF screen was in the ZZP HVTB when I swapped it on.
I'll have to check.

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DHP Powrtuner & Tech 2
1998 GTP Coupe
1999 SSEi 70k miles
2000 Silverado 2500 Ext Cab 4x4/6.0L/4L80E
2000 Trans Am WS6 LQ3 (LQ4 w/LS3 top end) 4L60E
2001 SLE
2002 Monte Carlo SS Top-swapped FSIC Gen V TOG's, Mezeire.....
2002 GTP 40th Anniv Ed Coupe
2004 GTP Comp G 'Bottom-swapped'

Very good info! I forgot until reading this, but the bottom injector orings will cause a lean condition when they go bad. The bottom orings go bad before the top, do to the heat they have to deal with. Use vasoline on the new orings when replacing them, this will keep them from rolling/tearing when installing them. But also look into the other things mensioned, fuel pressure and MAF screen installed. That MAF screen is very important. Without it, the MAF can not read air flow correctly because the air is too turbulant when flowing through the MAF. The screen straightens the air before it enters the MAF.