LTFT at idle will *NOT* come down.....

[fyi]

I give up....I've tried everything and I don't give up easily.

2002 C5 Corvette A4
427 Vortech T-trim S/C
236/242 115
Mototron 60 lb/hr
FAST fuel rails
Aeromotive FPR
Racetronix pump/fuel system
1997-98 fuel system
SS3200
3.42 gear
Headers, cats, etc, etc, etc.

Idles at 875 RPM, 50 MAP in park- 65 MAP in gear, Idle timing 21*, approx. 3500-3700 mhz MAF at idle, FTC 17 at idle. (more info if needed)

Tuned the calibration w/ RoadRunner, BEN's are at .9 or 1.0 across the board, MAF curved adjusted 11% positive to account for error, -2.5% to -3.3% o2 correction everywhere, idles smooth, added 6% to injector offset to correct error, used online calculator on this site to adjust IFR.

The car runs *excellent* in every-single area of the powerband and the calibration appears to be spot-on according to my logs, except at idle. No matter what I do, I cannot get the LTFT to come down from +11% at idle and it's driving me nuts.. I've even went as far as making HUGE adjustments to fueling in that area just to see if it made a difference, but it doesn't!

The o2's are new, there are no leaks with the A.I.R. system, (new check valves both sides) the headers are new, the engine is new and sound... I've built similar cars like this before and *NONE* of them gave me this kind of problem...

I'm completely out of answers, have tried everything under the sun I know of and haven't a clue of where to do next...

Maybe LTFT boundry MAP/RPM change?

Anyone like to take a stab at it to help please?

[purrvert]

Check your check valves, pull supply hose off and see if the check valve is being operated by the overlap on the cam. Some C5 owners still using AIR have inserted an orfice plate between check valve and exhaust manifold. I ended up removing the AIR system for that reason.

[fyi]

Thanks, I will check that in the morning.

Is it the erractic pulses in the exhaust port/pipe that activates the one-way poppet disc?

Just from my intial observation, I would guess so, because the left bank check-valve is closer to the exhaust port than the right, where the problem seems to be worse.. (left side)

I'll give it a whirl.

[purrvert]

Overlap in cam may be moving the poppet off seat allowing unmetered air into the intake.
I've tried an aftermarket replacement check valve from Auto-Zone but the spring was to strong for the air pump to overcome.

[fyi]

Well, I've got both check valves disconnected from the air injection hoses on both sides...no difference. I made a couple of calibration changes yesterday in the LTFT boundries and that brought the right-bank down to where it should be and it never varies any more than -2% anywhere in the powerband, even at idle, so i suspect the right-bank is accurate.

Knowing that now, the problem primarily exists on the left-bank and there obviously must be a leak somewhere on that side of the engine, outside of the now disconnected/plugged left check valve I just tested.

Currently, there is a 6%-8% split between the left and right bank..... I wonder if the header has a hair-line crack in it somewhere? Possibly at the header tube convergence welds??

[fyi]

Found the problem.....

Pinhole in the convergence of the 4 header tube primaries at the collector. Damn, that's going to be a real PITA to remove that whole thing again to fix it.....

Thanks for the assistance purrvert.


-john

[purrvert]

Great troubleshooting. Took me a bit longer to pinpoint mine.

[joecar]

Found the problem.....

Pinhole in the convergence of the 4 header tube primaries at the collector. Damn, that's going to be a real PITA to remove that whole thing again to fix it.....

How did you find this...?

[fyi]

With a mechanics stethoscope. I attached a long 3/16" tube to the rubber hose leading to the ear-piece. I then probed the lower portion of the header collector near the convergence, mounting flanges, etc. and when it passsed across the defective weld where the leak was, I could clearly hear the leak quite loudly. I've used that method before on old SBC's and it worked quite well then also.


-john

[joecar]

Good work, thanks for the stetho tip.