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LTFT's are +17% across the board on a stock '99 C5
A friend just bought a stock '99 C5 with 116,000 on the clock and we spent an afternoon doing some basic checks on engine health. We removed all the plugs which looked great, they were NGK PTR5D-13 platinum plugs. We did a compression test which showed 130 psi on all 8. Then I plugged in my laptop with hptuners and went for a drive. Car drives and feels perfect. Idle LTFT was at 0 but cruising and acceleration were at +15 to +20 in all cells. I'm trying to figure out why the PCM is adding so much fuel. Vacuum leak seems unlikely because it idles and drives so well. Can't hear an exhaust leak. Both banks are identical, it's not like one bank is +10 and the other is +20. The plugs didn't look rich at all. Now that I'm writing this I didn't check fuel pressure, I suppose there's a chance that could be low. Any other ideas?
Turn off LTFT's and use STFT's while you are driving around tuning. You can leave them off or turn them back on when you get thru tuning. Remember when you write a calibration, you have to drive around about 10 minutes before STFT's get back to normal. If you make adjustments quicker than that, you will bw chasing your tail.
Those miles and symptoms point to low fuel pressure under load. I would look at the fuel filter being possibly clogged as the first culprit to check. With that many years and miles that should always be suspect. No need to look for odd problems before checking the usual suspects.
Those miles and symptoms point to low fuel pressure under load. I would look at the fuel filter being possibly clogged as the first culprit to check. With that many years and miles that should always be suspect. No need to look for odd problems before checking the usual suspects.
Did some testing today and found the problem, it was the MAF!
First we connected a fuel pressure gauge and drove around with it taped to the windshield. Even under full throttle acceleration, the pressure never dropped below 55psi. The pump and filter are fine.
Then we cleaned the MAF and went for a drive. That brought the LTFT's down from +15-20% to +13% in the highest cell.
Then we unplugged the MAF and drove around in speed density. That showed a max of +5% in the highest cells.
Then we plugged in my spare MAF that I had used for almost 20 years before buying the new style for my LS3 swap. That one gave us -9% in the lowest cells.
Quite the spread! He's currently still using the spare MAF which I cleaned before running it. Neither one is visibly dirty but clearly something is very wrong with one of them (I suspect his original has the problem). I think the next step is to replace the O2 sensors to hopefully dial those trims in even more. Neither of us know how long his car has been running rich since he just bought it but I feel like it's probable his O2's are fouled to some degree.
06-24-2024, 02:42 PM
LTFT's are +17% across the board on a stock '99 C5 
