LTFT's are +17% across the board on a stock '99 C5
06-24-2024, 02:42 PM
#1
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?
06-24-2024, 03:01 PM
#2
If it's the same on both sides, I'd clean the MAF.
No vaccum leaks based is drives well, is kind of a ridiculous statement.
Spray carb cleaner around the ports and watch your STFT and 02 sensors. Other than a smoke machine, that's the best way to find one.
No vaccum leaks based is drives well, is kind of a ridiculous statement.
Spray carb cleaner around the ports and watch your STFT and 02 sensors. Other than a smoke machine, that's the best way to find one.
06-24-2024, 07:13 PM
#3
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.
06-26-2024, 02:30 AM
#5
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.
The following 2 users liked this post by gametech:
G Atsma (06-26-2024), Old Buzzard (06-26-2024)
06-26-2024, 12:32 PM
#6
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.