LTFT pegging high (Page 1/1)
TurboGN APR 29, 07:20 AM
I have a 1988 Fiero GT with the L32 swap. I just got HP Tuner and did a first log and every time I rolled into the throttle LTFT was pegging high. Until I can get a bigger fuel pump installed will raising the fuel pressure help, or just cause another problem?

Also, at times the MAP was reading 20 psi which has to be wrong as my boost gauge has only shown a max of 11 - 12 psi and I have the stock pulley.
Dennis LaGrua MAY 03, 07:48 AM
I assume your fuel trim numbers were all over 5 at every RPM and MAP reading? What do you mean by "pegging". There has to be a number in each cell on the HP Tuners scanner chart. If the LTFT's are all over 5 then the PCM is adding too much fuel and diagnostics are in order.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 05-03-2022).]

olejoedad MAY 03, 09:13 AM
What pump is in the car now?

If you need to replace it to get a properly sized pump, do it now!
TurboGN MAY 03, 11:03 AM

quote
Originally posted by Dennis LaGrua:

I assume your fuel trim numbers were all over 5 at every RPM and MAP reading? What do you mean by "pegging". There has to be a number in each cell on the HP Tuners scanner chart. If the LTFT's are all over 5 then the PCM is adding too much fuel and diagnostics are in order.



Sorry, I'm just starting out with HP Tuners.
LTFT was hitting 16.4 and yes, except for a couple of small areas that were 3.9 most of the readings were well over 5.
TurboGN MAY 03, 11:17 AM

quote
Originally posted by olejoedad:

What pump is in the car now?

If you need to replace it to get a properly sized pump, do it now!



Pump size is yet another unknown. I bought the car from a dealer that bought it at auction with no history except recent rebuilt engine, brakes, tires, and paint job.
The L32's were made with returnless fuel rails but when the previous owner did the engine swap they put in an adjustable fuel regulator next to the fuel filter with a return line.
I was told to set the fuel pressure around 52 with Key on Engine off, but can't due to the setup. With the Key on Engine off, the fuel pressure goes up to about 50 and drops immediately to around 30 and slowly goes to 0. I have checked for leaks, and no signs of injectors leaking down. I set it for 50 at idle and that's what I was running on the log. I have bumped it up to 58 which I saw on 3800 forum was where it should be, but haven't recorded another log.
olejoedad MAY 03, 02:29 PM
Do yourself a favor, drop the tank and verify what pump you have and that the rubber hose is certified for gasoline immersion.

And ditch the adjustable regulator. You need one with a vacuum fitting to allow fuel pressure to rise under boost.
TurboGN MAY 03, 09:26 PM

quote
Originally posted by Dennis LaGrua:

I assume your fuel trim numbers were all over 5 at every RPM and MAP reading? What do you mean by "pegging". There has to be a number in each cell on the HP Tuners scanner chart. If the LTFT's are all over 5 then the PCM is adding too much fuel and diagnostics are in order.



I made another log today with the fuel pressure adjusted to 58 psi at idle. The LTFT still would hit 16.4 at full throttle but everywhere else it dropped down between -2.4 and 7.0.
To make things more confusing the MAP which was showing 20.0 psi at times now went up to 24.7 in the same situation.
Dennis LaGrua MAY 05, 12:11 PM

quote
Originally posted by TurboGN:


I made another log today with the fuel pressure adjusted to 58 psi at idle. The LTFT still would hit 16.4 at full throttle but everywhere else it dropped down between -2.4 and 7.0.
To make things more confusing the MAP which was showing 20.0 psi at times now went up to 24.7 in the same situation.



Follow Joe's advise and replace with a 2 BAR fuel pressure regulator that will reference to engine vacuum and boost. You can't set the fuel pressure by hand to a fixed number and expect it to work. The fuel pressure must always rise above boost level.
At a fuel trim of 16 that is a lean condition. Continued operation will cause you to burn a piston.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

TurboGN MAY 08, 10:27 AM

quote
Originally posted by Dennis LaGrua:


Follow Joe's advise and replace with a 2 BAR fuel pressure regulator that will reference to engine vacuum and boost. You can't set the fuel pressure by hand to a fixed number and expect it to work. The fuel pressure must always rise above boost level.
At a fuel trim of 16 that is a lean condition. Continued operation will cause you to burn a piston.





The fuel pressure regulator is vacuum referenced. At full throttle it rises to about 72 psi.
I have a new 340 lph pump that I will be putting in.
olejoedad MAY 08, 10:29 AM
Good move, you can't troubleshoot a system without having the system completed with the correct components.