3800 Swapped, 1988 Fiero.

TL;DR - **** MPG.

Engine: 1999 Grand Prix GTP
Trans: 1999 GTP 4T65e-HD
PCM/Harness: Fieroflyer

So recently my mileage has gone down.. to 12mpg. I've never seen over 20 in the past two weeks of driving this thing.

I have a NEGATIVE, LTFT of between -14 and -17.1 while driving around.
Thing pisses through gas faster than I can imagine.

Here is what I KNOW:

Fuel pressure:
KOEO - 44psi at the rail
KOER - 40psi (no change with steady throttle, snap throttle = 45psi max)
Vacuum DC from FPR - 50psi

Engine vacuum - ~19.1

Replaced the o2 sensor, no change.

Nosrac believes a 1998 MAF was programmed into my 1999 PCM.
Another believes my Fuel pressure is too low, check for clogged fuel filter (Almost-new)

That's all I got right now. Too pissed to think of anything else.

IDLE reading in Park for LTFT is:
-2.3 to -3

Drive reading, with no throttle is -7 LTFT.
Drive reading, in any case under load. -14 to -17.1 LTFT

WHY is it dumping so much fuel that it has to try and pull that much away?

So this thing is OBDII, yeah?

I would personally start by logging the MAF to see what the PCM thinks is happening. It's either adding the amount of fuel it thinks is correct, or incapable of properly metering fuel. I'd start with the easy one - be sure it's reading the MAF properly.

2500hz @700-800rpm
3500-3800hz @16-1700rpm.
4800hz@ 1700rpm
5300hz@ 1300rpm mild throttle
5600hz@2100rpm @40mph
8100hz@3000rpm heavier throttle
5300hz@1800rpm @25mph

Not sure if this is meaning much I've been told my MAF readings are good, that it could be a '98 MAF table was programmed instead of a '99.

Cat clogged?

What are your running temps? Does it run very cool?

[This message has been edited by Steel (edited 06-14-2015).]

I bet my mileage is worse. I went through an entire tank in less than 100 miles.

Easy fix.............you have a hole in your gas tank!! JUST KIDDING GUYS, trying to add a little humor to what must be a frustrating situation.

Any DTC's show up on the scan? Your LTFT's are way off. I tune for numbers close to zero that would show that the PCM is not adding or subtracting fuel. +/- 5 is a good target +/- 10 is marginally acceptable but you should never see over that number.
Your engine is choking on fuel. Improper values in the MAF table/programming issues, defective sensor, clogged CAT or a leaking injector are possible causes.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, 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 "

Yeah... either your injectors are dumping fuel for no good reason (seriously oversized, seriously leaky, bad FPR, seriously high fuel pressure) or your maps are wrong. MAF readings seem okay, so it's either what the PCM is doing with that info or an inability to properly meter fuel.

a bad temp sensor would cause engine to dump fuel

Update:

No CAT.
Plugged in my '99 Unprogrammed ECU, no change in LTFT.
Temp gets to 195 and steady.

Everything is stock besides the ZZP power log.

It'll stay at 0.0 to -0.7 at idle UNTIL I touch the gas.
Then it'll jump to -7.0 to -3.1 but driving it still hits almost -22.

MAF unplugged:
LTFT climbs to +30 and beyond.

I've been told to remove FPR vac line and run without it quick.

I've noticed that my voltages drop to 8 (no light coming on) I have to hold my AUX gauges down for it to come back up.

quote Originally posted by 87FieroSport: I've noticed that my voltages drop to 8 (no light coming on) I have to hold my AUX gauges down for it to come back up.

Wait, what?

Dead serious lol. Around the time of **** mileage, r just a tad before (I have never gotten above 20mpg really)
My colts would shoot down to 8, if I tap on the AUX gauge it comes up to 11, sometimes back up to 13-14.

If I push the AUX gauges down, it keeps good voltage.

But anyhow, did a lot more tests earlier. No change in LTFT (idles at -12 now......)

Oh, got it. Maybe just a bad connection on the gauge. I thought you genuinely had 8v in the system!

This is just a 3800, no SC, yeah?

You should be able to drive around with the FPR unplugged ok. But, an easier test to connect a Mityvac to the FPR vacuum line and see if applying/removing vacuum from the FPR causes your wild AFR variations. Maybe you just have a bad/wrong FPR - although you're seeing 45psi max, which suggests it is behaving.

Do you have a noid light you could hook up the injectors to see how they're pulsing?

Or maybe build one of these?

http://www.impconcepts.com/...ormonitorforsale.htm

I had one years ago in my Saab - was handy when turning up boost!

[This message has been edited by thesameguy (edited 06-15-2015).]

Supercharged.

Okay, then you'll need a Mityvac that can do pressure and vacuum. Test the FPR with both and see how the AFR numbers change - you should be able to hear/feel the mixture richen as you apply pressure to the FPR.

Also, as Lou said, have you checked resistance on the CTS?

Another east test is disconnect FPR vacuum. Didn't get better, stayed the same.

Not the same thing. Just disconnecting the FPR causes it to return to atmosphere, which is potentially very different than its response under boost.

What is the part number stamped on your MAF? ends in 04 =98 and earlier 05 = 99+

Do a compression test. A blown head gasket will tank mpg.

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"DRIVE IT LIKE A FIERO"
'84 Fiero, engine to be determined '87 Duke (Sold)
'87 Quad 4 H.O.
'87Blue GT 3.4L Swap Completed!!!!!!!! Boosted!!!!!!!
^^^^ Now in the Construction Zone^^^^
Las Vegas Fiero Club Parts/Sales/Service/Club
Fiero Road Club Of Northern Nevada

Not a head gasket.
160psi across the board, no misfires, no KR.
I put the OEM MAF back in, and I put the untuned 99 PCM in.

With the 99 ECM:
-14.8 LTFT max, -10.9-11.7 at cruising

With the 01-03 "tuned" PCM:
-20.3, -21.8 LTFT, -16.4-17.8 cruising.

99 PCM feels better, doesn't burn gas as bad as the "tuned" PCM.