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High Positive LTFT by AndrewTHG
Started on: 05-04-2020 07:17 PM
Replies: 10 (231 views)
Last post by: Dennis LaGrua on 05-22-2020 07:44 PM
AndrewTHG
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Report this Post05-04-2020 07:17 PM Click Here to See the Profile for AndrewTHGSend a Private Message to AndrewTHGEdit/Delete MessageReply w/QuoteDirect Link to This Post
Hey all, I'm back with a new issue (this is why we love these cars...right...?) Anyway, I've been working with Darth (Ryan) quite a bit, and he has been more than helpful. But rather than continue to bother him, I figure I will try and bother some of you as well! So let me describe the problem, the car, what we've worked through, what the thoughts are, and maybe we can solve this!

The issue:
Right now, at idle, I have high positive LTFT (16.4). And even on the throttle/driving it always stays positive, but to varying degrees. Looking at my scans, as I get on the throttle, I can get the LTFT down to as low as 1.5% @ 3600 RPM and 37% throttle. But then as soon as I am off the gas, regardless of RPM, the LTFT start to go high again. And as I sit here and look through my scan, I can notice a pattern in all of the data. On the throttle, LTFT drops and trends toward 0. The second my throttle % starts to go back down (i.e. I am getting off the throttle), manifold pressure drops which I would expect, but the LTFT starts to climb back to 16. Even if the RPM's stay high. The car idles fine, no fluctuation, doesn't seem to miss or anything. But I want to fix this because it is not right. So that's why I turned to Ryan, and why I am posting here.

The Car:
-1987 Fiero GT
-Getrag 5 speeder
-2004 (Series 3) Engine
-1998 PCM from Ryan
-N* (Northstar) throttle body
-LQ4 MAF
-MAP sensor from series 2 vehicle (brand new)
-Series 2 O2 sensor, verified to be working and sending voltage signal
-Series 2 EVAP purge valve
-Boost bypass valve hooked up to vacuum, but no solenoid installed
-Connections made:
-Tree at top of S/C: Brake booster, boost bypass, and EVAP
-port on N* - goes to PCV port on S/C
-nipple on LIM: Tee'd to MAP and fuel pressure regulator

What's been done/checked
-Fuel pressure (~43psi at the rail, which is a series 3 rail flipped around. Aeromotive adjustable regulator installed)
-All injectors bench cleaned before going in the car. Also ran some injector cleaner just to double down on my work.
-Vacuum leaks - this is a big if. I had a bad IAC that had my idle hunting, but replaced that and idle came down and was steady. So now it idle beautifully. I sprayed some carb cleaner around a bunch of areas and didn't find anything. Also poked around the injector o-rings and double checked my clamps and everything on my intake tubing. Nothing seemed to fix it.
-Cleaned the MAF with CRC MAF cleaner. No change.

Thoughts
-My PCV is hooked up like the picture below, where it goes from the Gen 5 supercharger nipple, to the port on the N* throttle body. Is this okay? Or is it reintroducing the vapors post-MAF, allowing un-metered air in? Could it also be causing weird vacuum issues that would cause what I am seeing in my scans with the throttle vs LTFT?
-Ryan thought maybe a faulty MAF. I don't know much about MAFs, but this is an LQ4 and what I know about those is that they are expensive as heck online! If someone would be okay with me borrowing one, I would gladly pay for shipping. This could at least rule that out.
-Vacuum leak - like I mention above, I checked, but it is always possible. What makes me think it isn't is my idle and that I still haven't found it. What makes me think maybe it is, is that the LTFT drops as I get on the throttle. But why then, would it be so dependent on throttle position as the scans seem to point out?
-Am I an idiot (okay maybe don't answer that one) and hooked up my EVAP purge valve backwards? I blow into it and get no air movement either direction. Scientific, I know.

Any help before I go crazy(er) is greatly appreciated. Like I said, Ryan has gone above and beyond (try ~48 emails above and beyond), and so I figured I would reach out for some help here, especially if someone has a spare MAF I can borrow. I am in the Atlanta area too, if anyone knows somebody that would wanna smoke test this bad boy to check for leaks.

Thanks!
-Andrew

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Dennis LaGrua
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Report this Post05-05-2020 07:35 AM Click Here to See the Profile for Dennis LaGruaClick Here to Email Dennis LaGruaSend a Private Message to Dennis LaGruaEdit/Delete MessageReply w/QuoteDirect Link to This Post
Is your MAP sensor from a supercharged GTP? You need a 2 BAR unit. Confirm this before moving on.
High LTFT's as you have shows that the PCM is being called upon to add more fuel. You are running far too lean. This is dangerous especially for a boosted engine.
As in your setup I run a larger MAF sensor and throttle body changing all of the air flow values and the resulting fueling. This requires importing a totally different MAF table with different values , adjusting for the engine and adding compensation for the larger throttle body as in the Hot Power Enrich % opening vs RPM table. If all sensors are working properly then the tune is probably way off. Your LTFT's should hover near zero or at most +/- 5 .

------------------
" 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 05-05-2020).]

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AndrewTHG
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Report this Post05-05-2020 08:01 AM Click Here to See the Profile for AndrewTHGSend a Private Message to AndrewTHGEdit/Delete MessageReply w/QuoteDirect Link to This Post
Dennis, I assume you meant is the MAP sensor from a supercharged GTP? If so, yes. It is Delphi PS10082, which shows as a direct replacement for the 03 supercharged engine (which this is running as). Ryan did my tune, a year or so ago, and I told him the setup that was being run, so he did account for the larger throttle body, MAF, etc.
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Report this Post05-06-2020 12:47 PM Click Here to See the Profile for AndrewTHGSend a Private Message to AndrewTHGEdit/Delete MessageReply w/QuoteDirect Link to This Post
Just thinking out loud here....when my IAC went bad, my idle was pretty high, which would have made my LTFT fairly positive, right? Air was leaking past the IAC, not measured by the MAF. When I replaced my IAC, which solved the high idle, should I have unplugged my battery for an hour or so to let the fuel trims reset? Or should they have re-learned by now with some day to day driving? If this is as simple as me unplugging the battery so it forgets the old trims.....
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Report this Post05-06-2020 01:37 PM Click Here to See the Profile for Dennis LaGruaClick Here to Email Dennis LaGruaSend a Private Message to Dennis LaGruaEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by AndrewTHG:

Just thinking out loud here....when my IAC went bad, my idle was pretty high, which would have made my LTFT fairly positive, right? Air was leaking past the IAC, not measured by the MAF. When I replaced my IAC, which solved the high idle, should I have unplugged my battery for an hour or so to let the fuel trims reset? Or should they have re-learned by now with some day to day driving? If this is as simple as me unplugging the battery so it forgets the old trims.....


A vacuum leak can throw off the fuel trim numbers. When corrected, it takes about 30 minutes of driving to re-stabilize the fuel trim readings. If you want to disconnect the battery, you can do that to reset the PCM, but it would still take 30 minutes of mixed driving to get somewhat accurate fuel trim numbers. If you can scan while you drive that will be beneficial. Are you using a scanner that graphs fuel trim vs engine speed and MAP readings? . That would allow you to pinpoint where the trims are in one or all engine operating conditions.

------------------
" 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 "

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OntarioKev
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Report this Post05-07-2020 03:09 AM Click Here to See the Profile for OntarioKevSend a Private Message to OntarioKevEdit/Delete MessageReply w/QuoteDirect Link to This Post
High positive fuel trims only at idle/low loads is quite often unmetered air entering after the MAF.

The O2 sensor catches that the mix is off and uses trims to compensate for it, so the car isn't actually running lean, because the trims compensate for it.

What this can do is cause the engine to run rich when you enter power enrich mode. If your fuel trims are positive when you enter power enrich mode, that percentage is added to the delivered fuel. If your trims are 0 or negative when you enter PE mode, the fueling is not altered.

The best way to find any leaks is a pressurized smoke test before the MAF sensor (to make sure you catch any and all leaks after the MAF sensor).

You will be amazed at what you will find this way. Quite often you fix a leak, and then you find another one farther downstream. Spraying carb cleaner around can really only find very large leaks, and it doesn't take a very big leak to mess with idle trims, especially if you have larger fuel injectors, the larger the injector the smaller the leak has to be to make a difference.

I used to tune all kinds of Cobalts, and it got to the point where I wouldn't look at someones tune until a smoke test was performed.

I would offer to pay for the smoke test myself if there were no leaks found. Not once did I ever have to pay out BTW.

Most common spots for leaks was fuel injectors. Even wit brand new orings with clean injectors installed quite often don't seal the first time and need adjusting. I also had people guarantee there were no leaks because they had "checked over everything themselves", sprayed carb cleaner, and even pressurized the manifold and claimed it didn't drop pressure. Every one of them had leaks. Once the leaks were fixed, really easy to touch up the tune for perfect idle.
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AndrewTHG
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Report this Post05-07-2020 11:03 AM Click Here to See the Profile for AndrewTHGSend a Private Message to AndrewTHGEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thanks for the responses guys. Darth had suggested this as well which is what got me to poking around with some carb cleaner in the first place, but it may be time to go pay for a smoke test. If anyone has recommendations for a reputable garage around the Atlanta area, I'd appreciate it. In the meantime, I'll check around the injectors a bit and make sure they're properly seated. Might also head to the junk yard and grab a MAF to rule out me having a faulty MAF as well. It's brand new....but that doesn't mean much sometimes.
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Report this Post05-13-2020 05:43 PM Click Here to See the Profile for AndrewTHGSend a Private Message to AndrewTHGEdit/Delete MessageReply w/QuoteDirect Link to This Post
Okay, todays list of stuff done (still no change):
-removed fuel rail and cleaned the injector ports that seal the o-rings, as well as cleaning off all the injectors
-made sure LIM bolts were all tight
-made sure supercharger bolts were all tight
-pulled off N* throttle body, checked gasket
-pinched off boost bypass vacuum line (no change)
-pinched off EVAP line (no change)

One thing I am wondering, would a bad IAC cause this? I idle at ~900-950 RPM and MAF stays steady at 5g/s or so. If the pintle is not pushing all the way out, that could be an air leak....but the lack of a fluctuating RPM makes me lean toward no.

And does anybody know a good shop north of Atlanta? I am getting to my wits end messing with this. I know we discussed smoke test....that really should be next. I just need a good place to go to, that is willing to troubleshoot.
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Report this Post05-14-2020 09:48 PM Click Here to See the Profile for OntarioKevSend a Private Message to OntarioKevEdit/Delete MessageReply w/QuoteDirect Link to This Post
What injectors/transmission are you running? If manual what flywheel are you running?
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AndrewTHG
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Report this Post05-22-2020 11:45 AM Click Here to See the Profile for AndrewTHGSend a Private Message to AndrewTHGEdit/Delete MessageReply w/QuoteDirect Link to This Post
They're the original supercharged Series 3 injectors (P/N 0280156201) with the Getrag 5 speed and a Spec 3+ clutch mated to FieroRog machined flywheel
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Dennis LaGrua
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Report this Post05-22-2020 07:44 PM Click Here to See the Profile for Dennis LaGruaClick Here to Email Dennis LaGruaSend a Private Message to Dennis LaGruaEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by AndrewTHG:

They're the original supercharged Series 3 injectors (P/N 0280156201) with the Getrag 5 speed and a Spec 3+ clutch mated to FieroRog machined flywheel


There few different tables that control the fueling but MAF sensor output provides the main fueling parameter to the PCM in the 3800SC program. It could be defective. The VE tables are there just for a backup in case of MAF failure. Can you scan the MAF frequency vs RPM curve?

------------------
" 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 "

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