Blackbird

Been fighting off and on for about 2+ years.

Cannot figure out why the car goes pig rich under decel or any lifting of the throttle.

If I accelerate and let off the throttle and put the car in nuetral a/f will dive down to 10:1 for a second or two then work it's way back up to 14.7

If I decel in gear it will stay 10:1 until I touch the throttle and it will then pop back into line.

It annoys me because of 1. it's a waste of fuel, 2. makes dialing in ve / maf / etc.. a pain in the a$$ because of the massive swings, and 3. I think it is part of my light throttle bucking on occassion.

The car has done this NA, with the blower and with the turbo. Remember it does it with NO FI so don't say it's the blow off valve or something like that, I know it isn't.

Car also does it with maf or without. Currently I am SD mode with no maf.
I notice when it does this if I have ltrims enabled they go +15 or more. Keep in mind though I do see the behavior with or without ltrims enabled.

It's a 98, vacum/boost referenced FP regulator, 60 mototron injectors @ 50psi base pressure. (did this with Lucas 42's as well), LS6 intake with EGR, cleaned up heads with 915 springs, 212/218 .522/.529 114 cam, stock manifolds, cats, ported TB (does it with a stock TB), pulse dampner on the fuel rail is gutted, 255 in tank pump. all smog equipment in place.

I am pretty sure it's something mechanical as I can throw the same tune into my buddys 98 TA and his car behaves just fine. Let off the gas and DFCO kicks in and the car leans out as it should, touch the gas and boom right back to 14:7.

I have great vacum, can't find any leaks anywhere, only thing I notice fuel system wise is that the evap test never seems to complete, but I have no idea if or how that could effect this.

I do have HPtuners so I can send logs, etc...

If anybody has any theorys I am all ears and if somebody does find the fix I have a pair of used boxed hotchiss lca's they can have if they want them as a reward.

Typing this one thing I don't think I have tried is running 100% open loop.
Guess I can try that this week and see what behavior I get. O2's have been changed plenty of times in case your wondering, no difference in behavior.

SMOKINV8

iTrader: (6)

Double post.

SMOKINV8

iTrader: (6)

It sounds to me like its tune/injector related. I would say it was related to the minimum injector pulse width, but you say it works its way back up to 14:1, so the injectors must be able to compensate.

Is there any chance that the FPR is hanging up a little?

SHINER

iTrader: (18)

change your o2 sensors and reset your pcm by turning just the ignition to "on" remove PCM ign and PCM bat fuses wait about 10 min and then turn off igntion and replace fuses.

technical

First things first. Any engine will want to go rich under decel. Then let's add the large injectors. Now figure out what your minumum pulse width is and guess where your afr would be when the PCM wants to cut the fuel back but can only go to ~2.1ms with 60 lbs injectors or the old ones 42lbs. The PCM can't reduce the fuel enough.

SMOKINV8

iTrader: (6)

I agree, but the 42lb'ers shouldn't be running him as rich as 10:1, and he said the problem existed with both? And if it works its way back up, surely that means the injectors are some how ramping their flow down, IF they are the problem.

Blackbird

Shiner -O2's have been swapped before, no effect. PCM been reset numerous times over the years, no effect.

Technical - my buddys 98 TA acts fine with both 42's and now the same brand of 60's I have so I don't think thats it.

technical

If you want to use your friend's car as a sanity check, then start with the differences between yours and his...if there are diffs.

You mentioned Evap test nevers completes. The wiring or solenoid could be bad. That's a vacuum leak that wouldn't be easy to find.

Blackbird

Is the evap stuff that little black box and such mounted up by the fuel tank?

not really sure how to test the components. I will start looking into that.

kp

iTrader: (34)

evap shouldnt do anything on decel, you can just unplug the evap solenoid by the intake and that shuts it off if you want to test it. All evap does is suck the fumes out of the cannister at the rear when you start the car more or less.

I would look at you DFCO (de-acceleration fuel cutoff) tables and make sure something didnt get tweaked. Also if you still have EGR enabled and no egr valve it will go rich on decell. I would also make sure sure COT is shut off.

My car will go to 11:1 a/f for not even a second and then peg the wideband lean when I let off untill I give it some throttle.

Did you compare the file from the car that works fine to yours?

technical

He says the tune works fine on another car. Makes me wonder about the DFCO tables...they should be fine no?

EVAP runs during cruise. If he's cruising then lets off, maybe the solenoid bugs out.

ninobrn99

iTrader: (8)

so your FP is at 50psi right? can I see your bin and a log using that bin please?
nino@mobile-bay.navy.mil

kp

iTrader: (34)

I'm pretty sure evap should shut off on decell, if the solenoid was bad or the purge lines were open he would be getting all kinds of evap/tank pressure codes I would think but anything is possible.

I missed he can put his tune in another car. But its also possible there is just something tweaked in the PCM that his tuning software cant get to, or even evap solenoid driver or something. I had to throw away the old PCM from my car since after HPP3s, LS1edit, HPtuner and EFI live tunes over 8 years it just kept doing crazy stuff for no reason..

technical

I was thinking since he mentioned the EVAP stuff, and said the afr climbs back slowly in neutral that maybe the solenoid sticks open which would cause the PCM to slowly adjust. The PCM only runs the tests at predetermined intervals. Given the relatively small window of time this issues occurs, maybe he's "sneaking" past the DTCs.

I do like the thought of a bad PCM. That could throw someone for a loop seeing how the tune works in his buddy's car...with a different PCM.

kp

iTrader: (34)

Never know, GM really has that evap system covered as far as trouble codes though, I still get the 'small tank leak detected' code once in a while just from the bulkhead fitting I put in for the feed line not sealing real well - its pretty touchy..

I would try swapping PCMs first, then start checking everything that is hooked to the intake, even the PCV hooked up wrong and cause some strange part throttle/decell fueling.

SMOKINV8

iTrader: (6)

I'll have to respectfully disagree with you guys about the PCM thing. I could see it maybe being a software glitch that may be fixed with a complete rewrite, but not a physical problem with it.

Since the fuel pressure regulator is vacuum modulated, what kind of an impact would a vacuum leak elsewhere in the intake tract cause?

technical

Well...when vacuum goes away the fuel pressure goes up. During decel vacuum is supposed to be high, but if there was no vacuum then pressure would be higher than normal and possibly cause the PCM to have a hard time figuring out why a certain pulse width was still spraying too much fuel. That combined with the MAP sensor reporting more load since the vacuum was low would most likely keep you from entering DFCO and cause a fat mix.

SSpdDmon

iTrader: (10)

I think I'm with smokin' on this one. Under decel in gear, your MAP typically drops down into the mid~low 20's. This represents a high amount of vacuum behind the throttle body all the way through to the pistons and pushed past the O2's. I bet you those Hotchkis trailing arms you have a small leak somewhere behind the throttle body that's only big enough to make a difference in those high vacuum scenarios. Another supporting fact is the fact that in neutral you don't experience as much trouble. Usually aftermarket cam'd engines idle with higher MAPs/less vacuum. That would explain why it's not as obvious once you take it out of gear. Look over every possible opening air could seep into your cylinders or your exhaust pre-O2's (manifold gaskets, tb, exhaust gaskets, egr plug, etc.). That's what I've got my money on....or should I say lower control arms?

technical

That's why I still have a vacuum gauge/tester in my box.

Cyborg

iTrader: (8)

You might wanna try a new wideband O2 sensor...that is how my wideband O2 sensor behaved on its way out...put a new one in and see...it may suprise you