Ok, this is my second post on this site so you could say that I am a noob. I've been lurking on here for the past few months and have read a lot of very informative posts. I've got problems similiar to others that have posted here but would like to approach this subject in a different way. The specs are as follows:

1995 Impala SS LT1
52MM Aftermarket Throttle body
30 LB Injectors (I know they are not needed for a basically stock application)
Corvette Fuel Rail and FPR (stock)
Hedman Block Hugger Headers
No cats
Straight through muffler (Cherry Bomb I think. It looks like a glasspak)
New 02 sensors
New ECT sensors
New Ac Delco Optispark
New Taylor Wires
New AC Delco Platinum Plugs
New Starter
New Alternator
New PS Pump
New Coil
New Valve Covers
Carter P5000 Fuel Pump (rated at 95 psi)

The PCM was programmed to eliminate EGR, VATS etc and placed in Speed Density mode. The new injector size and throttle body values
were also alledgedly included in the reprogram.

On to the problem.

When it starts, it idles fine. If I go from idle to WOT smooth and gradually then no problems. In fact I can bounce off the rev limiter if so desired. If I do it smoothly.

If I try to go from idle to WOT instantly then the motor stumbles and slight backfire occurs through the intake (The backfire doesn't happen every time but enough to know that it does it). If I hold the throttle wide open it takes a second and then it wakes up and we are off to the races. I have a scanner and I can read the values real time. I have a pressure guage on the rail and while running it averages somewhere around 37-38. What do I need to look for during the scan. The values seem to be in line with the GM service manual.

Here's a quick crappy vid of what it is doing. I'll try and take a better one tonight.

http://s136.photobucket.com/albums/q...t=DSCN2545.flv

Thanks in advance for the help.

 

Done. Next problem?

My guess is you are quenching the flame at WOT with too much fuel.

Do you smell gas? You should.

 

check to make sure everything is hooked up....no loose wires anywhere

 

So you're telling me that the fix is as simple as finding a set of 24# stock injectors?

Hmmmm. The funny thing is my father-in-laws LT1 acts the same way and his motor is completely stock and used in the same application. The really funny thing is the person that programmed the pcm's did them both around the same time and neither one of ours were sent to him with a stock tune. What I mean by this is my pcm actually came out of a caprice with a L99 (very long story) and his was purchased unprogrammed from advance auto parts. Both of which would require a complete new program. Could this possibly be the problem?

 

been there done that.

 

I don't know if this will help or not. If the two PCM's are causing two different cars to do the same thing, then obviously the PCM is the culprit. From my tuning experience with OBD2 GM cars, there are a couple of tables that control fuel. One table, specifically, that I'm thinking about, if set too rich would cause this exact problem you're talking about. The table I'm referring to adds fuel immediately as you push the throttle down, to ensure no knock is seen immediately. Then the normal fuel table takes over. I wonder if the guy that programmed it has some sort of "tune" he's using?

Again, don't know if that helps, and I'm writing this from memory from a few years back.

 

Yeah, what he said.

When you included the second bit of info - two LT1s both tuned by the same guy and the same time with the same problem, you answered your own question.

 

As soon as I get my cable in I will get the info on the existing tune and work from there. My father-in-law has learned to live with. I'll be darned if I do the same. I know this thing is a beast in stock from and I want to run it using it's full potential. Who would have ever thought that the stock tune could be wrong? There's another one chalked up to the word assume.

Thanks for the help.

 

The backfire tells me that it is running lean when you crack the throttle...mine has the same problem, when the car had the stock tune in it the problem was horrid, but after the tune it was better and had a better response but the problem is still there but not as bad...I do not have this problem in open loop operation. So the next time I have the tune upgraded I will have them richen up the off idle settings. Shon herron's website says that a malfunctioning egr system can contribute to the problem, I can not confirm this.

 

The EGR has been deleted on this setup.

Here are some Scanner Values from this past weekend. Unless I missed something the values appear to be in range. I am really beginning to believe that my problem lies within the existing tune. As soon as I get the cable and get the current info, I'll let everyone know. Thanks for everyone's help. Oh yeah, the fuel pressure at the rail is 44 psi with the vacuum line unplugged and capped off and about 37-38 with the vacuum line on the regulator.

Engine Group 1

Codes Present 2 MIL, Trans Temp too Low
A.I.R. Pump Off
AC Clutch Off
AC Pressure -13.12
AC Request Off
Baro (v) 4.9
Battery (v) 13.5
LT Fuel Trim On
LT Fuel TR CL 16
LT Fuel TRM L 140
LT Fuel TRM R 138
CC On/Off SW Off
Evap Duty (%) 0
Coolant (ºF) 195
EGR Duty (%) 0
FC1 Relay On
FC2 Relay Off
IAC Position 3?
Idle Req RPM 550
Inj L PW (mV) 0
Inj R PW (mV) 0
ST Fuel Trim L 128
ST Fuel Trim R 127
Knock Ret (º) 0
Knock Sensor 76
Loop Status CLSD
MAF (gr/s) 0
MAP ("HG) 10.9
IAT (ºF) 113
O2S Left (mV) 71-839
O2S Right (mV) 71-839
PNP Switch P/N
PRNDL Switch DRV2
Run Time 16.45
Spark Advance 19-22
TCC Brake SW Off
TCC Control Off
TCC Solenoid Off
Veh Speed (MPH) 0

Engine Group 2

Engine RPM 540 avg.
Prom ID 858
TCC Control Off
TCC Duty Cyc (%) 0
TCC Slip Off
Trans Temp (ºF) -40
Veh Speed (MPH) 0

 

L & R injector PW at 0 I dont think so...mine start at 2.5ms at idle...and the timing set on mine is 24 to 26 sounds like it is running lean to me. I would say it is in the tune who tuned it? I would also say that your trans temp sensor is bad.

 

I failed to mention that I am not running the 4l60e tranny. I am running a TH350 so the tranny controls should have been disabled. Dana at www.lt1350.com did the pcm programming.

 

Did you get the software from PCMforless and program yourself or did you send him the PCM. What has been your experience with those guys?

 

My car is a 93 so he burned me a memcal chip and mailed it to me for 150.00...yours is a 95 so you would have to send him your whole pcm...or pay the extra and send back the core for credit...At first I had a problem with the Idle staying high but I sent it back to him and he fixed it, the car runs great and you can really feel the improvement, especially in the off idle accelleration...I would say it is worth the money!!!!

 

I ordered a cable from AKMCables and it arrived PDQ. After speaking with Bryan @ PCMFORLESS he said I should reinstall the MAF Sensor. This weekend I plan to reinstall the MAF and hopefully receive the tune from Bryan.

Thanks for everyone's help and I'll keep ya'll updated on the situation.

 

my advice would be to back off the timing! 19 degrees of advance is a lot unless you have high compression when you move slowly into the throttle the MAP senses this and tells the pcm not to adjuct timing as quickly. one way to find out if that is the culpret is to go from about 3500 and punch it and if it gets up and goes i would say thats your problem because you are at the most advanced spark in any engine at around that rpm.

 

What you are saying describes the situation perfectly. I do not even have to go to 3500 RPM. If I rev it up to 2000-2500 and nail it it takes off like a rocket. I ordered the tune from PCMFORLESS but htis weekend I may try playing with the timing a bit after I reinstall the MAF sensor.

Thanks for the info.

 

no problem, your knock sensor was probably detecting the knock and retarding timing so much that you are seeing almost no power on the power stroke of the cylinders and it will just be a dog, back it off about 7 degrees and see what you get, keep playing with it till you can go as far advance as you can before this happens again and that will give you the best performance available.

 

might want to check that knock sensor out too its really easy if you have a timing light, just point to the crank pulley and tap the block near the KS you should see timing retard. because with 19-22 degrees of timing at idle thats really close to detonation!!!

 

Here's an update on this situation. I reinstalled the MAF sensor and changed the ECM switch from Speed Density mode. The thing runs great now. Apparently the person that programmed my PCM initially just flipped switches on the ECM switch table I recieved my program from PCMFORLESS and will install it this weekend. I am hoping for tit to run even better. I would like to thank everyone in this post for their assistance. Now that it is running great I am selling it. Thinking about buying a 2002 Camaro SS or something I can have more fun in.