I have a 2002 corvette LS1 / TH400 swap in a 56 chevy belair. It idles fine, and dosne't throw any codes. Under anything more than light throttle / low-load driving, it will bog, almost as if it has a throttle stop or something. I have tried running it SD and disconnecting the MAF, and i only makes it worse. car has 54psi fuel pressure, good plugs, so i am lost on this one. If i keep a super light foot and just lightly cruise, it runs awsome, and has no issues. Any suggestions?

 

Also, the car is DBW, with corresponding vette pedal and TAC

 

Bring the fuel pressure up to 58-59 psi (where it should be) and see how much the situation improves. Hard stumbles are usually lean spots; closed loop operation is covering this up at light steady throttle. Transitions uncover the underlying issue.

 

Also... that PCM is setup for a six speed (to act properly with the th400 / no feedback) right?

 

Fuel pressure is a steady 58lbs, using a factory vette regulator/filter setup. Fuel pressure never drops below 55lbs at any time. It is setup as a manual trans for the TH400. I am getting code p0102 (Maf input circuit A low), but car does this while Mafless (speed densisity) also.

 

I checked for any vacuum leaks, found nothing. Car has 58psi fuel pressure, charging at 14.5 volts. I even put another MAF and MAP sensor on it (both new, from another motor), and it does the same crap. You can feel as soon as the motor moves into the next load cell, it falls on its face and just bogs. It has to be the tune. It still is throwing code p0102, low frequency MAF code. But it acts the same under speed density, with no maf, so i dont think that is whats causing the problem. Anybody have any idea or suggestions? Does this car just need to go back to the tuner? I just want to make sure that all mechanical issues on my part have been taken care of, so i dont waste the tuners time.

 

P101,(and or)102(and or),103 indicate Speed Density Air Measurements- are in operation (MAF Failure).

Probably beyond what you have availible but, What are the fuel trims saying?

Short of that, what do your ears, eyes and wrists say? Exhaust leaks within 16" of the header collector HO2 sensors? Short of all that, yeah tuning issue.

Sounds like a really cool project- good luck and let us know how you are doing.

Doc

 

Thanks for the reply! No exhaust leaks, O2 sensors are installed in header collectors, comparable to stock vette location, although that shouldnt matter anyways. Fuel trims are horrendous.....i was able to view freeze frame data from the DTC with the piece of crap scanner i have, and it had long term and short term fuel trim "3" in the +%50ish areas. The MAF malfunciton is probably being caused by a wiring glitch, i need to test the 3 MAF wires and check voltages tomorrow. So, you are saying basically that with the p0102 code that its running in speed density mode, and the speed density "tables" are way off? If the tables that correspond with the speed density calibration were way off, the fuel trims would be way out of wack (like they are), and the computer isnt capable of enrichening the 50% above and beyond that the fuel trims are reqesting. Does that sound right? Im trying to get a grasp on how the EFI operating system functions with these motors....thanks in advance for the help!

 

You are getting there...

First of all, short of a GM engineer stepping in and giving out propreitary information there are sevral pervasive ways to tune these cars with the popular aftermarket tuning software suites all yeilding somewhat similar results.

The basic premise that I employ is the pcm is measuring airflow in and out via the various sensors.

On the GAZINTA side you have the MAF and MAP sensors and on the GAZOUTA side you have HO2s recording the result.

I calibrate both the MAF and the MAP airflow measurement systems, some do not.

There are plenty of other methods employing different techniques that I won't debate anymore. The key factor for me is repeatable results.

 

Trims 3 & 4 mean nothing to the LS1 / LS6 motors.
You want LTFT and STFT for banks 1 & 2.

Since this is a fresh conversion, might chase down
the O2 sensor wires back to the PCM and make
double sure that they did not end up swapped
side-side, which would make them report back
"stuff" that has no relation to the PCM's trim
attempts (so over the top and into the ditch).
Observe the real time O2 voltages for activity,
if they are consistently stuck to one end or the
other.

However if this problem is present with a cold
motor as well, closed loop is not active and you
have to look elsewhere than the O2s and trimming
for the root problem.

Being as it's a DBW throttle, might have a look
and see if it actually follows the pedal and opens
up (neutral, try and hold 4000RPM, did it?) or if
there might be some problem there.

 

THe car runs fine when not under load. I can rev it in park, no problems. The throttle body opens fully when the pedal is depressed, and it isn't throwing any codes related to the DBW system. The 02 sensors are in the correct sides, and the LTFT and STFT 1 and 2 are within 1-3%. It does have the problem when hot and dead cold

 

UPDATE:

I was able to get the MAF code P0102 to go away, it was a internally broken connection in "signal" wire to the PCM.....tested the wire for continuity and it failed. Replaced the wire and all is good with the MAF meter. So......

The car dosen't throw any codes, idles great, has 58psi fuel pressure, DBW throttle is working correctly (will acheive WOT position when pedal is floored), 02's are connected, working, and not throwing any codes. Problem still persists, car will flat out bog when under anthing other than a exceptionally light load. 1/8 throttle light acceleration in first gear, works fine. Anything more, especially as the car shifts into second or third (increased load) and it will bog with anything more than a super light application of the accellerator. I have even replaced the MAF and MAP sensors with brand new factory sensors (I had a few sets). Is there anything left to check? I swear i can feel right when the 'bog" happens...it feels like right as the computer is transitioning to the next "cell"....its like flipping a switch and turning on the "bog". IS this most likely the tune, it seems that everything else checks out fine! I dont mean to beat a dead horse....but rack your brains and if you come up with anything, please let me know. -JH

 

Are you verifying 100% TPS while driving when this happens or just with the engine off key on? DBW systems have a Reduced Power Mode they will trip when certain conditions occur. This usually limits throttle opening which you may be interpreting as a bog. Since it is a transplant you probably don't have the DIC which tells you when Reduced Power Mode is activated. Just throwing it out there for something to check.

 

Will the computer store a code that will come up on a scanner for reduced power mode? That is a good idea, something to check for sure. Sounds like i need to hire a midget to sit under the hood and watch what my throttle body is doing while I make a pass!

 

Scanned the computer again, it is completely code free. MAF wiring is definatley fixed, its been though 5 ignition cycles and not throwing a single code. Would reduced engine power mode trigger a code?

 

Reduced Power Mode is not something that sets a DTC code. It is a mode the PCM invokes when it thinks something is awry with what various sensors are telling it as a safety check for the DBW system. Think of it as a mode to prevent a run away throttle when it thinks something isn't right since it is not controlled mechanically. Any type of scanner that will log TPS should tell you if the PCM is giving the throttle body what your foot is commanding while driving. Since your first post said it almost feels like a it's hitting a throttle stop of some sort, it is just something I thought of.

 

Yea, i need to get my hands on a real scanner that can show commanded throttle angle / etc. It does feel like a reduced power mode, i can drive the car freeway speeds if i wanted to, but i can only use a 1/8 of the throttle to get there slowly. Thanks for the insight.....something i hadn't thought of! I combed over the entire harness tonight, at least i fixed that damn MAF problem. If it is reduced, is it the Pedal, the TAC, or the TPS? How can you tell which component is the offender?

 

You can usually change something in the tune to make it less intrusive. It commonly shows up after blower installs where the airflow exceeds expected for given throttle angles and rpms. I think in those cases it can set a P1514 or P0068 code, but there are probably other cases I'm not aware of that may trigger it and not set a code. Everytime I've seen it it's been on a vehicle that has a DIC display that tells you you are in reduced power mode. It generally takes a key off/on cycle to reset it. Is that TH400 giving any VSS input to the PCM?

 

No, trans has no connection to PCM. But Pcm is programmed to run with TH400, supposed to be set up similar to stick shift car.

 

What vehicle type and year is the PCM programmed with?