3.1 LG8 Issues! I'm pretty stumped.

[ptcfast2]

A few months ago I replaced my 3.1 L82 with a crate turn key 3.1 LG8. I've found that the car will almost stall when downshifting. If I leave the AC on it's worse, but if I shut if off (as expected) it's a bit better. You can see the RPM drop on the tachometer and the engine recovers at the last minute. The SES light is not turned on during this.

If I unplug the EGR or the TPS the issue does NOT occur and the car runs fine (maybe because of the PCM trying to compensate by trying to dump more fuel into the engine?). The PCM technically still thinks it's controlling an L82...never tuned it after I installed the engine.

Fuel pressure is 40psi w/ engine off, 45 psi w/ vacuum removed. I left the old injectors/fuel rail/regulator from the L82 and used them in the LG8. I know that the LG8 was designed with 52+ psi injectors, so I'm wondering if the low pressure is causing the engine to stumble a bit when the all the sensors are plugged in and the PCM isn't using defaults which in turn provides enough fuel for the engine not so stall out.

So, any ideas as to what causes this...I hope I might be on the right track as I don't have much to go by. My current plan is to replace the FPR with the LG8 version.

[Azrael]

The flow rate on the injectors are slightly different between the L82 and LG8 (17-18 lbs/hr compared to 19 @ 43.5psi or 22.5 @ 55psi). Stick with what the PCM is programmed for. Using the 55 psi regulator will bump up the flowrate a few lbs/hr. I'm not sure by how much but it's likely more than the computer can compensate for.

I'm thinking it could be the idle air control valve. When turning the A/C on in park/neutral, does the RPM rise at all?

[ptcfast2]

The RPM jumps up about 150-200 when the AC is turned on. It goes back down when it's turned off. Unplugging the IAC causes the engine to rev up to the 2500RPM zone.

I guess the question might be what changes when you unplug the EGR or TPS...IE what does the PCM do by default when it realizes it can't communicate with those specific sensors? Of course unplugging the EGR would be the better starting point as unplugging the TPS makes the car run like crap (which is expected).

[ptcfast2]

After a few test drives I can add a few more pieces of information:

1) If I unplug the IAT or MAF the issue does not occur.
2) If I unplug the EVAP sensor/rectangle thing the issue still occurs.
3) If I shut off the A/C with all the sensors plugged in the issue does not occur.
4) If I keep the A/C on with all the sensors plugged in the issue occurs.
5) If I unplug either the EGR, TPS, IAT, or MAF sensors AND keep the A/C on the issue does not occur.

I might be full of crap with that...but I mean this is my NEW theory (I'm trying to put something together from thin air here with the above data).

My A/C compressor is making quite the loud repeating *clug* noise when on and you can feel the car idle rougher when it's turned on....A/C still works though. I possibly suspect that it's causing excess load on the car when the PCM is using default values - thus unplugging a sensor causes the PCM to boost/fall back to richer settings which in turn makes the car idle higher/run richer and in turn is able to compensate for the excess (not normal) compressor load. When the PCM has all of it's data it decides what's optimal and thus uses that figure...which in turn causes the compressor to drag down the engine a bit further.

Bleh, I don't know, but that kind of makes sense in a weird way. Compressor has 238,000 miles on it. Engine has about 2,000.

[Dpzelenak]

i would say its the MAF or the TPS,
try doing a voltage sweep of the tsp and still if its still reading good