[deus402]

I have a bone stock 85 Grand National. It idles around 1000rpm, and bogs under WOT. It has been like this or worse since i first got my hands on it.

First thing i did was checked the fuel pressure, and it is getting 60 psi at the rail at idle, i have no way to test it while driving because my gauge isn't long enough though.


the IAC counts were around 130, but now i have the minimum air screw (aka idle screw) backed all the way out to where it isn't even touching the throttle blade linkage, and the lowest i can get the IAC counts to is around 80.

After i adjusted the IAC the best i could, i reset the TPS to .45 at idle. I also pulled the EGR valve and made sure that it was clean and operational, since they tend to cause idle problems on GM's as well.

After i had all that set, i hooked up a vacuum/boost gauge, T'd into the MAP vacuum line, because the dash boost gauge shows it going to 15PSI almost instantly, which i knew couldn't be right.

took it out for a test spin, and noticed several things that seemed relevant. first, the engine bogs down when the pressure/vacuum goes to 0 psi/kPa (or maybe it goes to zero because it bogs... ) at idle i am around 20 in Hg / 50 cm Hg of vacuum, and see the vacuum drop (approach 0) as the throttle increases.

also, at WOT the O2 Sensor voltage drops to .1 V or less, sometimes 0, and stays there.


other notes:

my TPS seems to be working OK, shows .45 V at idle, and around 4.8V at WOT, which seems about dead on.

IAC counts jumps up 130+ under acceleration, but drop back down to around 80 when i let off again.

At idle, O2 counts are around 200, which seems like it might be a little high vs specs posted on the site, but i always understood that higher was better because it was reacting quicker.

I have also tried removing the fuel pressure regulator and plugging the vacuum line to get more fuel, but that really makes it run like crap at all rpm/throttle values.

did a QAD vacuum leak test with a can of ether, and nothing is immediately obvious by sight or sound either.

With a vacuum pump hooked up to the wastegate actuator, it does move, i didn't actually measure how much, but it seems like it's working right. however, it doesn't seem like the turbo is spooling up. it actually seems like the power cuts out right when the turbo should be starting to produce boost (however, i have never actually driven a turbo buick that ran right, so that is just speculation)

I can't quite wrap my head around what the actual problem is... not sure if my O2 readings are due to a bad sensor, or because of fuel starvation actually causing that lean of a condition. also, i'm not quite sure why i can't get my IAC counts into spec. possibly related to the O2 problem? maybe something else?

right now i am halfway into removing the throttle body to inspect the turbine and shaft to make sure it isn't seized/bent/otherwise hosed, and i have the O2 sensor soaking in PB blaster so i can bench test it with a torch. any other ideas would be much appreciated, i'm running out quick.

Disclaimers: all sensor values are from winaldl, using a mastach (mastach.com) cable. the actual prom isn't listed, but all numbers and codes "make sense." my IAC was replaced with a junkyard one, i just grabbed 5 or 6 off of various mid to late 80's PFI 3.8 gm cars. my original was completely shot, as well as the first two from the boneyard that i tried. also, sorry in advance for the rambling style of writing, lots of info to relay, and i'm not the best wordsmith...