New Here: Bought an 88 Laredo which failed smog- have ideas, welcome confirmation

I just bought an 88 Laredo 4.0 with a separeted harmonic balancer - easy fix for $60 and now I am happily driving it around. It seems to run great aside from a slightly rough idle. The previous owner spent a good sum of money replacing most of the hoses and sensors on the top end (egr, pcv, etc) as well as the brake vacuum booster. The last smog test failed HC(PPM) and CO(%) at 15-25 mph with the following results:


I also ran a compression test which ran from 118 to 135 psi accross all 6 with no two side by side of the same pressure.

The engine idles like it is searching around a bit, sometimes it seems a little low but it has not stalled out. I have looked, tested and listened for vacuum leaks but I can't find any. I have replaced all the remaining rubber hoses.

I am assuming from the smog results that the oxygen sensor and/or catalytic converter are bad but I have not thrown any engine codes yet so it is purely speculation. Timing on this is computer controlled so I cant set that. My concern is that I may have an injector problem which has perpetuated the results. I have also heard that there is an Idle Control Valve which sometimes requires cleaning to return a good idle (this may be related to a 2500 rpm idle I had once after starting it on an average SoCal day, a few revs and the idle dropped to normal and never did that again). I'm new to this setup and I'm hoping that some of you may have ideas into common problems I might be overlooking.

Thanks for your help, I have a feeling I'll be around here for a while!

- Mike

Check the sensor under the intake manifold on the engine block that tells the renix ecu what the coolant temp is.. If that is bad it will tell the engine it is cold and cause those kind of smog readings.. also most likely is the O2 sensor could be reading a little slow and not creating enough oxigen to lean the motor out. The Nox looks relatively normal so I would maybe rule out the cat converter... I hope this helps..

Scott.

Scott,
Thank you, I'll look right into that. The DMV said something about these years being Gross Poluters, hence the test only requirement. Is there any validity to this? I am thankfully registered in the desert where I only need to smog the vehicle once but I'd still like to keep it up to spec. .

BTW: Nice rig and great modifications you've made! I jumped to your pictures where the terrain looks familiar, TruckHaven Hills or Ocitillo?

Yes they can be gross poluters... I did back in 99 and had to test every year till 04... it sucked... So every year I would put fresh plugs, cap and rotor in before going and getting a pre-test. I finally figured out the coolant sensor was the cause.. and this year passed with flying colors... Thanks, some of the pics are at truckhaven, and others are by red canyon...

Scott.