1990 LT5 emissions fail

[Franke]

Thanks TOMTOM. I agree and will replace the O2's at the same time. I really don't like the way chevy did the pcv setup but it is what it is. I may redesign it with solid metal pipe back behind the motor and shorter sections of rubber hose. When I pull the plenum, I will replace every vaccumn hose under there. Also good idea about the zip tie too.

[tpr60]

I did the metal pipe assy. to replace the pcv hose, worked great. There is a thread on how to do it in the net registry files.

[lakemeadzmonster]

A change to 10-30 Amsoil or any synthetic and to be sure run a bottle of Amsoil P.I. thru your system will lower it that much no worries

[Franke]

All, just an update. Replaced all injectors, O2 sensors and all vaccumn hoses under manifold and some externals as well. The ones under the plenum i replaced with 3/16 inch brake lines and new short rubbe pieces to connect them. Brake line bends well and i had no problems. The Lterm trims are now sitting at 128. (histogram shows good fuel mix across the rpm range from idle to 5000 rpm vs. map output). Before repair the left side was 115 and the right was 130. Thanks to all for your great advice.

[Hog]

Sounds like you got it all under control now. Does it run any better at light throttle?

peace
Hog

[batchman]

Originally Posted by Franke

dealer ... believe the cats are failing and $4000.00 dollars could fix it...L98 engine exhaust

Am amazed, and then again not, that a dealer would prefer to shotgun $4k of the wrong parts rather than diagnose your actual problem.

Cats only catalyze HCs. While failing cats can lead to mixture problems that can show up at NOXs that's really only on an OBDII car where the after-cat sensor is getting out of range. I've not looked specifically at this recently enough to recall but I'm pretty sure these cars only have an O2 sensor before the cat so a failing cat that is not blocked won't have an effect on mixture, or anything other than HCs for that matter.

Glad you've got it running better and hope that clears your issue!
- Jeff

[Franke]

Actually Hog it is smoother idle and quicker response. Light throttle is very smooth and it feels good to be able to drive it like a regular car and then when i get on it it doesn't hesitate. One other note is that i discovered my map sensor to have a low output voltage after i replaced the injectors. I got a code 34 on a test run and the MAP was about 1 volt low at idle according to the FSM. Replaced the Map and all is well. Going in for emission next week - should pass ok.

Jeff, you are correct there is only 1 O2 ahead of the cats. I'm thinking of going back to the Chevy dealer to explain what they need to do to diagonse emissions issues. They probably ran the DTC codes only and that is not enough. My car didn't show any DTC's as it was barely out of compliance on NOX.

[mike100]

Originally Posted by Franke

.... My car didn't show any DTC's as it was barely out of compliance on NOX.

That's because the original O2 sensors were not completely accurate anymore. As was mine. I had a 50-60% reduction in NOx after O2 sensor replacement (no other changes). NOx production is very fuel mixture sensitive. Also 3-way catalysts do indeed handle oxides of nitrogen.

[Z51JEFF]

Originally Posted by SoCalZR1

I see, gpm vs. ppm is all. Just different units of measure.

Sounds like a plan. One sensor is easy and the other, I forget which, is a PITA. But like you said, cheap swap of old parts. Good luck!

Mike

I believe its the Pass side that a pain to replace.Its not so much the sensor but getting the plug to connect takes some work.

[SoCalZR1]

Originally Posted by Z51JEFF

I believe its the Pass side that a pain to replace.Its not so much the sensor but getting the plug to connect takes some work.

I think you are right. It was that connector that was the battle!