Failed smog on 1988 with 350 TBI

[Frankenchevy]

Hello, vehicle is a 1988 Suburban V20. Failed smog today due to high CO% (1.6 %). Looking for suggestions on the most likely culprit.

I have a new AC Delco o2 sensor that I’ll be installing. Also have a set of AC Delco plugs and CTS that I’ll install.

What I’ve recently installed:
AC Delco MAP sensor
AC Delco TPS
Standard Motor IAC
GM Genuine ignition coil
AC Delco cap and rotor

Manifold vacuum looks low. Maybe suspect new ACD MAP sensor?

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Don’t suspect vacuum leak because HC value was not high on smog test according to tech. Also, when setting base idle with IAC fully closed and disconnected, I can get the engine to idle below 400rpm.

Any help appreciated.

 

[gmbellew]

Tune up parts (plugs, cap, rotor), maybe O2, cat condition, maybe run some top end cleaner through it. Otherwise I don’t think MAP, TPS, CTS, or knock sensor are going to do much for emissions.

 

[Ricko1966]

Lower the base timing.

 

[Frankenchevy]

Tune up parts (plugs, cap, rotor), maybe O2, cat condition, maybe run some top end cleaner through it. Otherwise I don’t think MAP, TPS, CTS, or knock sensor are going to do much for emissions.

Cat is newer. I’m somewhat wondering if the ACD MAP sensor is messing it up. I’m going to put the old one back in to see if those readings normalize.

 

[Frankenchevy]

Lower the base timing.

I think it’s at 0* but I will double check.

 

[gmbellew]

Might also check the PCV and EGR for proper operation?

 

[skysurfer]

Cat is newer.

Still could be a problem though. I installed an aftermarket cat when I realized the oem unit was a burned-out shell. The readings went way down, but over the course of 3-5 years they crept back up. The aftermarket honeycomb units don’t last like the originals did. Bought yet another cat, but installed flanges on it this time and only mount it for testing. Readings stay well below the limits now.

 

[Ricko1966]

I think it’s at 0* but I will double check.

Even if it is at o drop it down a few degrees you'll lose some power but it will clean up emissions bump it back up after testing. 2-maybe 4 degrees. Anyway that was one of the big reasons for ported vacuum on later carbureted stuff more retard timing creates more heat,less power but the CO burns off more completely. Bump it back after your test.

 

[Frankenchevy]

Found the results from when I bought it. Only a couple thousand miles put on it since then… something changed like a switch, unless this was a buddy “hot smog”…

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[Frankenchevy]

Stick to .035” for plug gap?

I’m running AC Delco CR43TS.

 

[gmbellew]

Stick to .035” for plug gap?

I’m running AC Delco CR43TS.

thats the stock plug, and what I run

 

[DoubleDingo]

Good suggestions thus far, only thing I'd add is make sure it's good and hot when you arrive for the testing.

 

[Frankenchevy]

Good suggestions thus far, only thing I'd add is make sure it's good and hot when you arrive for the testing.

I remember you saying this in the past. So I drove around until I saw the bay open up and pulled straight in.

 

[HotRodPC]

Found the results from when I bought it. Only a couple thousand miles put on it since then… something changed like a switch, unless this was a buddy “hot smog”…

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And it could have been. I used to do those alot in the 80's. My race buddies would come to the station I worked at. I'd pull up a car I had in there for a tune up that I knew would pass and plug in their #'s. Happened all the time. I'm rusty on that **** now for sure, but I do remember an engine misfire will cause HI HC, so if your HC is good, then all your plugs, wires, cap, rotor all that is still good. No misfires, unless you're getting a lean misfire which IS NOT your case since it's HI CO which is in most cases, Rich AF mixture. How does your spray pattern look on the injectors? Do you know if it failed, Idle or high rpm or both??

 

[gmbellew]

Cat is newer. I’m somewhat wondering if the ACD MAP sensor is messing it up. I’m going to put the old one back in to see if those readings normalize.

Remember that MAP reports out absolute pressure, not gauge pressure. So subtract the MAP reading from atmospheric and that is gauge pressure (engine vacuum).