1976 L48 odometer reads 70K+ (not sure if it rolled over)
- Edlebrock Performer intake
- new distributor/cap, plugs, EGR, carb rebuild by SMI, all in 2016, 3K mi. since
- not sure most recent oil change, think it was 3/2018
- mostly highway miles, 3K MSL give or take (high desert), usually 16mpg
- needs about 1/2 qt oil approx. 1.5K mi.

First test 7/2018, ambient temps low 90's, humidity pretty low


Found #1 plug wire arcing to exhaust manifold, #6 boot crumbled,
so replaced all plug wires. Checked new wires' resistance, was
within specs (but didn't record values).


Second test 12/2018, ambient temps low 60's, humidity pretty low




Not sure what to make of the low HC at 25MPH, especially the second test. Tried to understand what all the gas values mean, but didn't have much luck (all the sites that came up seemed to be quoting from the same source). All the lambda calculators I could find seemed to require an entry for NOx, which apparently CA doesn't test for anymore.

Thinking the high HC value meant some kind of ignition failure, I pulled the distributor cap. Looked good (minimal pitting). I pulled the plugs; didn't find any that were wet (car's in garage, haven't been able to get doors open, so couldn't do procedure to properly read the plugs). #8 had some whiteish/light tan deposits. Pulled the EGR; both it and the intake manifold looked O.K. (failed a previous test because original manifold was plugged and EGR was bad).

Once I can get the garage open I will try to check the EGR, read the plugs, and do a compression test.

In the meantime, based upon the test results, any advice on what to target?

Thanks,
Mike

 

High hc means you have raw fuel in the echaust like a miss, and high co means the what is burning is also rich. I would check your idle afr and lean it out and also make sure your choke is not stuck on.

 

High HC is usually caused by incomplete combustion, which is typically an ignition issue but could be caused by low compression in one or more cylinders, a significant vacuum leak on one or more individual cylinders, timing being off (usually advanced too much). Your CO numbers are good so I would not consider the carb as a contributor at this point. Also, make sure you have a fresh oil change.

 

https://m.smogtips.com/failed-high-hydrocarbons-HC.cfm

 

Those 2500 RPM HC ppm's measurements being so low on a high mileage L-48 are VERY suspicious...11 PPM at 2,500 RPM? Those numbers are what you see with modern engines which are built much more efficiently for emissions and with fuel injection as well. The 327/502 at idle are MUCH more typical of a mid 70's V8....A couple of thoughts would be to lean the carb out at idle with the mixture screws as much as possible, make sure the engine gets to at least 200 degrees on the gauge, 210-220 is better for emissions, and using some "emissions pass" from a mass market store (mostly alcohol which burns very clean) with a few gallons left in the tank and you should pass.

 

My 77 l48 passed recently. The smog tech let it idle for a full hour before he tested it. I was worried it might over heat idling that long. In anycase it passed.
On other cars that have failed on hc I have done the following that seems to work well,
change the plugs and engine oil. The brand new plugs help it burn cleaner. Engine oil can apparently absorb hc. Fresh clean oil helps. Both things are a no brainer because the car will need them eventually anyways. This worked on two cars of mine that failed on hc. Recommended to me by my master mechanic friend.

 

we used to drip some water thru the carb,at a fast idle

cleaned them right out

 

Thanks for all the replies so far. I was able to find lots of basic info. The problem is that all of it referred to relative terms like "high" and "low". I couldn't find much that quantified what values were associated with those terms for each of the gases. So, other than knowing that HC is high (since I failed that), I can't tell what the other gasses are. There are lots of tables that if you know high/low for CO2, O2, HC and CO, you can narrow down the possible cause for the failure. Also, if I'm remembering what I read, if you can calculate lambda, that can aid in diagnosis as well as get a relatively accurate approximation of AFR.

Since I already failed once after fixing something, I'd like to not do that again. I was hoping the smog test details might aid in focusing my efforts. It wouldn't hurt to go through checking everything, but I don't have a lot of cycles so that might take awhile before I can get all that done.

Thanks,
Mike

P.S. Also, I failed to mention that it's a 20+ mile trip at highway speeds to the testing station, didn't get tested right away, but they let the car idle at least 10 minutes before the tests.

 

See post #35 by Lars:

https://www.corvetteforum.com/forums...results-2.html

 

Good reference by Peterbuilt.
I did the exact same procedure twice (2016 & 2018) since my original post and both times the car passed. Once I got home, I re-tuned it to Lars performance paper. Runs strong and fast!
Fran

 

How old are your cats?

 

so, just undo everything Lars said to do to make your car run well. and as Lars said at end of 35. car will run like crap but pass emissions. this is a LOT of the reduced power output from 69 to 75 or so.

 

Cat is 11 years old.

I had previously bookmarked that thread with the advice from Lars. Thanks for reminding me about that.

The car passed smog the previous two tests (going back 4 years). If in good running condition, it should pass now too. While the tips/tricks/etc. may be enough to get the car to pass, I'd rather find out what the issue(s) is(are) and resolve it(them).

Thanks,
Mike

 

I would richen up idle mixture to close to the 2% CO rich limit (13.76-1 A/F if read in front of the cat with air pump disabled) and see if the HC reading drop to within the limits. I think that your catalytic convertor may not be doing its job, an easy way to check a cat is to get temp reading on the in and out pipes of the cat if the temp increases the cat is doing something but if it stays the same or drops it is dead. FYI, If you use engine oil with zinc it will kill the cat over time thus most engine oil no longer contains zinc.


I hope this helps Henry @ ole's carb