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Old 09-11-2018, 02:23 PM
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1994 Chevy 1500 - Failed Smog Test

Hi All,


My 1994 Chevy 1500 failed the California smog test, and I was wondering if the knowledgeable folks here could shed a little more light on what to do based on the results.


The truck is a 350 TBI with auto transmission. The truck got a full tune up about two years ago and it passed smog just fine. Its been driven less than 500 miles since then.


Afterwards, I replaced coolant and engine temperature sensors and had to repaid the wires that connected them as well because the gauges kept showing a very low temperature.


This time around, I replaced the air filter, poured in a can of Seafoam, drove it around til the gas tank was almost empty, filled it back up and took it in for the test.


The technician said that the timing was off and the evap canister may be clogged. I've attached the results below.


I will be checking the timing first, and go from there. But I am also not trying to go overboard and replacing everything either. Right now, my shopping list includes: new evap canister, spark plugs, wires (Autozone's Duralast brand, so free under warranty), and any damaged vacuum hoses.


Thanks in advance!


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Old 09-11-2018, 06:06 PM
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Oxides of nitrogen being high indicates cylinder temps to high during combustion.

This can be cause by too high compression, no EGR operation when needed, or timing being too fast. (advanced)

I would straighten out the Evap first, as that can affect mixture control. Unspent Hydrocarbons are also high. This could be the culprit there.

Timing could be too fast if the Knock sensors arent contributing to timing control (active signals retard the timing). Timing could be pulling too far advance if the Knock sensors arent signalling the ECM that detonation is occuring (inaudible to the ear) .

Each time a knock signal is generated the timing is retarded by around 3 degrees, as the ECM tries to keep the timing advanced for economy.

Clean out the EGR valve and check to be sure that it operates. Recirculation exhaust gas cools the combustion process to help keep NO in check.
Pull a vacuum on the EGR valve with a pump. See if it holds open . If it leaks vacuum it will not open far enough ,or at all if bad enough
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Old 09-12-2018, 05:13 AM
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500 miles in 2 years?
Probably all carboned up....High test gas and drive the snot out of it up the freeway. Test when still good and hot.
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Old 09-12-2018, 05:51 PM
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I was also going to suggest a upper cylinder de carbon job.Like a seafoam drip into the intake to clean the piston tops off.

But running the snot out of it would work too.
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Old 09-12-2018, 06:40 PM
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Thank you LATECH and poncho62 for the feedback and tips!


Yes, the truck has been driven about 500 miles in the last two years, maybe less. It has spent more time idling in the driveway to keep the battery charged than it has on the road lol!


I am ordering the evap canister and spark plugs this week. While they are on their way, I'll check the timing. Just need to find out what the correct setting is.


Unfortunately, driving the snot out of it is not an option because the tags will not be issued until it passes smog. Any more info on that Seafoam drip job?


Thanks!
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Old 09-12-2018, 07:16 PM
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Warm up the engine to operating temp. Then shut engine off.

Then use a vacuum hose on the engine ,like the PCV hose to suck in the seafoam with the engine running By Taking the pcv valve out, (with engine shut off for safety) and adapting a small hose to the pcv hose.
Then use that to suck the seafoam into the engine.
Dont suck the whole can down at once. Give the engine a little then let some air go in the hose.Then some seafoam, then some air. Too much seafoam (liquid) at once could hydrolock the engine so let it suck it in slowly. Do this over a 10-15 minute period.
You will want to prop the throttle open with a shim of some sort so that it runs at about 1800-2000 rpm while doing the process.

If you let the engine idle a lot it can cause High HC as it will tend to pull oil past the valve guides from high engine vacuum, and it will build up and send the HC reading over the top as oil is a Hydrocarbon so.... It also crusts up the piston top Raisng Compression and causing hot spots which in turn can cause HIGH Oxides of Nitrogen .
Be sure to clean the EGR passage and check the valve operation as I explained as that will be the biggest / best factor for lowering NO
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Old 09-13-2018, 08:00 AM
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LaTECH is spot on but be warned, this will inevitably smoke down the neighborhood. I highly recommend playing some of your Dads Santana records and wearing a bandana.
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Old 09-13-2018, 01:55 PM
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Ah, so its that treatment. Done that a few times on a few different vehicles. Guess I'll be having a cookout at night lol!


The first time I did this with Seafoam, it was right before the school down the street let out. As I went out to drive out the smoke, every parent and child stared in horror lol!


I'll be sure to check the EGR valve as well. I wonder if it might just be old enough to need replacing, like several other things at this point.
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Old 09-17-2018, 04:56 PM
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Quote:
Originally Posted by lt1silverhawk View Post
Ah, so its that treatment. Done that a few times on a few different vehicles. Guess I'll be having a cookout at night lol!


The first time I did this with Seafoam, it was right before the school down the street let out. As I went out to drive out the smoke, every parent and child stared in horror lol!

Ahhh, an Uncle Buck moment.
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Old 04-24-2019, 05:17 PM
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Hi All,

Wanted to post an update...


The truck passed smog today.

I ran quite a bit of Seafoam through it the last few weeks (3 cans at least). I poured into the TBI, the gas tank, as well as through the PCV valve. Lots of blue smoke.

I also did the following:

- replaced spark plugs with the same AC Delco version that it originally came with

- replaced the spark plug wires

- replaced the EGR valve after the original failed when tested with a vacuum pump

- replaced the EVAP canister

- adjusted the timing to 0 degrees after pulling the tan wire in the passenger side of the dash

- found two bolts in the rear part of the exhaust manifold on the driver side sticking out, so I tightened them up. Tech said that's probably caused the NOx to go up.

- replaced the TPS with an AC Delco unit. Probably not related to the smog issues, but the truck won;t shift into 4th gear, and that was one of the things I tried.

Thank you everyone for all your help!

Last edited by lt1silverhawk; 04-24-2019 at 05:31 PM.
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Old 04-24-2019, 07:59 PM
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KOOL....:thumbu p:
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350 chevy, chevy, enging timing, evap canister, smog test

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