Question: 1990 L98: Failed California SMOG gold shield NOx tests

[AV8R]

Hello,

I have a 1990 C4 L98 automatic transmission California Corvette that has failed only on the NOx numbers (all other tests passed):

NO (ppm)
15mph: 1325 (541 max)
25mph: 1207 (498 max)

2 years ago the Cats were replaced by a local shop, as well as the EGR valve, and the exhaust headers were cleaned out. Then it passed the non gold shield SMOG tests.
Now I have to pass the gold shield SMOG and its failing. Ive read that a EGR valve is often the culprit for very high NOx numbers. Also, Im told that cats going bad wont show up as just high NOx values.

So my questions are thus:
a. Do you agree that a new EGR valve is the way to go?

b. Positive or negative pressure (I believe Ive read negative for high NOx values)?

c. What GM Delco part number(s) would I search for? (ACDelco 214-5535)

d. And if its not the EGR, what else could it be? (O2 sensor, mixture?)
I recently replaced the fuel injectors because the originals shorted out - as they do eventually Im told.


Now on a related topic, the shop that replaced the precats and cats may have put in the wrong cats (as the gold shield US Smog shop told me when the vette failed recently)
They state that the cats put on my car were not those called out by the State Air Resources Board, Executive Order D-193-87. Rather what is on the car are D-280-80, which are for a truck. Does any of this make sense and ring true? (remember, I did pass the non-gold shield SMOG test with these earlier).

Before I take my vette to another and more trusted shop, can I get some insights?
Thank you.

 

[boomdriver]

Hello,

I have a 1990 C4 L98 automatic transmission California Corvette that has failed only on the NOx numbers (all other tests passed):

NO (ppm)
15mph: 1325 (541 max)
25mph: 1207 (498 max)

2 years ago the Cats were replaced by a local shop, as well as the EGR valve, and the exhaust headers were cleaned out. Then it passed the non gold shield SMOG tests.
Now I have to pass the gold shield SMOG and its failing. Ive read that a EGR valve is often the culprit for very high NOx numbers. Also, Im told that cats going bad wont show up as just high NOx values.

So my questions are thus:
a. Do you agree that a new EGR valve is the way to go?

b. Positive or negative pressure (I believe Ive read negative for high NOx values)?

c. What GM Delco part number(s) would I search for? (ACDelco 214-5535)

d. And if its not the EGR, what else could it be? (O2 sensor, mixture?)
I recently replaced the fuel injectors because the originals shorted out - as they do eventually Im told.


Now on a related topic, the shop that replaced the precats and cats may have put in the wrong cats (as the gold shield US Smog shop told me when the vette failed recently)
They state that the cats put on my car were not those called out by the State Air Resources Board, Executive Order D-193-87. Rather what is on the car are D-280-80, which are for a truck. Does any of this make sense and ring true? (remember, I did pass the non-gold shield SMOG test with these earlier).

Before I take my vette to another and more trusted shop, can I get some insights?
Thank you.

High NOX is the opposite of high hydrocarbons....a new cat will fix a hi-HC, a new EGR or o2 will fix the hi NOX,. NOX is caused by excessively hot combustion. Too lean a burn or low fuel pressure.
What was the HC number? Ign timing may help, but I suspect that its got something to do with the new injectors...are these the same size as before? smaller?
you can dial UP the fuel pressure and try that.

Hi NOX means something is LEAN. Hi HC means something is rich... The balance is what you strive to get.

Up the fuel pressure, retard the ign timing 1 degree, and test a bit cooler. Try this at a test only station, tell them its your practice test for tuning... Use a good grade of premium fuel as well.

 

[Hib Halverson]

You have the car registered in an area of So Cal which requires the enhanced smog check. With respect to that test, there is no such thing as a "Gold Shield" test and a "non-Gold Shield" test.

What's probably happended is you've been ordered to go to a test only station and you can satisfy that requirement two ways 1) go to a "standard" test-only station or 2) go to a "Gold Shield" station which can servce as a "test only" station as well as other roles. The test itself is the same whether you go to a regular test station (ie: not a test-only) station, a test-only station or a Gold Shield station.

While an excessively lean air-fuel ratio can cause high oxides of nitrogen emissions, it is incorrect to assume that the only reason you can have high NOX is lean air fuel ratio. High NOX can occur for other reasons.

In the last test--the one you flunked--what were the test numbers other than NOX?

Before you start buying parts, I'd troubleshoot the engine's EGR system using the information in the factory service manual to find out if the system is faulty.

As for the car's catylitic converter(s). The Pup cats (or "precats") make no contribution to the emissions as measured in the enhanced smog check. The critical part is the main cat. On a 90, there's only one and it's underneath the center of the car. That's the one you need to replace if you have a bad cat.

 

[boomdriver]

[SUB]yep...

Gold Shield means that the station can test as well as repair, and is certified by the state. The "test only" stations won;t allow even YOU to so much as turn the distributer....

Many folks are wary of the gold shield deal, because it seems like its too easy for the shop to write their own work order. Gold Shield is supposed to be an honest, proven shop. Like all the others...:eyerole[/SUB]

 

[AV8R]

Other results from the Test Only (US SMOG CHECK station), sorry its not a gold shield station as I said before, my bad.

The other ASM Emission Test Results (forgive the formatting misalignment, this forum removes more than one space in a row)
________CO2%_____O2%_
test ____RPM meas meas
15 mph 1358 13.16 2.02
25 mph 1235 13.20 1.84
_____________HC(ppm)_____
test ____RPM max ave meas
15 mph 1358 77 21 22
25 mph 1235 59 18 14
______________CO(%)______
test ___RPM_max ave meas
15 mph 1358 0.50 0.05 0.01
25 mph 1235 0.41 0.04 0.02
___________NO (ppm)______
test____ RPM max ave meas
15 mph 1358 541 222 1325 FAIL
25 mph 1235 498 198 1207 FAIL


Emission control systems visual inspection/functional checks
RESULT ECS
modified cat conv
pass visual EGR
pass fuel injection
pass vacuum line connections
pass fuel cap functional
pass MIL
pass fuel evap test
n/a thermostatic air cleaner
pass pump air injection
pass ignition spark controls
pass O2 senors and connectors
pass other emission related
n/a functional EGR
n/a fill pipe restrictor
pass fuel evaporative controls
pass fuel tank cap visual
n/a carburetor
pass wiring of other sensors or switches
pass ignition timing: 8 BTDC
pass liquid fuel leaks


The fuel injectors were GM OEM correct parts for the 90 vette. This mechanic is fastidious with using new OEM parts.
I dont believe they did a smog test after replacing and adjusting them though.

Any insight on the positive or negative pressure EGR valve?

 

[TedC]

b. Positive or negative pressure (I believe Ive read negative for high NOx values)?

Not sure I understand your question. L98s use negative backpressure EGRs. These EGRs operate in a similar manner to positive EGRs but in reverse. Here is a link for more info http://www.tomco-inc.com/Tech_Tips/ttt27.pdf . If the EGR isn't working correctly by flowing exhaust when demanded, it should trip a code 32. I have found that these negative pressure EGRs are very sensitive to several factors. If the exhaust flow is restricted, or the EGR is lazy (this was how my EGR was acting) then the engine may ping due to a lean condition. Start the engine and force the EGR lower diaphragm at idle. Idle quality should drop. This will at least tell you that the EGR ports are open, not the degree of restriction if any.

 

[AV8R]

Boomer and Hib,

What does the full smog test tell you?



TedC,

She's not pinging at all. Very smooth and strong at 87K miles. I wouldnt know how to mess with the EGR on a running vet. My electrical and electronic engineering degree I earned at SUNY at Buffalo didn't cover this topic. ...

By the way, my other vet is a 1974 triple black I bought in Tonawanda right after graduation back in 1982. "Go in debt and buy a vette" was my mantra back then. It has no California smog hardware and doesn't need any. The 2005 law states 1975 and older dont require smog, but no longer do we have a rolling 30 year window to exempt aging vehicles. Makes these antiques even more unique now. Doesn't have A/C either, which can be hot this time of year. ...

So yea, I still have my first car, a Buffalo 74 that resides out here in La-La land.
See attachment for picture



(note to forum admin, note that the text carriage returns go away when I enter this with my iPad, but it comes out fine when I use my Apple PowerBook Pro?)

 

[AV8R]

HIb,

I typed all the info you requested. Does it reveal anything more to you?

Thanks!

You have the car registered in an area of So Cal which requires the enhanced smog check. With respect to that test, there is no such thing as a "Gold Shield" test and a "non-Gold Shield" test.

What's probably happended is you've been ordered to go to a test only station and you can satisfy that requirement two ways 1) go to a "standard" test-only station or 2) go to a "Gold Shield" station which can servce as a "test only" station as well as other roles. The test itself is the same whether you go to a regular test station (ie: not a test-only) station, a test-only station or a Gold Shield station.

While an excessively lean air-fuel ratio can cause high oxides of nitrogen emissions, it is incorrect to assume that the only reason you can have high NOX is lean air fuel ratio. High NOX can occur for other reasons.

In the last test--the one you flunked--what were the test numbers other than NOX?

Before you start buying parts, I'd troubleshoot the engine's EGR system using the information in the factory service manual to find out if the system is faulty.

As for the car's catylitic converter(s). The Pup cats (or "precats") make no contribution to the emissions as measured in the enhanced smog check. The critical part is the main cat. On a 90, there's only one and it's underneath the center of the car. That's the one you need to replace if you have a bad cat.

 

[WhalePirot]

I believe there is a vacuum operated control for the EGR; electrical input from the ECM opens/closes a vacuum switch that feeds the EGR valve's operation. Using the FSM (you have one?) and a vacuum pump connected ot that EGR vacuum control line should tell you right away if the EGR valve can operate. Working backwards, a vacuum gauge on that control line should register a reading under decel.

I won't claim to be a smog expert, but those numbers tell me the cat is working great. It is catalyzing the CO into CO2 and knocking down the hydrocarbons, too. You might want to Scroogle 'NOX High' to get more understanding of the causes.

With the other work in that engine area, it may be as simple as the ECM feed wire unplugged from the solenoid. Wouldn't that be sweet. If you need to cc some pages from the FSM, drop down to S.C. and we'll make some from my '90 FSM.

 

[AV8R]

After some scroogling, here is my understanding of how converters work, and the impact of temperature and other factors on the output of the NOx and other gases:

The converter is supposed to work like this:
Perfect combustion: Air + Fuel -> CO2 + H2O (and nothing else)
Real World combustion: Air + Fuel -> CO2 + H2O + NOx + CO (and unburned HC, O2 & N2)

However, temperature affects the catylizing of the of the air/fuel into its exhaust gases. So the higher the temperature the more NOx are created, which rob the O2 from the CO2 and H20 creation, hence the high NOx condition. My car has a high NOx and normal HC and CO2 emmisions. Something is causing the high creation of NOx.

The purpose of the EGR is to feedback cooler exhaust back into the hot exhaust to lower the creation of NOx. However there is a push-pull tug of war going on between reducing the NOx and the HC/CO. High amounts of O2 are needed to clean up the HC/CO, but the opposite is required to clean up the NOx. So some balance needs to be set, thus we have the emissions and air/fuel mixture specs we are required to meet.

At the end of the day, it could be mixture or EGR or timing or anything that affects the NOx oxidation process. This is where the techs and their experience comes into play I would presume.

 

[Hib Halverson]

Sorry I haven't been back here sooner.

No question...you have a poop-load of oxides of nitrogen in the exhaust, however, the CO2, O2, HC and CO numbers tell me the engine is, otherwise, running ok.

Could you verify the EGR system is working? Please, trouble shoot the EGR per the FSM and then post to this thread the results.

When you went for the smog check did the car have fresh fuel in the tank and were the cats hot...I mean really, really hot?

Lastly in re: Gold Shield smog stations...while I'm not at all a fan of the State's nitorous "Smog Check", to be fair-and-blanced, I'll tell you that Gold Shield stations are rare because the requirements to get the certification are very stringent and the BAR constantly monitors their performance both overtly and covertly. A Gold Shield station which is "writing its own work orders" with bogus smog checks is not going to have a Gold Shield license very long and the penalties for "misbehavior" are so costly there's not economic incentive to do it.

 

[boomdriver]

After some scroogling, here is my understanding of how converters work, and the impact of temperature and other factors on the output of the NOx and other gases:

The converter is supposed to work like this:
Perfect combustion: Air + Fuel -> CO2 + H2O (and nothing else)
Real World combustion: Air + Fuel -> CO2 + H2O + NOx + CO (and unburned HC, O2 & N2)

However, temperature affects the catylizing of the of the air/fuel into its exhaust gases. So the higher the temperature the more NOx are created, which rob the O2 from the CO2 and H20 creation, hence the high NOx condition. My car has a high NOx and normal HC and CO2 emmisions. Something is causing the high creation of NOx.

The purpose of the EGR is to feedback cooler exhaust back into the hot exhaust to lower the creation of NOx. However there is a push-pull tug of war going on between reducing the NOx and the HC/CO. High amounts of O2 are needed to clean up the HC/CO, but the opposite is required to clean up the NOx. So some balance needs to be set, thus we have the emissions and air/fuel mixture specs we are required to meet.

At the end of the day, it could be mixture or EGR or timing or anything that affects the NOx oxidation process. This is where the techs and their experience comes into play I would presume.

Nope....

the purpose of EGR is to introduce burnt gases that contain little or NO o2 back into the COMBUSTION to cool it in the head so that the actual firing and burning/combustion process is effected, not as stated in the exhaust where its too late. The exhaust is managed by the Air Injection Reaction system, or AIR system ( not part of the EGR system) , that will either add room air to the exhaust OR to the CATs,or neither, depending on conditions and whatever the ECM decides based on temps and other sensor input.

Adding this colder exhaust gas to combustion also helps eliminate pinging and/or a lot of knocking by cooling the over-heated combustion chamber and saving your little aluminum pistons from beating themselves to death against the cylinder wall. They are designed to go up & down, not to rattle around in the hole. Thats when they break, EGR helps stop that from happening.

As the combustion is cooled, ign timing can return to a high degree of advance since knock is being managed, instead of the only alternative which is retarding the timing which reduces power and increases emissions. The knock sensor does the same thing by making adjustments in ign timing in miliseconds and only for milliseconds where EGR combustion cooling can last much longer and continue until the excessive loading on the engine is relieved.

And yes,
you need to test every aspect of EGR operation, as the EGR is NOT working well enough...
Your symptoms are what we would see if the EGR was removed and capped off. More EGR would lower your Oxides of Nitrogen.

 

[c4c5specialist]

Hi there,

EGR valves are NOT the only thing that can cause NOX failures in emissions tests.

Carbon buildup is ALSO a big contributor, even if the EGR VALVE IS WORKING CORRECTLY>

I would suggest a simple way to diagnose this issue.

Vacuum test the EGR valve by putting actual engine vacuum directly to the EGR vacuum port while the engine is idling. When you do this, the engine should STALL. This is due to the fact that EGR never works at idle due to overwhelming the engine with exhaust, diluting the combustible air to run the engine.

If it does stall, you KNOW the EGR is working and you must look elsewhere for your failure.

I have seen L98 ports within the lower intake clog solid, creating failure of NOX even though the valve works correctly. Those I have had to clean out, it was not easy. BG44k and 1050002 GM top engine cleaner are both excellent decarbonization chemicals.

I have also seen broken plastic vacuum lines and frozen EGR vacuum solenoids create the condition.

Allthebest, c4c5

 

[AV8R]

I really appreciate the feedback and infos provided. To be able to perform these "simple tests", I would need a step by step procedure.

Some things I can just jump into and figure out, especially if its electrical or electronic; but modern automotive mechanical - I just dont have the skill set.

At the very least, the infos youve all provided helps me when I do take my car in to a trusted mechanic. Im not going clueless.

 

[Hib Halverson]

In what part of Southern California are you located?

 

[boomdriver]

I really appreciate the feedback and infos provided. To be able to perform these "simple tests", I would need a step by step procedure.

Some things I can just jump into and figure out, especially if its electrical or electronic; but modern automotive mechanical - I just dont have the skill set.

At the very least, the infos youve all provided helps me when I do take my car in to a trusted mechanic. Im not going clueless.

This is where the FSM (factory manual) is so valuable,. Its written in english, has flow charts to follow along with detailed diagnostic proceedures that even a GM tech could understand

Seriously, The books are worth the price. I paid $75 for a used set on FleaBay.

 

[AV8R]

This is where the FSM (factory manual) is so valuable,. Its written in english, has flow charts to follow along with detailed diagnostic proceedures that even a GM tech could understand

Seriously, The books are worth the price. I paid $75 for a used set on FleaBay.

I take it youre not talking the Chilton, Clymer, Hayes books for mechanical idiots books that I own.

Thomas
Chino Hills CA

 

[AV8R]

90 Vette, new EGR and 3 Cats, not enough back pressure to run EGR... why?

OK Guys,

Its 6 months later, and I have to get my 90 coupe SMOGed or else I can't driver her without getting fined to death. Since last I posted on this, I have had a good mechanic look at it and we replaced the EGR and cats (the goofball I went to before put on dodge/ford cats) and we doubt that it ever really passed... yes I have a BAR complaint filed after the garage owner rejected fixing the situation even after proving the wrong cats were put on..

Wrong cats that were on the car:
MagnaFlow Catalytic Converters - 49-STATE & CANADA Catalytic Converters For Trucks, Suv's, American Muscle, Diesel, & Sport Compact Vehicles


What is on it nowre-cat: Magnaflow 37105
back-cats: Magnaflow 38019

MagnaFlow Catalytic Converters - 49-STATE & CANADA Catalytic Converters For Trucks, Suv's, American Muscle, Diesel, & Sport Compact Vehicles
MagnaFlow Catalytic Converters - 49-STATE & CANADA Catalytic Converters For Trucks, Suv's, American Muscle, Diesel, & Sport Compact Vehicles

As for the new EGR valve, according the Chevy the correct part number should be 19210674 – production number 17090106.
I don't know if this is the right EGR other than what I was told. Is it right? Is it supposed to be a negative or positive pressure valve?

Now with all that, we are still failing the NOx because the EGR valve isn't getting enough back pressure. When the mechanic puts some back pressure on (with rags), then the NOx goes down to where it needs to be to pass the tests. Question is, why isn't there enough back pressure? The heads and EGR ports were cleaned out by the first mechanic (I believe anyways), so whats up?

Ive read that engines are like air pumps (air in = air out), so its not the engine that requires back pressure (it actually reduces torque and power by restriction); but its the emission system that uses it.

Could it be the new replacement cats don't provide the necessary pressure that the originals required?
This thread here touches on the back pressure topic, but not specific to my situation:
http://www.corvetteactioncenter.com/forums/c4-technical-performance/125762-catalic-converter-1990-corvette-3.html

With further reading has discussed the AIR pump and check valve that go into the cat...
http://forums.corvetteforum.com/c4-tech-performance/1480555-failed-emission-inspection-1990-l98.html

Does anyone have experience with this?
I would appreciate your assistance.

 

[Hib Halverson]

The reason the NOx goes down when your mechanic restricts the exhaust is that, when you restrict the exhaust, more exhaust gas gets left in the combustion chamber after the exhaust valve closes. More exhaust left in the cylinder is the same thing as exhaust gas recirculated through the EGR valve. The effect of exhaust gas in the chamber after exhaust valve closing is the same whether the gas is recirculated back into the intake and ingested through the intake valves or is left in the cylinder after the exhaust valve closes.

Either way, NOx goes down.

c4c5Specialist addressed other reasons, besides a faulty EGR valve, which cause high NOx. I'd be looking at some of those ideas as well as continuing to research a faulty EGR.

My gut feeling is the problem is not lack of back pressure.

The first thing to do is see if the EGR system if functioning properly per the Service Manual. If it is, then the engine is getting the correct amount of EGR and the high NOx is being caused by some other problem. Again, see c4c5Specialist's earlier post.

 

[AV8R]

@HIB,

OK Ive passed that along to my mechanic here in Chino Hills CA.
Note that the heads were cleaned out, new EGR and Cats.

Hardware saintiy check:
Do I now have the correct part numbers for the cats and EGR?
Am I correct that this L98 90 coupe has to have an AIR cat check valve?