LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

Failed smog High NOX again

Old 05-17-2019, 02:45 PM
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Failed smog High NOX again

Hi. I have been struggling to get through the California smog test every 2 years for the last 5 smogs. The problem is always high NOX. The car is a 1994 Camaro Z28 and is bone stock except for an LT4 Knock module and I bypassed the coolant hoses to the throttle body. I now have about 455,000 miles on the car and the engine was replaced with a Jasper 1995 engine and new GM vented about 100,000 miles ago. The spark plugs, wires and O2 sensors were replaced about 30,000 miles ago. I am using Iridium spark plugs and the wires and sensors are AC/ Delco. The air filter is new and I just cleaned the throttle. The car runs great and never misses a beat, I average 19 MPG. .
During the manual testing the NOX cycles from under 100 to over 800. Just moving he throttle to temporarily make it more rich it drops the NOX to a low level but that cant be done during the test. It seems that the car is slightly lean, but I do not see that on the scan? Over the years we have replaced: Injectors, ECU, Vacuum hoses, Several EGR valves, and solenoids, Air mass meter, MAP sensor, Knock sensor and about 6 cats (Currently a Ca approved Magnaflo, last one was a Walker). I replaced the first cat for high HC about 10 years ago and have had this problem at ever smog test since. Many tell me that the original cat was better and that is why I am having an issue now but I have trouble believing that is the case or this would be an issue for everyone. If I could find a new one I would try it. My wife and I both love this car but we are very frustrated. It is our daily driver. The car is meticulously maintained. Mobil 1 oil is changed every 5000 miles. I am going to check the EGR ports tomorrow.

Attached are 2 smog tests from yesterday before and after the latest Catalyst replacement and a scan I ran this morning.

Any and all help / advice will be greatly appreciated. Thank you, Walter


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Old 05-17-2019, 03:34 PM
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Re: Failed smog High NOX again

Downloaded it, setting up in the format I find most useful. I'll try and get something together in the next day or so. No guarantees I can find the problem, but I'll try. Thanks for the detailed info on the car and the test.

I see you show your location as Canyon Country. I remember it well. We lived on Sunbird Court in Santa Clarita from 1993-1995. Wife and kids bailed after the Northridge earthquake, returned to NJ. I stayed until 1995, fixing up the house, and looking for a job on the east coast. I was working for Parsons in Pasadena. Bought my 94 Formula at a dealer in Pasadena in August 1994. Added an NOS nitrous kit about 6 month later. Had it shipped back to NJ.

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Old 05-17-2019, 03:44 PM
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Re: Failed smog High NOX again

Forgot to ask.... did the problem exist both with the original engine, and with the Jasper engine?
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Old 05-17-2019, 03:51 PM
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Re: Failed smog High NOX again

well new CAT sure cleaned up HC

I have a 383 with the MagnaFlo CATS I put on about 8 years ago to replace the original 100k mi ones. My HC has been creeping up over the years to now be at the max allowed level to pass. Granted I have a H/C/Header, etc. motor with a "tune" which is not helping but I have always passed CA smog with the car since new. I had my spare PCM tuned with stock timing and fan on temps which GREATLY lowered my HC. I use it for smog checks now. I also had the IM monitors set to read "ready" all the time so I can swap PCM, go get test, and swap back my regular PCM and have no issue when the smog shop plugs in as the IM's are all set to be "ready"

At 100k mi on the replacement motor and a new CAT you should be good for several years but may need to repeat as the HC's climb again. Maybe there is some truth to the lifespan of original factory CAT vs aftermarket ones in terms of longevity

There are reasons you can have high HC. Unburned fuel entering exhaust. Intake or exhaust leak pre 02 causing PCM to add fuel. failing ignition components (coil. plugs, wires). injectors leaking to much gas, to much timing.

So you need to confirm those are not issues "causing" higher HC

If you have a scan tool read what STFT are. If they are high you are running rich and need to find out why

Bottom line CA wants these cars off the road so keeping them compliant to the emission test parameters is getting harder

If your motor does not push oil past rings, has a good tune, ignition components are working correctly and CATS are replaced the car should be able to pass CA emission

You always want to bring in the car after a good 30 min Hwy run. Get those CATS as hot as you can. I always left my car running when I came to smog shop. I have to go to "test only" ones. Now the guy says he has to shut off car and then start it...fine I said but you need to wait a minute for the car to enter closed loop before turning on your machine...he was clueless as to why
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Old 05-17-2019, 04:46 PM
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Re: Failed smog High NOX again

His LTFT's are the most stable, correct values I have ever seen. Range for all the cells he drove in is 126 -128. Haven't seen one that good before. NOx is generally a lean or hot condition. The coolant temp is stable at about 195°F, looks good. IAT is only 60°F. Doesn't seem to be a case of running hot. The high HC in the “before” test, coupled w/ the high NOx seemed like a contradiction. But, as noted, old cat may have been deteriorated in terms of the oxidation function. Thanks for pointing that out.

I'm going to focus on the EGR, which is shown operating when the car is traveling at 25 MPH. There is a DTC for low flow during EGR operating, but the OBD-1 diagnostic is not very accurate. Seems like most people can completely delete EGR, and not trip the code. Probably a good idea to check the corrugated tube from #8 primary to the back of the intake manifold. Make sure it's not cracked or loose. Press the diaphragm on the EGR valve with the engine idling to see if the engine stalls, to prove there is actual EGR flow.

A question for Walt.... did the Jasper engine include an intake manifold, or did you reuse the manifold from the 350K mile engine?

I need more time to look through the details. Also have to spend some time looking through the old threads for previous failures.

There is also a bit of knock retard at low speeds with gradual application of the throttle.

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Old 05-17-2019, 04:47 PM
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Re: Failed smog High NOX again

Originally Posted by Injuneer View Post
Forgot to ask.... did the problem exist both with the original engine, and with the Jasper engine?
Yes, engine is about 4 years old. Same issue with NOX

Interesting you started in Ca and ended up in NJ. I was born in NY, grew up on the upper west side and we moved to Morristown NJ. Where I met my wife. Moved out here and worked at Rusnak Jaguar in Pasadena 89 to 2003. Small world.
Thank you!
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Old 05-17-2019, 04:59 PM
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Re: Failed smog High NOX again

For what it is worth, I found the aftermarket cats did not work well at all for CA smog check. I got lucky and found a guy in San Diego who had a NOS Cat and that worked wonders in clearing up all of my out of spec smog numbers. Trouble is finding a NOS Cat. Good luck.
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Old 05-17-2019, 05:55 PM
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Re: Failed smog High NOX again

Originally Posted by Injuneer View Post
His LTFT's are the most stable, correct values I have ever seen. Range for all the cells he drove in is 126 -128. Haven't seen one that good before. NOx is generally a lean or hot condition. The coolant temp is stable at about 195°F, looks good. IAT is only 60°F. Doesn't seem to be a case of running hot. The high HC in the “before” test, coupled w/ the high NOx seemed like a contradiction. But, as noted, old cat may have been deteriorated in terms of the oxidation function. Thanks for pointing that out.

I'm going to focus on the EGR, which is shown operating when the car is traveling at 25 MPH. There is a DTC for low flow during EGR operating, but the OBD-1 diagnostic is not very accurate. Seems like most people can completely delete EGR, and not trip the code. Probably a good idea to check the corrugated tube from #8 primary to the back of the intake manifold. Make sure it's not cracked or loose. Press the diaphragm on the EGR valve with the engine idling to see if the engine stalls, to prove there is actual EGR flow.

A question for Walt.... did the Jasper engine include an intake manifold, or did you reuse the manifold from the 350K mile engine?

I need more time to look through the details. Also have to spend some time looking through the old threads for previous failures.

There is also a bit of knock retard at low speeds with gradual application of the throttle.
The intake is original. Fully depressing the EGR will make it run rough but not stall. More like 2 cylinder misfire.

I did notice the scan B+ was around 12.6v. Is this normal?

Thank you all 🙏

Last edited by wbacall; 05-17-2019 at 06:50 PM.
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Old 05-18-2019, 11:38 AM
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Re: Failed smog High NOX again

Key on/engine off the B+ is sitting at ~11.6 V. When the starter starts to crank it drops to 7.2 - 8.9 V, on the low side. When the engine starts, within 1.1 seconds, the B+ stabilizes at ~13.3 - 13.5 V. Would seem to indicate a weak battery and a healthy alternator.

In open loop, engine idling, the B+ holds in the 13 range. In closed loop, out on the road, max B+ is 13.4 V, average B+ is 13.3 V and min B+ is 12.8 V. That minimum appears with the engine at 3,000+ RPM, when the PCM is operating in power enrichment (PE) mode at 60+ MPH. Some load pulled the voltage down, but not to a problem level. That drop does not occur at the 25 MPH levels, so not something that would affect the emissions test.

Looking at the range/average for each measurement is the first thing I look at, to see if there are any significant anomalies.

I have been looking at the areas in the 20-30 MPH range, where the PCM has the EGR at 100% duty cycle. My first thought was to look for a sudden leaning (indicated by increasing STFT's) due to a possible leak in the corrugated riser tube, but there is no evidence of leaning at all. I have never seen an engine so stable with regard to LTFT's and STFT's.

If the problem is related to EGR, it would be a problem with flow. The fact the intake manifold has 455,000 miles on it could be a problem, and it's not an easy one to track down. The fact opening the EGR only causes a mild unsteady state may be pointing to poor flow. Do you have a vacuum pump (e.g. Mity Vac)? If so, attach it to the vacuum line to the EGR valve. Engine off, first pull a vacuum on the line and see if it holds vacuum, to verify no leak in that length of line or the EGR valve. There may be a bleed off provision in the valve, so it can close when the vacuum solenoid closes, but that should produce a relatively low leak down. Then start the engine, and open the valve using vacuum to see it the stumble changes compared to pressing on the diaphragm. Also use the vacuum pump to apply vacuum to the EGR vacuum line at the manifold, to see if there is a leak in the line between the manifold and the solenoid.

I'll keep looking for other things, but this isn't an easy one......
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Old 05-18-2019, 05:19 PM
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Re: Failed smog High NOX again

[QUOTE=Injuneer;700289

If the problem is related to EGR, it would be a problem with flow. The fact the intake manifold has 455,000 miles on it could be a problem, and it's not an easy one to track down. The fact opening the EGR only causes a mild unsteady state may be pointing to poor flow. Do you have a vacuum pump (e.g. Mity Vac)? If so, attach it to the vacuum line to the EGR valve. Engine off, first pull a vacuum on the line and see if it holds vacuum, to verify no leak in that length of line or the EGR valve. There may be a bleed off provision in the valve, so it can close when the vacuum solenoid closes, but that should produce a relatively low leak down. Then start the engine, and open the valve using vacuum to see it the stumble changes compared to pressing on the diaphragm. Also use the vacuum pump to apply vacuum to the EGR vacuum line at the manifold, to see if there is a leak in the line between the manifold and the solenoid.

I'll keep looking for other things, but this isn't an easy one...... [/QUOTE]

Spent the afternoon checking the car over. Plenty of vacuum at egr port. Moderate exhaust at port. Cleaned best possible. Scrubbed the throttle body. Egr valve a bit crusty. Swapped to a new valve I had. Tested per service manual code 32. All passed. Egr only opens with mityvac when engine is off. Drops as soon as started. Ok per manual. When commanding 100% egr with 9495 and engine running valve only opens slightly. Not sure how to make it open on it’s own.
I did swap the iac with another one I had just because and i did notice the iac count is very low. 0 -2. Might need to try another one.

I did notice an exhaust tick when I started the engine and found the rear exhaust manifold bolts were a bit loose. I torqued the manifold and exhaust flange hardware. 🤞
Thank you!
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Old 05-19-2019, 11:06 AM
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Re: Failed smog High NOX again

Good morning Great forum. So once the EGR is commanded 100%The rest is up to exhaust back pressure. You're cleaning of the throttle body is probably why the IAC is low. Is 0 count only means that Idle is where the computer wants it. If all the California catalytic Converters Are too high flowThis could be part of The problem. The bars definition of good EGR flow is 5" of vacuum drop at 2000 rpm Valve fully open. You mentioned in an earlier post that you have a different Spark control Module. It is separate from the ECM?Did you monitor the timing advanceAnd does it cycle like the Knox?
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Old 05-19-2019, 12:00 PM
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Re: Failed smog High NOX again

Originally Posted by Pkthoegersen View Post
. Is 0 count only means that Idle is where the computer wants it. ?
"0"….My understanding "normal" IAC counts want to be 32 (30-35) at idle with motor at full operating temp.
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Old 05-19-2019, 01:06 PM
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Re: Failed smog High NOX again

I guess if we had a high idle Issue I would be more concerned about the IAC value. Since fuel trims are normal we're not having any unmetered airMaybe the throttle Body is worn out Or adjusted to Far open. Obviously with a 0 count we can't lower the idle anymore But this car doesn't sit at 0 consistently
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Old 05-19-2019, 01:23 PM
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Re: Failed smog High NOX again

Originally Posted by Pkthoegersen View Post
Good morning Great forum. So once the EGR is commanded 100%The rest is up to exhaust back pressure.
The EGR valve pintle is exposed to manifold vacuum, which is also pulling the exhaust gas into the manifold. Typical intake manifold vacuum is as high as 20”Hg at the moderate throttle opening used for low RPM cruise. And that's when EGR is most commonly used, particularly during the 25 MPH constant RPM emissions check.


You're cleaning of the throttle body is probably why the IAC is low. Is 0 count only means that Idle is where the computer wants it.
At idle, you generally want to see IAC counts in the range of 20 to 40. The “counts” you see on the scanner tells you the position the PCM is commanding. There is no feedback as to the actual IAC position. When the idle RPM is too high, the PCM starts dropping the IAC counts to close down flow. If the counts hit “0”, it means the PCM has commanded the IAC valve closed, and it still may not have brought the idle RPM down to the programmed target value.

At the end of his data log, it shows that at idle the IAC counts are in the recommended range. Not sure where he saw 0 counts.

If all the California catalytic Converters Are too high flowThis could be part of The problem. The bars definition of good EGR flow is 5" of vacuum drop at 2000 rpm Valve fully open. You mentioned in an earlier post that you have a different Spark control Module. It is separate from the ECM?Did you monitor the timing advanceAnd does it cycle like the Knox?
That's why GM went to a “negative back pressure”. EGR valve when they went to the dual cats on the LT1 (1995 for CA emissions A4 vehicles, and 1996 for all 50-states).

He mentioned he has an LT4 knock module. That is in the PCM, and is a filter to filter out the noise from the LT4 engine's full roller rockers. It has no effect on the spark timing, as long as there is no knock detected.

He attached a 5,000+ line PCM data log of the engine operating under various operating conditions. We are in the process of doing a thorough review.
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Old 05-19-2019, 02:07 PM
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Re: Failed smog High NOX again

My current egr valve is a negative back pressure valve as original. I have a single cat. Could that be an issue?

The 0 idle count is probably due to the iac I installed yesterday. I will change it out as soon as i get a chance. Idle speed is still on the money so not too concerned. Thank you 🙏

Last edited by wbacall; 05-19-2019 at 02:52 PM.
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