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EGR Help ... High NOX

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Old 02-10-2009, 08:51 PM
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EGR Help ... High NOX

First some basic info.
I have a 1994 Camaro LT1 installed in a Jeep CJ7.
Due to CA emissions involved with an engine swap,
I was forced to change to an OBD-1 1993 speed density system.
The computer and harness was made by Howell efi.
The harness was adapted to all the 1994 engine sensors with a 1993 ECU.
The swap has been flawless for the last 5 years.
I go to get the thing smogged and failed to high NOX
Here are my emission readings in both 15 & 25 mph on the dyno rollers.

15 MPH @ 1500 rpm
HC max 84 measured 19 = pass
CO max 0.57 measured 0.01 = pass
NOX max 482 measured 931 = Fail

25 MPH @ 1936 rpm
HC max 49 measured 19 = pass
CO max 0.70 measured 0.04 = pass
NOX max 735 measured 244 = pass

I have done some basic tests to the EGR system.
1. EGR valve holds vacuum
2. When vacuum is applied, rpm's drop and engine stumbles.
3. EGR vacuum switch has good vacuum from intake.
4. When ALDL connector is jumped and ECU is in test mode, the EGR vacuum switch closes.
When the switch is closed, I connected a vacuum gauge to the EGR side and apply vacuum to the intake side of the switch and I get vacuum through the switch, there for from what I have read, this means the switch itself is good.
5. When everything is connected properly, I connect a vacuum gauge to the EGR vacuum hose and ran it inside to the dash and took it for a test drive.
No fluctuation or movement at all on the vacuum gauge.
6. I sent my cal-prom in and had it reprogrammed to make sure the EGR functions were enabled. I got it back and still have the same problem.
7. I also tested the EGR vacuum switch wires. One has power and one is switched ECM ground, which should ground and close the circuit when it is told to.
8. No codes are present and no runability / driveability problems.

Anyone have an idea of what the problem is, or another test to perform
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Old 02-10-2009, 10:18 PM
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Looks like you have checked the EGR sytem over good. The EGR needs the pcm to command the solenoid but also the valve requires backpressure to open. I had this problem with longtube headers not creating enough back pressure. Possibly too much timing at lower rpms causing higher Nox also.
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Old 02-11-2009, 02:10 AM
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Since Nox is caused when combustion temperatures are too high (i.e. too lean), I would also maybe check for anything that can cause it to lean out... low fuel pressure (pump and/or regulator), dirty fuel filter? or if its possible to pull timing at that rpm/map reading?
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Old 02-11-2009, 12:17 PM
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I think my problem is that the ECM is not commanding the EGR valve vacuum switch to open. I understand that the EGR valve needing a certain amount of back pressure for it to open,
But Regardless of back pressure, I think there should be a vacuum reading at the EGR side of the EGR vacuum switch when I go on a test drive, Yes ?
How would one go about taking a few degrees of timing out at the lower rpm's ?
Removal and programming the cal-prom ?
I don't know what my fuel pressure is, But if I was running low pressure wouldn't I have a lean miss as well ?
Thanks for the info and other things to look at.
Please keep them comming.
Injuneer, If you can hear me, Please help
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Old 02-11-2009, 04:46 PM
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Originally Posted by LT1CJ View Post
I think my problem is that the ECM is not commanding the EGR valve vacuum switch to open.
Wire a switch to open it so you can pass emissions
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Old 02-11-2009, 04:56 PM
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Originally Posted by MikeGyver View Post
Wire a switch to open it so you can pass emissions
I thought about that.
How would you wire in a switch to open & close the EGR vacuum switch knowing when it needs to be on or off ?
I would really like to fix it correctly, so my combustion temps are under control.
I also went to extreme lengths to make this thing CA smog compliant.
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Old 02-11-2009, 06:31 PM
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LT1CJ:
More than a few LT1 owners (both stock and modified) have removed or disabled their EGR systems and have no inspection NOx difficulties. Be sure you don't have any air leaks, especially the unmetered kind at the intake manifold that might cause an excessively lean condition at or near idle.
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Old 02-11-2009, 09:46 PM
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Originally Posted by NJ-LE View Post
LT1CJ:
More than a few LT1 owners (both stock and modified) have removed or disabled their EGR systems and have no inspection NOx difficulties.
I have to disagree with that one. When my car was only bolt ons I failed on Nox with brand new cats because the EGR valve would not open because the longtubes didn't provide enough back pressure for the valve to open. Once I swapped cams I had no prob because the cam created its own EGR function. Depending on the limits you may be able to pass without EGR but most will fail.

To the OP you can modify the EGR enable rpm and change timing with programming.
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Old 02-11-2009, 10:18 PM
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Originally Posted by 96flame View Post
To the OP you can modify the EGR enable rpm and change timing with programming.
I just had my cal-prom programmed to enable the EGR via coolant temp, speed sensor, and tps and it still isn't operating the EGR vacuum switch.
The ECU should command the switch to close and operate when all of the parameters are met between 900 & 3000 rpm's from what I understand.
I was also told that the ECU might be bad and it might not be picking up the programmed EGR command due to a possible bad circuit board in the ECU itself.
Any thoughts on that ?
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Old 02-12-2009, 12:23 AM
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Originally Posted by LT1CJ View Post
I think my problem is that the ECM is not commanding the EGR valve vacuum switch to open.
Easy to check what the ECM is telling the EGR valve to do with a scanner. Look for EGR duty cycle.

I understand that the EGR valve needing a certain amount of back pressure for it to open,
It doesn't need back pressure to open..... the vacuum does that. It needs backpressure to get adequate flow of EG into the manifold passages.

But Regardless of back pressure, I think there should be a vacuum reading at the EGR side of the EGR vacuum switch when I go on a test drive, Yes ?
Yes... the exhaust and the vacuum are two separate systems. They don't interface. And its not exactly a "switch". Its a solenoid that is rapidly cycled on and off to provide a variable degree of opening, based on the ECM's commanded "duty cycle". When the ECM commands 100% DC, the solenoid is staying open, not cycling.

How would one go about taking a few degrees of timing out at the lower rpm's ?
Removal and programming the cal-prom ?
Disable the knock sensor circuit. That will cause the ECM to retard timing at a variable rate, almost constantly, even at idle.

I don't know what my fuel pressure is, But if I was running low pressure wouldn't I have a lean miss as well ?
It would take very low fuel pressure to cause the A/F ratio to lean out. If the fuel pressure is low, the fuel flow drops, the O2 sensor picks up the lean mixture, and the ECM adjusts the long term fuel corrections to bring it back to 14.7:1. The ECM can add up 25% extra fuel using the long terms (BLM's)

The low CO may indicate its running lean though. A faulty O2 sensor could account for that. Where are your O2 sensors located in the exhaust? If they are too remote, they may not be getting hot enough, partucluarly if the 93 speed-density conversion was done with the single-wire O2 sensors used in the 93's. How close it your cat to the engine? The further away the cat is, the lower the temperature. Get it too cool and it doesn't work as well. The fact you failed at lower RPM, and passed at slightly higher RPM may be pointing in this direction.

What does your coolant temp look like? High coolant temp or high intake air temp will elevate NOx.
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Old 02-12-2009, 11:48 AM
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The O2's are in the SLP headers.
The SLP's are the 1996 direct cat flange shorty headers,
that I milled to a standard header flange.
The cat is at the end of the Y-pipe
My coolant temps are Max 210* fan on & 185* fan off.
I also have a 160* stat in it.

Here is the funny part I forgot to mention.
This has flew through CA smog check every year with no problems.
The last smog check was 2 years ago.
I have only driven it 29 miles in 2 years and now it failed.
I have been told that bad gas would cause high NOx.
It had about 3 gallons of two year old gas in it and I put 10 gallons of fresh gas in it be for I took it to get smogged.
After doing some testing, this is when I found my EGR vacuum switch/solenoid not to be functioning.
I contacted Howell efi and they said they never programmed the cal-prom to turn on the EGR.
My first thought was, How did it pass smog for the previous 5 years ?
Well, I thought that's why my NOx was so high.
After getting the cal-prom programmed to turn the EGR function on,
The ECM still isn't commanding the EGR vacuum switch solenoid.
Another thing is, My HC and CO emissions are almost identical to the smog test from 2 years ago, Just that the NOx is higher this year.
The engine has all sensors, switches and valves with complete emissions.
Nothing removed or modified.
The engine is basically stock with 6,000 miles with some bolt-ons.
They are: SLP coated shorty's, K&N CAI, 1.6 crane RR, TPIS 58mm throttle body with milled air foil, 160* stat, SLP AIT sensor, Bosh Plat +4 plugs, Taylor spiral Pro wires and a throttle body bypass.
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Old 02-13-2009, 08:06 PM
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Here's a couple pics of the O2's


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Old 02-14-2009, 03:06 PM
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Is that O2 wire right up against the plug wire? If so, move it. The plug wire can induce voltage pulses into the O2 wire. It doesn't take much spurious voltage to throw off a signal that averages 500mV.
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Old 02-14-2009, 04:37 PM
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Originally Posted by Injuneer View Post
Is that O2 wire right up against the plug wire?
There's about 2" between the plug wire and the O2 wire.
I just received another mem-cal today that was supposed to be a factory stock program with VATS deleted.
Well, I guess the VATS delete didn't take to the mem-cal, since it will not start and now I have a code 46
Also, the last program that was done to my first mem-cal to turn the EGR function on didn't work either.
Is it normal for the programming not to take to the mem-cal ?
Maybe I should take it to a shop somewhere in So. Cal to do the programming and do a dyno tune while I'm at it.
Does anyone know of a good shop to do this.
Remember, It's a 1993 speed density system.
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Old 02-18-2009, 10:32 PM
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Been several days.....bump
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