Hi all,

I have 91 TPI 5.7L. It has 1 3/4 inch SLP headers w/airtubes, 6Al MSD and blaster coil. All legal upgrades with the CA carb #'s, etc. It has all of these new parts: GM EGR, EGR solenoid, GM O2 sensor, IAC valve, TPS, MAP, sensor, dual cats, new GM injectors, basics such as plug wires, cap and rotor all new as well.

Parts that are not new or old that I can think of: temp sensor, ECM.

Car runs fine and no other issues.

The car passes everything else but Nox. Failed 15mph/1213 and 25mph/1187.

A friend passed the car by running the test with the EST disconnected. Nox was 15mph/188 and 25mph/181. All other numbers about the same.

I still want to be able to get the car to pass legally and not really sure where else to look.

Could I possibly have a vacuum leak? Bad ECM? EGR not opening enough maybe?

Any suggestions would be greatly appreciated.

Thanks in advance.

 

what were the numbers for everything else?

make sure the ignition timing is right.

check the EGR system for proper operation.
normally if the ECM doesn't turn the CEL on and doesn't have a code 32 stored, the EGR system is working ok, but not always.

if the timing and EGR system seems to be right, the next thing i would check is the balancer. its pretty a common problem for the outer ring to slip.
if the outer ring has slipped the ignition timing would be off when set with a timing light.
i've seen balancers slip in either direction. one way and the timing is too low, the other way and the timing is too high when the timing is set with a light.
too high will cause high Nox.

how to check the balancer,
http://www.crankshaftcoalition.com/w...with_heads_off

if the balancer has slipped, it needs to be replaced.

 

I will go ahead and check the balancer tomorrow, just in case. On the subject of timing, I thought that maybe my MSD box was causing a timing problem. I did three checks.

1) disconnected the MSD box and unplugged the EST to check timing. It was at 6 degrees as required at idle. Then I reved the engine while checking the timing and notice that the timing remained at 6 degrees. Not sure if the engine was still cold but the rpm seemed to surge somewhat while in idle. Does that seem normal?
2) check the timing with the MSD still disconnected but EST plugged in. Timing at idle was about 14-16 degrees, no surging like before. Then timing jumped to about 20 degrees while reving the engine. Is that normal too?
3) The I plugged the MSD back up and had exact same results as in #2. 14-16 degrees at idle and 20 degrees when reving. Only real difference with the MSD plugged in was a quicker start up, smoother idle.

I'm inclined to think that the EGR is probably functioning correctly becuase it's new and I think if it we're bad, car may have not passed with the EST disconnected. Just my guess.

thanks for the advice, I get back once I check.

 

no problem

occasionally i've seen cars that had a surge with the EST unplugged, but most have a pretty stable idle speed. for now i wouldn't worry about it too much.

i agree the EGR is probably working because normally NOx is very high without it. close to 2000 PPM or higher. but its possible the EGR passages are partly clogged. there may be enough EGR flow so it doesn't set a EGR code, but not enough flow to fully bring NOx down.
a couple of other things, a weak cat could cause NOx to be high.
also, some aftermarket cats don't work very good on NOx. i've seen some that would only reduce NOx by about 10~20 PPM.
excessive carbon deposits in the combustion chambers can cause high NOx.
the octane being too low or bad gas can do it too.

 

Here's a good one other than the typical EGR valve have you checked out your AIR system? Our cats are three way type cats to help keep the NOX under control and if the AIR system is not working right it will not allow the cats to do their job.

 

You need a heated O2 sensor. Not, that that's the problem, but you still need one.
The following may help find the problem:
Within a narrow fuel/air ratio band surrounding stoichiometry, conversion of all three pollutants is nearly complete. However, outside of that band, conversion efficiency falls off very rapidly. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and hydrocarbons) are favored, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NOx is favored, at the expense of CO and HC oxidation.

Basically this means that the O2 sensor should fluctuate from rich to lean about 1 per second (cross counts) to keep the reactions balanced. If the EGR and A.I.R. systems are functioning properly then the engine seems to be running lean.

 

Thanks for all the replies so far.

I noticed I typoed the NOX on my first post. Here are the whole numbers of the legit smog fail.

15 mph / 1426 rpm
CO2% = 12.6
O2% = 2.8
HC = 49 – max 108 - PASS
CO% = .01 – max .71 - PASS
NO = 1872 – max 767 - FAIL

25mph / 1357 rpm
CO2% = 12.4
O2% = 3.1
HC = 45 – max 108- PASS
CO% = .01 – max .71- PASS
NO = 1630 – max 767- FAIL

Doing some research on this board, I found a thread where Dyno Don explains that “Where your problem lies, is in the fact that with a better flowing system you will need a little more vacuum to the EGR to make it work right.

Remember, I told you to the drill hole where the vacuum line goes on about .020 larger. That will help to open the EGR better, lowering the NOX.”

Here’s the thread.

https://www.thirdgen.org/forums/exhaust/460555-increasing-back-pressure.html

Make sense to me since. So I drilled my EGR valve from the stock .020 hole, to a .041 hole. I did have a .040 drill bit but chose the .041 just for good measure. Put everything back together and went this time for a pretest since I already have my sticker anyways.
Here are those results:

15 mph / 1372 rpm
CO2% = 13.1
O2% = 2.7
HC = 101 – max 108 – PASS BARELY
CO% = .31 – max .71 - PASS
NO = 779 – max 767 – FAIL

25mph / 1357 rpm
CO2% = 12.4
O2% = 3.1
HC = 83 – max 83 - PASS BARELY
CO% = .16 – max .71 - PASS
NO = 639 – max 767 - PASS

So after drilling the EGR hole, NOX came way down to almost passing at both speeds, but HC went up to almost failing. Logic would tell me that if I drilled yet a little larger, I would probably pass NOX but would probably fail HC. CO% went up also, but not enough to be a concern. CO2% and O2% also improved slightly. So now I have to address bringing the HC back down and NOX still a little lower. I will have to digest these numbers and do a little more research I think before I do anything else.

The balancer was fine but I’ll read up on the heated O2 sensor.

Any thoughts are appreciated.

Thanks.

 

what about a smog pump? if you need one let me know. i have a brand new one off my 89 tbi.

 

The EGR Valve & Solenoid uses ported vacuum not manifold vacuum. Ported vacuum will increase with a better flowing engine.

 

Then why did the NOX go down more then 100%? There must be some direct correlation or drilling the larger opening should've had no effect then.


Also what is a heated O2 sensor and why do I need it?

thanks.

 

Where do you live? I live in Southern Cali and I didnt have to take emissions. So I just put in a 3" pipe in place of the cat.

 

I have the same 1-3/4" SLP Headers, at high speeds they cool off too much, which affects the O2 sensor.

My reply wasn't to prove, or disprove the modification, it was to clarify that for the EGR valve the source of the vacuum is what's important because manifold vacuum decreases with rpm. At the rpm required to enable the EGR Ported vacuum is much higher than manifold vacuum.

Maybe I should just have said, "Verify that the vacuum hose to the EGR solenoid is connected to ported vacuum."

 

emissions tests don't always give the same readings on the same car, even back to back tests can sometimes give different results.
any little thing can cause a car to pass or fail.
an example, higher humidity in the air helps to cool combustion a little. that will help to lower NOx.
if you noticed, there is a 50 RPM difference between your 2 low speed tests.


lets see if i can put this into words correctly.
i take it the hole you drilled out is where the vacuum hose plugs on the EGR
valve. there is an orifice there. that orifice slows down the amount of vacuum the diaphragm inside the EGR valve sees.
under most conditions when its commanded on, the ECM cycles the EGR valve solenoid on and off.
the hose between the solenoid and valve acts like an accumulator and stores
vacuum.
when its off, the solenoid bleeds off the vacuum in the hose going to the EGR
valve. by drilling out the orifice in the valve, the diaphragm inside the EGR valve sees more vacuum and opens the valve farther.
you've basically increased the on time of the solenoid.
this gives the motor more EGR.
exhaust gas won't burn, that lowers combustion chamber temps which lower NOx.
too much EGR displaces more fuel air mixture in the cylinder which makes the
fuel mixture a little leaner which drives up HC a little.
its a fine line between just enough EGR to lower temps, and too much which can cause a lean mixture.


for its next test, i would do a decarb treatment a few days before its time to take it in and pull 1 degree of base timing. instead of 6 BTDC, set it at 5 BTDC.
pulling a little timing normally reduces CO, HC, and NOx.
if you pull too much, CO tends to start rising, quickly.

 

i like this idea, run a can of seafoam through the brake booster hose and take her back, thank me later......

 

Thanks, already been done. 2 cans to be exact.

 

thank you for the well explained response.

Good point about the rpm. 50 rpm might have made a difference in this case maybe.

You are correct I drilled the orifice where the vacuum hose plugs in from .02 to .041.

I'm a little confused about this part. I thought HC was unburnt fuel going out the tail pipe, a sign of being too rich.

Even though I have 2 years now to play with this, my goal is to nail this down in the next few weeks just to get it done. I intend on making the necessary adjustments until the car passes easy or as clean as possible. I don't want to leave the car in a "barely pass state" since I won't be making any more mods, ever again. II'l just keep going to pre-test till I get the results I want. So having said that, what other adjustment can I make before having to resort to changing the timing to 5 BTDC, as I know we are allowed +/- 3 degrees? I seems I'm pretty close. (side note: I'm going to check all the vacuum hoses and change any that are old or new this weekend just in case.)

Please tell me if my thought are off on this; As far as adjustments, I was thinking that maybe I could now drill the orfice in the EGR from .041 to .05 or .06. Then I can richen up the fuel at the adjustable FPR. Does that sound right or will the ECM override the fuel mixture? Now I not sure since I'm a little confused about the HC's now.

This is almost like a good puzzle...

thanks for the help.

 

i need the smog pump for my 86 FB/TA will it still work if i hae carbed engine?? and how much?

 

Actually I think this is backwards more EGR makes the car run rich in this case i would think. Whats happened is as stated in theory already burned exhaust gasses don't burn again. Whats been done is increased the amount of exhaust gasses the car takes in however the ECM dosnt know this. The ECM being that its speed density is going to pump in the same amount of fuel however (or possibly more detecting lower vacuum due to the EGR valve bleeding off some the pressure but ide think this would be pretty negligible) because more of the air is being displaced with already burned exhaust gasses that dont burn this would make me thing that it would go a little rich. If it was MAF it might compensate for this as EGR gas increase as the incoming air is actually measured but a MAP system wouldn't be able to compensate I wouldn't think. Or at least thats my take on how this would work. Course im still not convinced that there is not some underlying problem causing this all. For the NOX to be this far out with street legal parts (which are tested and proven to not adversely effect emission) while everything else is pretty good I just dont see it as a problem with the EGR system being inadequate. Then when you consider how many threads there are with people with even stock motors complaining with these exact same symptoms and emissions numbers that are shockingly similar i would be very skeptical before i started drilling things out. Try doing a search and you'll see that failing emissions tests in this fashion isnt that uncommon even the numbers look the same. Last time my Firebird failed it failed with very similar numbers. Granted I get that its modified but still i don't think the cause of this failure is something exotic. Its always best to start with the simple stuff as thats usually what it is.

 

That kinda of make sense. I added vacuum to the EGR by drilling the orifice bigger. That made NOx come down but HC's go up. If HC's went up and HC's = unburned fuel making it out the tail pipe, then a rich condition make sense. So If I add even more vacuum, then it will raise HC's even more, and will need to reduce fuel pressure via the fuel pressure regulator?

 

you can have high HC being either too rich or too lean.
CO is the main indicator of rich or lean.
too rich you have high CO. you have high HC because all the fuel doesn't get
burned and also have very little to no O2, unless its so rich that the motor is
misfiring from excessive fuel. at that point you'll be able to easily see black smoke out the tail pipe. (i should mention here that a leaking fuel pressure regulator can cause white tail pipe smoke and it will have a very strong raw gasoline odor.) too lean and you have low CO and high HC because of not enough fuel to completely burn the mixture. there is also high O2.
too lean can also raise NOx. too rich tends to lower NOx.
if the motor is running lean, it can cause the cat to not process NOx properly.
you can't really go by post cat CO numbers for being lean. a good working cat can take care of the CO if the motor is only a little bit too rich.

without pre cat readings its hard to know just what is coming out of the motor.
with post cat readings, you can use a lambda calculator to help figure out if its rich or lean, but the AIR injection must be disabled and it can't have any exhaust leaks.

im really leaning more toward the EGR passages being partly clogged.
the 2 openings into the plenum right behind the throttle body being restricted are pretty common. you can clean that part of it by pulling the throttle body and use a stiff piece of wire bent to a 90 degree angle to get the 2 ports pretty clean. its best to pull the runner that has the small tube, then put a rag into each side of the plenum where the throttle body goes to prevent any carbon from getting into the motor. then with compressed air blow into the EGR port where the small tube on the runner bolts to the plenum.
but EGR passage along the bottom of the plenum back to the runners isn't real easy to clean.
soaking the plenum in carb cleaner and then blowing the passage out with
compressed air will do it, but it takes a couple of days of soaking and blowing it out to get it clean.



Rolling Thunder, you're like the third or fourth person to ever question me on this as being a lean misfire. nice catch
if you go by oxygen to fuel content, then it would be rich.
but with too much EGR, the fuel will still mix with the exhaust gas, so by volume the mixture is ~14.7:1.
so lean misfire isn't the right terminology to use, but niether is it rich.
dilution misfire is what that type of misfire really is.
i use lean misfire because most people can understand that and are ok with it
without me needing to spend a lot of time with them explaining the difference.
of course people some can get it right away, but other people don't.

 

MOST smog problems on modified vehicles come from the fact the computer is not tuned for the engine. This is the result of a properly tuned engine running with a good catalytic converter...The shortblock had 250K miles on it at the time. Vortec heads, Edelbrock 7116 intake (NO EGR), Doug Thorley Headers, Duals, mild Crane 272H10 (216/216 @ .050, .454/.454" lift, 110*LSA) Cam. Just because it was a 1983, was fairly modified, making 2x the stock power, and had 250K miles on it, doesn't mean it has to run dirty. This was taken back before Texas emissions machines could link to the CAN database. We ran a 2006 Honda Civic on the dyno immediately after and the old small block TBI 350 ran cleaner than the modern honda...LMAO...

Fractions of the allowed readings...

25 MPH
Emission----STD-------READING
HC---------326ppm-----42ppm
CO---------2.62%-------0.03%
NOX-------2320ppm------47ppm

15 MPH
Emission----STD-------READING
HC---------332ppm-----50ppm
CO---------1.89%-------0.10%
NOX-------2490ppm------72ppm