aammcc

Equipment - The car: 1991 z28 L98 - SLP 1 3/4 headers, MSD AL6, FPR set at 43 PSI. EGR is drilled out to .041, fan switches, heated O2 sensor.

As part of an ongoing project to get my car to pass emissions, I went for another pretest. Every thing passed well with in the requirements except for Nox at the 15 mph. It failed by about 35 PPM. Temperature was just passed the 220* as far as the dash gauge is concerned. The pretest numbers say the AFR is at about 16, according to the Lambda Calculator.

Given the above information, I see 1 choice that will guarntee passing, and 3 choices that I that I can try.

1) Current timing is at 6*. Since it is still allowed to be off by 3*, I can smog the car at 3* and I'm 99% sure all the numbers would be cut by half, and therefore pass easily. But I would like to see if I can get it to pass while leaving the timing at 6*.

2) Since I know that AFR is at 16 (according to the Lamba Calculator), a slightly lean condition, which = heat = higher Nox. The temp above 220* is probably the result of the slightly lean AFR of 16 and consequently the slightly higher Nox. I can try increasing fuel pressure by a couple psi. Increasing the fuel may improve the AFR and reduce the heat thus reducing Nox and maybe improving the other numbers with out having to back off the timing.

3) Since I can control both fans, I can turn them on earlier to keep the car cooler, thus reducing the heat and hopefull reducing Nox some more. This option however, feels more like curing the symptom rather then curring the disease.

4) I can try to drill out the EGR a little more to give the car more EGR. Although I think this may give me a leaner condition but not sure. This option feels more like a guess.

For the experts, am I on the right track? What would you do?

aammcc

I guess I'm going to try option # 2. I can hook up my scanner to check BLM's and INT first to see where those numbers are at. I maybe able to bump up the fuel pressure this way to maybe 44-46 psi I'm guessing. This should improve the AFR and improve the Nox as well as the other readings. We'll see how that goes.

ASE doc

Hi aammcc, Youre right on in your thinking around NOX production. Anything that increases combustion temp will increase NOX, and visa versa.

I am in agreement that you should look at BLM and Int for signs of a lean condition before making any changes. Lean operation would explain the high NOX and is not healthy for your motor.

I assume that you have a healthy cat. The 3 way cat reduces NOX.

Cooler fan temp may help some although, without lowering thermostat temp, lower fan temp will have minimal effect. And lower engine temp may increase HC emmissions as well.

SLP 1 3/4 heh? Those are the headers I run on my high revving, high horsepower 355 TPI. You know that such large primaries are intended for high rev horsepower and may actually hinder low rpm performance. Not knowing what other mods youve made to the engine, Im not sure what affect those headers may have on flow, and thus AFR. Your 91 is speed density and any change in air flow will affect AFR. Adjusting fuel pressure may be just what you need. Take care though not to push CO high.

aammcc

Thanks for the reply. Yes I have dual healthy cats and no other engine mods. I do have 180* thermostat which probably helps a bit. Gutted the intake as well, airfoil, and that's it I think.


Yes 1 3/4", it was the SLP's recommendation for a 350, I chose those based on that. And yes, it does feel like there maybe a little less at the lower end, not much less though I think since the wheels will chirp if I give it a quick step of the gas. Since overall I have a much better flowing system, it does make sense I would need more fuel.

I hear you about the CO. The CO in my test was really low. I think this reinforces the lean AFR number. The HC reading was on the north side of the middle. I know that a high HC typically means unburnt fuel, but with a really low CO (burned fuel) I think that not enough fuel could also cause a high HC because the fuel atoms could be too far apart and not burn completely thus causing a higher HC. So probably the introduction of a little more fuel will raise the CO a tiny bit but reduce the HC and NOX at the same time.

I'll give it a shot tonight but it will be a couple weeks before I can go to another pretest to verify the results. I'm sure I'm just about there though.

ASE doc

Absolutely correct. Lean AFR will cause high HC & high NOX. The chemical reaction is just as you describe it. The mixture becomes so lean(such sparse fuel atoms) that they pass through the chamber unburnt. Im impressd. Not too many have this depth of understanding on the chemistry of combustion.

Your thermostat choice is as low as I would go on an emmissions vehicle. The high temps(194 thermostat) from the OE are intended to reduce HC. You see, by running higher tempurature, especially at the exhaust, we actually process alot of HC from the exhaust before it even reaches the cat. This not only reduces emmissions but also extends cat life.

The higher temps are an unfortunate compromise for the sake of emmissions compliance. We all know that heat is the enemy. If we can lower temps by a few degrees and still maintain compliance, were doing the motor good.

Knowing now that HC was also high, I would definitely say youre running lean. If you could put the car on a five gas analyser it would be interesting to watch the effect as you raise fuel pressure a few pounds. I know that HC will come down along with NOX. Again just watch that CO doesnt come up too much.

A funny thing happens when we richen AFR just a bit too much. We actually see HC come up along with CO. This is more the case on catalyst cars than on non catalyst. I am quite sure that what were seeing is the cat being stressed and losing some of its effectiveness. Find that balance(stoich) and youll be fine.

aammcc

thanks for confirming some of my thoughts as I never really know whats correct. funny you mention the 5 gas analyzer as I had looked into the cost of one but seems they cost a few grand. Not worth it.

So with my scanner hooked up, I confirmed a BLM of 132 before any adjustments. Fuel PSI was at 42. I raised fuel pressure to 44ish psi and that brought the BLM down to 128.

Before the fuel pressure adjustment, the higher HC reading at 15 mph was 58ppm, with 81 being the max. As soon as I get a chance in the next couple of weeks, I'll go for another pretest and see where the numbers land. Should be really interesting...

ASE doc

The 5 gas analyser is invaluable for emmisions and drivability repairs. They can be found used relatively cheap. I have an old Bear machine that was purchased for $1200. Look around at old shops that are closing or check the classifieds.

ASE doc

I just noticed, you say HC was at 81 max? Thats not high. In fact thats low. Before you call this repair good, have you actually tested the EGR? Perform the stall test. When in doubt I go one step further and actually connect a vacuum gauge in the line to the valve and drive the car to verify operation. The timing trick does work well and is free and easy. You may end up going there as well.

aammcc

Actually no, my reading was 58ppm, the max allowable is 81. I have a really old smog test, before headers and any other mods and the HC was about 14-16. All numbers infact we're very low and far within passing range.

EGR and solenoid are working fine. Remember my BLM did drop to 128 after bumping a couple PSI. Won't know for sure until I get a chance to pretest again, maybe this weekend if time permits.

$1,200 is a good deal but then I don't plan on needing it that much after I'm where I need to be. Besides, $1,200 will give my 60 pretests. thanks for the tip though.

ASE doc

Wow! 81 ppm is strict. Oregon's limit is 220ppm.