Hey Thirdgener’s,

See attached emissions test results. Still passed by was approaching the limit on the HC at idle so only passed for 1 year instead of 2. What can cause the HC at idle to be so high when it is so low at 2500 RPM? It’s a 1990 IROC with a 383 TPI. It’s got fresh oil/filter, air filter, NGK plugs gapped at .044 (in vortex heads), Jacobs ignition, taylor 8mm wires, fresh 91 octane fuel with octane added, dual high flow cats into single 3” exhaust. I live in NM at 5,000 feet. It runs very well but I like to tinker/maintain it to the best of my ability so I was going to replace the cap and rotor and check the timing. What Should the timing be set at and can you all recommend anything else to check/replace.

Thanks in advance,
Ted

 

Make sure your ignition is 100%, HC is unburnt fuel essentially.

Stocking timing is 6BTDC for TPI. You have a 383 and no idea what your tune is, if any, so those are factors. But stock is 6BTDC

Another consideration is the Magnaflow high flow cats don't last but 10-20k miles in my experience. After 4 years they're rattlin or failin a smog test. Not sure why.

 

As tanner stated, HC is liquid fuel, as the term implies, that passed through the combustion chambers unburned. HC is normal from any engine, as no engine has perfect combustion. HC in the exhaust gas is normally burned off in the exhaust manifolds. What passes the manifolds is burned off in the cat on emission controlled vehicles. The AIR system's purpose is to decrease the amount of HC that gets past the manifolds and cat. On cold start, the AIR system injects oxygen rich fresh air into the exhaust manifolds which increases the combustion of HC and CO, which are both high during cold start enrichment. The added oxygen also helps heat up the catalyst. Then, after a short warm up, the AIR system switches to feeding fresh air to the catalyst which, again, increases the combustion of HC.

Does your engine still have AIR? If so, make sure it is functioning properly. If no AIR, you may need to do some tweeking to bring HC down for smog testing. Ignition timing has a big effect since burning HC in the manifolds depends on the manifolds being hot. Retarding base timing 2 degrees can reduce HC at the tailpipe by as much as 100ppm by raising cyl head and exhaust temps. Of course, heat is the key. The engine needs to be at full operating temp and the engine should be run at cruise rpm for several minutes prior to testing. If you have to sit in line waiting to test. Hold the engine at 1,500 rpm for several seconds every couple of minutes to keep the pipes hot.

If NM is a state where independent shops perform the tests, You won't be able to do anything about the car sitting before the test. You can however pull up to 4 degrees of timing without any significant reduction in driveability. That should bring HC well below the limit. Naturally, after you have passed emissions testing, the engine should be tuned for best performance.

 

Ill go ahead and order a new cap and rotor. I’ll check if any visible wear/damage when replacing. New cats have a couple thousand miles on them. I’ll check the timing too.

 

It’s equipped with SLP ceramic coated shorty headers with air. The vehicle was basically sitting at idle for 10 minutes before it got pulled into the bay for the test.

 

Hopefully NM isn't like CA where they actually put a timing light on the crankshaft to verify that the timing is set to the sticker under the hood - any other setting and it's an automatic fail. There are ways, but it typically isn't easy to get around the emissions laws here. You're at the mercy of the referee station when it fails.

Some of the OBDII vehicles skip the rollers and just get plugged in at the ALDL connector and scanned.

 

At curb idle (643 RPM) the engine is running too rich.
The high HC and low O2 readings on your test form is a nice clue.
Have you checked your fuel map and the O2 sensor readings at curb idle?
You should be in closed loop at curb idle speed once engine is warmed up. sitting in the lane waiting to test is no different than sitting in traffic.

• At closed loop your O2 readings should be cycling from .450 to .800 Volts (indicating STOICH mixture) perfect STOICH mixture would cycle from .450 to .500 volts.
• If O2 readings at idle are steady at closed loop you have a bad O2 sensor.
• If O2 readings at idle are cycling higher than .800 volts say .800 to 1.000 Volts then your A/F is very rich.

Did you have a custom PROM made? adjusting the fuel curve at idle to get O2 voltage in correct range would be better and would be permanent fix (you can leave your timing alone)

IDK what your cam specs are, but if you are using the factory TPI it cannot be too radical or the ECM would not be happy with it.

Keep us posted. this graph has A/F ratios (with Lambda conversion) and O2 voltage

 

Yep. Need to watch that the O2 is in closed loop at idle. Often they will fall out of closed loop if they are cold or the sensor is old or slow. If you fall into open loop at idle, the fuel mixture goes about 15% richer by default. This will easily cause you to fail or be very high on HC.

GD

 

FRMULA88, your description is over simplified. With headers and non heated (single wire) O2 sensor, it's unlikely that the system will stay in closed loop any longer than 2 or 3 minutes at idle. Cast manifold cars will barely maintain CL for more than a few minutes. Closed loop, the O2 sensor toggles between .200 and .800. May go as low as .100 and as high as .950. It will stay above .450 if the fuel trims are at full rich and the ECM can't pull enough fuel to reach stoich. The same goes for sticking low. You cannot use narrow band O2 sensor voltage to determine rich or lean AFR. As long as the system is in closed loop, the O2 voltage will toggle high and low across the .450 volt threshold. Unless the O2 sensor is faulty, but again, a non heated O2 sensor will not maintain closed loop at idle with headers installed. The operating range of the narrow band O2 sensor is 14:1 - 15:1. At any richer mixture, such as full throttle power enrichment, it will read .850 - .950V. Any leaner mixture, such as during decel fuel cut, it will read .050-.150. To determine rich or lean AFR, you must use fuel trims.

.29% CO is not rich. It would be lean for a non catalyst vehicle. For a catalyst vehicle, it is just about right. Fuel trims are not the cause of the HC problem. In fact 155ppm HC isn't really a problem, except for some over eager test tech. As I said, 2 degrees of timing, even one degree, will make a substantial difference. Make sure your AIR system is functioning correctly and feeding air to the catalyst. High HC is rarely caused by rich AFR, unless the system is very rich. HC is usually caused by lean AFR.

 

Not really, You just read what you wanted to read... and that's OK, MOST people do that.

O2 sensor placement is important with headers and it will cycle in closed loop.
But what do I know I was running projection 4Di on my 383 with Hooker 2210 headers and I did my own tuning using the narrow band O2 sensor and fuel map

 

I didn't mean to come off like I was criticizing you, only trying to educate. I've been tuning and diagnosing factory fuel and emissions systems since 1990 when I worked for GM. I tune carbs to pass DEQ testing regularly using a 4 gas analyzer. That high HC is caused by rich AFR is a common misconception. HC will go high but only when CO is past 4%. The real challenge is usually to bring CO down into the correct area without pushing HC out of range. As you bring CO down under 2%, on a non catalyst vehicle, HC wants to rise due to the poorer combustion of the leaner mixture. Keeping the manifolds hot by keeping timing down at idle is part of it. Making sure the AIR system is functioning well is another part of it. Overall engine tune is the most important part. One thing to remember on any TBI or carb application is the importance of fuel atomization and the part that the manifold hot bed plays in that. Most high performance builds delete the exhaust crossover to help keep the air fuel charge cool and dense. Of course with headers there is no EFE system, This unfortunately works against fuel atomization at idle. Factory EFI systems are different since there are no adjustments. On these we can only diagnose and determine what is missing when emissions are excessive. When a vehicle is modified, we have to look at ways to work with those modifications to pass emissions inspection.

I'm glad to hear that you were successful tuning your motor using the narrow band O2 sensor. Of course, you can only trust the narrow band readings for the central range of AFR. The engine will operate from 12.5:1 at full load to 18.5:1 during decel. Aside from some guesstimation based on the bottom and top voltage of the sensor, the narrow band can't be trusted to read those low and high ranges.

We do use the narrow band for some full load fuel delivery examination. On a hard pull uphill we expect to see the sensor(s) hold at least .900mV. On alot of GM vehicles the fuel trims default to 25% at full throttle and the O2 sensor is all we have to determine if there is adequate fuel. From there I look at MAF data vs. VE along with fuel press if O2 voltage doesn't hold above .900 at full load. Even then, that is only a basic indication of full rich AFR. It isn't an accurate reading. The chart you posted from the Bosch book shows how the narrow band sensor becomes very hard to quantify beyond its 14:1-15:1 range.

Mostly, narrow O2 sensor voltage and switching rate is used to diagnose a suspected faulty sensor.