I just failed the idle part of my emissions (barely passed on the cruise portion, but did).

I had 1229 PPM of HC, and the limit is 220. So it wasn't even close. It's a 396 LT1 w/ 236/236 cam, long tube headers and no cats.

I leaded it out as much as I could for the retest (I have a FAST system with wideband) and the headers cooked my brake booster which now leaks (Arrrgh!). That only brought it to 1000.

Given that it got a little better when I went even leaner, I don't think it’s a lean misfire.

I'm running 28 degrees of timing at idle. I think I can improve some by doing down to 20 deg. I can add a gallon or two of alcohol to the tank, but I don't think that's going to help.

Anything else I can do to get a moderately big cam and no cats to pass at 220 ppm?

Thanks!
- Dave

 

Wow I am amazed it passed the cruise test with no cats, very impressive. I really dont see it passing without some type of cat in there. When I had my stock 87 I tried like h ell to get through without the cat and no way. You could show the smog tech a couple C notes. Good luck.

 

Did you try running some portion of alcohol? That was recommended a few times in threads I searched, but no one seemed to have tried it!

 

Dave whats the CO reading at idle? That will tell you how rich it is. If you take too much fuel out at idle the HC count will go ballistic. To lean of a mixture will cause high HC readings. Its a fine line in balancing to get a cammed up engine to pass.

You may have to put cats on the car to make it pass, as that would drop the number considerably. Also you can do a 20% mix of denatured alcohol in the fuel mix to dramatically drop the readings. Start with a 10% mix then go NO more than 20% if you need to. I have used this trick to get cammed up cars to pass and thats one of the ways I can get even a 572 crate motor to pass I know I am a baaaad boy

 

How can too lean cause increased HC unless you go so lean as to cause a lean misfire? From the graphs in "Corvette Fuel Injection", HC drops forever as you go leaner.

My throught was that since going from 14.7 to 15.9 improved it, it wasn't too lean, but it's only $15 to try again :-)

How much does the alcohol help? If 10% only nets you a 10% improvement, it won't be enough...

 

Going too lean causes a lean miss and that is where you get the high HC counts. Unburned fuel = high HC count I would gurantee you that if you drop that AFR down to 14 or 13.8 the HC count would drop, but go to far then the CO goes up, its a see saw effect. Also raising the idle speed to within the testing stations limit is another trick. And somethings retarding the timing in the idle regions helps as well.

Alcohol works because there is no hydrocarbons in its molecular chain. I got a 572 crate motor to pass a few weeks ago with one gallon of denatured alcohol in a 8 gallon mixture. Its reading was 6% CO and 450 PPM of HC when I was done the CO went down to 1.7% and 180 PPM HC well into the passing zone for a 1986 one ton pick up truck. This thing had one of them new 572 620Hp crate engines with a rather large Barry Grant 950 CFM Demon carb mounted on a single plane manifold. If you want to talk about dirty emissions this truck with the 572 was it. Making your car pass would be a piece of cake

How far is Redmond WA from Vancouver? I bet I can make it pass

 

I am going through this exact same thing as we speak. I have the DFI system and our test consists of 15mph cruise and 25mph cruise on rollers. I went into the VE table and found the four or so cells that those MPHs fell in. I kept leaning it out and watched the numbers fall. I did not have any misfires as I leaned it out. However, which my single mass flywheel coupled with leaning it out, the car was bucking on the rollers and we couldn't keep the MPH within 2-3 MPH as required by the test. Good luck! I have an extra high flow magnaflow laying around with cheesy little Y pipes on either side of it. You can have it just for shipping $ if interested.

 

Exhaust gas recirculation (EGR) systems were introduced in the early '70s to reduce an exhaust emission that was not being cleaned by the other smog controls. Oxides of nitrogen (NOx) are formed when temperatures in the combustion chamber get too hot. At 2500 degrees Fahrenheit or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons (HCs) and the presence of sunlight, produces an ugly haze in our skies known commonly as smog.

How to reduce NOx NOx formation can be reduced by:

Enriching the air fuel (A/F) mixture to reduce combustion temperatures. However, this increases HC and carbon monoxide (CO) emissions.
Lowering the compression ratio and retarding ignition timing; but this leads to reduced performance and fuel economy.
Recirculating some exhaust gases.
How EGR systems work The EGR valve recirculates exhaust into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are recirculated. These gases displace some of the normal intake charge. This chemically slows and cools the combustion process by several hundred degrees, thus reducing NOx formation.

The design challenge The EGR system of today must precisely control the flow of recirculated exhaust. Too much flow will retard engine performance and cause a hesitation on acceleration. Too little flow will increase NOx and cause engine ping. A well-designed system will actually increase engine performance and economy. Why? As the combustion chamber temperature is reduced, engine detonation potential is also reduced. This factor enabled the software engineers to write a more aggressive timing advance curve into the spark timing program. If the EGR valve is not flowing, onboard diagnostics (OBD) systems will set a code and the power control module (PCM) will use a backup timing curve that has less advance to prevent engine ping. Less timing advance means less performance and economy. Do your customer a favor and fix those EGR codes that you may have previously deemed as unimportant.

Evolution of the EGR systems The first EGR valves appeared in 1973 on GM cars. Bolted to the intake manifold next to the carburetor, it has ports to the intake and exhaust manifolds. It has a diaphragm that pulls open a valve stem, which allows exhaust to enter the intake manifold when ported vacuum is applied to it. Ported vacuum increases with throttle opening. A thermal vacuum switch prevents vacuum from reaching the EGR during cold engine starts. This system had many problems. It would often open too soon or too much, which caused a hesitation on acceleration as massive amounts of recirculated exhaust hit the combustion chamber. Many people simply disconnected it when it began to cause problems because they did not understand its importance or design. By 1975, if you unplugged an EGR valve, you'd have a driveability complaint of engine ping. Manufacturers and technicians of that era experimented with vacuum orifice restrictors and vacuum delay valves to try to find a happy medium between clean air and performance.

Closed loop systems By 1981, closed loop computer controls were in place. EGR flow was now more carefully controlled with dual diaphragm and back-pressure EGR valves. Modulating the vacuum to the EGR valve's pull, open diaphragm controlled the flow of recirculated ex- haust. Called by various names such as amplifiers, transducers and modulators, both remote and integral vacuum modulated devices were used. The flow of vacuum was further controlled by solenoids that blocked the vacuum ports until certain criteria were met such as engine temperature, rpm and manifold absolute pressure (MAP).

As the manufacturers began to use these complex schemes with vacuum amplifiers, delay valves and solenoids, they added a lot of "spaghetti" to the engine compartment. Plastic vacuum connections would break and rot with age and were not very reliable. Vacuum diagrams were invented and became essential to the smog driveability technicians of the day. As these systems evolved, they had fewer parts and less vacuum tubing. This was achieved by the use of pulse width modulated EGR solenoids. The PCM controlled EGR flow through the use of these solenoids to modulate vacuum to the EGR valve instead of just turning it on or off periodically.

What is pulse width modulation? Let's take a moment to discuss how computers think so we can better understand this common form of PCM control. Computers are binary. The machine language they operate in consists of only two variables: on or off, true or false, high or low. That's the only way a PCM can think. As a result, computer controlled outputs are always on or off, high (system voltage) or low (ground). Therefore, a computer output is always a square wave, or an on-off step when viewed on a lab scope. The high portion of the waveform will usually be battery voltage or PCM voltage of approximately 5 volts, with a few exceptions where the PCM operates at a different voltage.

Once the PCM receives its inputs, such as rpm, throttle angle, coolant temperature and MAP, it then calculates a response based on the software program that is embedded into it. Next, it makes its decision and sends a command in the form of a pulse width modulated signal to turn the EGR solenoid on and off rapidly. The EGR solenoid has two vacuum nipples. One side gets either manifold or ported engine vacuum. The other nipple goes to the EGR valve. Its default position is to block vacuum to the EGR valve. A vent is incorporated to bleed off vacuum when the solenoid is being pulsed. Vacuum flows to the EGR in rapid on-off pulses as the solenoid is commanded by the PCM.

OBD I systems With each succeeding year, the EGR designs became more refined. The California Air Resources Board (CARB) liked GM and Chrysler's onboard diagnostic systems. In 1988, CARB required that all cars sold in California be equipped with an onboard diagnostic system and a "check engine" light to notify the driver of emission system failure. By this time, all manufacturers had to have an EGR system that was capable of alerting the driver if it was not working. OBD I diagnostics and trouble codes were added in to flag opens, shorts and sticking solenoids.

OBD II EGR systems OBD II requires that the EGR system be monitored for abnormally low or high flow rate malfunctions. The EGR is considered malfunctioning when an EGR component fails or a fault in the flow rate results in the vehicle exceeding the Federal Test Procedure (FTP) by 1.5 times. FTP is the government-mandated drive cycle smog test that all new cars must pass and adhere to.

The diagnostic executive, also called the diagnostic task manager by Chrysler, controls the EGR monitor. The executive is an OBD II software agent given the task of managing all the onboard monitors and the scan tool interface. The executive coordinates the sequencing and actuation of all the monitor's test routines. There are eight main monitors whose sole function is to directly monitor and test the components assigned to them to ensure they meet FTP standards for life. These monitors are:

Catalyst monitor
EGR monitor
EVAP monitor
Fuel system monitor
Misfire monitor
Oxygen monitor
Oxygen heater monitor
Secondary air injection monitor
A closer look at the EGR monitor Monitor tests are both intrusive and non-intrusive. An example of an intrusive test is when the EGR monitor cycles the EGR valve during a condition when it normally would be closed. In some cases, the customer may feel an intrusive test as a slight miss.
The method of testing used by the EGR monitor varies according to the manufacturer, but there are three main types.

One method includes looking for a change in manifold pressure as the EGR valve is actuated on and off.

A second method involves cycling the EGR valve and looking for a change in short-term fuel trim. When the EGR valve is opened, it displaces some of the air fuel mixture. When the EGR valve is closed, more oxygen enters the combustion chamber, which then leans the mixture somewhat. The O2 sensor will respond with a lean signal to the PCM, which in turn increases pulse width. This is called short-term fuel trim compensation. The EGR monitor looks to see that all these things are occurring as they should. It repeats the tests and averages the results. Before the EGR monitor can begin its testing, it must first receive clearance from the diagnostic executive. The executive ensures that there are no conflicting conditions that would invalidate the EGR monitor's tests. For example, if the car had a lazy O2 sensor, fuel trim compensation to the EGR opening and closing would be inaccurate. Therefore, there are many safeguards built into OBD II to prevent this type of occurrence from happening. OBD II also has rationality checks. In other words, it uses deductive logic and constantly compares its inputs against each other to make sure all are in sync with one another. After the EGR monitor gets the OK to run its tests, it uses strict enabling criteria to ensure accurate testing such as:

Engine temperature more than 170 F.
Ambient air temperature more than 20 F.
Engine run time more than three minutes since 170 F.
Engine speed 2248-2688 (auto. trans.), 1952-2400 (manual trans.).
Manifold absolute pressure from 5-20 hg.
Short Term Adaptive Fuel Trim is adjusting pulse width by less than +7 percent and more than -8 percent.
TP sensor from 0.6 to 1.8 volts.
Vehicle speed sensor more than 40 mph.
The above is used for illustrative purposes only. Refer to your manual or CD-ROM information system for specifics to the car you are working on.

The third type of EGR monitoring design includes monitoring an EGR position sensor and a back-pressure sensor. Some Fords use a differential pressure feedback sensor that reads exhaust back-pressure upstream and downstream of the EGR valve to determine its flow rate and operation.

While OBD I systems would usually flag an inoperative EGR system, OBD II systems are given the task of determining the correct amount of EGR flow to keep the car running clean.