Too much or too little EGR? Either way, drivability and emissions will suffer.
NOx, or nitrogen oxides, are a combustion by-product caused when nitrogen molecules and oxygen molecules combine under high heat, generally more than 2,500 degrees Fahrenheit. Lowering these smog causing emissions is relatively easy. All we need do is control the combustion chamber temperature, and we can do that in a few different ways.
One way is to add more fuel, enriching the air/fuel mixture, but that would increase fuel consumption and raise hydrocarbon (HC) and carbon monoxide (CO) emissions. Another means to reduce combustion chamber temperature is to lower compression ratio and retard ignition timing. Not a great alternative, though, because we'd lose power and, again, reduce fuel economy.A third way is to introduce already combusted exhaust gasses back into the combustion chamber. These inert gasses displace the fresh air/fuel mixture and chemically slow and cool the combustion process by several hundred degrees. This is the process we know as EGR, or exhaust gas recirculation. The design challenge is adding the right amount of these inert gasses and at the right time. Too much flow will reduce engine performance and cause a hesitation on acceleration. Added prematurely and EGR will cause a rough idle and misfire. Too little flow when needed will result in high NOx emissions and engine ping, or spark knock.
Exhaust gasses can be added to the combustion chamber in two ways. The most commonly used method is external EGR, where a passage between the exhaust manifold and the intake manifold is controlled by either a vacuum operated or electrically operated valve assembly. Another method gaining favor on gasoline engines is internal regulation, accomplished by altering the valve timing, increasing the amount of valve overlap and preventing complete scavenging of the exhaust gasses at the end of the exhaust stroke.
A Little Background
EGR began making its first appearance on cars in the early 1970s. Some of these early systems used ported vacuum to open the vacuum controlled valve. Since ported vacuum increases with the opening of the carburetor's throttle plates, more EGR is fed as throttle opening widens. They also used a thermal vacuum switch to prevent the addition of EGR during cold engine start. Most of them had a variety of problems, typically adding too much of the recirculated exhaust, or adding it too soon. Customers came in complaining of severe hesitation as massive amounts of EGR hit the combustion chamber.These systems were simple in operation, but relied on a lot of plumbing to work. Vacuum connectors and lines rotted, broke with age and were not reliable. That all changed when modulated vacuum control was introduced. On these designs, the ECM controlled an electronic vacuum supply solenoid with a pulse width modulation based on a variety of engine inputs.
Typically, there are two ports on this type of solenoid with one connected to intake manifold vacuum and the other connected to the EGR valve. The ECM calculates the appropriate amount of EGR needed based on the current operating conditions of the engine. It then calculates the amount of vacuum needed for the valve to deliver that amount. By sending rapid, on-off electrical pulses to the vacuum solenoid it can effectively regulate that vacuum supply.
Now, most applications use an electronically controlled EGR valve. Linear EGR, digital EGR and electronic EGR are all names for this next step in emissions control. In these designs, the ECM directly controls the amount of opening via electronic valve(s) in the component itself, no vacuum needed. In later designs of the vacuum-controlled, pulse-width modulated EGR and the more current electrical EGR valves; you'll usually find some type of position sensor incorporated to provide the feedback the ECM needs to make minor adjustments to its calculations. Techs also can use that feedback when diagnosing an EGR flow code complaint.
OBDII Monitoring
The ECM on OBDII compliant vehicles monitors the EGR system for problems in flow rate. It will illuminate the malfunction indicator lamp (MIL) when it detects a low flow or excessive flow condition. If programmed, it also will set related codes for faults found in the system components, like problems in the position sensor or vacuum solenoid and their related circuits.Honda/Acura EGR Issues
This is the Asian-focused issue of Motor Age, so let's switch gears and talk a bit about EGR related issues many Honda/Acura V6s suffer.
The ECM intrusively detects problems with flow on these engines. The ECM opens and closes the EGR valve when the engine is decelerating and in fuel cut-off mode. It looks for the corresponding changes in manifold absolute pressure. If no change is seen on the first test, the ECM records the fault as a pending code. If the second consecutive test also fails, the ECM will mature the code and turn on the MIL.While the most common problem is a restricted EGR passage, you should make a few tests before ripping off the manifold and performing the procedure I'll share with you in a minute. Are there any codes stored related to the position sensor? If so, check those first. Next, manually operate the valve with the engine running at idle and watch the scan data PID for the position sensor to make sure the valve is moving. An open EGR valve at idle should cause the engine to at least run rough as all get out, if not stall completely. No change with a confirmed valve opening points to a clogged intake.
Exhaust gasses are nasty stuff. Not only do they carry out the charred remains of the air/fuel mix, they also contain remnants of the oil used to lubricate the cylinders. These hot gasses are then fed back into the cooler air of the intake, where they begin to solidify and collect. So look for the restriction to start at the point where the gasses enter the intake and back up the passage from there.Early, single port design: The earlier design of the V6 used a single port entering the intake just behind the throttle plate. Typically, cleaning this port requires removing the upper intake manifold from the engine. Both a technical service bulletin (TSB) and warranty extension was released to cover problems with premature blockage of the port with the fix being the addition of a sleeve to the passage to minimize deposit buildup. Here's how it's done.
Following the directions in the service bulletin (00-004, release date Dec. 17, 2001), remove the upper intake and set it on your work bench. Remove the end plates, intake air temperature (IAT) sensor and boost plate from the manifold. Cover the six intake ports with duct tape to protect them during the sleeve installation process.
Use an 8 mm drill bit in a hand jig to clean out most of the deposits in the EGR passage. Turn only by hand — no drill, please! The next step requires a special installation tool kit that you can still find under Honda part number 07ZAD-P8AA000. While some techs have reported they've had success just cleaning the passage and sending the customer on their way, I've always been a stickler for following the OEM recommendations.
Using the included stepped drill bit, carefully over-size the existing passage. Do not go too far and drill into the other side of the manifold. Next, thoroughly clean out the intake manifold and remove all the drill shavings. Last, start the new sleeve into the drilled passage, aligning it as shown in the bulletin and use the installation tool that came with the kit to seat the sleeve in the intake manifold. Reinstall the manifold and verify your repair.
Later, multiple port design: Several manufacturers have gone to multiple EGR port designs that dump the EGR gasses right at each individual cylinders' intake runner. These designs create unique symptoms, with clogged passages generally resulting in drivability issues under higher loads, like high speed highway cruise or passing at moderate to high speed. The ports do not clog uniformly, and some cylinders still receive the EGR they expect while the others receive none. This results in cylinder misfires caused by the imbalanced EGR delivery. This problem is not limited to just the Honda/Acura V6s. Look for this issue an any design using multiple ports.
The Honda/Acura designs did, however, make this a little easier to inspect and deal with by including a removable inspection plate on the intake. (Many of their 4-cylinders have this access plate, too.) But according to TSB ASN 0603-05, released in June 2003, sometimes it's a tech's fault that the cylinders aren't getting an equal share. The design is not symmetrical, and installing the cover's gasket upside down can cause some of the ports to be covered.
To check for this possibility, look for the gasket's locating tab at the throttle body end of the intake manifold, toward the rear cylinder head. No tab? The gasket is on upside-down.
Troubleshooting the cause of EGR flow codes is relatively simple. But consider the possibility of improper EGR flow, whether it be too little or too much, when dealing with any related drivability complaint, code or no, and you may find yourself fixing idle quality and misfire issues you weren't fixing before.
