EGR valve question

I have a '98 Mazda Protoge with one known issue and some symptoms. First the known issue: Every once in a while the check engine light comes on and the code comes back as an EGR fault (the code is on-again/off-again usually a week at a time). The guys at Autozone said it was a bad/going-bad valve which probably needs to be replaced. This is an expensive part ($250!) and in an inconvenient spot (the 4-6 inches between the block and firewall). Now for the symptoms: The car chatters and surges in first gear and can idle rough at stoplights. Are the symptoms related to the EGR issue? I’m a young mechanical engineer for a company that produces EGR sensors, which means I’m just dangerous enough to start making a scenario in my head where a blocked EGR could be causing reduced/erratic performance. The car gets great mileage (32 city/36 hwy), and while a little coarse around the edges, works as a great commuter vehicle. Should I just let this go and live with the symptoms? Is there a simple (and relatively inexpensive) solution to the EGR issue? Any advice is appreciated.

I think your car’s symptoms have more than one cause. I think the only thing a bad EGR valve will do is cause your car to fail an emission test. If I’m not mistaken, cars didn’t have EGR valves before the EPA was founded.

A clogged EGR valve can cause slight pinging on light acceleration. A stuck open EGR valve can cause rough idle and poor performance.

What is the exact OBDII code?
Often EGR related problems are not the valve. It could be clogged passages, and/or a bad flow rate sensor (common with Ford).

I’d be inclined to want to repair it.

When you nail the gas pedal, a large amount of oxygen is allowed to suddenly enter the air intsream. This has the same effect as using a bellows on a fireplace. Feeding a burst of oxygen into the chamber causes the combustion temps to rise. The EGR valve allows a bit of inert exhaust gas to be fed back into the air instream to displace a bit of oxygen in order to prevent the cylinder temperatures from getting too high.

High cylinder temps can cause two problems: pinging (or preignition) and elevated NOx production. The EGR valve prevents these problems.

If the valve is stuck closed, pinging and elevated NOx production can occur.
If the valve is stuck open, erratic idle and poor performance and mileage can occur.

Sometimes the egr valve can be cleaned to fix the problem. You might remove the valve (if possible) and look for carbon build up where it mounts to the block. This carbon must be removed by physical means. It might save you $250 at best, you’ve done no harm at the least. Ensure line between egr valve and egr sensor (if it has one) is carbon free also.

I agree with tardis to check out the DPFE sensor that controls vacuum to the EGR valve. My Fords usually developed this problem. Don’t know if it’s the same configuration as Mazda.

The printout is as follows:

BBEGR system malfunction- insufficient or excessive BBflow detected
Probable cause
BB1.- Blocked EGR passage
BB2.- Blocked exhaust system
BB3.- EGR faulty

Will carburetor cleaner suffice to clear out any blockages?

Yes, but I’d use sensor-safe throttle body cleaner instead. Non-sensor-safe carb cleaner may damage some sensors. Make sure you clean out the EGR module and the passages in the head it attaches to. You may also need to remove the exhaust manifold to get to the other side of the passage the exhaust gasses travel through to thoroughly clean it.


Thanks for your suggestions. I took BustedKnuckles advice and picked up some throttle body cleaner. I shot it in all the different places I could easily reach. After a few minutes letting the solvent work its magic, I started it up and took her across town. The car runs much smoother, has more zip and exhibits better response. While the light was off immediately prior to the treatment, I’ll have to wait and see if comes back on in the next week or so. Thanks again. -Domer08

Over the years, thousands of EGR valves have been removed and the hole covered with a plate made for that purpose. Sometimes these plates are custom made. That usually solves ALL EGR related problems (except CEL’s)… If an EGR were to fail, you would want it to fail in the closed position…The ONLY time they are open is under steady state cruising speed. They are normally CLOSED at idle and during acceleration…Their purpose is to limit NOx emissions by reducing combustion temperature. When the driver demands acceleration, high NOx emissions are tolerated for that brief period…

This is not a legal fix.
This is not a good fix.
They do also open during light acceleration. Without it, the combustion temperatures will go up and the OP may get light pinging.
Pumping loses will go up and so mileage will go down.

High combustion temperatures mean high efficiency. That’s how most people like it. Those living west of the Mississippi are still living in a semi-free country and can do what they want with their cars. Nobody has ever been fined or is serving time in prison for blocking off an EGR valve…If that were true, I would be serving a life sentence…

When EGR valves first came out (back in the 70s IIRC), I used to plug them as Caddyman noted. I even plugged many in new cars at the dealership I worked in. In practice I never noticed any increase in power or mpg.

I remember learning my auto mechanics school that the EGR valve was designed to dilute the incoming mixture with burned gasses to reduce peak combustion temps. Nitrogen and Oxygen combine more easily at high temps - forming NOx. However peak temps also means peak cylinder pressure (ie peak push on the piston).

I never learned that a design goal of EGR valves was to reduce pinging. I can see how they would do that as an added secondary benefit. If ping-reduction is a design goal, that’s new to me.

Joe, I didn’t say the original design goal for the EGR valve was to prvent pinging…I only said it did.

Some of us had a lengthy debate about this very issue awhile ago, debating about exactly what was the original reason for the EGR valve. I believed at the time that it was implemented (along with the cat converter) to reduce NOx output, others argued that it was to reduce pinging. They pointed out that at the time the valve was implemented engine operating temperatures were increased for purposes of reduction of CO and HC. Both arguments have merit. I lived through the pinging problems early on in the implementation of emissions reduction efforts.

In truth, it really doesn’t matter. The EGR system effectively helps with both problems.

9 to 1 compression “unleaded fuel only” engines seldom if ever have “pinging” problems…But if they do, it’s controlled by adjusting the ignition advance, not EGR flow…

I agree that ignition timing is also a cause of pinging, especially on distributor based ignition systems. But I think we’ll have to agree to disagree as to whether it can be a problem on stock engines for reasons other than timing, specifically a nonfunctioning EGR system.

Anyway, it’s good to see that the OP got his problem resolved. And cheaply, too!

Have a beautiful holiday.