Could detonation throw a EGR code?


Mar 31, 2005
Boston, MA
I'm starting to think about getting my car ready for the spring. Right before I put it away, I bumped the timing and my car started throwing some EGR codes for the EVP sensor. I got a new sensor, and I have a valve I got from Ebay that is ready to go in. before I put the car away, my mileage dropped from 17 mpg to 13 mpg, all of a sudden.

Here's what I was thinking, let me know if its possible... I want to still be able to run 87 octane, so maybe my car was MILDLY detonating for a short period of time. Since one of the purposes of the EGR is to cool the combustion temperatures, could the computer think the EGR isn't working properly if it was detonating?
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The EGR not performing up to par can cause detonation at part throttle. If the EGR is not working (or not working well) it can also cause a drop in fuel economy.

It sounds to me like you have more than one thing going on. I don't see how bumping the timing would cause the egr codes. But non functional egr has been know to cause part throttle detonation, and a slight drop in economy. But Hissin already told you that.

Other things that can cause bad economy.
vac leak - this probably will not show on codes
bad O2s - usually would show in codes, but I don't think they always do
bad ect - mine took a dump the other day, and economy has been horrid
Some basic theory to clarify how things work is in order…

The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine to ping. It can also reduce HC emissions by reducing fuel consumption.

The EGR system has a vacuum source (line from the intake manifold) that goes to the EVR, computer operated electronic vacuum regulator. The EVR is located on the back of the passenger side shock strut tower. The EGR valve and the passages in the heads and intake manifold route exhaust gas to the EGR spacer (throttle body spacer). The computer uses RPM, Load. and some other factors to tell the EVR to pass vacuum to open the EGR valve. The EGR sensor tells the computer how far the EGR valve is open. Then computer adjusts the signal sent to the EVR to hold, increase or decrease the vacuum. The computer adds spark advance to compensate for the recirculated gases and the slower rate they burn at.

Diagram courtesy of Tmoss & Stang&2birds.

The EGR sensor is basically a variable resistor, like the volume control on a radio. One end is 5 volt VREF power from the computer (red/orange wire). One end is computer signal ground (black/white), and the middle wire (brown/lt green) is the signal output from the EGR sensor. It is designed to always have some small voltage output from it anytime the ignition switch is the Run position. That way the computer knows the sensor & the wiring is OK. No voltage on computer pin 27 (brown/lt green wire) and the computer thinks the sensor is bad or the wire is broken and sets code 31. The voltage output can range from approximately .6-.85 volt.

EGR test procedure courtesy of cjones

to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 5in vacuum?

if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500 RPM.
should read about 5in vacuum
Sounds like a plan, thanks guys. One more quick question? I was just searching online for a vacuum pump to see if parts stores have them, it didn't look like it. Anyone know where to get a vacuum pump?
Parts stores have them. THey used to be a little over 30 bucks, but I have not checked lately. AZ and HF probably have the cheapest generic ones, though MitiVac's are more expensive and might be of better quality.

Good luck.
sunil6784 said:
Sounds like a plan, thanks guys. One more quick question? I was just searching online for a vacuum pump to see if parts stores have them, it didn't look like it. Anyone know where to get a vacuum pump?

You do mean a mitivac type right?