I just got a '90 LX Hatchback a couple of weeks ago (new to the forum) and its pretty mint except for this one problem which has just begun. Last week sometime, when I started the car up in the morning the check engine light came on briefly. I ignored it and then the following day when I was driving to work, it flashed on again briefly. Again I didn't pay it much attention and made a mental note to flash the codes out of the MIL later. Then a couple days later when I was on my way back from a store in the evening I noticed that the Voltmeter was reading pretty low. Not in the red, about two notches above the 8. Usually it's right in the middle of the gauge and I didn't have the HVAC blower on, or the rear window defroster, or any other major electrical load. When I got home I did the KOEO and KOER tests and produced a code 31 in the continuous memory. This is what the Chilton manual describes under code 31-PFE, EVP or EVR Circuit below minimum voltage. Since then the check engine light has come on and off in the same manner, Usually for only 2 or 3 seconds and then goes off. One night it stayed on for probably 15 minutes while I way driving home on the highway, but shut itself off before I got home. Before I had gotten on the Highway the light was already on and the system voltage was pretty low at idle and low speeds, but came back up to normal once I was going. The low voltage and the MIL light being on sometimes accompany one another and sometimes not. The problem seems to be affected by heavy acceleration. A couple times when I have had my foot to the floor and am rowing through the gears the MIL light will come on while I am still accelerating and will come off as soon as slow down. The base battery voltage of the car is about 12.05V, the system voltage with the car running is about 14.5V. If I run the car and turn on the blower, rear defrost, and high beams and then shut them off and let it charge for a couple seconds and then shut if off, I get a battery reading of about 12.7V. So, the car seems to be charging properly most of the time. This doesn't seem like a huge problem right now, but it's kind of irritating and I want to catch it before it develops into something worse leaving my stranded somewhere. Has anyone ever had similar problems with their 5.0L? Or is this the first you have ever heard of such a problem? I don't want to just start replacing parts. Thanks for reading this far, and any input would be greatly appreciated.
alternators do go bad sometimes not all at once. turn the car on and let it idle or have a friend give little gas wile you listen to the alternator. you should be able to hear some kind of whooshing or shooshing noise if the brushes are going bad.
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Just reading through some info on this site, on the EEC IV system, I came across a list of acronyms that included the ones under the code 31 description. It seems that EVR in this case stands for Egr Vaccum Regulator and not electronic voltage regulator as I had thought. So it seems as if I have an entirely different problem to deal with. Although It still leaves me wondering if the charging system is functioning properly. What kind of readings do you guys get on the stock gauge normally?
When I figure out more about the EGR problem I'll start another thread.
That EVR is mounted on the rear of your passenger side shock/strut tower. It's the larger of the three electronicly controlled vacuum valves. The computer tells the EVR to open and close, thereby regulating vacuum to the EGR valve.
But my book says that 31 is EVP below minimum voltage.
I'd check to make sure that none of the vacuum lines from the engine to the EVR to the EGR valve are leaking. They do like to get brittle and break.
You can test the EVP with an ohm meter and a vacuum pump (or by sucking on a vacuum line connected to the EGR valve. It should range from 5500 ohms with the valve closed, to 100 ohms with the valve open - with the EVP unplugged, and the ohm meter connected to VREF and Sig Rtn.
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Charging System Problem 
