99 Miata EGR light - only when tank is 1/2 full

Is this a US model 99? The US models have OBDII, which if the ECU light is on, sets a code. This code will show what the computer thinks is the cause. The problem may have to do with the pressure in the tank, or the vapor recovery system and some stuff associated with it. To add insult to injury, the fuel bipass regulator is located about halfway up the tank. When was the fuel filter under the car last changed?

Thank you for responding....yes...it's a 99 US model. It drives me nuts. It was off for the longest time since I kept the tank full, but now it just went on with about 2/3 of a tank. If I fill it up and turn it off (disconnect the battery for 20 seconds), it'll probably stay off, assuming I keep the tank full or close to that. My repair guy wants to replace the EGR valve but I think that's his choice of repairs. I'm not sure that's the problem. The car runs great with the light on....I'm tempted to just disconnect the bulb. I've already had a sensor replaced to the tune of $240!!....that didn't fix it.

Good question on the fuel filter....answer is never in 65K miles.

The ECM sets the EGR light because is senses high oxygen levels in the exhaust.

This is usually caused by either a bad EGR (test the valve with a vacuum gage, it should hold a vacuum and then test the EGR actuator, see if is working ok.). If the valve is ok, it could be the passage to the intake manifold. Check for carbon buildup. However, this would cause it to fail all the time.)

The other possibility is that you are running lean. Test you fuel pump delivery with the tank empty, see if the delivery or the pressure is ok. Besides the filter, you could also have a clogged pickup in the fuel tank

There is no EGR light on my '99. Are you refering to the CEL? When you replace the cap on the fuel filler after fueling, do you twist it tight for at least one click? If not you can get the CEL.

Shouldn't that show up before a half a tank is used?

Dunno, I tighten the cap properly. And I'm not gonna tempt the CEL gods by leaving it loose.

Yes....I do tighten it.....now it's become intermittent. It comes on for a few days and then goes off for a few days (Check Engine showing EGR failure). Seems to be no rhyme or reason. If it was the EGR valve wouldn't it come on and stay on? My repair person wants to change the EGR valve - $300 but I hate to do that if it's not a fix. Also he checked the exhaust ports and they're clean. I'm at the point of just putting electric tape over the light so I can't see it....but my remorse is that I don't want to be responsible for polluting the air if that's what is happening. Any other thoughts of why it comes and goes?

The EGR is not monitored as an individual item.

Its performance is inferred by other engine parameters. The test is along the lines of engine at operating temp, rpm at a constant 2000 or above so that there is enough back pressure to let the exhaust flow into the intake manifold, the ECM opens egr solenoid to put vacuum on the egr and monitor O2 sensor output and fuel flow or rpm. If the test parameters are not in range, CEL light turns on and code is stored. The light will go off the moment the operating condition of the engine changes.

You did not mention if your mechanic tested the EGR solenoid and for Viacom leaks. They also can trigger the code.

Perhaps I missed the answer to this question someplace in this thread: Did someone check for codes when the light was on? You can do this with an relatively inexpensive code reader or rent one for free from your local Autozone store. That would be a good first pass at determining what is wrong.

Ken

cantera snipped-for-privacy@removethis.netzero.com wrote:

Good feedback....thank you very much. Will check it all...quite desparate at this point.