Confused by EGR valve operation and CEL

[Mr. Guy]

This is on a '90 (latter half; has a timing chain) Iron Duke.

I've read that the EGR valve operates like so:

Idle - 45mph, valve is closed. No/very little exhaust gas is allowed into the intake air stream. The O2 sensor ignores readings for the EGR portion of the ECU until...
45mph+, the valve is opened via vacuum, allowing a small amount of exhaust to dilute the air/fuel mixture, which in turn produces a cooler burn and prevents the engine from heating up too much. The O2 sensor monitors the exhaust stream for the correct exhaust composition, and lights the CEL if it's wrong.

But, it seems to me like a completely passive system like what appears to be mounted on this engine, would operate more like:

Idle - 2500rpms, valve is initially closed, and opens more depending on engine speed and resulting vacuum underneath the throttle plate. The O2 sensor constantly monitors the exhaust stream to ensure the valve is delivering the proper amount of inert gasses to the intake stream, and lights the CEL if readings are off.
2500rpms+, the valve is fully open, allowing the most exhaust as is safe to dilute the air/fuel mixture, which in turn produces a cooler burn and prevents the engine from heating up too much. The O2 sensor continues monitoring the exhaust stream for the correct exhaust composition, and lights the CEL if it's wrong.


So which is right? I'm pretty sure the seal on my valve is bad (not maintaining proper vacuum), so it doesn't like to open until I hit around 63mph. But if the first "way it works" is true, then why would my CEL light during in-city driving, around ~25mph?

Furthermore, how would the EGR valve know how fast the vehicle is traveling, and how would it be controlled even if it did? It's a simple vacuum system, after all :\

[xeroinfinity]

What code(s) are you getting exactly for the cel?

If it's a EGR code, then it could be bad or the tubing is clogged with carbon. Have you tested the EGR or had it off to check the tubing for cloggs?

Not exactly sure right off which EGR type your 2.5 has.
Thier's a ported one, runs off vacum, and negative back pressure, pretty sure your 1990 is the second one, negative back pressure.
Thier were also to other EGRs, but not used on the 2.5L.

[Mr. Guy]

Code 32; check EGR System/EGR System failure (my book isn't specific)

It's a negative backpressure type, yup.

The vacuum line is clear. So is the port in the throttle body. I just had the EGR itself off a week ago, cleaned it out so it would quit its leaking at idle. Which it didn't, the stupid thing. Though, I guess if I'd been forced to put up with the abuse it has, I wouldn't seal too well anymore either
Checked the port on the valve too; clear, I could suck air through it. Which...I shouldn't be able to, should I? At least not easily?

I did try testing it using the engine (increasing RPMs until it opened)---sometimes it would open, sometimes not. I don't have a vacuum pump or anything to test it with, unfortunately...

Maybe I'll take it out later and coat the upper diaphram with a thin layer of hi-temp silicon sealant. I have a feeling it's not sealed properly anymore, and so isn't letting the vacuum build up properly...and it's not like doing so can hurt anything at this point.

[xeroinfinity]

I'd bet its the egr thats bad.

[Airjer_]

Originally Posted by Mr. Guy

I did try testing it using the engine (increasing RPMs until it opened)---sometimes it would open, sometimes not. I don't have a vacuum pump or anything to test it with, unfortunately...

But you do have an engine that produces vacuum while its running. You can tap into a vacuum hose off the intake and run a hose to the egr to see how it reacts with vacuum applied. At least you would now if it is opening and allowing exhaust gas through.