Hello,

When putting the mity vac to my EGR diaphragm beneath the carb of my 81' F100 300-6, I noticed I could not move the valve or even develop a vacuum. I thought the diaphragm must be leaking in some way and ordered another EGR unit. Just to check, I put a vacuum signal to the new one and the same thing happened. All air suckage, no pintle movement at all. I exchanged it for another at NAPA...same deal. Later on I ordered one from Advance thinking perhaps a different vendor might ruin the chances of getting a non-defective part line. Same deal.

What might I be missing?

I know to control the EGR to get variable positions there are bleed-off's and extra valving. These are usually in the hose circuits before the final valve. How is this EGR supposed to ever work if it sucks wind right up to the very end?


FYI:
I know that opening a post like this is going to open a flood of questions like: "What do you care about an EGR on a 25+ year old vehicle, just bypass it..." Right now it is bypassed. With all due respect to the great tech writers on here, I don't want to distract from my original question, I'll just say that my experience with Ford's earlier EEC systems have led me to believe the EGR is also used to prevent knocking by cooling the combustion chamber. (as we know cooling the cc IS the main purpose of the EGR. It gets rid of NOx.) Where other systems use a knock sensor and retard the timing to get rid of ping, the DPFE controlled EGR on the EEC-IV system behaved like this: Bad DPFE, pings under load, even at highway speeds...Working DPFE, (and then) EGR flow, no pings, not even at take off. Maybe the accountants at Ford pushed the engineers to develop this cheaper solution?

I have some excessive dieseling on this '81 when I use anything below 93 octane and shut the truck off after a nice warm drive. You have to stall it with the clutch and brake sometimes to get it to shut down. I think if the EGR is functional, it will cool the chambers a bit and fight this off. Yes, I do know that pre-detonation can come from other factors. I am running a larger radiator and the cooling system and temp's are pretty darn cool. It doesn't burn oil and has only 80k. Ignition timing in spec, but remember its after I kill that anyway. Carb is clean and smooth, choke is dialed in. Not much carbon I would think. Starts and runs excellent, but when its up to operating temp it will sometimes ping under a take off. I don't really want to turn this article into a debate over whether or not my theory is right, I just want to know what I'm missing about this strange vacuum leakage on the EGR. I'll be glad to sink in the money and time after we answer this question and let you know what I find. No worries, its a cheap and quick fix overall and might save money at the pump.

 

I think what you have is a positive backpressure EGR valve. There has to be some exhaust backpressure on the valve for it to operate when the engine is running. You can't test this type with a vacuum with the engine off. See link, you'll have to scroll down about one page.

AGCO Automotive Repair Service - Baton Rouge, LA - Detailed Auto Topics - How Does an EGR Valve Work

 

Yep, I could not figure it out either so I took one apart. I put it on a grinder and ground away the edge all the way around where it was crimped.

That's when I discovered they have a metal diaphragm inside with a high temp silicone rubber piece. With no exhaust pressure on this metal diaphragm, it sits up, and the vacuum flows right through. If you turn the valves over you will notice around the edge a piece of felt. This is the filter where the vacuum sucks through when there is no back pressure. When there is backpressure it pushes this metal diaphragm down and seals off the chamber and it lets the main diaphragm work and open the valve. But it doesn't just slam shut, this system actually meters the amount of egr according to the amount of backpressure.

This is why any type of exhaust modifications, mostly in the front of the catalytic convertor are deemed illegal by the EPA. It fouls the EGR system up (among other things) and you don't get hardly any egr if you get a free flowing exhaust going on one of these units. I put headers on this one particular engine and that's when I found out why it wasn't working and I was getting pinging, even with the EGR hooked up.

 

And here I thought they required back-pressure simply to shove enough exhaust in there... Exactly the kind of responses I had hoped for. Thanks Gents!