On a 78 L48- should the EGR valve "hold" vacuum? Specifically, if I apply vacuum to the valve (with a pimp) should the valve hold it? Mine move when I apply it but unless I keep applying it, it closes to its original position.

Thanks,
Dan

 

It should hold vacuum.

 

Should hold vacuum, time for a new one.

Gary

 

you got a pimp to suck on it???? sorry couldn't resist

 

Simple, I could not find a "ho", so settled for the next best thing. I knew I should have taken those typing classes in high school!!

Thanks everyone!

Dan

 

Not neccesarily. At least some 1978 Corvettes used a Positive backpressure EGR valve, as opposed to a ported EGR valve it doesnt hold vacuum without the engine running and without some exhaust backpressure built up.

For example My 78 L82 came with just that type of EGR. Now some cars were equipped with a ported EGR which indeed should hold vaccuum. It is necessary to know exactly which type you are dealing with before the question can be answered.

A search on NAPA auto parts will give you some idea which cars came with what, if you have the original part number you can cross reference it as at least the Echlin EGR valves seem to all provide the OEM part number matches.

The positive back pressure EGR valve is easy to spot because its pintle shaft is much thicker than the single diaphragm type. This is because the shaft is hollow. The hollow design allows exhaust gases to flow into the shaft and push up on it. When positive back pressure in the exhaust system is sufficient, the shaft raises up and seals the built-in control valve. Once the control valve is closed, it allows applied vacuum to pull up on the diaphragm. Without back pressure to lift the hollow shaft and close the control valve opening, the EGR valve will not hold vacuum. It is bled off to the atmosphere.

To test this valve, bring the engine up to 2,000 rpms to create back pressure, then apply vacuum. EGR should open and cause a 100 rpm drop or more. Exhaust leaks or a modified exhaust system can create havoc here.

 

fauxrs (or anyone else), I read the article you linked to in another thread.

http://www.asashop.org/autoinc/dec97/egr.htm

Having a '78 L-82 like you, it appears that the correct EGR is the back pressure type vs port vacuum, am curious on your take on the EGR impact of headers and a true dual with x-pipe exhaust system?

Specifically in relation to my thread I'm stumped: Timing, Vacuum & Secondaries

The above linked article states:

Mine does not have the "P" nor does it hold a vacuum. Yet, in researching the part number (17056495) indicates it is a back pressure type EGR.

Can a port type EGR be used as a replacement for a back pressure type to overcome changes made to the exhaust system.

 

Well I'm no expert in that regard however I really dont see why not. The backpressure EGR was just an attempt to make the device a bit more selective when running, more sensitive as it were. I see no reason why a ported version couldnt be used.

It certainly can be tried

 

Am assuming that the EGR for 78 L-82s was ported vs back pressure, but have not been able to fully confirm. Additionally, it appears that the L-48 EGR is definitely a ported style and assume that the dimensions are the same.

Am thinking that with the headers and dual exhaust that I'm not generating enough back pressure to fully activate the EGR which is adding to my cylinder temps and detonation/pinging.