I'm in the stages of putting my motor back together after a cam swap/ head refurbish. I started this work because my intake manifold had a pretty bad coolant leak so I decided to buy an edlebrock intake, then a new cam, water pump etc etc and a bunch of things while I pulled the engine.


Anyways, because I was doing major cleaning/painting, I was cleaning up the old EGR valve and came up with a question.


Is the EGR valve on my 91 305 TBI motor supposed to hold vacuum when applied?

I don't suspect I had issues in regards to my EGR valve before, and don't recall any engine lights/codes, but the valve doesn't seem to be holding vacuum.


What I did to test was push up on the diaphragm (I feel the air rush out the vacuum nipple on the EGR valve), then try and plug the port with my finger. The valve didn't seem to stay open at all when I tried that. I then grabbed my silicon vacuum caps and tried it with those. The valve still wouldn't stay open, but it closed slower with the caps.


I have seen several posts on these forums about people saying it should hold vacuum, but I have also seen a few where people say they bleed the vacuum off, so I don't know what to think.


Maybe this is partially the cause of my lower MPG than I was expecting. My friend who bought an 88 with a 305 TBI T5 3.08 gears is getting around 25 MPG, while I (91 305 TBI T5(V6 T5 ratios) with 3.23) was getting around 18, then all the way down to 15 mainly highway driving. I know based on my rear end and slightly different transmission ratios I would get worse mileage, but not by that much.

 

In general: Three different kinds of EGR valves:

1. Ordinary. These should hold vacuum

2. Positive back-pressure. They'll only open when there's positive back pressure in the exhaust system. Probably won't hold vacuum.

3. Negative back-pressure. They'll only open when there's negative back pressure in the exhaust system. Probably won't hold vacuum.

Newer cars have the EGR controlled with solenoids, so there's no vacuum to the "EGR valve" at all.

Unless someone else can help, you'll need a service manual to figure out what style of EGR valve your vehicle is supposed to have.

 

Thank you for the info, I completely forgot to reference my Chilton manual...

I'll try and find it and see what it tells me. I have been doing more searching and there still seems to be contradicting info on exactly what type of valve to use for my setup (91 tbi 305).

I did purchase a new valve today but don't know what type it is yet (positive or negative back-pressure), and my motor won't be back together and in my car until earliest this weekend.

Anyways I'll look up my manual and see if it does tell me exactly what type, otherwise I'll just put the one I bought at Oreilly's on because it probably won't hurt to replace a 25 year old part.