Question : Negative Pressure EGR Valve Diagnosis - 84 2.8L

[pgtr]

Negative Pressure EGR Valve Diagnosis - 84 2.8L

I have an '84 Firebird w/ 2.8L V6 Auto. I've suddenly started experiencing considerable spark knock (detonation) after replacing the intake manif gaskets. I had to remove the EGR stuff and reinstall but didn't pay any attention to it during the process. According to the manual one of the areas to investigate is the EGR system remaining closed causing spark knock.

I have the original EGR valve (180k mi) installed part # 17084729. It has an "N" stamped on it which is supposed to indicate it's a negative pressure EGR valve (vs a positive or ported EGR valve). Supposedly it will open up if vacuum is applied while the engine is turned off. I tried this with my mightyvac pump and it doesn't move. It will however move relatively easily by hand. When I do actuate it manually by hand - it does run rough and stumble. I also hookedit up directly to manifold vacuum for kicks and this time w/ the engine running - no movement detected at various RPMs while in neutral w/ engine warmed up.

Q: Sound like a bad unit, should be replaced? (I ask because there are different EGR systems in this year/model and was curious about experienced advice regarding the '84 2.8L carb motor specifically)


Also I tried to test the EGR control solenoid (electric switch mounted on valve cover w/ wire plug and 3 vac ports...). I noticed it's always one (energized) when the engine is running in park regardless of temperature, RPMs. But virtually no vacuum is present at the EGR valve regardless of the state of the solenoid.

Q: Why is the EGR valve solenoid energized all the time? Cold? Hot? Various RPMs (in Park, not under load)... Shouldn't it only be energize under certain running conditions after the engine is warmed up?

Q: Does this also sound suspect? If a new EGR valve solenoid were installed - it would seem to have a 'new' EGR valve open all the time...?

[geoffinbc]

The switch will have 2 ports. One receives vacuum from the motor (manifold) at all times. The other is the control port which is attached to the EGR valve. When the valve is commanded open the switch will open and apply vacuum to the control port.

Your EGR valve seems to have a leaky diaphragm you just need to replace it.

[pgtr]

Agreed!

I think I'll MODIFY my vacuum hoses so that it does have manifold vacuum coming into the switch.

FYI1 It's a 2-way switch - that is it has 1 source for vacuum and 2 possible outputs for the vacuum to pull through. It's also identical to the idle compensator vacuum solenoid (for AC etc...)

FYI2 my solenoid switch does not have manifold vacuum going into it. In factory stock config it has the usual squirrels nest of hoses and it would seem to receive vacuum from a variety of sources under a variety of conditions that include several ported* sources off the carb body as well as the other vacuum switch that controls the faster idle position (for AC etc...). But no source of manifold vacuum is routed to the switch...

*Unfortunately I cannot detect ANY vacuum (ported or otherwise) from ANY of the PORTED sources on my stock E2SE carb under a variety of RPMs... I've verified all hoses to the factory emissions sticker (the carb is even conveniently labeled with letters for each port). And I've also checked all hoses for blockage as well as the various ported vacuum connections on the carb - again no blockage.

So at this point I'll likely tee a known good vacuum source off the manifold or at the throttle plates to ensure good vacuum to the solenoid. At this point I think it may be easier than trying to tediously infer rationality from vintage emissions gizmos that were intended to function under inobvious conditions. At least it will serve as a 'test' until I feel like further debugging why vacuum is not making it to the switch in stock config...