Engine runs bad with EGR hooked up

[124Fan]

I had to replace the head on my '77.

I got everything put back together. Valve clearances are perfect, ignition timing is at 5 BTDC, etc. At idle, with the EGR hooked up, the engine will barely run. I have to crank the idle speed (Carb is a EMPI DFEV from Mark) way up to keep it from stalling. It clears out a bit once I transition to part throttle.

If I unhook EGR vacuum hose from the intake manifold and cap the nipple, the car purrs like a kitten. Runs nice and smooth though the idle speed goes way up.

At some point, it ran well with the EGR hooked up but it no longer does. Not sure what to make of this?

I have to run the EGR since the '77 is not smog exempt in California and it's due for smog.

Thanks,
Sonny

[So Cal Mark]

the egr must have ported vacuum only

[124Fan]

Hi Mark,

Thanks for the tip. It has been about a year since I worked on this and of course don't remember how things were hooked up. So much for not forgetting.

If I remember correctly, the EGR was setup before using this lower switch in conjunction with a T. A vacuum line came off of the switch to a T. One leg of the T went to the EGR valve and the other leg went to the intake manifold. That is how the previous owner set it up, but is that the correct method? I can't see how the switch would do anything unless there was some sort of vacuum signal present (hence, the T)


Assuming that the EGR does connect to this lower switch, I don't quite understand how this thing works. The back side of it vents to the atmosphere. Is it just bleeding vacuum from the EGR line to limit exhaust flow?


Sonny

[124Fan]

Ok, I think I have it figured out.

The thermoswitch between the carb and intake cam box is the vacuum source for the EGR. However, it is dead. Hot or cold, idle or throttle..there's never any vacuum there.

The EGR can't see full vacuum at idle or the engine runs really poor.

So, I hooked the EGR vacuum line up to the vacuum advance port on the EMPI DFEV carb. It seems to be doing the trick. There's no vacuum at idle, but as the throttle opens, vacuum is present and pulls the EGR open.

There is a definite stumble at "tip in" with the EGR hooked up. Just need to get through smog and then I'll unhook it.

Sonny

[So Cal Mark]

the thermovalve is not a vacuum source, it merely opens at a specified temperature and allows vacuum to pass through it. The intake manifold is always the vacuum source. for a few bucks you can buy a vacuum delay valve and install it in the line to the egr valve. That delays the valve opening for a few seconds so you don't get that stumble off idle