My 305 is running rough at idle & acceleration.
Doesn't cut off, just runs rough.
I disconnected the bypass wire to set base timing, and it runs like normal.. once plugged back, runs rough again.
I did a smoke test for vacuum leaks and noticed smoke coming out of the EGR valve.
Is that normal? or is it leaking?
Can that cause the problem?

I’ve changed MAP sensor, IAC, TPS, & Spark Plugs.
Fuel pump & Filter are also new.

 

It's not normal. Either the EGR valve diaphragm is leaking or the EGR valve gasket is leaking.

Yes

Disconnect the vacuum hose going to the nipple on the EGR valve, lift up on the vacuum diaphragm, put your finger over the vacuum nipple on the EGR valve to seal it off and let go of the vacuum diaphragm. If it holds vacuum it's good. If it doesn't hold vacuum and the vacuum diaphragm goes back to its normal position then it is bad and will need to be replaced.

You can also test the EGR vacuum diaphragm with a MITYVAC Hand Vacuum Pump or something like it.

 

If the engine chugged and stumbled regardless of EST being active, I would suspect a leaking EGR. If there is even a doubt, it would be helpful to verify that the EGR pintle is sealing against the seat, and not allowing vacuum leakage to the intake manifold. If the diaphragm is holding and EGR control valve is intact, cleaning out any carbon from the pintle and seat could possibly completely solve the question of a leaking EGR. One advantage for you is that it is NOT a TPI system with the EGR buried under the plenum.

However, since you report that the idle is NOT rough with the EST in bypass (base timing only) I would suspect that there may be other issues. I believe that when the EST is bypassed the ECM enters Backup Fuel Mode to permit the engine to function well enough to set base timing, minimum air, and other adjustments. As part of this, some of the typical closed-loop ECM functions are suspended, including any EVAP purge, any EGR cycling, any block learn function, and also a reversion to the "limp home" fuel tables based solely on RPM and any reasonable sensor inputs the ECM might have. As a result, if there IS a vacuum leak, the approximate 12:1 fuel mixture the ECM tries to maintain in Backup Mode could mask any lean condition when the EST system is in bypass.

If you suspect this might be the case, it is also possibly to invoke Backup Fuel Mode once the engine is running and warmed up by inserting a 3.9K (3,900Ω) resistor between terminals "A" and "B" of the ALDL under the dash. If this also causes the idle to become smooth, vacuum leakage is still a suspect.

It could also be useful to know what the idle vacuum is measuring and at what idle RPM.

 

That's a completely valid test, provided the EGR is a negative backpressure type. By 1992, the factory systems were experimenting with all types of valves, and it's not really simple to exactly identify whether the installed valve is negative, positive, or straight port-type valve, and if it has EVER been changed all bets are off.

In case no one has looked yet, Bernie Tripp has a very good article on general EGR diagnosis: https://www.thirdgen.org/egrdiagnose/

To add to the confusion, many parts listings show both positive- and negative-backpressure types as replacements for a 1992 305 TBI application.

 

Thanks. I'll read it.

 

Who has EGR’s hook up still .