my 89 coupe started surging badly only while it was cool.after checking afew things i checked the vacume line to the egr and there wasn't any vacume at idle or when i reved the motor up.
the solenoid on the passenger side i believe it is called evpr could be bad it is a commonly replaced part according to the guys at ford parts counter.just follow your vacum line from the egr to the evpr cant miss it
you can also pull vacuum on the egr valve at idle (get a vac line that has vac at idle, but that is not important to idling - i.e.,the FPR line is not a good choice), and hook it to the egr (you could use a miti vac too). when you apply vacuum to the egr valve, the idle should turn to poo. if this happens, that is good -indicates that the egr is functional and that it is not stuck.
good luck.
Here's an EGR test proceedure I copied from cjones
to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 rpms (remember snap the throttle don't hold it there).
did the vacuum gauge show about 5in vacuum?
if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500rpms.
should read about 5in vacuum