I posted this before about my 86 fwd 4100 deville running rough on the highway above 55. It runs 100% perfect under 55. I changed the rotor, cap, ignition coil, plugs and wires and it still runs rough on the highway.

It's definately not the front end or wheels, it's engine related.

What about an egr valve? I checked it at idle and pushed up on the diaphram and it started to stall like it should.

Then, I had someone put it in drive, hold the brake, and give it gas while I felt the diaphram but it didn't move up and down. The car was cold though....does that matter?

What about the egr tubes? I attempted to clean them but the injectors were in the way and I couldn't get a coat hanger down there with the screen in the way. Am I supposed to take the throttle body apart?

I have no clue as to what's wrong :banghead:

The EGR will not activate with the engine cold. You are checking it the correct way but you must warm up the engine thoroughly first.

You likely do not need to clean the EGR tubes. Those seldom if ever plugged up on the TBI cars. The fuel sluicing down from the TBI injectors acts as an excellent "solvent" to keep the EGR tubes clean with TBI. The EGR tube plugging on high mileage 4.5 and 4.9 engines is on the 90 4.5 and 91 and later 4.9 with port fuel injection. Besides, sounds like the EGR system is flowing okay if you pulled the EGR valve diaphragm up manually and it stalled the engine.

You might get a vacuum gauge and tee it into the vacuum line between the EGR valve and the EGR solenoid. Drive the car and watch the vacuum level to the valve. You should see the EGR solenoid turn on when you tip into the throttle and supply vaccum to the EGR valve . If the vacuum to the valve is disappearing when the roughness starts then it may be the vacuum supply to the solenoid/valve or the solenoid is failing.

An 86 car had a vacuum pump that ran all the time to supply extra vacuum to the various systems on the car (cruise, HVAC, etcetera) including the EGR system. If that is not working then the EGR will not have sufficient vacuum to perform correctly. You might check that the vacuum pump is running and operational.