best for last: right here on this forum.
This 95 engine 16v , I am told , uses the MAP drop method.
ecu commands EGR open.
ecu looks a map , and looks for pressure drop ( less vacuum).
caused by exhaust invading the intake plenum.
if the drop doesnt happen, code 51 sets,.
Code 51 says you have a gross failure to detect that EGR action.
I says Exhaust gases are not entering the Intake , enough.
(clogged passages, bad solenoid valve, egr valve, lines or bad MAP sensor)
when code 51 also sets the engine managment to go to low ignition advance and or rich.
called back up strategy. Poor Gas mileage results every time.
This is to prevent pinging or detonation. very bad to have that.
it is just a go /no go test.
later Kicks 96+ on can calculate flow rates (OBD2 system monitors)
on newer cars its a code P0400.
also on the 96+ we have:
The Manifold Differential Pressure (MDP) sensor and the EGR Bypass (EGR
valve (mounted in the vacuum line between the EGR modulator and the EGR valve.) The ECM uses these components to compare the pressure in the EGR system with the pressure in the intake manifold to check for a blockage."
look for these parts, California cars.
back to 1994 EGR:
Can you push in on the EGR diaphram while the engine is idling and
the engine stalls? ( or very close, it will stumble and die)
IT MUST DO THAT !!!
Also you can attach a hose to the EGR vac port and
suck on it and get the same results. "MityVac time"
This will tell us all of the EGR passages are clear and working
if you car starts and runs with the EGR valve removed , your passages are in fact clogged and the ECU is not Lying.
Inspect ever inch of the EGR path Exh to and Inside Intake EGR passage.
I think this is 99 percentile failure mode.