4.9 EGR modification

[adallak]

I was checking the EGR system on my 1991 Seville 4.9 with 143.000 miles and found that the EGR valve was not that contaminated with carbon and was functional. Nevertheless, the vacuum delivered to the valve by EGR vacuum solenoid was not enough to open the EGR valve. I have got same results with new EGR solenoid. The solenoid is completely open when you unplug it. It works like a proportional valve. In other words if you have 20" of vacuum at idle from throttle body (TB) the solenoid will supply the EGR valve only with 5-10".

When I bypassed the solenoid, the EGR valve opened and the engine stalled immediately. Drove the car hot with gas pedal slightly depressed and there was no spark knock. I believe the main cause of the failure of EGR system in older cars is that the threshold of the EGR valve increases for number of reasons (carbon contamination, heat, age etc.). In this connection I believe the system might be slightly modified so that the conductive ability of the EGR solenoid would be higher (say 1:1 with respect to the input from the TB) BUT somewhere before or after the solenoid would be a simple $1.00 valve similar to one you have in your kitchen to provide with opportunity to INCREASE the vacuum supply to match the changed threshold of the EGR valve. This simple step would save a lot of time and money if one takes into account the time associated with diagnostics, part search and replacement and , of course, the cost of the parts .

Just an idea.... I know the real life maybe more complicated!

[adallak]

Guru,

Thanks for the reply. The two solenoids are exactly the same and act similarly (same proportion of the vacuum to the valve). There is one orifice in the solenoid and none in the valve.

Well ... if one can open the valve with increased vacuum it does mean the valve's threshold increased (whatever the reason is). If matching the new threshold will open the valve but make it uncontrolable (perhaps this statement maybe proved with some experimenting)... it means the life of the EGR solenoids is limited to a few years or say 40.000 miles.

BTW, there was nothing in the valve to clean. Just a film of carbon deposits, no chunks.

Did that film change the conditions? I do not know, might be.