EGR Vacuum Modulator Wierdness
 

My 1987 Supra Turbo was running funny with the following symptoms. It ran normal when cold and at idle, but after it warmed up and the car was at speed with no load, it felt like the drive train was pulling forward and pushing back in a jerky fashion. Not quite like a miss but similar. I thought it could be a bad spark plug or at worst uneven compression but it turned out to be the vacuum modulator for the EGR valve. I did the diagnostic tests in the book and it failed the second part where you check the vacuum at 2500 RPM with the car warmed up and a T fitting in the vacuum line going to the EGR valve. The vacuum gauge needle was swinging back and forth wildly which is not normal. There was vacuum coming up from the VSV valve at 2500 RPM but it wasn't making it through the vacuum modulator properly. I took the P Q plastic cap off and noticed the filter was dark and wet with moisture under the cap. (Be sure you put it back on the correct orientation. P and Q is printed on the vacuum modulator body and the cap.) I blew air by mouth in the "bottom" of the vacuum modulator that connects to the EGR body and found that the diaphragm must have torn because air could go through it. I replaced the vacuum modulator and all is well but I was curious to see what was inside the broken one. The diaphragm had a tear in it causing the EGR valve open and close at speed hence the push pull feeling in the drive train. Check out the photos. The way the EGR valve opens is when the throttle is opened to about 2500 RPM, vacuum is applied to one side of the vacuum modulator at the same time the exhaust flow applies enough pressure to overcome the spring inside. The diaphragm inside moves up blocking the vacuum bleed hole and allows the vacuum to pass through to the EGR valve which opens proportionally to the amount of vacuum applied. Under boost conditions the EGR remains closed since there is no vacuum. Fuel economy has increased a few MPG which is nice to see. The way to check that the EGR valve is capable of working is to apply vacuum to thin hose that goes down there at idle. The car should run rough or stall. Same test thats in the book. I got my vacuum modulator from eBay through J. C. Whitney catalog. The turbo one is VS96 from Standard Motor Products or BDW EGR3034 from Intermotor (both are the same product). I thought it would be an aftermarket part but it turned out to be a genuine ND Toyota part from Japan. The Turbo vacuum modulator has 2 vacuum lines: P and Q. The non-turbo Vacuum modulator has 3 vacuum lines: P, Q, R.
Never use any cleaning solvent inside a vacuum modulator, you'll only wreck it.