car has 102,000 plus miles and an automatic trans. any time i drive at speeds over 40 the engine rpm falls aprox 200-300 rpm momentarily and then goes back to where it was. it happens every 10 seconds like clockwork, and the rpm always falls then returns. this causes the transmission to jerk fairly hard. autozone obd II code reader shows po190 ( fuel rail pressure regulator circuit). the fuel pressure regulator is all plastic with three wires attached, 1 fuel line and 1 vac line.it mounts at the end of the fuel rail in line on the passenger side of engine. chilton manual, oreilys, autozone all show an all steel one with no wires attached.i unplugged the connector and started the engine, it ran rough. i bought a factory wire diagram for contour/mystique it doesnt show this part or the wires. ive tried driving with the o/d off, the cruise control on.ive pulled out the air cond relay,no change. checked the wiring on iac valve and several grounds for the pcm as well as engine grounds. checked igniton coil, primary was ok. secondary coils checked 11.2 ohms, should be 11.8-13.2.i unplugged the coil capacitor, changed fuel filter and put a new strainer in fuel tank, checked the plugs. they are the correct double platinum and gapped properly. car runs fine otherwise except starter cranks slow and on cold days it may take a couple tries to start it . after car is warm starts fine. this is my 16 yr old daughters car and i need it to be dependable for her. any help is appreciated.
99 mercury 2.0 auto surges at high way speed. on 1999 Mercury Mystique
by Visitor in Carthage, MO on April 28, 2011
ANSWER by DaveJM , April 28, 2011
My initial thought is that there is a wiring issue that is shorting to ground or to power and causing the fuel control problem. This car had wiring harness issues, and so it may take some effort to figure out where things have gone awry. It's not going to be an easy diagnosis, in any case. I don't have any great direction to offer via the Internet. You may want to get a sharp Ford driveability technician to help out.