My 1991 VW Jetta lost power and it was the fuel pressure regulator and also a bad head gasket. The car would lose power on the freeway. This went on for years. And it was intermittent. Also the exhaust pipe was pulling the spring loaded header coming out of the exhaust manifold and this would cause the car to go into a bad idle condition and hard starting. Im still troubleshooting the exhaust leak but I think the spring cant have any pulling from the exhaust pipe whatsoever which causes exhaust to leak. I have replaced the hydraulic lifters due to a ticking noise which solved the problem. I replaced the head gasket which solved the heavy smoke coming out of the tailpipe. I also replaced the valve stem seals which eliminated the rest of the smoke. I had a problem with the starter catching on the flywheel and seizing the motor until I took a socket and put on the crankshaft to free it then removed the starter and filed down the teeth on the starter very slightly which may have solved the problem. I am still working on the exhaust leak behind the engine and have removed the spring loaded pipe coming out of the manifold to see if it leaks so I have not started the car much since i have done this most recent work but I idled the car to operating temperature and the car ran very well except the exhaust leak coming from under and behind the exhaust manifold. If anyone knows about the spring loaded pipe and do they leak or should I buy an alternative pipe because they arent available through VW. If anyone knows is the leak coming from this spring loaded peice of pipe. Also the rear hanger was broken and I used a piece of wire to pull the exhaust pipe and this caused the car to stall so I will have to adjust the exhause to not pull on the spring at the front where this spring loaded pipe is and hope this works.