I'm going to replace my fuel pressure regulator this afternoon- the part will be in later, and I wanted to remove and test the old unit first.

The car is an '86 745 turbo. The O2 sensor reading seems to indicate a rich mixture (voltage .8 to .9 volts). When I create a vacuum leak, I see the O2 sensor voltage lower. Removing vacuum at the fpr does not result in a higher idle- I believe I read somewhere that it should. Fuel mileage is poor, and this came along with a reduction in performance recently.

I assume that after removing the two bolts securing the fpr to the intake, the fpr will simply release from the fuel rail.

I read in the FAQ that one way to reduce fuel pressure at the rail is to apply vacuum to the fpr so it will dump the fuel into the tank. I don't think this will work in my case as the FPR may have failed closed. I also read you can simply run the pumps with the motor off, but don't understand why the pressure still wouldn't stay high with the pump working against the closed FPR.

[Edit- just re-read FAQ on relieving fuel pressure. Pulling the fuse on the pump and running the motor dry makes much more sense. Thanks to all for that great resource.]

Any advice regarding this diagnosis or replacement is appreciated.

-Dylan

Remove the ( I think) # 1 fuse and run the engine til it dies if it will start at all. Your fuel pressure will be releived. Just remove the two bolts from the FPR and pull it out as is just a press fit in the rail with an o ring. Wrap it in a rag to catch the small amount of gas that runs out. Put a little vaseline or white lithium grease on the o ring of the new one as it's difficult to start it in the rail dry. Bolt it down and your're good to go.

bl

Yeah I just did that- thanks for the advice, it came out fine. I pulled fuses #1 and #11 - the latter being the in-tank pump. Car wouldn't start, just crank.

I tried testing the fpr with a mityvac- I applied vacuum to the fpr and hooked up a long *unused* length of hose to the return fitting. With or without vacuum applied, I couldn't blow air through the FPR. I'm guessing it should have passed air with vacuum applied. Sound right to you?

Thanks for the quick response and the install tips. Let's just hope this was the problem. If not, at least it looks like I'm replacing a bad part.

-Dylan

Yes the FPR should open up as vacuum is applied. Max manifold vacuum is probably in the range of 20" Hg and you probably got close to that with the Mityvac. It definitely sounds like you have the culprit.
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Rob Bareiss, New London CT ::: 'Roterande Fläkt Och Drivremmar!'

FPR will always appear closed when removed from car with or without vacume applied. It requires fuel rail head pressure before it starts to open.

The main pump is capable of pressures of over 80 psi. When operating correctly on the car the FPR will bypass fuel all the time thereby regulating rail pressure at around 30 to 39 psi depending on amount of vacume. As vacume increases it boosts up the pressure by about 5 to 7 psi. This assists in enriching the mixture under load, the O2 sensor fine tunes the mixture.

Have you established that mass air flow sensor is good?
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David Hunter

Yeah, I haven't driven the car enough post fpr replacement to test mileage, but performance hasn't changed. The O2 sensor voltage lowered somewhat. Once a few weeks earlier I drove the car with the AMM disconnected (accidently) and remembered that it was disconnected a few hundred feet down the road. It was night and day between disconnected/connected. Limp home mode is aptly named. So I know the AMM is doing something, and I'm considering checking CO and base idle settings right after I do this timing belt/crank pulley swap this weekend.

Thanks-
Dylan