Ok, this has me really going on what it is. My milage SUCKS, not 10 mpg but about 17 on average with very easy driving. My highway is about 21 at most. Here is what I have done,

Oxygen sensor
Air filter
Oil is 15-50w mobil 1/filter
Wires/cap/rotor/plugs
New tranny fluid
Always marathon gas, high grade

So here are a few things I THINK it might be. Fuel injectors, OE I think, so they might be getting clogged but I have ran fuel injector cleaner though it, the red line stuff. The other thing I am THINKING is my cat on the car. There isn't any vacuum leaks, exhaust is all good (just had it in the shop), cat does LOOK good but after 18 years, who knows. Is there any way to test these? What else could it be? Up at Steinel's they did a look over and couldn't see anything out of the place. I do a lot of highway driving so I would like to know what the problem is. My motor is strong so it isn't that. Runs REALLY good but just have bad milage. I hear a lot of you guys getting like 27-30 on the highway, pass some over this way!!

 

A failed FPR might cause too much gas to be delivered through the injectors. You might notice some smoke at times with this and lower gas mileage.

I drove my '83 NA on a round trip of 1000 mi last summer and got just over 30 mpg. I don't know if a 951 should do that well.

 

A really bad FPR will make the bottom of your rear bumper black. My 87 S get 21.5/22.5 mpg no matter how I drive.

 

I replaced my FPR after basically doing everything you mentioned and that did the trick. I could almost watch my fuel gauge drop before. Getting really good mileage now. I also pulled, cleaned my injectors, and replaced the seals.

 

.. and my bumper is no longer black after driving 10 miles. I could wipe my hand across it and be covered in carbon.

 

OH yea, thanks, but I already replaced that with my guru chip set. I really don't see much black smoke coming out of the back either. Bumper is NOT black either. What else?

 

Can you get a pressure gauge on the fuel rail? Have you tried the screw driver stethoscope (sp?) trick on each injector to possibly narrow down a bad one? Clark's has a write up on the FPR that may be helpful.

I would pull the injectors and clean them. Just soak them in parts cleaner, make yourself a momentary on switch with a 9v battery, get a garden sprayer and run some parts cleaner through them. There are probably a few shops with injector cleaners that would clean them "professionally" for a few bucks and be able to tell if you have one failing. Seal kits are dirt cheap. Lube the end with ATF to get the caps back on without snapping them. If you have not done this before, it probably needs done any way.

Hope that helps.

 

Oh, I just noticed you're running a 3 bar FPR. Maybe that's the problem. Can you lower the pressure? Isn't 2.5 normal?

Think is it this way. If drive 15000 mi/yr at 20 mpg you use 750 gal/yr. If you got 30 mpg you'd use 500 gal/yr. At $2/gal, it costs you $500 year for the perfomance.