Low fuel pressure = poor fuel mileage

[billcu]

When I bought my new black 94 Mark VIII with 90K miles, I expected 30 mpg highway, just like all my other stock 94's.

I was getting around 21, worse than my 97 with a 373 rear end (around 23-24 MPG). The car actually ran OK, but it was taking 2 times to start.

The return fuel line was pinched, I opened it back up by wrapping it with leather and squeezing it open with vice grips.

I checked the fuel pressure, when I keyed on, it slowly went to around 4 psi, then keyed on again, it went slowly up to around 8. Then I started it. When it started, it very slowly went up to 30 psi and stayed there. When I revved it, it stayed at 30 (it should have gone to around 40 then).

I dropped the tank and pulled the pump assembly. It was actually a split fuel line at the pump, but I already bought the pump so, I put the new one in with the supplied new pump fuel line. I actually took the pump assembly over to Jeremi's as he was placing an order anyway, and ordered the pump for me. He even put the pump on the assembly for me, Thanks again Jeremi. :wave:

It's back up to 30 MPG and starts on the first crank. :thumbsup:

Here's a photo of the split line:

 

[driller]

Good troubleshooting Bill!

You've got to admit 30 mpg highway in a 20 year old luxury coupe is pretty spectacular!

 

[95blklsc]

What fuel pump did you use? I need to do mine in my 95. The fuel pressure during start up is causing a slow start I believe (sometimes almost a no start)(. The pressure actually bounces around when I'm trying to start, it could also be an electrical connection issue. With nearly 20 years of age and 190k it's on borrowed time!

 

[Lvnmarks]

Never seen over 28 mpg in any of my stock Mark's. Makes me want to go a fuel pressure test now.

 

[tonybrooklyn]

Great thread thanks!

 

[billcu]

That 30 mpg I mention is perfect conditions on the expressway of course, I don't get that all the time.

28 and lower for spirited driving is more common.

I drove the car to work today, 50 miles each way. The car runs great, and it's getting great mileage. I've got a 3.5 hour expressway drive coming up, I'll see how it does then. :thumbsup:

 

[billcu]

Oops, I forgot, I used a Walbro fuel pump, I think it was a 155. We had to open up the bottom of where the pump mounts with a grinder, and change the electrical connectors.

 

[Gau8mechanic]

Bet you it was a 255lph pump. I think that is the normal flow rate for the stock pump (or is it?). My 93 got 29 on all highway driving. This 97 I have now never got better than 24.5 on a 430 mile trip. The only difference between the two models was the 3.07 in the 93 and the 3.27 in the 97. Maybe I need to check line pressure too.

 

[billcu]

It's not the 255, I've got that one on my green Mark VIII, that's overkill for a stock car and makes too much noise.

 

[Gau8mechanic]

I put a 255 in my '93. Never heard it. Hmmm

I checked my fuel pressure the other day after reading all this too. Before starting the car I tried to prime the pump. Took 3 attempts to get to 30+ psi. Started fine. Ran steady at 32 psi. Accelerated it and saw a spike to 44 and then immediately returned to 32. Replaced the fuel filter and then retested. Maybe 1 psi more at both measurements. Not sure if it even helped.

 

[billcu]

The 255 is really not that loud, I can just hear it more than the other ones.

I don't think changing the fuel pump will help your mileage unless the pump is really bad, or has a bad hose like mine did. Changing the filter is a good maintenance thing of course.

 

[Roadboss]

If you have a new pump you probably need to check for vacuum leaks and the DPFE.