Running rich, 17.5 mpg on last tank

[Irish Pride]

The car is a 91 b6d 5 speed. Getting very poor mileage. Went thru a tank over the past 2 days. 8 gallons got me right at 140 miles. I bought the car not too long ago wanting a car to drive as a DD that got better mileage than my other cars. As of right now it gets the worst mileage of all 4 of my cars. When i bought it i averaged 20mpg on the highway.

What it does - when first started in the morning it will not idle. If i start it and dont give it gas it will die. It will continue to do this for the first couple minutes of driving. Once warmed up it will idle nice and smooth. Once warmed up, if i turn it off and then restart it, it will die again. I have to give it gas for a few seconds and then it will idle smooth again. While driving, if i am accelerating, it runs very smooth and pulls strong. If i am just trying to maintain speed it will start to jerk a little. When it starts to jerk i either have to downshift or accelerate to get it to smooth out. It does not have a smell of gas so i dont think it is leaking. The tailpipe is coated in black soot. After sitting at a traffic light, while accelerating thru 1st and 2nd i can see a cloud of something behind the car coming from the exhaust. At WOT i can see the same cloud behind me. As soon as i let off the throttle it will go away.

What has been done - the car came with a fairly new exhaust system from the PO. I have replaced the o2 sensor, plugs, wires, dist. cap, and rotor button. When i pulled the plugs, all were pretty black and one was completely fouled and covered in gunk. Cleaned the MAF. Checked the injectors to make sure they were firing properly and run Seafoam thru it with the last 2 fill ups.

I have not checked for vaccum leaks yet. Will be doing that with carb cleaner today.

What else do i need to check? I was expecting double the mpg when i bought this thing.

[Pu241]

Whoa!!!!!
My BP returns a steady low 40's MPG and with a little problem I discovered recently and semi-corrected, I'm shooting to break my record of 44.5.

What do you know, if anything, about the engine swap that was done?

Wondering if correct ECU is being used.

[FB71]

check your fuel pressure. Sounds like the return line is blocked.

[Irish Pride]

Whoa!!!!!
My BP returns a steady low 40's MPG and with a little problem I discovered recently and semi-corrected, I'm shooting to break my record of 44.5.

What do you know, if anything, about the engine swap that was done?

Wondering if correct ECU is being used.

i dont know anything about the swap. i bought it this way and the PO said he bought it that way too. I asked the PO about the mileage before i bought it and he said he had never tried to calculate it. I did some searches on here before hand to see what i could expect and then bought it in good faith. I'm starting to think i made a big mistake.

[TominMO]

i dont know anything about the swap. i bought it this way and the PO said he bought it that way too. I asked the PO about the mileage before i bought it and he said he had never tried to calculate it. I did some searches on here before hand to see what i could expect and then bought it in good faith. I'm starting to think i made a big mistake.

Chad/Irish bought this car from me. Chad, I'm sorry about the mileage. I hope someone here can help you with the diagnosis. I'm sure it's not a big expensive issue to fix. I had driven it very little, probably only a couple of tanks, and honestly never paid any attention to the gas mileage. This should be easily fixable, then you won't feel like it's a big mistake.

One possibility: there are two green wires spliced into the wires going into the MAF. These had come loose when I took off the air filter, because they were poorly spliced in the first place, and I wasn't sure which went to which. I got the car running tho, so thought all was OK. Try reversing them; i.e. if one green wire is spliced to a red wire and one to a yellow wire for example, splice the green-to-red wire to the yellow one, and vice versa. Solving the mileage issue might be that simple.

If nothing you try works, some swap expert here should be able to help you to go back to the basics and trace the wiring from the ECU forward, to verify that it was done right in the first place, or what corrections to make. Maybe even come by your place when convenient to get hands-on with it. Good luck!

[Pu241]

check your fuel pressure. Sounds like the return line is blocked.

I was wondering about that, but doesn't the fuel pump have a built in cut off pressure or does it keep pumping but at a reduced volume?

[Pu241]

I'm starting to think i made a big mistake

No, I don't think so.
It runs, that's way better than some swaps you've read about here.
We've got some good diagnosticians here!
If FB71 says check fuel return line, there is a good chance he is correct!
He isn't 100%, but I'd put money on his suggestions rather than on someone else's "facts"!

[FB71]

I was wondering about that, but doesn't the fuel pump have a built in cut off pressure or does it keep pumping but at a reduced volume?

dead-head pressure is at least 80psi, fuel will cavitate around the impeller until flow is permitted (inj opens), but it will maintain 80+psi. running pressure is 30-40psi.

[Pu241]

dead-head pressure is at least 80psi, fuel will cavitate around the impeller until flow is permitted (inj opens), but it will maintain 80+psi. running pressure is 30-40psi.

And with the doubling of the pressure you can be sure the amount of fuel injected is well above, if not nearly double, what it should be!

[Irish Pride]

How do i go about checking the fuel pressure?

Tom, i will swap those wires and see what that does.

[Damkid]

How do i go about checking the fuel pressure?

Tom, i will swap those wires and see what that does.

If it was me, I would just hook up a fuel pressure gauge and see what pressure is there when it's running

[Festiva Time]

That poor of mileage sounds like a fuel leak. You should be getting double that mileage. All that fuel has to be going some where and it's not the tank. Fuel injector stuck open? You should easily see black smoke out the tailpipe if the engine is running that rich. Maybe, someone stealing your gas?

[TominMO]

That poor of mileage sounds like a fuel leak. You should be getting double that mileage. All that fuel has to be going some where and it's not the tank. Fuel injector stuck open? You should easily see black smoke out the tailpipe if the engine is running that rich. Maybe, someone stealing your gas?

They'd have to be pretty good to steal his gas while he's driving down the highway!

[Irish Pride]

OK, i have a fuel pressure gauge now but i cannot find the port/valve to hook it up to. On my SHOs the port is directly on top of the intake. I have looked all over the intake area, traced both ends of the fuel rail and just cannot see where it needs to go.

I did run my hand along both the fuel line coming from the filter and the return line and both felt as though fuel was flowing thru them. At the rate the fuel was flowing thru the return line it does not seem like there is a blockage, but then again i have no idea what the pressure is yet either.

Sprayed carb cleaner over every possible place there could be a vacuum leak and found nothing.

Today when starting it i did notice it was smoking black smoke from the tail pipe. This is the first time i've actually noticed the black smoke but just confirms that it is running rich.

[Irish Pride]

I guess i'll have to take the end cap off the fuel rail and get an adapter to connect the gauge to the rail. Is that the way i should do it or is the a port to connect to?