Help '87 Volvo 740 Turbo terrible gas mileage only on highway or freeway ?

I have an '87 Volvo 740 Wagon which I dread getting on the freeway for.
It seems like I'm getting 10 mpg on the freeway at speed limit of 65.
I'm not even using the turbo. When the throttle TPS sensor became
defective, it was like this, so I replaced it, but it's still using up
a lot of gas.

Is it logical to think that because I'm giving it more acceleration,
that more gas would be used?
I don't smell fumes when driving or stopping. It's like 5-10mpg on the
freeway!

I replaced the O2 sensor which was full of carbon and black, but it
hasn't seemed to help. I replaced spark plugs and replaced the air
filter. It's just ridiculous--I can see the guage going down little
by little as I'm driving on the freeway. It's as if I can't go
anywhere unless it's by street lights taking forever to get there, but
at least I'm not using up so much gas.

Maybe the catalytic converter? I do hear something rattling sometimes
when I'm driving, but I'm not sure if it's that or not. Any help,
please!

 

james,
did you ever replace that wiring harness that was giving you so much
trouble? perhaps something electronic controlling your fuel mixture
and/or ratio is shot. I hope someone else will chime in, but you posted
in February all of the problems you had with your wiring harness.

Jamie

 

No, not yet. I know, I know. I really need to get that fixed. On a
side note, it does idle like it's cold (kinda shaking) when I'm at a
stop. I'll look into that harness wiring. I just haven't been able to
come across it recently at a decent price. I'll look into that.

I will say this, though, It hasn't died one me once yet : ) like it
used to. I isolated the shorts which were causing it to die, at least.

 

Well replace or rebuild the wiring harness first, until you do that
you're just chasing your tail.

 

Contact Dave Barton. I can't tell you his prices at this very moment,
but I know that there are times when he has harnesses for around
$100-$130. Sometimes he has some in decent order that require minor
repairs and he can sell these cheaper.

You can always e-mail him and ask to be put on a "waiting list" for a
$100 harness.

Good luck.

 

A malfunctioning turbo caused poor milage on my daughter's Saab.

 

Shaking at an idle could be from a lot of things, including a vacuum
leak, bad valves, etc. You should really have the car looked at by
someone with the proper diagnostic equipment rather than guessing and
swapping out parts.

 

You need to carefully check all the hoses running from the MAF to the
intake manifold. One of them has a split or blow out on the bottom. Any
air leak at all between the MAF and the intake manifold will cause the
fuel injection to be very rich. As a side note if the green wire from
the O2 sensor is horted to ground the system will go full rich.

Bob

 

I'll check all hoses, thanks everyone. As far as having it looked at a
Volvo mechanic or someone with experience and the equipment---I'll
first invest in that wiring harness first---I know that is something
that does NEED replacing. Thanks for all the replies, and suggestion,
people ! : )

 

I rebuilt my existing harness by splicing new wires into the old
connectors and putting heatshrink tubing over everything. Took a lot of
patience and I was freezing my butt off in January doing it but I had to
get the car back on the road and it worked.

 

freezing your butt off in January sounds about right this time of year
( the West Coast is warming up right now).

 

Come on up to Northeast Texas - a cool 100 degrees. Thank goodness for
low humidity and this drought. Dust held my old harness together for I
think the last 6 months. I think in some places the wires rotted out
and the electricity had to arc to make a connection.

Damn tough cars!

 

Yuck, we're pushing up into the high 70's here off and on, figure I'll
have to start running the AC later this week.

 

High 70's? We call that a "cold front." LOL

We're about to go into water restrictions - serious drought here. I
moved here 1 year ago and if we've gotten 10 days of rain in that time
I'd be surprised.

I'll be happy when fall rolls around. My wife and 5 year old son are
ready for some camping.

 

Once when I lived in Phoenix my wife and I were doing a Cub Scout thing
outdoors the first weekend in Spring... the temperature hit 101 degrees.
Only four months there have not recorded 100 degrees: November through
February. My wife and I married in the middle of the longest unbroken run of
temperatures above 110F recorded there: 21 days. Most of those days were
115F and above.

I'm so glad to be in Flagstaff now, where the summer temperatures are
flirting with 90 degrees in the hottest part of the day.

Mike, the reformed desert rat

 

I had some suspicions about a BMW Motronic harness. It was easy to
split open to trace the doubtful wires. There's lots of ways to stich
it up again.