gas mileage in 91 SC ?

quicktsi

Registered User
My 91 SC w/ aod is only averaging 18 mpg on the highway. The trip is on a flat 20 mile stretch of highway. I average about 70-80 mph on the highway. The car does not run bad at all, even when the tach drops out. The car gets only 91 octane premium gas. What do I need to do? Will a bad cam sensor affect gas mileage?

Here is a list of what I have done.

New tires
Alignment
4 month old fuel filter
Mobil 1 changed 1k ago
redid the DIS with artic silver 5.

Things to check/replace

Cam sensor
boost leak test<--and install teflon gaskets if needed
check air pressure in tires
air filter <--can not see light when you hold it up to sun light
plugs and wires<--original owner said they have been changed
02 sensors
 
high flow K&N panel
New plugs/wires
do 70MPH or 68
cruise contorl if flat ground
i have an 89 auto and i get roughly 25-28 on roadtrips doin about 75

oh and do new o2's...motorcraft
 
Yes a bad cam sensor will affect gas mileage - but you usually get other problems like the engine cutting out or stumbling as well. I replaced mine and cured the engine problems and the gas mileage increased by 20 to 25%. It's an easy replacement for $25 to $30, as I recall. I did all of the other stuff too like O2 sensors, fuel & air filter, coolant temp sensor, etc beforehand but the cam sensor was the problem.
 
What's your temp gage read going down the road? if its not warm enough you will get what your getting for MPG. I just changed 1 that stuck open when it opened past the retainer & got stuck (poor design). so you need to see about that among the other things above. check your T-stat output line with a IR thermometer to verify 180 or more too.
 
JD,

Check your tire pressure. Don't assume that it is set correctly by whomever installed them. My 96 Ram 2500 runs 10 ply tires than need 80 psi. When the tire shop installed them, the tire pressure was set to 35 psi.
 
Flex

I checked the tire pressure and it was down around 25 and I upped it to 32 psi.

35thauto

I did notice that the dummy gauge is not reaching the 1/2 way point, but I did not know if was due to outside temps since it was between 40-50 degrees every morning last week. I will borrow a friends laser temp and check it.

Colo Bird

Did you replace just the little cam sensor on top of the shaft or did you replace both the shaft and sensor? If replacing just the sensor, how important is it to get the engine at the correct ATDC?
 
I would replace the 02's first before considering the cam sensor. As Colo said, usually there are other symptoms with the cam sensor.

Any idea how old are the plugs and how many miles? Have you looked at the transmission?
 
The original owner said he had replaced them fairly recently, but I am going to swap in a new set of wires and spark plugs shortly. I guess I will swap O2 sensors along with a new air filter at the same time. I will also be doing a boost leak test and fix.

What do you mean by checking the tranny?
 
I know the tranny is leaking from the cooler rubber line section and going in to the radiator. I am thinking of doing the dremel cut off and place a new section of hose with clamps. I need to check and fill it. It slips when the car is cold. When it warms up it acts fine.
 
Slipping is a first sign of trouble. Could just be a plugged filter or low on fluid. Have the oil and filter replaced. It is well worth the money and may improve mileage.
 
Yes on the filter. No for the oil if there is a drain plug. Otherwise yes for that as well.
 
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