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7mpg average! Help if anyone can.

4K views 25 replies 6 participants last post by  Sevillian273 
#1 ·
OK so I am averaging around 7mpg total driving city and highway. I notice whenever I use the gas pedal to accelerate I'm getting around 2mpg or 4mpg.

I don't have a lead foot by any means. I drive pretty much like an old man so I'm told. I just dont know why Im getting such terrible gas mileage.

I also only use premium gas from Shell or Exxon or Hess so that's not the case.

I know the EGR valve needs to be replaced due to a E48 error code.

I also have a E49 code.

Water pump and Idler pulley also needs to be replaced and I have a hole in my exhaust manifold.

Can any of these cause such horrible gas mileage? What would be the reason for the vehicle running so rough? It also shakes pretty violently during hard acceleration. It does have abit of hesitation to it.

I'm just pretty frustrated with the vehicle at the moment but its a work in progress I guess I love Cadillacs and DeVilles and refuse to give up on this car. Its a 90 Deville with the 4.5.

Any thoughts or help or advise would gladly be appreciated.

Thanks.
 
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#2 ·
I notice whenever I use the gas pedal to accelerate I'm getting around 2mpg or 4mpg.
Its normal to see that on the Inst Mpg display. Take your foot off on the hwy and it will read 70mpg.


Sounds like you have alot of issues going on at once. I would start with a tune up (plugs, wires, cap, rotor) Then onto the E048 EGR. Also check the resistance of your fuel injectors. All of the above systems can cause your symptoms although I would think something serious would be going on to cause such low mileage. Have you checked the actual fuel mileage via the trip odometer between fill-ups? There would be gray/black smoke coming out of the tailpipe all the time if that much gas was going through the engine.
 
#3 ·
Yes I have checked it, No I have no smoke. I already did a full tune up, Plugs, Wires, Filters, cap and rotor.

Im not sure what the problem is with the gas mileage. Yea I did notice that on the highway If I'm going 65 and take my foot off the gas it will read 70mpg. But once I get back on it just to coast the car or cruise I will get about 9mpg on the highway.

The car shakes violently at those speeds as well.
 
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