UGHHH 20 MPG! What is going on mostly open highway also
 

20 MPG! I was in shock when I seen this, the car was driven 175 miles on the turnpike at cruise speed of 80ish using the cruise. I am guessing there is something major wrong that isn’t affecting my SES light, as I would expect? the rest of the 240 miles was a mix of freeway and city also.

I am changing the plugs/air filter and fuel filter in the am. The car seems to run a lot better when it has closer to a full tank (fuel pump issues right?) How much would I expect to pay to have the fuel pump replaced also?

I wish it were as simple as my Imports (remove the rear lower seat cushion and unplug and then replace 30 minutes later back on the road and all good as new) the car has stopped doing the chugging and the wanting to stall around curves now also. The car has somewhat a sputter-delayed effect upon take off till it gets to right around 3,000 and then it pulls well. I noticed that around 70-75 on the open turnpike today it seemed to feel as it chugged a little also, but upon going right around 80 it didn’t seem to do it or not to the noticeable effect anyhow.


Any ideals and help or hints?

Also is our fuel filter under the car, I haven’t looked for it yet and I think I recall my friend telling me back in 2000 his was maybe not cant recall for sure.


Thanks,

Donnie H.

I you are going by your average mph this could be normal depending on your foot pressure and amount of time spent stopped idling or accelerating.
80 mph will also reduce the average a bit. ;)

It seems low to me. I easily get 25-30 on the highway. I only dip below 20 in winter driving, mostly short trips.

Seems like a fuel pump problem would not cause low MPG, since that would require the engine burining more fuel. Same with the fuel filter theory.

O2 sensor? Plugs and wires? Air filter? Tire air pressure? When's the last time you checked or replaced all of the above?

The torque converter clutch is what gives you such good mileage on these cars. Its possible yours is not engaging due to some other problems, such as engine will not go into closed loop.

On the highway, go to 100 kph. Allow the engine to relax, and see if your rpms go to 1500-1600. They should stay there, and not move, unless you blip the throttle a bit. Try it at 80 kph, the engine should be at 1200-1300 rpm, and should stay there, unless you blip the throttle a bit.

Before I go any further, have you changed the thermostat? A stuck open thermostat will not allow the engine to go into closed loop, and thereby keeps your tcc from engaging. I found when I replaced mine, the engine warmed up quicker, and i got better mileage.

Hey everyone,

Thanks for the quick answers; I have no clue as to what’s been done to the car as I just got the car on Monday the 10th. I have got new plugs and new filters to put on the car in the AM.

The TCL (TQ converter lock up) is working fine, however on hills it does drop out a lot or seems more then what I would think it should, My car very seldom ever- ever gets past the first line on the temp gauge (but I have warm-hot air from the heater) and what’s up with when you set the heater to hot it comes out on feet only! That’s a little weird and don’t make for a good heating situation (have had several Pontiacs and all had awesome heat and ac in there)

My ac don’t work of course, and still cant seem to get the passenger side door lights to work when you open the doors as well as the over head lights and the keyhole lights also (just going to learn to live with it I guess) I looked at the thing down by the door under the carpet but couldn’t tell what part I was needing to check to make sure was clean (but looked rather clean to me)

My car also had right at 130,000 on the clock. My Z32 (300ZX) gets better fuel mileage when I drive it 3 times harder than my Bonneville does so far and the Z32 is a auto also!


Thanks again everyone

Donnie H

Replace the O2 as well... This can be a major killer if its shot... Hopefully its not a Bosch unit in there now...

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That could be the problem. If the gauge is accurate, you may have a faulty thermostat that is sticking open and not allowing your engine to get out of closed loop warm up mode. So it would run rich and produce poor mileage.

Short trips are what does it for me. Most of my driving is 10 minutes runs. Just gets out of closed loop when I arrive at my destination.

Its possible that the gage is innaccurate...

But for the TCC to apply on a Vin C 3800, the engine must be at a minimum of 150F, if the engine does not reach this temperature the TCC will not apply...

I had that problem with my 91 a long time ago... I found that the Thermostat was stuck open, and the engine was only getting to 130-140F so the TCC never did apply... Took the thermostat out and the whole story was explained... ;)

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These revs may differ for the GX3 3.33 final drive ratio that's on the 90/91 SSE. I do reckon I do just a hair under 2000rpm at 100km/h.

I have the weird idle issues under control and the missing while cruising taken care of after replacing the plugs; the old ones didn’t look very bad at all, just old. I am thinking this might take care of the very poor mpg as well. I will know soon as I am going out to south Oklahoma (120 mile trip one way Monday night) I will let everyone know what happens on this.


Thanks for all the help this far its great!

Donnie H.