Getting Horrible Gas Mileage - Any Solutions?

[topgun129]

Hello All,

I got a 93 LS400 with about 142000 miles on it. I did a tune up probably 15-20K miles ago, but recently I've been noticing the car is burning gas a lot faster than usual.

My MPG is around the low teens! I did change the fuel filter about 20K ago and 8-10K miles ago I put a fuel injector cleaner bottle in with a gas fill up. One idea that is coming to mind is that maybe the fuel injector cleaner left behind some small debris that it cleaned and that might be blocking some injectors.

One thing I've noticed with the car is that the RPM once in a while fluctuates downward at idle which causes the car to vibrate and shake for a few moments. Also when driving I notice some subtle hesitation (it feels like something is holding the car back) when accelerating.

Thanks!

[landar]

One simple thing you could try is to replace the thermostat. Sometimes as they age, they stay open somewhat resulting in lower engine temps and lower MPG. Again, it may not be your issue but its fairly cheap and easy to replace if you have not done so recently.

What did your recent tune-up consist of? Plugs, wires, rotors?

[topgun129]

Hi,

During the tuneup I did the wires(NGK), spark plugs (NGK), rotors , caps, PCV valve, and cleaned out the throttle body. The thermostat might be an issue.

[lex460]

Hi,

During the tuneup I did the wires(NGK), spark plugs (NGK), rotors , caps, PCV valve, and cleaned out the throttle body. The thermostat might be an issue.

Your O2's must be slow by now Cats clogging? Time to check more than plugs and wires.

[killerFatty]

I bet it's your coolant temp. sensor. I was getting around 7-9 mpg when I first got my car, and once I replaced it, I am at normal mpg. If it's faulty it will cause your engine to stay in warm-up mode and richen the fuel mix. Thus causeing terrable mpg and poor performance.

[topgun129]

Hi,

What are you referring to when you say 'cats'? Do you mean the catalytic converter? I looked in some other threads and people mentioned they are glowing red if the engine is hot which is a sign of a problem there. Where are they located under the car? How can I check them?

The 'cats' I think are 2nd on my list of possible problems. I'm going with the ignition coils for this.

Thanks!

[curiousB]

It could get expensive if you just start replacing components until the problem is fixed. You might want to spend a little more time diagnosing before doing too much swap and replace.

If it’s ignition you should see a sizeable performance hit from before the "tune up". Most common failure on this car is loss of a coil hence 4 cylinders go dead. That is more than noticeable. If you see OK performance with no major shake at idle or lack of power at WOT then it is more subtle than loss of 4 cylinders.

Could you have mis-wired plugs to distributor thereby killing 2 of 8 cylinders? Bad wires, or bad plugs? Maybe the new ones weren’t perfect.

How accurate is your MPG measurement? Was it based on a nearly full tank consumed and at the mileage off the odometer traveled?

Checked tire air pressure? Low tires cost you in MPG. Dragging brake pads? That would result in very hot rotors after a short drive.

The coolant sensor is also a good suggestion. Maybe check ECU for error codes. An out of range temp sensor should “throw a code” in the ecu log.

[topgun129]

when you refer to the ecu, what do you mean by that? how do i check it for error codes? are you referring to doing the following:

http://www.lexls.com/tutorials/air/accodes.html

Thanks!

[killerFatty]

I totally agree with curiousB. Spend much more time in diegnosis, and save much more in the end. Pulling the codes like lexls.com suggests is a good placce to start. However the coolant temp. sensor will not throw a code until it is completely dead, which never really happens. Mine never threw a code even when it had cut my mpg in half. Check all spark plug wires and make sure there is no crossed connecctions. With the car warmed up a bit, get out and look under the car and see if your catylitic converters are glowing. It should pretty noticable if they are. Does your car pull to one side or another? Go to a road with minimul traffic, and coast to a stop. Put it in park or nuetral set the E-brake. Get out and check to see if any rotors are overly hot. That should be about all the diegnosis you need to do. If all that comes back negative, I would go ahead and swap out the coolant temp sensor. For $30 it's not to rediculous, and its a simple job.

[topgun129]

Hi,

I did a little drive yesterday and looked underneath the car and didn't see any glowing cats. I'm leaning in the direction of the sensor b/c compared to the coils, I would be noticing a huge drop in power if there was an issue there.

I'll be doing the ECU checking very soon.

Also I don't notice any sticking of the brakes (I've seen this before and when this happens the rotors are red and very hot).

Thanks!

[curiousB]

OK sounds good. The coolant tempsensor isn't too expensive so if that isn't the problem you're not out too much.

Forgot to ask is temp gauge reading low? If thermostat is stuck open car may be running too cold and causing richer fuel usage than it should be. Thermostat is just a few dollars so might want to consider that too.

[topgun129]

Hi,

I ran a diagnostic scanner on the ECU but nothing turned up, and besides the check engine light is not on.

Do you know where the coolant temp. sensor is located in the engine and how much time it takes to replace?

Thanks!

[killerFatty]

It's pretty simple, yet hard to explain. It's near the passanger side ignition coil behind the plug wires that cross between the two distributer caps. You will need a deep well socket to get it out I believe it 3/4" or maybe 1/2". It will have a greenish clip on top of it, and should be goldish in color around the base I believe. You have to take off the passanger side coil to make enough room to wigle the plug wires around enough to get a socket on it. The manual says to drain coolant and disconect the water inlet housing, but that is a bit excessive. Just make sure and do it when your engine is COLD. You want as little pressure in the cooling system as possible if you don't drain the coolant. It sounds much harder than it is. It took me about 20 or so minutes to do. If your unsure of what your doing just take your time and go slow. If you have a buddy that knows a thing or two about cars, see if he can help you out. Good Luck BTW here is the link to all data that tells you the round about way to do it.

http://www.alldatadiy.com/alldata/AFI~V19914195~C41277~R0~OD~N/0/41746052/56620557/56620562/110767916/34853741/100411974/34856156/101367121/14585324/147387059