What happened to my MPG!?
 

I love this site. It has helped me a lot! So I bought my 88 XJ a year ago for $500.00 and replaced quite a bit. All new fuel injectors, plugs, wires, distributor, cap & rotor, o2 sensor IAC sensor, battery, and I am getting 9-10 MPG. Why!? I even put a Spectra cold air intake and a Dyno Max exuast system. Still I have no better MPG. Any one have an idea. I don't think my catilatice coverter is bad.

Cracked exhaust manifold? Anything blocking aerodynamics? I know its a flying brick but it can still get worse. Extra weight in the vehicle? Any cracks in the exhaust? Exhaust blockage?

what kind of tires you got on it?

If your MAP sensor up there on the firewall isn't getting good vacuum it will dump extra gas. (thinking you have your foot in it). Besides the tube being free and not leaking, low manifold vacuum will give it a lower reading.

My vacuum drops an inch when I turn on my defroster.(tube or diaphragm's shot). I'm not using it lately because gas is so high!

The intake air temp and the coolant temp sensors might be checked as well.

What kind of driving are you doing?

My gas tank was leaking, but I couldn't tell because it would pool in the skid plate and evaporate. It rusted out along the tank straps.
That was killing my gas mileage...

Do you have a lift and larger tires?

I just put on a brand new set of BFG's All teranins (Great tire!) I have thought about the MAP Sensor... I can smell fuel sometimes but I can't see a leak. I also put new o-rings in when I did the fuel injectors. So how should I check to see if that is it for sure? I don't think my tank is leaking I leave is parked for weeks sometimes and never see or smell anything.

Still stock hieght and tires are 3.x9.50

1. Is this "calculated" miles per gallon? Gotta do the math. Miles driven divided by gallons consumed so there is no guess work or estimation.

2. Map sensor can be tested with a meter and a manual. And be darn sure that the vacuum line leading to the map sensor is pulling good vacuum and the vacuum line isn't cracked where it connects. MAP is directly involved with fuel/air ratio. Coolant sensor should also be tested.

3. Find the source of that gas smell! It could be related and even if it isn't, any fuel leaks can lead to fires. And those are never pleasant.

4. Below is a link to testing many things on your RENIX. It was written by an associate of mine on Jeeps Unlimited, where we are moderators. Get your meter out and start testing!

http://www.lunghd.com/Tech_Articles/...iagnostics.htm

I have a gas smell too. And I get 11 or 12 mpgs. Granted I'm lifted with 33s and stock gears. Reason I have a gas smell is my exhaust manifold has multiple cracks in it. So as stated above, check that

tjwalker, Thank you for the tip and link. They are extremly helpful! I can't wait to figure this out now. I should have posted this inintialy but I forgot, sometimes when I start it the jeep revs to 3000 rpm and will not stop until I turn it off and then on again. Sometimes I have to do this two to three times in order for the engine to idel normally befor I drive off. Does that give you any more clues?...

Sounds like you need to test your throttle position sensor!

Flinstone, that's your que!

I've up and down, inside and out trying to squeeze a few miles more out of my 92.

Not a chance. If there is a bad sensor or component you'll see a big jump.

Otherwise, like the rest of us your gonna have to live with it.

I'd verify the TPS as well as clean both the throttle body and idle air control.
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RENIX (87-90) TPS ADJUSTMENT

Before attempting to adjust your TPS be sure the throttle body has been recently cleaned.
It's especially important that the edges of the throttle butterfly are free of any carbon build-up.

IMPORTANT NOTE: Using one lead of your ohmmeter, probe the B terminal of the flat 3 wire connector
of the TPS . The letters are embossed on the connector itself.
Touch the other lead of your meter to the negative battery post.
If you see more than 1 ohm of resistance some modifications to the sensor ground harness will be
necessary. The harness repair must be performed before proceeding.
I can provide an instruction sheet for that if needed.

MANUAL TRANSMISSION:

RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body.
This manual transmission vehicle TPS provides data input to the ECU. The manual transmission
TPS has three wires in the connector and they're clearly embossed with the letters A,B, and C.

Wire "A" is positive.
Wire "B" is ground.

Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors..
Note the voltage reading--this is your REFERENCE voltage.

Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your
OUTPUT voltage.
Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For
example: 4.82 volts X .17=.82 volts. Adjust the TPS until you have achieved this percentage. If
you can't achieve the correct output voltage replace the TPS and start over.

AUTOMATIC TRANSMISSION:

RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a three-
wire connector, same as the manual transmission vehicles have, and it is tested the same as the
manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES.

However, the automatic TPS also has a four-wire connector clearly embossed with the letters
A,B,C, and D.. For the automatic transmission equipped vehicles the four-wire connector
provides data to the TCU.

Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your
REFERENCE voltage.

Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT
voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage.
For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage.
If you can't, replace the TPS and start over.

So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and
the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire
connector side of the TPS. If you have ENGINE issues check the three-wire connector side of
the TPS.

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is
stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.

Great,thanks everyone for there help.