what common faults gives low mpg

just had a drop in mpg from 17 down to 11 this last week in my standard 97 xj

just trying to see if i can do anything?

done the leads, plugs, rotor, filters and oil

checked to see if any codes, but none

had a new cat a few weeks ago so not sure if it could be the O2 sensor?

any ideas what are likely problems i need to check that may cause low mpg?

thanks

Start with the HEGO sensors - they provide feedback for the fuel metering system.

Other engine management sensors can cause trouble - but you'll usually see a more gradual decline in fuel mileage, since they tend to fail gradually. I'd still start with the HEGO sensors, and probably the TPS as well.

5-90

thanks for that, but how many hego are there? i thought it was just the one (upstream)
or are there others?

is there a way to check the TPS ?

thanks again

qwerty247 said:

just had a drop in mpg from 17 down to 11 this last week in my standard 97 xj

just trying to see if i can do anything?

done the leads, plugs, rotor, filters and oil

checked to see if any codes, but none

had a new cat a few weeks ago so not sure if it could be the O2 sensor?

any ideas what are likely problems i need to check that may cause low mpg?

thanks

Click to expand...

The most likely culprit is the upstream O2 sensor that sends a voltage signal to the ECU. The downstream O2 sensor merely monitors the function of the cat and when it goes bad, it triggers the MIL but doesn't affect fuel mileage.

Is it automatic or stick shift?

He sez "standard," so I'm assuming he's stirring the gearbox himself. And, he sez it's a 1997...

The easiest way to test the TPS is to get an analogue (sweep needle) ohmmeter and connect it to the centre terminal and either side terminal (after disconnecting the wiring harness,) and operating the throttle by hand (engine OFF) and watching the needle sweep. You should get smooth, cosistent action, with no "flat spots" or "skips."

Also, check for +5VDC (nominal) at terminal 3 with the key ON (engine running or not.)

The TPS proper may be tested thusly (from 1997XJ FSM, no distinction made between RHD and LHD):

"The TPS can be tested with a digital voltmeter. The centre terminal of the TPS is the output terminal.
"With the ignition key in the ON position, check the TPS output voltage at the centre termain wire of the connector. (sic - cumbersome, no?) Check this at idle (throttle plate closed) and at Wide Open Throttle (WOT.). At idle, TPS output voltage should be greater than 350 millivolts" (350mVDC, .350VDC - 5-90) " but less than 900 millivolts. (900mVDC, .900VDC - 5-90) "At wide open throttle, TPS output voltage must be less than 4.5 volts. The output voltage should increase gradually as the throttle plate is slowly opened from idle to WOT."
(1997XJ FSM, p.14-43)

All well and very proper, however I will maintain until the end that the BEST way, bar none, to check action of a potentiometer/variable resistance (which is exactly what the TPS happens to be!) is to use an analogue ohmmeter. A digital ohmmeter/DVM/DMM contains a "buffer circuit" that slows response time, and can actually hide the beginnings of a flat spot in a sensor like this. Also, the sweep of the analogue meter is direct, which will also allow you to see "skips" as well as "flats" in the sense arc.

No differentiation was noted for domestic/export production, left-hand/right-hand drive, or for automatic/standard transmissions. Nothing against Langer, but his question most likely has its roots in RENIX, where the manual transmissions had a single TPS, and the automatics had a gang of two (two sensors in one unit, with two harness connectors.) ChryCo did away with that in 1991, when they installed their own OBD-I compliant electronics.

5-90

although there where 2 connectors they where the same thing. only one went to ecu and the other went to the auto tranny comp.
this would only be valid prior to 91

thanks for the very detailed and useful instructions how to test it, i will have a go and see what happens.

also ordered a new upstream O2 so will put that in and hope i get a few more mpg, especially that here in UK it cost now £1 per litre...


thanks again for all the advise !

No problem. Report back on results from swapping the HEGO sensor and what results your testing yielded.

5-90

qwerty247 said:

thanks for the very detailed and useful instructions how to test it, i will have a go and see what happens.

also ordered a new upstream O2 so will put that in and hope i get a few more mpg, especially that here in UK it cost now £1 per litre...


thanks again for all the advise !

Click to expand...

how much is it in U.S dollars?

90xj06 said:

how much is it in U.S dollars?

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Look up the current exchange rate, and equate one litre to one quart (close enough for government work.)

Or, just look up the exchange rate, and multiply by four to get rough $US/gallon.

Either way, it's hideously expensive. On the other end of the scale, Venezuela is still paying somewhere around thirty cents per gallon, and even the UAE, about a month ago, was somewhere under a buck-fifty...

5-90

looks like im driving to venezuela for gas. ill get 300 gallons of it and it will still be cheaper for that than it is to fill this hog

90xj06 said:

how much is it in U.S dollars?

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Last time i was there about 1.65 or 1.66 US to one Pound. 3.78 liters (i think) in one gallon should be around 6.25 a gallon cheap eh ?

5-90 said:

Look up the current exchange rate, and equate one litre to one quart (close enough for government work.)

Or, just look up the exchange rate, and multiply by four to get rough $US/gallon.

Either way, it's hideously expensive. On the other end of the scale, Venezuela is still paying somewhere around thirty cents per gallon, and even the UAE, about a month ago, was somewhere under a buck-fifty...

5-90

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That gas is cheap shit though. Detergents suck ect. If your going to an area liek that you better go deisel.

um i feel poor now. thats expensive. im squaking over $3.05

90xj06 said:

um i feel poor now. thats expensive. im squaking over $3.05

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crap i was squaking at $2.25 :roll:

if you're in the UK, wouldn't it be KMPG not MPG?

BrettM said:

if you're in the UK, wouldn't it be KMPG not MPG?

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Actually it would be how many Liters per 100km.

BrettM said:

if you're in the UK, wouldn't it be KMPG not MPG?

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Na the UK split the metric system down the middle, they use what they want.

ie. kilos not pounds
Litres not gallons
MM not inches
Celcius not farenheight

But they still use miles not KM, some use inches not MM, some use pounds not kilos.

Ya it's confusing.

So it would be Litres to the mile (MPL)

in the UK we have always used miles for travel
but a few years back they changed everything to metric but some people still want to use inches....



the £1 is currently equal to $1.90 so i believe 1 gallon is 4.6 litres?

so 1 litre is $1.90 therefore a gallon is $8.74 !! which i thick is VERY expensive ?

so i am doing 11 miles for $8.74 which is why i realy need to find out what happened as i was kinda happy with 16...

90xj06 said:

although there where 2 connectors they where the same thing. only one went to ecu and the other went to the auto tranny comp.
this would only be valid prior to 91

Click to expand...

On the Renix auto TPS, there are two wipers inside the TPS. The two connectors are not the same!