The Dreaded "Bad MPG Tips"
BAD FUEL MILEAGE TIPS & HINTS
OK, let’s assume that you’ve just bought your beloved JEEP, You’ve had it a short time now and already you started to notice poor MPG (Low double figures) lets look first at what may cause this:
1/ Are you aware now that you have a vehicle that is driving two heavy axles all the time? It’s no longer a two wheel drive (Unless some of you already have only that application or type) the vehicle itself weighs close on two tonnes and is a heavy piece of kit to lug around.
2/ Assuming that you knew this when you purchased the vehicle, have you had a good look around the vehicle to check that both the foot brakes and emergency park brake are not binding on?? Early models with Teves Calipers were prone to this binding or seizing. Makes sure that the guide pins are free and that the pistons are working. , make sure all wheels run free and are not interfering in any way. This will result in poor gas mileage if not corrected and is a common issue.
3/ Have you checked all your tyre pressures and equalized them to ride on the same PSI?? Low tyre pressures also play a part on low MPG.
Assuming that the above has been carried out and that the mileage is still low – When was the vehicle last serviced?? What was done to it during that service?
A routine service, plugs, oil filter, air filter is going to help so make sure before we start to delve, that this has been carried out to manufacturers spec. and that the car still runs smooth through the rev band and load. On high mileage vehicles a fuel cleaner would be recommended through the tank to clean the lines and the injectors. A recent test carried out proved as much as 6bhp can be regained.
A Good source of information is to carry out a compression test to see how healthy the engine actually is. Remember if the compressions are low the tuning gains for better MPG are going to be minimal…
Okay, now we have to start digging a little deeper into the bowels of the engine. A good on tune spark plug should be a nice golden brown, any moisture or sooting indicates that already there’s a problem that needs investigating. Has the injector failed and over fuelling, does it have a nice even spray pattern, Has your MAF (Mass Air Flow) Meter expired (If you have one.), some vehicles have this on the airflow intake others have a small sensor, either way it needs checking out.. Check that all 6 or 8 pending on, are golden coloured… an emissions test done by a portable gas analyser on the exhaust will confirm this.(Gunsons -Can be hired) When was the last time the fuel filter changed? A Choked filter can reduce performance and restrict the engine by as much as 3bhp..
Some of the forum members talk of “Stuttering or spitting”. What condition are the coilpacks in? When were they last replaced? A rule of thumb is to replace at ever 80k miles and is fairly norm for the manufacturer to advise this. Don’t forget they’ve been on there since the vehicle was born so by the time you’re getting it, they may well have degraded to a weak spark. Another common problem is heat failure. The heat of the engine and where they are located will of course cause failure over time as they expand in heat and contract on cooling or cold. Always carry a new one or known good one to bail you out. Another point… when were the leads changed?? I only used Magnecor leads as the come in various sizes (Diameter) and colour.. and found no performance issues at all and better, cleaner response on pick up.
So now we’ve looked at fuelling and spark, so our engine is starting to feel a little bit healthier than before, but still we need to keep looking.
A key are during service is the air filter, often overlooked. A choked air filter can make all the difference to fuel mileage, so when did you say you last changed it?? REMEMBER: It takes three things to make combustion – AIR, FUEL, SPARK/IGNITION. If none or one of them is not healthy then we can almost certainly expect issues. Again a choked or dirty filter can have a detrimental effect by 2bhp…
So let’s move over to some of the not obvious areas:
A dirty throttle will also cause poor fuel consumption. Remove the Throttle body and examine the butterfly and its action. Is it black and sticking? The butterfly (Disc) should be spotless, using carburettor cleaner, spray until absolutely spotless. You should be able the hold the body up to the light and see a very thin line of light around the disc. Make sure that the disc and bore is perfectly clean and has no carbon ridges on its surface, lubricate with light oil (3 in 1) and work in and re install. You may find that this will result in a lower rpm and will require the TPS to be recalibrated accordingly or replaced.
Another, area of guidance is to do the “key dance” and read the codes. Common issues are the O2 sensors, remember they don’t last for ever and chances are that they will need replacing, these if faulty will send out false reading to the ECU asking for more fuel. Resulting in overfuelling and poor mileage. Only use MOPAR parts at this point as the Jeep is temperamental with spurious parts and false readings can occur..
An area often overlooked is the condition of the wiring to the sensors. Don’t forget that over time these have been subjected to heat, water and dampness, so have a good look at the wiring and don’t just assume that because they are all there that they are OK. I’ve had wiring on an O2 sensor look fine but in fact needed replace for a good three feet all because it had deteriorated so badly it was like pencil lead. It just kept breaking!! Most commonly they just snap at the connector and it’s a matter of splicing and soldering in a new piece followed by shrink wrapping and vulcanised tape for good measure.
A dodgy fuel pump can also be a culprit. Remember: AIR/FUEL/ SPARK? So what happens if this fails? Most commonly are three symptoms: Firstly on start-up there is a loud whine or hiss, a strong indicator that the pump is heading south. Secondly it can be accompanied by stuttering performance or surging as it fails to deliver properly, and thirdly it can just fail!! Resulting in non-starting. A less common issue is that it performs normally but on cruise or heavy load it manifests itself on cutting out (Low Fuel Pressure) or stuttering, again the need to replace is necessary
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