Richard Potter

I replaced sensor attached to exhaust pipe and garage replaced a vacuum tube. But still excessive consumption ( 10-15mpg) and 'kangaroo petrol' syndrome sporadically.

Ralph77

Has the computer been cleared. Maybe if so it is relearning everything. If not you should. Do this:


Jeep Electronic Control Unit Reset

When installing new power or performance accessories how does the computer for the Jeep know that it has a new accessory? Most of the time the computer does not. The computer would need to be reset to take full potential of the performance accessories you have added.

The Electronic Control Module, or ECU as its commonly called, stores the information for your vehicle and your driving habits in its memory. The memory is maintaned by a series of capacitors that hold a charge and maintain power for the ECU when the vehicle is turned off. The following procedure will allow you to reset the memory and allow the computer to 're-learn' how to drive by adjusting to the drivers habits and the vehicles modifications.


1. Disconnect the battery by removing the positive battery cable. Remember, the positive battery cable.

2. Ground the positive cable to the negative cable for 30 seconds to erase the electric charge in the capacitors.

3. Reconnect the positive cable to the battery.

4. Turn the key to on but do not start.

5. Turn the headlights on.

6. Turn the headlights off.

7. Turn the key off.


The ECU has now been reset to its factory settings. For the next 50 warm-up periods the ECU will re-learn the engine and driving habits for your driving conditions and the newly installed parts.

BlueRidgeMark

Which one?

There are two; one in front of the cat on the down-pipe, and one immediately behind the cat. If you replace the one behind the cat, it will have zero effect on the way the engine runs.

Also, vacuum leaks can happen in many places. One way to find them is to spray something like brake cleaner or WD-40 around the engine at places where leaks can occur (intake manifold, for example). If there is a leak, the flammable liquid will be sucked in, and you'll hear the engine speed change.

bad_idea

What is 'kangaroo petrol' syndrome?

EZEARL

+1 on Mark's O2 comment. Mine went bad on a trip and tanked my MPG.

How long since a tune-up? What brand of plugs?

CurrySoSpicy

Australian for bad gas. Car lurches, has rough idle, jumps around. Hence the "kangaroo-ing".

Zephyr77

(Apologies for old thread revival but this seemed like the most appropriate place to ask my question.)

2000 XJ 4.0, one day wouldn't start, fuel and battery gauges inop which indicated crank position sensor, could only source aftermarket part (yes, I know, forgive me Father for I have sinned, but I'm in the UK, OEM parts are either v. expensive or v. hard to source, and I needed the car running for work), fitted aftermarket part, she now starts and runs OK but slightly lumpy idle and occasional ECU light on.

Would the ECU reset detailed in original post be beneficial after a CPS change, or now that she's up and running again am I best off to wait for payday and try to get an OEM CPS?

Also could this ECU reset procedure potentially cause any harm or other issues?

Thanks for reading.

dave1123

Loss of gauges can also mean low voltage supply to the ECM (ECU, PCM, computer). If there is a bit of corrosion inside the battery cable terminal on the wire that feeds the main fuse/relay box (PDC), this may not effect how it cranks since the other wire feeds the starter itself. I'm not saying the problem wasn't the CkPS, but you may still have a low voltage situation that is intermittent. A "lumpy" idle can be caused by an occasional misfire at idle from carbon buildup on the spark plugs or intake valve seats. The occasional ECM light I suspect is caused by improper signals because of a low voltage condition.

Now I may be off the mark, but these are my opinions and you can take them with a grain of salt, if you know what I mean. The Jeep 4.0 is susceptible to carbon buildup because of it's crankcase ventilation system. Oil laden vapors are vented into the intake manifold to be burnt in the engine and this can cause carbon to build up on the intake valves and spark plugs, particularly if the engine is operated below 2000 rpm most of the time. Short low speed trips and long periods of idling make this worse.

All the ECM reset talked about does is clear the current performance memory, as in your personal habits about acceleration, duration of high and low speed driving, and how much load you normally carry to help the computer supply the proper fuel trims. This is totally different from a "reflash" which is to clear the entire programming from the computer and install a new one. A memory reset will only make the computer resort to it's default settings until it relearns your particular style of driving.

Zephyr77

Cheers for the reply.

I'll try changing the plugs, but I didn't have either the engine management light or the lumpy misfire before I swapped out the CPS. I'll definitely check the fuse box feed though. So to clarify - if it just holds info about performance memory, you don't think an ECU reset would be worth doing after a replaced CPS? Someone else on here (somewhere) advised doing the reset after any replaced sensor?

agreen

Unfortunately, you may be at the mercy of having to wait for a Mopar crank sensor. It's been shown, with hard scientific proof, that aftermarket CPS aren't up to the task. Whereas the Mopar sensor will show a defined square wave on an oscilloscope, the aftermarket ones show a rounded square wave and don't give the ECU the proper signal. I carry a known good used with me at all times so I won't be stranded if mine ever goes out.

00t444e

10-15 MPG is quite a wide range and pretty normal fuel milage for an XJ.

Roler

Inoperable fuel and battery gauges do not relate to a faulty CPS. You also say that before you changed the CPS, to didn't have the symptoms.

Sounds to me you swapped out non faulty CPS and replaced it with a incorrect working after market CPS..

Zephyr77

Car was cranking endlessly and not firing, new battery, good earths/leads, this is why I changed the CPS. I read on here that fuel and battery gauges are also a symptom of a faulty CPS. I'll certainly try putting the old one back on and see what happens.

I've been digging around for CPS prices, and have found -

Genuine Mopar jeep :£162.91
Blueprint £143.02
NTK £41.62
Intermotor £25.90

Can't believe the price jump in the middle there.

Battle

Yeah, maybe for a poorly geared one with too much weight but I definitely wouldn't call that "normal" - I'd personally call that unacceptable.

Roler

CPS sends it info to the pcm, nothing related to gauges for fuel and battery. I can't even think of how they would relate, looking at what does what.
The only relation the CPS has, indirectly though, is with fuel delivery. Once the PCM receives the correct signals from the CPS ( and other sensors involved) , it will allow the fuel delivery to start)run the engine.

Yes, the prices differ. The best bet for a proper working CPS would be the Mopar. It sure is costly, but cheap aftermarket might be making you think you have a new cps in there so it can't be that anymore. However many claim to have had faulty sensors out of the box.

If no Mopar available, I'd probably go NTK, they tend to make decent quality but that's a person preference. The others I don't know.