XJR over fueling when hot
 

Hi all

From New Zealand, 1996 XJR. Lately its been overfueling after its been run for a while. From cold, it will idle fine for about 45 mins, cycling through many fan activations. It doesnt overheat, but it will start sputtering amd eventually cut out. Its then impossible to start it, until its left alone to cool down for an o
hour or 2.

Works done to deal with this issue as follows:

New coils
New crank sensor
New coolant temp sensor
New intake air temp sensor
New cold air intake
New intercooler pump

There are no codes thrown at all. There is a slight crack in the exhaust manifold, however, thats been there for a long time and hasn't been an issue.

Any ideas?

Thanks in anticipation

Any of the new components can be bad off the shelf

A few bad coils as new could be expected, GETS WORST AS THEY HEAT UP

A good CKPS will read 1300 ohms and the issue is when warmed will expand , place in oven before testing

The 2 wire coolant temp sensor can be calibrated against page 78 the same time oven check at 200 f it could be the ECT drops out bringing the engine out of closed loop regulation ( best lean tune )

Same as for the inlet temp sensor with a different calibration chart page 86

These last 2 sensors are thermistors as a solid state device that has to be powered , so a 9 volt battery powered meter

The short term fuel trim should be target 0 % in all ranges with + or - 3 % realistic

jagrepair.com/images/Training Guides/801S - 2000.pdf :icon_banana:

There is device in the supercharged intake that was negated in later XJRs on page 108

EGR equipped ? as not all SC have and maybe just don't

I do not have the supercharged

Thanks heaps, all are tested OEM parts and made zero difference to the issue. Im left wondering if it might be the fuel pressure regulator?
The car literally floods

G'day Kiwi,

I had a XJ6 SIII years ago which ran way rich when warmed up and it turned out to be a broken wire in the coolant temp sensor (CTS) line which made the ECU think that the engine was always cold and so overfuelled the engine after it had warmed up.

The coolant temp sensor has high resistance cold and it decreases as the engine warms up so a quick test is to, with the engine warm, disconnect the CTS plug and bridge the two connections with a paperclip or something similar. This should send a warm engine signal to the ECU and fuelling should then not be over rich.

As has been previously mentioned even a new component can be faulty.

I will attach some info about CTS function and testing which was written specificlly about my car, a 1995 V12, but the general principles will still apply.

Cheers and good luck,
Jeff.

Thats a great tip, I will definitely try it and see if it works. I also noticed today parking on a steep slope nose up caused it to run rough when cold.

Overfuelling
 

Mine turned out to be partly a new but faulty temperature sensor. No codes.
It read correctly until almost at operating temperature then reverted to showing an erroneous low temperature.
I mean the sensor the ECU uses to tell when to stop enriching the mixture.

What was the other issue?

Inoperative camshaft position sensor tho that was more starting than fuelling.

I also replaced the fuel pressure regulator.

Hope you find the source of the problems

Miss our X300.

FPR IS next on the list haha

Can you pull the little hose off the FPR and see if it looks wet or smells of fuel? If it does, you’ve most certainly found your culprit.

​​​​​​….if it doesn’t, still doesn’t confirm it’s working well.

After FPR, you could have leaking fuel injectors.

I can’t really see how failing oxygen sensors can cause this issue. Not without a really rough idle/drive cycle.

I would think a faulty TPS would also force strange RPM readings.

Cheers mate, yup pulled the hose and was dry, but FPR looks pretty old. When the engine runs from cold, even after closed loop, its fine. But at some point it starts over fueling. Then floods.

If i drive the car and not stop anywhere, it runs and wont cut out. I haven't driven it long distance for a couple of months now, but driving it around the neighborhood, doesn't make it die. Its only at idle, and after quite a few fan activations.

The fuel pressure regulator if regulating to bypass over pressure amount will give a sound and hum

the regulated pressure is 43 psi at idle and the pump should put out more than that , the bypassed fuel will recirculate to the tank on rubber hose X

Fittings are hard to find to get pressure reading

It's a rubber diaphragm so won't last forever but a diaphragm should leak into the upper small vacuum hose into the intake manifold causing cylinders to be over rich

https://cimg9.ibsrv.net/gimg/www.jag...85129afcd7.jpg

I am just waiting for the parts to arrive, but will also replace the vac hose.

Very helpful, thanks heaps

The vac hose will change the fuel pressure regulated by the spring inside from base 43 to 47 when vac applied , throttled up engine drawing vac

This is normal already accounted for by the ECU controlling injectors bandwidth ( open time / squirts )

With a leaking vac hose it will under fuel in theory and not develope full power

Sweet, thanks for that. Its definitely not short on power

I'm out for a while

Real quick O2 sensors heaters inside each , only 1 fuse for all 4 total sensors

The O2 sensors are not used until coolant above 165 F

right engine bay fuse box either # 10 ,,12 . 14 , or 16

The sensors are heated by this for them to output correctly , O2 sensor physics

The correct different titania based O2 sensors as not the usual

Are you saying pull the fuse? See if it over fuels?

fuse must be good and always in

Sweet as, I shall check and probably replace for good measure