XJ XJ8 / XJR ( X308 ) 1997 - 2003

RP with No Codes...

Old 05-08-2019, 06:46 PM
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Default RP with No Codes...

OK, with apologies to those who want a nice brief question to answer, and with thanks to all those stopping in who may have wisdom to share, I'll firstly give a little background on this car, which may or may not be relevant to the problem in hand..

So, I recently bought a 1999 XJ8, which was on its way to The Happy Hunting Ground when I intercepted it. Apparently it had sat for a looong time, the previous owner had taken the fuel tank out (I think to replace the fuel pump), and had problems getting the pipes back in underneath, before finally losing patience with the whole thing. With a bit of swearing and scraped knuckles, I got them back in, and the car seemed to be heading back to life.

A further problem that became immediately apparent was a "Gearbox Fault" message and the problem of it thumping into Drive (yet reversing beautifully) - which I gather from reading here could be a cracked drum. A lack of any debris or swarf in the sump pan led me to replacing the valve block - finding a nice crack in the casting in the process, so hopefully that will cure it - the "Gearbox Fault" message has cleared now anyway.

I haven't driven the car yet, because although it starts beautifully, first turn of the key, and sounds lovely, nice and smooth, once it warms up, it starts sounding lumpy, definitely not right, and I get a "Restricted Performance" message, along with limited revs. One of the coil packs did seem to be cracked and distorted, so that was changed, but no difference seemed evident. My neighbour plugged in his SnapOn diagnostic thang but no codes were thrown up.

Any clues as to how to proceed? I'm wondering if this has been a long-time problem and why the previous owner was doing things with tank/fuel pump. Strange that it fires up wonderfully at first, then after 5 minutes just doesn't sound right. It barely had the guts to get out of the workshop once it was warm.

All answers gratefully received, cheers.
Old 05-08-2019, 10:26 PM
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A couple of points:

Check the vacuum hose to the fuel pressure regulator (FPR) for any loose connection or cracks. If this hose is bad, it will cause higher fuel pressure. If it is good, check the fuel pressure with a gauge - it should be 38 psi when the engine is running. If the pressure is higher than this, the FPR might be bad. Higher fuel pressure than the specified 38 psi will not affect the engine running much while it is cold but will cause rough engine running when warmed-up due to too rich mixture. This should show fault codes; perhaps the SnapOn tool cannot read Jaguar codes, try with a Jaguar specific diagnostic tool.

Another part that can cause too rich fueling is the coolant temperature sensor. You can check its resistances against the table below.

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Old 05-08-2019, 10:35 PM
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Hi MrGrey,

Rough running once the engine has warmed up suggests a problem either with a component the Engine Control Module (ECM) ignores at first, or a problem with a component that is providing an incorrect signal once the engine is hot.

The ECM ignores the O2 sensors until they have reached operating temperature, but typically problems with O2 sensors will trigger Diagnostic Trouble Codes (DTCs).

One component that comes to mind is the Engine Coolant Temperature Sensor (ECTS), which is a primary sensor for ECM fuel metering. If the ECTS sticks at a resistance that tells the ECM the coolant is cold even after the engine is warm, the ECM will continue to apply cold-start fuel enrichment, leading to rich running. The situation is compounded if the coolant thermostat is stuck open, preventing the engine from reaching operating temperature.

Your neighbor's Snap On scan tool can probably read Live Data. With the engine running, watch the signal from the ECTS to see if it is plausible. I've seen many that reported irrational signals such as 32 degrees F when the engine was hot.

Other things to watch with Live Data are the Short-Term and Long-Term Fuel Trims (STFT, LTFT), which will tell you if the engine is running lean (positive fuel trims indicating the ECM is adding fuel to compensate for too much air) or rich (negative fuel trims indicating the ECM is reducing fuel).

Plenty of other sensors you can monitor for plausibility, but those would be good places to start.

Please keep us informed.


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MrGrey (05-09-2019)
Old 05-09-2019, 08:59 AM
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First place to start is a scanner that will read codes that are there . Most generic scanners do not always read codes that are in fact there. Even my own generic scanner i use to avoid firing up sdd will say no codes when in fact there are a bunch and or the cel is on. Hook up sdd and yep there they are. Untill then purely conjecture. See what the ecm sees for rp. Then go from there.
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Old 05-09-2019, 09:11 AM
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My Actron OBDII will sometimes read P codes with a 1xxx but will always read P0xxx DTCs.

The 1 prefix is an auto makers specific code that sometimes can be read as 'enhanced' code and the reader needs a MAKE MODEL YEAR to display the DTC.

WDS, IDS always finds the complete list in all the modules.

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Old 05-09-2019, 04:24 PM
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Thanks for the tips chaps, I'll keep you posted...
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