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Excessive rich mixture - Fuel pressure regulator?

XJ XJ6 / XJR6 ( X300 ) 1995-1997

Excessive rich mixture - Fuel pressure regulator?

 
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Old 03-01-2019, 06:57 AM
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Default Excessive rich mixture - Fuel pressure regulator?

Hi Guys, My '95 XJR6 has started running very rich to the point that it occasionally misfires and generally smells of petrol. It runs smoothly on a drive, but seems down on power. There are no fault codes listed on my diagnostic and the sensor display shows the oxygen sensors switching, the MAF sensible numbers which change as you rev the engine, sensible coolant and intake air temperatures, short term fuel trims are stuck max one way, cant remember the figures but think the long term trims were always stuck at 77.
I put a new aftermarket MAF on it a few years ago and the crank sensor has been replaced maybe five years ago. I suspect that the fuel pressure regulator may have failed and plan to have a look this weekend, ie look for fuel / leaks in the vacuum line. My Dad also has a wide band oxygen sensor - so will have a go with setting that up when I get chance. Hopefully it will show any changes so I will know if it is fixed before I book an MOT.
Any comments or suggestions welcome.

Thanks,

John
 
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Old 03-01-2019, 07:16 AM
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A fuel pressure test would confirm a faulty pressure regulator.

Also, if the vacuum hose to the regulator has raw fuel in it, the regulator is leaking.

My 1995 XJR suffered fixed (or nonsensical) fuel trim readings for years, even when it was running very well. I never could get to the bottom of it. And, it often would not set trouble codes even when a fault was clearly present. I think these very early OBDII cars were still in the "We're still ironing out the wrinkles" stage of development

Cheers
DD
 
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Old 03-01-2019, 08:55 AM
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Hi John,

Another common cause of rich running is an Engine Coolant Temperature Sensor that is stuck on a "cold" reading. This can cause the Engine Control Module (ECM) to continue to apply cold-start fuel enrichment even after the engine has achieved full operating temperature. You can measure the resistance across the terminals of the sensor when the engine is cold, then again when the engine is hot. Off the top of my head, the reading should be a few ohms when cold, and a small fraction of one ohm when hot. Also check the wiring harness to the ECTS, since high resistance in the circuit can mimic a "cold" sensor reading.

If you haven't checked your air filter recently that's easy, and it's worth cleaning the MAFS wire with a good zero-residue MAFS cleaner spray.

Leaking fuel injectors could be contributors. Also, a stuck-open carbon canister purge valve can contribute to rich running, though it will typically trigger a diagnostic trouble code.

Please keep us informed.

Cheers,

Don
 
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Old 03-01-2019, 02:23 PM
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1. Check your coils. They could be going bad. When was the last time they were replaced? While you're at it, spark plugs, and possibly camcover/spark plug/camcover bolt gaskets.
2.. I had LTFT issues for a long time, and I tried throwing every part I could at it...except a TPS, because it tested well on the bench and it is super expensive. Guess what solved it? A new TPS. To this day there is absolutely no way to have known that was the problem until I bit the bullet and replaced it.
2. You seem to have a vacuum leak. Has anyone checked it for vacuum? Had two different shops check my car, on the neighborhood go-to guy and a Jag specialist. A third jag specialist found it in 5 minutes....it was the intake manifold gasket. If that happens, make sure they change the ******* hose.
3. I have had lazy oxygen sensors. They would switch, but not quick enough and I had issues until I replaced them.
4. Could have a faulty ECU.

Your car is reluctant to throw codes. I'd go after new coils first.



 
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Old 03-04-2019, 06:46 AM
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Thanks for the suggestions, I will have a look through them. So far I tried swapping the coils with used spares but to no avail. The coils were replaced in my ownership but several years ago. Charging the battery massively improved and seems to have nearly cured the misfire, only noticeable occasionally now. Plugs a bit dark as you would expect, I am wondering does it seem rich because it is missing, or is it missing because it is rich? The scanner showed sensible coolant temperature and I had previously replaced the sensor to make sure.
I had a look at the fuel pressure regulator vacuum hose over the weekend - no petrol came out when I pulled it off so may be ok. A fuel pressure test kit with set of fittings which I hope would include one to fit my car looks to cost around 50 and the regulator costs 60, so not sure which to buy, probably the tester as useful in the future. Does anyone know the correct description for the fuel rail fittings?
I have a boost gauge which shows vacuum on idle, but not sure if it is reading any less than it used to or not. Idle speed ok - increases slightly when I removed the vacuum hose and let some air in.
I guess I would check the carbon canister valve by removing the hose to the inlet and plugging it.
The oxygen sensors are only a few years old.
I was wondering if anything would need to be reconfigured as I had the battery off for a while.

Thanks,

John
 
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Old 03-04-2019, 02:46 PM
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Poorly charged batteries do cause unexpected problems. My door locks start acting wonky when the battery is about six months out. It wouldn't surprise me if a long lay up created a situation where the car had to readapt to some thing. The throttle body is known to gum up and cause issues when the car has not been driven. You'll want to take a look at that. Your butterfly may not be closing all the way, or perhaps that TPS is flaking out.

As long as both of your banks are reporting the same LTFT, your 02 sensors are probably fine. I doubt they would both go equally bad simultaneously.

Drive it and see what happens. If it gets better, its adaptation. If not, take a closer look at that thottle body. At time I was successful in unplugging and replugging the TPS while the engine was running. You'll notice an immediate difference when plugging the TPS back in.
 
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Old 03-22-2019, 07:44 AM
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Does anyone know how to describe the fittings on the fuel rail (inlet) so I can make sure the correct adaptors are in any kit I buy?
 
 
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