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

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

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.

Also, a coolant thermostat that is stuck open can prevent the engine from reaching full operating temperature, leading to the same fuel enrichment from the ECM.

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

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.

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

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.

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?

What would be the best way to test for vacuum leaks? I have heard of squirting easy start around potential leak sites and listening for a change in revs. Also I got the Torque app diagnostic and the MAF output did not seem to be changing very much, I understand they are no longer produced. I just missed out on one on ebay, anyone got one to sell?

The fuel rail that the regulator traps the fuel in will be at 43 psi at idle and go up to 47 at throttle up with the vacuum hose attached in the back of the FPR

When the FPR fails it usually internally bypasses dropping the fuel pressure in the rail

The fittings for the line test are hard to come by and the time can be spent on a vacuum leak

First vacuum line is the one on top on the back side of the FPG top the intake , you can cap this one off at the intake but your throttle up revs will be compromised

https://cimg8.ibsrv.net/gimg/www.jag...cbe7ba7628.jpg


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There's another way to check for leaks.

Take some plastic wrap and insert it so it creates a barrier where the air filter meets the MAF. (Anything upstream from the MAF is irrelevant)

Light a cigar.

Pull the hose off of the camcover on the engine. (Did I mention this should be done when the engine is cold?) Take a drag and blow the smoke into the hose, then quickly block off the hose with your thumb. Then do that again, until you've pressurized the hose with your smokey breath.

Can you see any smoke coming out of your engine bay? That's your leak.

This will only help find leaks on the intake side of your system.

Something happened to my writings on my earlier post and will have to edit it later in the day

I suffered from rich running for quite some time and replaced just about everything over the years.

Last month we changed the water rail gaskets and in doing so, had to remove the intake manifold. The gasket had stains where unmetered air had been entering the engine so the system compensated as best it could by enriching the mixture trying and balance the extra air intake.

After fitting new a intake manifold gasket, my mileage improved by about 10% and she doesn't run rich anymore.

Larry

Cracked exhaust manifolds or leaks in the downpipe can cause rich running. Ask me how I know.......!
Rather counterintuitively, Oxygen is drawn IN through the crack between exhaust pulses, and confuses the oxygen sensors into thinking the mixture is too weak, so the ECU enriches it. No codes, as there is no fault as such

Thanks Gents,
It might be worth a try with the smoke, as looks not such a quick job to pull the manifold on an XJR with the extra plumbing. I have previously had cracked exhaust manifolds, which I have replaced some years ago with fabricated replacements. I can't hear or feel any exhaust leaks upstream of the oxygen sensors.

Regards,

John

Hi John,

Did you ever change your coolant thermostat and check the signal from your engine coolant temperature sensor (ECTS)? As I mentioned before, these are very common causes of rich running and will rarely trigger diagnostic trouble codes. They're both inexpensive parts that are relatively easy to change so I consider them routine maintenance items. The insulation on the wiring at the ECTS and its electrical connector are also prone to becoming brittle and cracking, so have a good look.

Cheers,

Don

Thanks Guys,

To follow up.
I had suspected the MAF see other posts on this topic, but I think changing the fuel pressure regulator has solved the problem, even though the old one wasn't obviously leaking from the vacuum port. The plugs now look to be turning gray, despite being heavily fouled from rich running. Some new plugs and then book an MOT and see if I am back on the road.

Regards,

John