Hi Folks,

Hoping someone has run across this before on an X350. My 2006 XJR started showing "Low Coolant Level" in the dash with the red light several months ago. It only happens when it's been up to full running temperature for about 10 minutes (started more like 1/2 hr, but has shortened). At first startup when the engine is cold (or cool), it goes away. I replaced the coolant level sensor on the bottom of the expansion tank last weekend when I replaced the thermostat, but it made no difference. The coolant level is consistent at just over (1/16") the max - no leaks, temp sensor stays consistent at midpoint. I've checked the grounds to the vehicle body (had these rot through twice in the past - don't you love galvanic corrosion 101) - not those. Have wiggled the wires from the connector down to the main harness and banged on the expansion tank while I had someone watching the warning light to see if there was some sort of a corroded wire or stuck float. No change.

So, I've reached the point where I'm out of good ideas. Either something is sticking or the resistance is rising as the heat rises to the point where the car thinks the sensor has been tripped. I'm still thinking it's somewhere in the wiring, but can't find it.

Any advice or suggestions would be extremely welcome.

Thanks very much!!

Chris.

 

Looks like it is just a float switch that when coolant level is low it grounds out IP5-5 plug to instrument cluster.
If you get it up to temp and the low coolant level warning appears, disconnect the coolant level switch to see if the message ceases. (not sure if it would immediately disappear or whether you need to turn car off and on again to reset warning). If it does, you need to check the new switch isn't faulty or your actual coolant level isn't dipping sufficiently somehow.
If the message doesn't disappear, then you might simply have some intermittent lower resistance appearing to ground on the Orange/Green striped wire to the switch which you should be able to get a reading on with a multi-meter (car off, coolant level switch disconnected).
Check resistance to earth at CP1 pin 6 (10 way Black connector that is RHS of radiator), Disconnect CP1 and then continue to measure resistance upstream.
That then goes back to IP8 pin7 (14 way Grey connector - cabin RHS A post), another disconnection point to help you isolate which portion of the loom might be causing you issues.
Beyond IP8 plug is just the remaining loom up to the instrument cluster.

Good luck.

 

Thanks ever so much Mark. I'll go and try all of this and report back.

Chris.

 

Quick update: When I disconnect the sensor to force open circuit, the "Low Coolant Level" message and light comes on. Connect it back up and it goes away, after restarting the car.
Just for the heck of it, I added a bit more coolant. The level was 1mm above the max. level before I added, it is now approx. 6mm above the max. level (daren't go higher than that). I've been out for another drive and after about 25 minutes, the warning has not come back on. Wonder if something on the float magnet has moved just slightly. Guess we'll find out - I've got to drive it 1,500 miles in the next 4 days.
Will let you know. Thanks for all the help!!

 

Hi Mark,

So, I'm back from my long trip and with the level approx.10mm over the max., the warning light does not come on. So, I have to believe it's extremely unlikely to be an electrical issue. I'm now 99% sure I know what's happened. Back in September, I had to fix a coolant leak under the supercharger (fun job getting the front bolt on the intake off - not......). To do that, almost all of the coolant pipes and expansion tank had to come off. I put the expansion tank on top of my chest freezer in the garage. At some point, I'll bet one of my kids dropped it on the floor as I found it with a crack in the plastic on the sensor. I wonder if when it was dropped, it upset the position of the magnet that triggers the open circuit, that coincides with when the problem started. I guess a new expansion tank will tell me that - a job for another day before it gets warm.

Anyway, that's no longer the biggest problem. You may ask me to start a new thread to describe this one as it's not related to the coolant system. My XJR is officially poorly.

It's a fuel pressure problem - I think.

About 1,000 miles ago, I started to feel a hesitation and then felt the car misfiring. I had my OBDII reader with me, so I was able to get some data to help diagnose the problem. Here's what I know:

- No issues at idle. Smooth as you like. Fuel pressure fluctuating from 47 to 72psi. Short term and long term trims all normal in the single digits.
- Fine under acceleration, even hard acceleration (75% throttle) for as long as I can hold it before I have to slow down. Fuel pressure remains good.
- Under steady load, approx. 60mph, fuel pressure slowly falls to below 20psi at which point the misfiring starts. Take my foot off the throttle, pressure rises, misfire goes away. Put my foot back down, power comes back. Short term trim goes heavily negative, consistent on both banks (-30%)
- If I try steady load, 2,000rpm in park, it won't do the same thing - pressure stays normal, no misfires.
- OBDII codes - random misfire on all 8 cylinders. P0193 - Fuel Rail Pressure Sensor High Input, P0088 - Fuel Rail/System Pressure Too High, P0087 - Fuel Rail/System Pressure Too Low, P0171 - System Too Lean (Bank 1), P0174 - System Too Lean (Bank 2),

With all of this, I really hoped it was simple. First I tried Sea Foam in the last 2 tanks of gas. I also changed the fuel filter today. Neither of these have made any difference. If the car will still accelerate hard for a sustained period, it's unlikely to be the main fuel pumps in the tank. Could it be a sensor fault?

Once again, I'm hoping that the experts out there can help point this amateur in the right direction. Very grateful for any suggestions as to what to try next.

Thanks very much,

Chris.

 

Possible that your fuel pressure sensor is on its way out.
It also has a small air tube that comes off it which you might want to check hasn't been broken off at one end or the other since you have dome some intake work recently.
Start there and rule out malfunction in FPS air bleed or sensor,

The SC variants have two fuel pumps, calling the second one in on high throttle demand. (mine is NA model so don't have this feature)
As your issue is on low demand, my last thought is you might have a main fuel pump or filter issue.

Perhaps someone else with a SC model can chime in here with their experience.

 

Thanks Mark,

Kind of you to get back to me so quickly.

That'll be a fun job, it's going to be about 12 hours for me. On the SC 4.2, pretty sure I'm going to have to pull the supercharger back out and take off the intake to get to the FPS. If I'm going that deep, I may as well just replace it unless I find something obvious. I really hoped I'd never have to do that again. At least I cut a v-notch in the back of the SC where it bolts to the air intake at the back, so that won't take three hours to undo one M6 bolt.

Do you think it's worth replacing all the injectors while I'm in there as mine now has 205,000 miles on it and they've never been changed?

Thanks,

Chris.

 

Replace the cracked expansion tank by all means, and A.S.A.P. No, don't replace anything on the Jag unless it's defective as those Jags are finicky as it is, and you would only be putting money into the parts dealers' pockets.

Regarding the coolant level problem, I would dare bet that there is an air pocket somewhere in the coolant system.

 

Thanks again for the advice! If I have to pull the supercharger and intake manifolds out to replace the fuel pressure sensor, the expansion tank has to come out anyway, so I'll replace it.

I looked further into the fuel pump. Apparently they changed from twin pumps in mid-2005 to just a single one, even for the XJR. I wonder if that changes the diagnosis? Probably not. As the pump seems to deliver under high load, it's probably OK. The pressure is definitely falling causing the misfires and lack of power. It's just a question of what's causing the pressure to fall, starving the engine.

Thanks again.

 

I don't see how you can keep a cracked expansion tank. Or kids

Expansion tanks should be placed in a bank vault whilst not in use. In my case, that pip breaking off was enough to replace the expansion tank, even though I could have potentially DIY'd.

Thanks for the amusing "admission".

 

My 1/8" hose barb on the expansion was a constant source o a weep, so I removed he expansion tank, tapped the hole, threaded in a heavier hose barb with two part epoxy, made a two pass wire clamp and problem solved. I know people replacing the expansions with "uprated" ones only to have leaks reappear at that hose barb

Now the deteriorating hose barbs on the radiator tanks main hoses themselves will really bite you just when you have your foot into the throttle, mine stranded me in the middle of I-80 in Wyoming . That means a new radiator ASAP because once those main hose connections to the radiator tanks begin to turn to cheese, you will never get a hose to stay in place.

 

I should have known better......never put anything between late model teenagers and food.

Fortunately, it was the sensor that cracked, not the tank. So no leaks, just a dodgy level reading.

 

HI Chris
Your fuel pressure problem could be due to one of the plastic
valves on the fuel pipework inside the tank eg NRV (US check valve)
or the pump itself.
I doubt that the pipework etc is available separately.

 

Thanks for the suggestion. Now I have to decide which to try first, Fuel pump and pipework or Fuel Pressure Sensor? The first is easier to get to under the back seat, but will cost a lot more.

Crazy question, but is there anything like a fuel pressure regulator in front of the fuel rails that might be failing and causing the pressure fluctuations? Not sure how this part of the system works.

Thanks again for everyone's suggestions.

Chris.

 

Hi Chris,
Sorry for the late reply....been a busy day at work today.
You're right....the later SC production is indeed single fuel pump.
I recalled the dual pumps from prior schematic reviews of earlier production models.....sorry for the misdirect.

As you say, the pump can deliver high flow when required. The only nagging question would be if the lower flow rate was somehow being inhibited by something like a fuel filter or valve, but when the high rate flow was called for it is still managing to push enough through.
Not likely an ECM issue or a wiring loom issue from ECM to fuel pump module, as either would likely produce a nogo or intermittent nogo scenario.
That leaves the fuel pressure sensor and the fuel pump module (supplies the pump).
Again, the fuel pump module is receiving a variable PWM command signal from the ECM, to tell it how much voltage to send to the pump to regulate the output fuel pressure. The fuel pump module is less likely to manufacture a decaying output by itself if the PWM signal into it is remaining somewhat constant.
In my mind the ECM is likely to be transmitting a decaying PWM signal to the fuel pump module, which in turn is faithfully responding to that command.
The ECM will be relying upon the feedback it gets from the fuel pressure sensor etc. to tell it how hard to drive the pump.

All well and good me giving you this advice from afar, not having to dig into the task myself. I am mindful that
I would still like to have any other members add their "two cents worth" in before you embark on a 12 hour excavation.

Monitoring the output voltage to the pump motor while the car is going through its fault behavior could provide a clue.
If your code reader can also tell you live fuel pressure, and you see that pressure declining, you should see the fuel pump module output try to increase its output to the pump in response to address the imbalance.

 

Hi Mark,

Absolutely no need to apologize. I'm extremely grateful for anything you can offer, whenever you can offer it.

What you're suggesting actually makes an awful lot of sense. It makes me add an extra piece of information that I thought wasn't pertinent - when I was driving 400 miles or so on Friday, the car was having the problem initially for the first 100 miles. Then, I stopped for a few minutes and cleared all of the system codes with my OBDII scanner. For the next two drive cycles (about 250 miles), the car ran fine with no obvious issues. Then, after the third stop, the problem immediately re-appeared and persisted all the way home.

The good news is that, although my scanner can't read the voltage to the pump, my friend's can, plus it can run diagnostic testing on it through the fuel pump module. He's coming over tomorrow morning and we're going to see what we find. Also, just to appease my curiosity, I lifted the rear seat and popped out the two rubber discs just to confirm there's definitely only one pump in there. The RHS (driver's side for you, and where I'm originally from) has hoses and wiring coming through the screw cap, the LHS just has a plain cap.

Once we've done some diagnostics tomorrow, I'll report back.

Thanks again,

Chris.

 

The fuel pressure sensor has engine vacuum on one side of the diaphragm and fuel pressure on the other. Missing vacuum will lower the sensor's pressure reading because one side of the diaphragm is atmospheric instead of vacuum, which tells the ECU to increase fuel flow (because it thinks it's too low) and that reduction leads to improper combustion, and probably rich readings at the O2 sensors. That rich reading will have the ECU reduce fuel flow. There would actually be nothing wrong with the flow, the sensor, or anything else, just an incorrect reading at the sensor because of the lack of expected vacuum on one side.

Fix the vacuum lines first.

 

Hello, Chris.

Don't know how many miles are on your car, but @ 139k, after a high speed burst in a faraway place, my 04 XJR had a random misfire on all 8 cylinders. But, it would always go into limp home mode after a full throttle run up to the end of 2nd or 3rd gear. It was the cats. Apparently that first high speed adventure cooked them. Could also smell them after a full throttle burst. Just a thought.

 

Thanks for all of the great suggestions.

Some good news. After a dig around under the hood, I see that I CAN get to the Fuel Pressure Sensor without pulling the SC off. It's just behind it, not under it as the diagrams showed. So, for the sake of that, I've ordered one that'll be here tomorrow.

We also ran diagnostic testing on the fuel pump with my friend's scanner. Didn't seem to be any issues at multiple loading conditions. Interesting that after clearing all the codes again, at idle the fuel pressure was randomly jumping between 30psi and 70 psi with nothing in-between - same problems under load as before. Before clearing the codes, it held 50psi constant at idle. There's definitely something going on in the feedback loop so maybe the sensor is a good starting place. At least it's cheap.

Thanks,

Chris.

 

I'm not sure what I said earlier about the vacuum line applies to your car... I did not even see that it was a supercharged engine, because read is hard.