Hi there, I am looking for some input from other technicians on the forum, or members that have dealt with this issue on an AJ27 equipped XJ8 or XK8, both naturally aspirated. I have had 3 vehicles in the past year with the P0172, P0175 codes, and the fix is proving to be elusive.

The current vehicle, a 2000 XK8, presented with restricted performance and the codes I mentioned logged. It also had the P1111 so I know its been at least a few days since anyone cleared codes. From the Freeze Frame Data, the MAF, ECT, IAT, TPS are all at logical reasonable values for the conditions (it set at idle, in gear).

I have also checked fuel pressure, 47psi with vacuum unplugged (no fuel present in hose either).

On road testing, the LTFTs are around -20% at idle, improving slightly as engine speed/load increases (but still subtracting significant amounts of fuel). The Oxygen Sensor Equivalence Ratio is at 0.8 lambda, unless I accelerate hard enough to put the vehicle in open loop, then it moves to 1.0 lambda. The downstream sensor voltages trend rich and are reluctant to toggle. My feeling is they are reporting the condition correctly as it is very unlikely for all 4 Oxygen sensors to fail at once.

I have even substituted a MAF sensor from a known good car to eliminate that.

I have tested the car with the purge blocked with no difference (eliminating the EVAP system as a source of extra fuel).

So it comes down to this question. What is providing the extra fuel? The injectors? Hard to imagine them getting bigger, and I don't have pressure leakdown that would point to a leaky injector. (Even then, it is hard for me to imagine a leaky injector providing 20% extra fuel across both banks).

Could it be the ECU? Is the driver stage some how turning the injector on for too long? (I currently don't have a similar car in the shop to compare).

Your thoughts?

 

Wow, excellent groundwork my friend, not often we get such good information to work with.
You correctly noted the action of the O2 sensors, the trend and lack of switching of the downstream sensors is the give-away that it really is running rich, and not just a rich indication. I'm good with the open line fuel pressure, but I'd like to know the pressure when connected. It should be about 39PSI then. I know you substituted the MAF, but I'm still suspicious if it.
First thing I want to know is what the ECT is actually doing. I would check it starting cold and make sure it reads the prevailing ambient temp. Then I would watch it as the engine warms up, hopefully it goes up [nice and linear] to the usual 90-100 deg Celsius and levels off. Then take a drive and see that it does not fall while you are moving. If it does, change the thermostat, temp that falls will make the engine run rich. Now back to the MAF. Check the MAF PID at the 4 conditions in the attached image. If it varies more than 3-4 g/sec at any condition, it probably has to go. Obviously the checks in Drive need to be carried out in a safe area with the brakes held tight, not in a garage.
Oh, I can't forget to mention to substitute the air filter, too; that can throw the MAF output out of wack.

Good luck!

 

I had a similar problem on an XK8 that turned out to be the connection at the MAF. I had swapped the MAF to no avail as you did. After cleaning and tightening the connections (I slightly bent the pins on the female harness connector to make a tighter connection) it solved the problem. If the connection is not good the ECU defaults to full rich. Had the same thing on an XJ8 and fixed it the same way.

 

Thanks for your response, more details follow. BTW, I have also flashed this car with the latest firmware this morning.

Fuel pressure is around 39psi when vac is connected.

The ECT operates correctly, I have monitored that.

Unfortunately, being a new member, my privileges do not yet allow me to view images on the forum. Can you give me the specs in text?

FWIW, the freeze frame data gave the MAF as 0.80 lb/min which translates to 6.047 g/s, which, from my experience, is in the normal range for idle in drive (647rpm, 0mph).

The WOT just before the 1-2 shift is around 240 g/s which is also in the ballpark.

Thanks again.

 

At this point, I don't suspect the MAF input to the ECM based on the data I am getting from the MAF PID, both in freeze frame and in the datastream.

However, I will keep your tip in mind.

I'm off to give the vehicle another drivecycle right now!

 

I was thinking that your TB was under the service action S514 but that only covered 1997 – 1999 cars, however, I would not rule out a TB problem. Before I would go into R&R the TB I would first add a fuel additive BG 44K and see what that does. Along with that I would take a little time to clean the TB plugs. I know that you would normally get a P0121 code for that but I have seen stranger things. Give it a try and see what tales place. I hope this helps!

 

How does that work? Doesn't the MAF just account for how much air it actually flows?

 

Yes, but there is more to it than that. It measures by sampling the air stream, not measuring ALL the air. The MAF takes a sample from the center of the air stream, and from that the system CALCULATES total air flow. These are hot wire sensors, the sensor measures how much current it takes to keep the wire at a given temp. Air flow in, wire cools down, more current to keep it hot = more total air flow.

A guy from Denso explained it this way. Stick your face in your AC vent, point it right at your nose [the air sample] see how a tiny deflection of the vent vanes make your cheek bone the cool spot instead of your nose. Suddenly your nose [the sample] isn't the cool spot anymore. He said anything that makes the air flow a little different than the way it is with the expected air filter, air box etc., will likely affect the air measurement. When this sample is multiplied to calculate the total, any error becomes almost exponential. That is why all the tech info for the rich codes lists restricted air filter at the top of the list of possible causes for P0172 and P0175.

When he put it that way, it began to start making sense.

I know it seems almost counter-intuitive, I originally felt that way too, but that's the facts.

Edit: I ran across another description, it's attached.

Cheers,

 

How nice .. preemptive attachments

What the Denso guy said is all true.

But, that would have to be one severely buggered air filter. Maybe that's why the installation of modified air intake systems is so chancy.

Instinct wants to say that one of the functions of having the air filter is to reduce turbulence in the path to the MAF.

It would be interesting to observe fuel trims upon changing to a new filter.

BTW, one way to big power gains on turbo Supras was to change to a bigger Lexus airhorn, bypass the extra air *around* the MAF, and install fatter injectors. The ECU never knew the difference.

 

Air will follow an ideal path thru the airbox & filter, and on these spots the filter will become clogged quicker. Then the air will flow more from other (cleaner) spots, and that will alter the flow over the maf of course, and could cause the rich faults.

 

I was ******* down the same rich codes earlier and did some testing and measurement with old/new air filter and old/new MAS sensor. I learned that the filter can make a real difference in readings and even a "clean" looking one can push you over the edge if your MAFS is not 100%.

 

I substituted the TB yesterday morning with a known good unit and it fixed the trims. I will post with an update when I install the new throttle body on Monday morning. I will also be performing bulletin 303-58 for the mismatched connectors.

 

Thanks for all the input. Gus got me on the right track along with Stanley over on the IATN forums. I can't explain why, but replacing the throttle body did the trick.

I took MAF readings to compare with XJRGUY's chart.
Park-idle - 5.1g/s
Park-2500 - 16.2
Drive-idle - 6.3
36.0

I had earlier on, in my diagnostic efforts, removed and cleaned the connectors on the throttle body and had noted the connector material mismatch that often causes a P0121. I also cleaned the MAF connection.

Interestingly, on my road test after replacing the throttle body, the car stumbled at idle and I watched the trims go negative. I looked at the MAF data and it was high at 13 g/, so I jumped out of the car and wiggled the MAF connector. The car instantly smoothed out and the MAF reading returned to normal.

So, in closing, I replaced the throttle body with a new one from Jaguar, soldered in the new gold plated connectors for the TPS, and soldered the connectors at the MAF sensor.

After three complete drive cycles, with flawless trim and MAF numbers, and smooth running, I returned the car to the customer.

Thanks to all of you for your time!

 

Wow--What a fantastic thread. I learned alot in these posts and realize how valuable this forum is to both owners and folks who work on these cars. I salute you all!

 

Kudos to you! Back on the road again!

 

Great documentation jagman, thanks for sticking with us all the way to the succesful conclusion post. Don't be a stranger either, you may have some advice to share even though you don't have a jaguar in your shop to work on at the moment

We should send you a jaguarforums.com window sticker, proudly display it on your businesses door.

 

2000 XKR*****Kept Low idle and stalls at stops after HIGHWAY use after winter storage. I was suspicious of fuel pump knowing they are notorious for going bad after winter storage. I had P0175(system Rich bank 2), p0506(RPM lower then expected). I limped it back to my garage and decided to unplug the MAF while the engine was running bad with very low RPM. As soon as I unplugged the car smoothed out and ran great. I'm thinking that I put the car in open loop but it would of ran the same if I had a fuel pump issue. Am I replacing the MAF or did I cover the real problem and could possibly be a TB. I also have read that this car has 2 fuel pumps. Please advise

 

cleaning the MAF more the once can often have a positive effect. One cleaning, two cleanings, and I once still had the rich running codes on both banks. Despite the sensor looking OK, I cleaned it a 3rd time. Bingo.

The long term trim numbers settled down to near perfect, and not a problem since.

Z.