2000 XJ8 gets Restricted Performance indicator when accelerating... generally lights up when entering highway.

38,000 miles, lives in garage.

A few years ago, this would light up extremely rarely, now is pretty much any hard acceleration. Codes are P0171 & P0174. Have read all the threads I can find on this. Engine runs smoothly at idle and under all conditions. Acceleration *might* be sluggish below 40mph.

Cleaned MAF sensor thoroughly with spray, checked for vacuum leaks to the best of my ability. Have a few questions before I give up and bring to repair shop.

- Can the MAF sensor fail without a specific MAF error code?
- Does the brake cleaner spray vacuum test really work (I'm limited to listening for diagnostics on this one)
- If I buy a fuel pressure gauge, what pressure(s) would be significant in diagnosis?
- Is there something else I should be looking at/trying?

Sorry if this has been answered elsewhere, and thanks for any insight you can provide.

 

Most likely a leak in the air filter to throttle body duct. Did you check the bottom of the pleated section of the duct for cracks? That duct sits directly above the exhaust manifold and is exposed to far more heat than it should be. Common problem area not seen unless you bend the duct to open the splits like the engine might do under acceleration.

Spray cleaner is not flammable and reduces the rpm when introduced into a leak opening. A propane torch, unlit, is also used. Being flammable the rpm increases.

Yes, the MAFS can cause other codes. Other than complete failure it doesn't set any direct codes. Replacing the MAFS probably should be the last thing to try.

The fuel rail pressure should be 41 psi plus/minus a couple, mostly meter error.

Also check the part load breather tube (left side) valve cover. You should be able to pass a 5/32nd drill bit through the opening. The opening is about an inch below the top of the fitting. Be careful, easy to break 10 year old plastic!

Have you cleaned the throttle body?

 

I REALLY wanted this to be the problem, but I have taken the whole assembly off, examined it closely while flexing, and also while shining a light from inside in a dark room. It looks perfect.

I will probably try this at some point just for completeness.

Sounds good.

Thank you... this should be easy to check.

I saw reference to this elsewhere, but didn't know where to go with it. If I'm looking at the left/driver's side valve cover, is the breather tube on the left (front), middle, or right (rear) part of the valve cover? Any description you can provide to aim me in the right direction would be great! I think I know my way around an engine relatively well, but can't seem to get traction on this one for some reason.

No. I can try to find a thread on it if you think it's worth pursuing with these symptoms/codes. Would a injector cleaning have any potential effect?

Thanks so much for taking the time on this with me!

 

The part load breather tube is at the front top of the left (sitting in the driver's seat) valve cover. Half inch tube attached to a fitting in the valve cover disappearing under the intake manifold.

There is an excellent pictorial on cleaning the throttle body on this forum somewhere. If you cannot find it here try here: http://www.maxperformancecars.com/in...ora&Itemid=132

Under 'Articles' at the top. 'Texas Dan' was supposed to add it to this forum but I cannot find it at the moment.

Injector cleaning is probably not the problem.

 

Thanks again! I had this tube off when I was checking the vacuum hoses, but didn't think to look inside the fitting. I found 3/32nds was about right for this hole. Seemed completely clear and I got no gunk on the drill bit. It's possible I may have knocked some stuff off back into the valve cover area, but it wasn't obvious to me.

Will do the throttle body cleaning and check the fuel pressure and will report back tomorrow.

 

Sounds like a leak between the MAF sensor and the throttle body. See post linked below for the source of my P0171 and P0174 problem.

https://www.jaguarforums.com/forum/s...187#post225187

 

I have a 01 XJ and had the identical problem. RP would light up every morning on my way to work. When I hit 65 or 70 on the interstate. I cleaned the MAF to the point it looked spotless and still had the issue. After trying every recomendation, I took it to the shop and they basically cleaned the MAF again and the lights went off. It has been over a month with no lights and the cat is puring.

On another note a friend had similar issues with a 98 vdp and he replaced the rubber seal on the throttle body for 20.00. Everything is fine.

 

Dan's throttle body, and part load breather instructional is in the FAQ section of the XK8/R forums.

 

Thanks very much! I think I've been able to rule out the MAF sensor seal and definitely the sound attenuator.

 

I used half a can of cleaner on the MAF sensor, with no reduction in the problem. Maybe I need to use the other half!

The throttle body seal sounds interesting. When I pulled the TB seal off the duct, I notice a line of fine dust on the duct side of the seal, as if the dust was being pulled in part way and then being stopped by the seal. I can't get a new one right away but maybe I'll try taping things up on the outside to see if it makes a difference.

Another thing I noticed (full load breather tube in particular, where it fits to the valve cover) when I took things apart, was that some of the o-rings didn't look dark and shiny on the outside circumference. In other words, they didn't look like they had been sitting tight against the inside of the female fitting. Since the o-rings were still supple and they felt like they were grabbing when I put the fitting together, I assumed they were good and that I may have just gotten them dirty when disassembling. Do these o-rings ever turn out to be the problem?

Off to get the throttle body cleaner!

Tim

 

Thanks! I'll take a look at it.

 

Ok. Cleaned the throttle body. It was actually clean as a whistle to start with, other than a black ring around the place where the throttle plate sits closed. But I took care of it since I was there. One thing I should point out to others who may follow in this path is that the MPC directions did not indicate that the ignition needed to be turned on for the plate to open, even with the gas pedal floored.

I taped up the duct to throttle body seal, probably 100% effective on the duct side and maybe 70-85% effective on the TB side, and went for a test drive. Out of about 14 accelerations that I would have expected an RP light, only two caused the light, and those were in the first 20 minutes of a 45 minute test drive. (Temp gauge showed normal operating temp, but the engine hadn't been fully soaked, so to speak.)

I'm thinking the duct to TB seal may be the culprit. I still need to test the fuel pressure and do a carb cleaner spray test, but I'm a little leery of spraying that stuff around all that plastic in the engine compartment. Is it my imagination, or is the intake manifold plastic?

 

Did the final two tests.

The solvent didn't faze the plastic, nor did it affect the idle speed.

Fuel pressure read out at 38 psi while idling, and down to 34 psi when running at about 4k rpm, no load. Bumped up to 41 psi when the throttle was released. I wasn't completely confident in the accuracy of the meter, so I tested it on a couple tires and found it to actually read about 4 psi high. If the gauge is compatible with both air and fuel, then my fuel readings would really be about 34 and 30. I think I'm going to replace the fuel filter and see what happens.

Where's the fuel filter so I don't have to crawl around too much? Thanks!

 

Replaced fuel filter. Idle fuel pressure about same, 4k rpm pressure maybe a little higher. Test drive yielded same results as last time. A couple of RP warnings early on, then nothing.

Weird thing. First RP warning was when I was cruising at 60, maybe uphill. Second warning was after highway entry acceleration. I alway assumed there was a lag in the warning, but maybe it's having the problem when I let up on the gas at the end of the acceleration? I'm putting this stuff out here in case there's a clue buried in it.

 

I replaced the air pipe to throttle body seal and find that the warning light seems to happen less, but it still came on once during a half hour test drive.

It doesn't seem to be happening during acceleration anymore. When it came on today, the car was going up a slight incline cruising at 75 mph. Any further thoughts beyond a vacuum leak?

Does the Restricted Performance warning come on when the condition is present, or is it delayed at all?

Thanks.

 

Did you check your throttle body harness connector, making sure it is not loose at all and all pins are in good condition (also not loose?) I still think air leak is most likely. Is your air filter housing sealed, no broken tabs to cause a opening post-filter? You could always swap out a MAF sensor from another car, just to be sure. I'm sure with the wide range of early 4.0 xk8 & xj8 owners around, you may be able to 'borrow' one to use as diagnostic.

I heard that the heater wire in the MAF must also be cleaned, not just the sensor itself. You can't get to it w/o removal of the unit from the air pipe.

I'm about out of ideas.

 

No I did not. Definitely will look tomorrow.
The engine compartment is very pristine. Don't even have brittle hose connecters. The housing is 100% intact, and I've been over the air pipe several times.

I've always wondered if a vacuum leak could cause performance problems, why wouldn't a filter that breathed too easily cause the same problem?

I took the MAF sensor out when I cleaned it. Unless the sensor itself needs to be disassembled, I think I did a pretty thorough job. I wish I could get a MAF sensor to swap out, but being in South Dakota has it's pros (cruise at 75 legally) and cons (not many other owners).

I very much appreciate you sticking with me this far. If it ever gets resolved, I'll be sure to report back. Since there aren't any new ideas other than the harness, I'll try that and if it doesn't fix things will probably have to dive all the way back into all the vacuum plumbing. May replace the o-rings on the breather tubes.

Still interested in hearing if the RP indicator is real time or delayed due to some timing cycle.

Also thinking there's some temperature factor involved because I get the light without fail on one stretch of road about 12 minutes out on the test drive, but never again as I go over it multiple times. Maybe I need to drive around for 20 minutes then try it for the first time.

 

Ok, looks like the problem is solved. (Keeping fingers crossed.) I put off working on this because I thought I might take the opportunity to examine the vacuum hoses more closely when I did a secondary timing chain tensioner replacement.

I did not find anything wrong with the hoses, but I did remove the MAF sensor and clean it again. The first time around last year, I sprayed the amber headed component in the first picture below to within an inch of its life.

When I redid the cleaning, I found these little components shown in the second picture below. They were buried a little deeper in the sensor housing and I completely missed them the first time. They were a little fuzzy, and spraying them with the MAF sensor cleaner removed the dirt easily.

When I went for a test drive today, there was no hint of an RP code. I'm hoping that this settles things. It was only 5 degrees F out today, though, so I won't declare complete victory until the RP light stays away in more reasonable outside temperatures.

Edit: h20boy, I guess this would be the heater wire.