I've been chasing a P0171P0174 codes on my 2004 XJ8 (US model).

Long term trims are 19.5 with short term at ~0. Smoke tested everything. Had a leak in the intake hose (I broke the resonator a while ago and siliconed in a plug and this was leaking) and bought a used hose and replaced it. The code remained. Pulled the check valve from the brake booster (was looking for anything that could cause the leak). Check valve works (I could blow through it but not suck through it). Kept it disconnected and plugged it. Started the car an long term trims quickly decreased to 12 with short terms at -8. I assume if I drove it (or ran it longer than 5 minutes) the trims would all drop to close to zero.

Other issues with the brakes are with engine off I only get one application before losing boost and if the car is off for a while (1/2 a day) there is no boost (until I restart it of course). Several years ago I needed to install a spring to keep the brake pedal high enough to use the cruise control (and keep the brake lights off) because while driving the pedal would go down slightly by it self and randomly shut off the cruise control.

Before I change the booster I had a question. On the early boosters - prior to VIN:G45703 there is a brake master cylinder active booster solenoid (which mine has). What does this solenoid do? Could this be the issue? Any idea how to test this solenoid? Does the group think I am on the right track changing the booster?

Thanks

 

No one had new ones (other than $1000 ones). Several places offered rebuilding of mine. So I ordered a used one on line. If its no good I'll look into having it rebuilt.

 

It does sound like a vacuum leak, I assume you have had no issues with lose of brake fluid, The pedal falling is wierd, and the fact you have no other braking issue while moving would point to a vacuum leak. As you have the parts now, I would be tempted to try and swap out the solenoid first, if it's easy access, see how that goes, then do the booster itself if that doesn't cure it, hope you get it fixed.

 

Used parts arrive next Tuesday (the 30th). Pedal does not fall much just enough to turn on the brake lights and cancel the cruise control. Spring on the pedal two years ago solved that. Reading up on it the solenoid is suppose to add more braking based on signals from the stability control. So I think its either the solenoid not sealing or the seal on the brake booster valve (around the pedal shaft) is shot. The fact that it generates the lean code tells me its passing air. The fact that I don't have power brakes after the engine is shut also indicates its a vacuum leak. I'm going to install the used booster and then play around with the one that comes out to see if I can tell what the issue is.
Mostly I posted so others chasing the lean codes would know to check the booster. With a good check valve the smoke never gets into the booster so its a tough leak to find.

 

Dear Pragmatic
I have been looking an an 04 XJ8 to buy from a friend here in Columbus OH-IO.
He now has 115K on car but bought 1.5 years ago at 105K.
At purchase he had a VERY through check over of the car at perhaps the best indie exotics shop here in Columbus.
Yesterday I looked the inspection over at length - the codes you chase P0171 and P0174 are related to the PCV valve according to the report.
PCV valve was replace and codes went away.
This shop takes video of everything they do - and when they went in to replace PCV there was a puddle of oil sitting in the intake manifold which tech states clearly and shows on the video.
Front of engine was off to replace thermostat and amount of oil was shocking, also showed oil on spark plugs etc.

this shops code reader shows the following text on codes
P0171system too lean bank 1
P0174 system too lean bank 2
P0193 fuel rail pressure sensor circuit high input
P1571 Brake switch malfunction
C1445 brake switch circuit failure
he also had low fuel rail pressure and fuel rail temp sensor problem fuel pressure should be 58 to 72.5 psi 9his was 55)
Also - my friends other codes showed Brake pressure warning switch was bad, had high water content in brake fluid,

I don't do my own work but I recognized these codes and thought this input might help.

As I mentioned he's now driven the car 10 K since this inspection and engine work and it's been trouble free although he did have some O2 sensors recently replaced as preventive.
I've driven the car and it is really nice.

I'm taking my 05 super V8 to this shop next week as I've been told to scrap my car due to a leak from the timing chain cover - we'll see.
It's only money.

Mike D.

 

Junk yard booster fixed the P0171 and P0174 codes. Now have no DSC. Run ABS codes and there is a fault with the "active booster solenoid"

 

What is the code number? Looks like the solenoid is a 2-wire sensor, module should check for open, shorted, or normal.

 

Thanks for the diagram. The problem is the solenoid can not be removed from the booster. Its both the solenoid and the pedal force switch which is wired to the diaphragm inside the booster. So a bad one requires a new booster. I just got a rebuilt booster from RockAuto ($200 with shipping and $30 core). I'll install it tonight after work and see what I get.