So I got the P0174 yesterday and checked for obvious air leaks today, using the tissue paper method. No luck.

Pulled the MAF and cleaned it. While I was waiting for it to dry, I poked around some more and saw this.

It's towards the back of the bay. No idea what it does. Plugged it back in and cleared the codes. Nothing so far.

Any idea what this thing is/does?

 

Vacuum pipe

 

Could have been my problem?

 

Yes probably your problem it looks like the vacum pipe going to fuel regulator.
more of an issue is why its come off? the elbows are prone to splitting so may need to be replaced.
check it again in a week.

 

You need to fix it in place with a clamp or cable tie, it also sees manifold pressure so 13psi or so.

 

I had po171 and po174. lean on both banks. Usually a intake leak. Replaced all intake O rings. replaced the crankcase vent valve. Not the problem. Check for leaks. (Smoke the car) none! Add a o ring to the end of oil dipstick. A lot of times the seal is bad. Also check Oil cap

After all that I still pulled the codes. Replaced MAF. Reset the ECU battery off 30 mins. Been about 6 months since no more codes. I guess the faulty MAF was giving incorrect readings because there was no un metered air entering the intake.

During the time it had these codes, I got 22-25 mpg in the city. So not a fuel problem, ruled that out.

 

Thanks guys.

Doesn't seem to be a fuel problem as I'm getting 22mpg. I'll double check the o rings as suggested and cable tie the connection for safe measure.

 

Sorry but is the car supercharged?
if so yes there would be pressure in manifold but if not it is vacum? and wont need to be clamped.

 

Yes anindyams car is supercharged.

That hose connects the outlet of the supercharger to the reference port of the fuel pressure sensor.

On my XJR that hose was in 3 pieces with barb fittings, I also had a leak from that line. All the joins are now held on tight with cable ties, and i've got no leaks

 

Yes I'm supercharged.

Been driving it around for the weekend and it triggered once, but not since then. So far so good.

 

Did you reset the engine light with a scan tool or battery off method? The codes are stored in the ecu so sometimes when you dont do the battery off method it will still pull the code. My light came back numerous times until I reset the ecu. You can take both terminals off the battery touch them together for 30 secs to speed things up. Of course program your windows again

Good luck buddy

 

That hard reset has the advantage of clearing the (bad) learned fuel trims etc. It should relearn anyway but a hard reset zeroes them which is a better starting point after a fix.

 

Spoke to a jag tech the first thing they do after reading the codes is a hard reset then go from there

 

Thanks guys. The code keeps coming back after I first clear it using my generic obdii scanner. I'll do the hard reset next.

So just to be clear, take off both battery terminals and touch them together for 30 secs, and then reattach to the battery?

 

Yes but if it (P0174 or other fault code) keeps coming back it's not fixed.

 

what side of the engine was this vacuum plug on?? on and did you remove the engine cover? i will assume you did. i have the same code and it keeps coming back even after changing both O2 sensors .. i will also have to do the hard reset thing, that may also help. ( even i can do that! hahahahaha ) my Mechanic did not do that. He used his meter thing.

My car is a 2006 XJ8L with 156K on it and i love it.. just need to stop this pesky code. Mechanic said he found no exhaust leaks so that is not causing it. i have printed out the pictures so he can check for that plug as well. thank you.

 

If you're facing the car, it's on the right side back towards the windshield. You have to take the cover off to see it. I ziptied it and cleared the code with my obdii reader. Hasn't come back since.

Had the car in at the Indy mechanic for some other stuff and had him smoke test it. Found no other leaks.

When I plugged it back on, it stayed, but leak was still there. After ziptying it, it seemed to have fixed the issue.

 

P0174 causes (according to the book):

Engine misfire
Air intake leak between MAF Sensor and cylinder head
Fuel filter / system restriction
Fuel injector restriction
IP Sensor fault (lowfuel pressure)
Low fuel pump output
HO2 Sensor(s) (2/1, 2/2) harness wiring condition fault
EFT Sensor fault (lowfuel temperature)
MAF Sensor fault (lowintakeair flow)
Exhaust leak (before catalyst)
ECM receiving incorrect signal from one or more of the following components:
ECT Sensor, MAF Sensor, IAT Sensor, IP Sensor, EFT Sensor, TP Sensor

 

I think what Patrick the Cat is saying is the code could come from a lot of sources, but it's probably a leak somewhere.

I got lucky, but I confirmed the fix after having the smoke test done that there were no other leaks. I never did the rest, btw.

If you leave it too long, I'm told you'll burn out your catalytic converter, and that will be very expensive.

 

His is a SC engine so your regular XJ8L won't be setup the same way.

As was posted above that is the fuel pressure reference point for intake pressure/vacuum for the fuel injectors. So the fuel pressure will increase with the boost.

I am surprised you did not get more codes or some kind of drivability problems with it unhooked? Under boost your fuel pressure will be too low.
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