Hi everyone,
I've got a 2000 XJR with 64 K miles. I've been having problems with the check engine light staying on and giving me a PO171 and PO174 code along with the occasional restricted performance message on the dash. The car runs ok for the most part. I have been using a Launch diagnostic tool to watch the short term and long term fuel trims. Mine are running right around -1 for both banks on the short term and around plus 3 or 4 for both banks on long term. It seems like these are acceptable numbers, but I'm still getting the CEL and RP.

So far, I've replaced the fuel filter, checked for vacuum leaks ( on the large plastic intake hose and the rubber gasket where it connects to the TB), cleaned the TB and replaced the purge valve under the car near the charcoal canister ( was getting a code for that but not anymore). The MAF sensor was replaced right before I got the car and the Launch tool shows it to be working correctly (5-6 cfms at idle). I have noticed very slight oil seepage at both valve covers and was wondering if this would cause the CEL to be coming back on? I'd really appreciate any info as I'm running out of ideas.

Thanks in advance,
Dave

 

I did not see that you specifically looked at the part load and full load breathers. After that, I would try and chart STFT under increasing load conditions and see if idle, part loaded or full load is the problem. Another weak link is the rubber donuts between the intercooler and the supercharger.

 

Hi Ross,
I forgot to mention that I checked the full load breather but not the part load (assuming that's the one on the driver's side that runs from the cam cover and under the SC to the TB). Looks like it might be tough to inspect so maybe I should plan on replacing it. Any ideas as to what kind of STFT numbers to expect under load? I'll also look at the rubber donuts on the SC that you mentioned.
Thanks!
Dave

 

You are looking to see trends up or down, one side vs. the other. In a perfect world, ST and LT trims would remain near zero. BUT, the world is far from perfect! From the trends,you can tell if relatively more or less excess air is happening under lower or higher vacuum or flow, giving an idea if your leak is relatively large or small and whether it is before or after the TB. Remember, the LTFT trims the fuel mixture slowly, so it can "wind up" in one direction, causing the opposite effect for the opposite condition.

The first suspect is the MAF for these faults, but you say it was changed. And the idle measurement is only a first pass as to whether it is working correctly or not. I would give it a good cleaning unless I was sure that it was replaced with a brand new unit recently