T4 engine diagnosis

[jtravers]

Excuse the vague title but this post covers quite a bit. Been having a running problem with my 2000 T4 for a while which I have been trying to pin down, and have come to a point where I could do with some advice. I have already used a lot of information on this site, so thanks to all posters.

The symptom I am seeing is persistently maxed out positive fuel trim (LT FTRM or LTFT) to which is added high positive short-term fuel trim in closed-loop. When in open-loop the lack of extra trim seems to make it almost cut-out and is undrivable until the oxygen sensor heats up. The codes the ECU throws are EFI-435 and EFI-512 (read with VOL-FCR) which only really indicate that the trims are out of range. So it seems the possibilities are: unmetered air in the combustion mix, too little fuel in the mix, or a faulty sensor reading giving the wrong ECU map.

Here's what I have tried so far:

• Suspected an air leak so got a vacuum gauge and tested values on the throttle body - perfect
• Repaced the fuel pump with a Walbro 255. The old one was whining loudly and had 120k miles so not a wasted exercise anyway. Thanks to 960kg for a great post on this one.
• Got a fuel pressure tester and performed all the tests detailed on Vadis - all values spot-on.
• Not satisfied with that result I routed a fuel line from the fuel rail into the driver's compartment and measured pressure while driving - this also transforms your V40 into a handy crematorium in the case of a crash. The fuel pressure was fine under all conditions, anyway.
• Wanted to get Jim on the forum here to do a professional cleaning job on my injectors, but unfortunately he didn't have a temporary replacement set and I don't have alternative transport. Set up a makeshift cleaning rig anyway and backflowed the injectors with solvent - probably not a patch on the proper service but should have removed large blockages. No change.
• Replaced the front oxygen sensor - no change.
• Checked all other sensor values with diagnostic tool (Uiftech U581 - highly recommended, btw). TPS, IAT, coolant temperature sensor, all good.
• Performed several tests on the MAF sensor as I presumed the fuel and air supply to be ok: Checked idle value against Vadis - right in middle of range
• Read somewhere that BHP * 0.8 can be used as a rule of thumb for maximum airflow. I got 156g/s flat out so looks good.
• Routed a cable from the MAF signal to a multimeter in the car to see values while driving - varied from 0.6 to 4.6 volts which is exactly as specified in Vadis
• Checked the exhaust upstream of the oxygen sensor for a hole causing a false lean reading - none found.
• Back to the air-leak idea, I set up a portable compressor on the air intake at the MAF to see if I could find a hole. Couldn't get any pressure to build up though and tried blowing through it myself - found all the air was coming out of the oil-filler cap and the dipstick tube. With either of these removed there was almost no resistance - the air just went straight through!
• Compression tested all cylinders, although admittedly this was a long shot give that the car runs ok in closed-loop.

So, can anyone confirm that very little air-resistance between the air-cleaner and the crankcase indicates a fault - the only route I can think of for this air is through an open PCV valve? Also, if the valve is blown, could this cause the symptoms I am seeing? Given that the crankcase pressure is fed to the intake manifold anyway, I don't see how the valve being open in the opposite direction would make a difference other than allowing boost pressure to enter the crankcase. Unless the PCV valve also provides some resistance to restrict the volume of air and this function is also blown out.

Any takers?

By the way, I now have quite a collection of gadgets amassed from this saga - if anyone needs any info on these or any of the procedures I have tried just shout:

• USB to OBDII cable
• OBDII diagnostic tool
• Vacuum/boost gauge
• Fuel pressure gauge
• Compression gauge
• Home-made injector cleaning rig