Xantia V6 fuel pressure regulator upgrade.

[xantia_v6]

Someone on club-xm.co.uk has tried the 3.5 bar regulator on a 2.0 TCT and claims that (with 95 E5 fuel): "fuel consumption is about the same.
On the plus side, the engine pulls better from lower revs and is more eager to rev past 4000rpm".

I have noticed that my Xantia (auto), with original regulator, running E10 fuel has a bit of a stumble on acceleration when cold, I never noticed this with the (manual) XM, so I am suspecting that the manual ECU might always have been a bit richer in open loop mode at part throttle.

[Mandrake]

Interesting on the TCT engine - is the original regulator the same model 3.0 bar regulator as the V6 ? I don't know much about the TCT engine so can't really comment much further than it sounds like what I'd expect from a slightly richer mixture.

Regarding the stumble you see on your new Auto - do you mean only if you punch the throttle heavily from a stationary start, or do you also see it it while already driving up if you give the throttle a poke ?

If it also happens when not idling then I'd suspect something like an ignition misfire. I'm trying to remember but I think I had this problem at one point on my old V6 and it was ignition related, possibly bad ignition leads to the rear cassette. I haven't seen this problem on the current V6.

If you're only seeing it when you punch the throttle from idle then if its any consolation, both of my V6's have done this despite my efforts to eliminate it. As I've never driven a new one I don't know if this is a normal quirk or something that only happens on an aged engine...

The old V6 did it much worse, especially towards the end of its time with me, while the current one only does it a small amount. On both cars, the increase in fuel pressure significantly reduced the stumble when punching the throttle off idle but didn't cure it, which does suggest it is a lean mixture stumble, but that the real cause is not fuel pressure and that the richer open loop mixture from the higher fuel just helps disguise the problem rather than fixing it.

Both cars have had new coil packs, new rear ignition cassette/leads, new spark plugs, new air filter, new upper inlet manifold gaskets, all inlet pipes checked for leaks, and throttle plate and idle control valves thoroughly cleaned. The old V6 also had a new MAP sensor and a TPS sensor. While other symptoms were cured by various items above the stumble off idle remained.

On the current V6 I've tried swapping the MAP sensor for the one of the old V6 but no change was noticed. I have not tried a new TPS on this one yet, and a TPS with a deadspot could in theory cause these symptoms as the TPS is the first signal the ECU will get when you mash the throttle to warn it that it needs to enrich the mixture. (The MAP reading always lags behind throttle movement as it responds to pressure buildup with some degree of smoothing, and the pressure buildup take a finite time as well)

Interestingly the idle control valve on the current V6 was seized solid about half way when I got it and I don't remember it having this stumble with the seized ICV, I don't think it was there until I cleaned the ICV, but that may have been due to it idling too fast.

So I'm not really sure what causes the stumble off idle, but I have a suspicion that it may be related to the ICV. Even if it's not, I'm fairly sure a worn ICV is responsible for the annoying dip below idle which then surges to recover that you can sometimes get. Again - this problem was a lot worse on the old V6 which did have badly worn (slack) bearings in the ICV.

[xantia_v6]

You have misunderstood my description, the stumble occurs at about 20 MPH when accelerating VERY gently to 30 MPH, and only when the engine is cold, and (I believe) only when running E10.

[Mandrake]

Ah ok. That's something else then.

Is the oxygen sensor OK ? Have you put it on the Lexia yet and verified oxygen sensor, MAP and TPS are all working ?

A faulty oxygen sensor doesn't set the engine management warning light on this engine ECU (nor does faulty TPS or MAP for that matter!) and could cause the hesitation you describe...

[Koukku]

Very interesting thread. Especially the part about the ES9J4 ECU not having long term fuel trim. I've always thought that it's a basic feature of EFI systems. Maybe it's there, but doesn't get stored anywhere, so the system starts over on the next start up?

I've just put in an order for the Bosch 0280160562 on amazon.de. It was only 17.09€ shipped (to Finland). I'm doing a flexfuel/E85 conversion on my V6 in the near future, so this is a sort of supporting mod (of course it'll have bigger injectors as well). But if this also improves driveability it's a win-win.

I have an Innovate LC-1 wideband controller kit (got replaced by the MTX-L in my R53 Cooper S, also running on E85) to go on the Activa, so I can keep track of the AFR. That might also reveal something about the LTFT on this ECU.