Yes, the hose that comes from the thin metal line coming from underneath, should be going to the intake boot. The fuel pressure should not be dropping (Oscillating means nothing) but it's not really clear about how you are checking the pressure. It would be best to check the pressure under load (and rpm) with fuel gauge taped to the window and driving to rule some things out. At this point though, if you know there has been an update exactly for this problem and the update wasn't done, then maybe lean towards that as well.

I had the fuel pressure gage connected to the service port on the fuel rail and held up by mechanic's wire so I could see it through the windshield, just doing deep stabs of the throttle at idle. Not ideal from a diagnostic perspective but I wouldn't feel safe driving around like this.

My fuel pressure gauge hose is just long enough to feed up to the window and close the hood. That said, you should definitely not see a 5-# drop from a throttle stab in neutral. In my second video I'm doing redline punch to get less than 1psi drop. Any less throttle and it didn't drop 1/2 psi.

I would take apart the FPR and check those two o-ring though i think just the inner one matters. I think the outer one goes to atmosphere. (So would come out to ground)

Somewhere i have a picture showing the size it's like 8mm OD 4mm id but that's mostly guessing.

Throttle tip-in issue/stumble ([emoji2391].0 [emoji2[emoji2391]9[emoji2391]]-speed)
 

https://uploads.tapatalk-cdn.com/202...d214a9d181.jpg
https://uploads.tapatalk-cdn.com/202...88773255b5.jpg
https://uploads.tapatalk-cdn.com/202...fe9326dc06.jpg
https://uploads.tapatalk-cdn.com/202...24397e8f9c.jpg
https://uploads.tapatalk-cdn.com/202...4cb48fb7a9.jpg
Look at the o-ring. You can immediately tell it will no longer do its job.

8/5 mm very close.

It looks like on my model year the regulator is integrated into the filter, no seals or anything. But I have a new filter+regulator coming to rule this out. So far almost everything on the vehicle has been original so the fuel filter probably is too.

Very few had and still have the separate regulator, anyway if there is a problem it will be with the fuel filter not the regulator. If you would have checked the fuel pressure under load you could have ruled out the pump and filter but not a bad plan to change it if it has never been done.

Agreed. I wish I had a setup (or safe place) I could do this test properly under load.
Although bad fuel filter would also probably affect high power, high RPM operation - no? That's when fuel demand would be the highest. Either way this seems like a maintenance item to rule out.

Exactly, any problem with fuel delivery would show up more under load.

Most likely the off idle tip-in was on the lean side from the factory and even a small amount of carbon build up and engine wear turned out to be a problem which is why there was an update.

You still shouldn't have the pressure drop like you are saying but maybe that has something to with with how you are looking at the gauge ?? And still no codes ??

Combined 2-in-1 replace filter also FPR.

Mine that was serviceable was from 2001. I don't know when they changed.

FPRs "main job"* is tob hide a failing fuel pump. A pump not putting out enough pressure will be hidden due to the FPR regulating the output.

* Tongue in cheek, it's collateral pain in the butt however plenty of times I've seen a weak fuel pump not caught due to FPR hiding the output dropping.

It's too bad can't easily measure psi/flow at the pump output.

I was in similar shoes i replaced the filter and FPR as a unit only after teardown did i realize that the filter tough plenty dirty probably had 200,000 miles left they are huge, and that the small o-ring was toast most likely the cause of my woes.

I never had any codes or even lean warning just very very hard starts occasionally.

Wife never hit that car hard WOT so the weak psi full throttle never caused a problem driving.

FPR/filter is a maintenance thing ay this age. I didn't see if you replaced the pump but do some math:

Odometer in miles divide by average speed in mph. That will show you estimate of hours.

If using ethanol gas you should get about 5000 hours, pure gas 4000 hours.

In mine and wife's case it was very close.

132000/27=4888.889 was one of them the other was crazy close.

Hope you find and resolve. I wished my 3.0 was three pedal would be much more responsive.

Okay, so I've managed to do a few things:

• changed the fuel filter and regulator with a Hengst branded one
• replaced the throttle body with a new OE (VDO/Continental) one. I found one on clearance at Rockauto so went ahead and did it.
• Did a visual boroscope inspection of both catalysts through the O2 sensor ports to see if they were clogged. Not obviously melted or anything.

Unfortunately the problem persists. It's hard to diagnose anything through INPA because the refresh rate is so low it totally hides transient issues like this one. Are there faster/better monitoring or logging options? I'm running out of things to try. I could go ahead and put a new fuel pump on since the original is probably on borrowed time and it's not that expensive or hard to do.

Will try to get the software update next week.