Fuel pumps - or?

[deilenberger]

I see a lot (well enough) people replacing fuel pumps on their Cayenne's..

My '06 S exhibits a bit of lean start.. by that I mean when cold it takes longer then I'd expect for it to catch when cranking. It also has a very slight hesitation when gently accelerating and it's in a higher gear. It has no lack of top end, and no lack of power - which to me means it has adequate fuel supply for the engine's needs.. but it's been suggested to me it could be fuel pumps.

Questions:

- Anyone else have these symptoms?

- If so - any resolution to them?

- If it involved fuel pump replacement, was the fuel filter replaced at the same time?

- Anyone try JUST replacing the fuel filter?

The reason I'm asking - the positive displacement type fuel pump gets unhappy run dry (since fuel is used to cool it) and unhappy if there is backpressure - which can be caused by a plugged fuel filter. I find it hard to understand putting the filter in the tank where it's extremely difficult to get to (most vehicles have it external) - so I'm guessing that when Porsche replaces the pumps, it's probably automatic to replace the fuel filter.

There are other possibilities for the lean start:

- BMW M3's ('01-06) have this happen every spring as the east coast fuel mixtures are changed to summer blend. Apparently the stuff is different enough that the car isn't that happy starting. It frequently can and does take two cranking sessions to start them. Like swallows returning to Capistrano - the M3 forums are regularly deluged in the spring with people asking questions and complaining about this behavior. It seems to go away about September.

- BMW was replacing fuel pumps for this on cars under warranty - with the claim that the check valve in the fuel pump (that maintains pressure in the system when the car isn't running) was allowing the pressure to bleed off. The test was an overnight pressure test (put the gage on before the mechanic went home at night, then check it again in the AM - more then some BAR number loss was considered new pump time.)

- Anyone checked the fuel pressure on the Porsche? The manuals want 2BAR - but didn't give a bleed off time test that I could find. It appears from the manuals that the check valve is in the top mount to the tank for one of the pump assemblies. Dunno if that part is available separately.

Or is the lean start condition just normal on these engines with certain fuel mixes..?

Lots of questions.. but that's the only "problem" I can find with my new to me Cayenne, and it will bug me until I resolve it, or decide it can't be resolved and "they all do that.."

[wvicary]

It wouldn't hurt to check the fuel pressure but I can't remember the spec. i would first start by cleaning the throttle bodies and check for engine codes. Do you know the service history of the vehicle?

[deilenberger]

wvicary,

Interesting on the throttle bodies - and doesn't look difficult to do since the TB appears to be right up front. No idea on engine codes yet - no OBD-II codes are stored, but awaiting my Porsche specific tool..

The service was done by a dealer. They've promised to give it to me, I just have to get there when the service department is open. The pumps haven't been touched since the carpeting hasn't been cut.

Thanks!

Don

[wvicary]

I mention the TB as they are easy to clean and I have had that solve some hard starting/ poor cold idle / drivability problems. I have never seen a cayenne pump give the symptons you describe, I have had them work properly or not at all in the Cayenne, not saying it couldn't be at fault. Also a low reading MAF sensor can cause simliar issues.

[deilenberger]

I cleaned the TB this AM (and it is easy, once I found the page showing the positions of the lock-goobers for removal of the intake Y plumbing, the Ebay manual I got has 9980 pages.. luckily Acrobat is pretty good at searching.. :) ) but - I digress..

It was certainly not awful. I did dirty a rag (used carb cleaner on the rag rather then spraying it) on it, so there was some stuff. It seemed to start fine after that, but that was only one try. Will update if this improves things. Also have new plugs arriving sometime this week (56k on the ones in there as far as I know..) so I'll be installing them.

Thanks for the nudge toward the TB - certainly worth the try, and I'll update the results after a few more cold starts.

Don

Edited April 3, 2011 by deilenberger

[deilenberger]

wvicary,

After about 20 stops and restarts (we went out trucking today in our local national park) - it's fixed! Started flawlessly each and every time. That one suggestion made membership cost for this forum insignificant. MUCH happier with the Cayenne now!

Thanks much!

[wvicary]

Glad it's sorted. I think TB cleaning should be done as part of regular maintenance.

[deilenberger]

Glad it's sorted. I think TB cleaning should be done as part of regular maintenance.

I think it will become part of the replace the air-filter process for me.. Be much easier to do both at the same time.

[PKN]

Has anyone tried sea foam on this motor? It is a common way of cleaning the TBs on Dodge trucks. Just disconnect the vacuum tube from the brake servo and let the running engine drink the contents of the bottle. It worked great on my hemi but not sure where I would connect it on my Cayenne and I also dont know if the turbos would like it

[deilenberger]

Has anyone tried sea foam on this motor? It is a common way of cleaning the TBs on Dodge trucks. Just disconnect the vacuum tube from the brake servo and let the running engine drink the contents of the bottle. It worked great on my hemi but not sure where I would connect it on my Cayenne and I also dont know if the turbos would like it

I'm VERY leary about something that has as much magic attached to it as SeaFoam. The other difficulty I see - I didn't see a vacuum line running to the TB, although there could be one. The ease of getting to the throttle body (especially if you do it while doing an air-filter change) makes it a no brainer for me to do it the right way, with carb-cleaning solvent on a rag. I opened the butterfly on the TB, and held it open with a soft-handle screwdriver handle so I could clean the backside of it and the area of the bore past the butterfly. Once you know how to remove the Y intake pipe (and I didn't remove it entirely since the hose on the bottom didn't want to unclip - I just moved it out of the way) - the rest of the job takes about 5 minutes.

Next time I'll take some photos for a DIY..