stutter and hesitation if fuel level above 1/4 tank!

[lewisweller]

If I run the car at less than 1/4 tank full it's very smooth great acceleration and responsiveness. With engine running I went and filled up with 99 octane as usual and straightaway the engine stutter comes back!!!!!!!! Fyi it is NOT fuel quality. 

I've started a new thread for this but it's an old problem mentioned in previous posts and unresolved. 

Basically the ctt 06 with 114,000km has been hesitating and stuttering for a while when accelerating particularly under boost in lower gears through the rev range of 2k 3k 4k ish rpm, it also will vibrate and stutter revving the same rpm in neutral/park when stationary. (which is very weird). 

So after a lot of finding issues which helped the car overall none of them sorted this stutter out. 

Quick list, 

New oem Coils, plugs, n75 valve, air filters, purge valve, and Dv's,

Fixed a lot of vac and pcv pipes which leaked 

Smoke tested intake no leaks boost is +0.8

Head Gaskets done a few 1,000 km all new parts gasket Seals etc. (problem was present before and after head Gasket work).

 

I had a suspicion that the lower fuel level was playing a part in this but I can't work out why a fuller tank would cause running issues? 

[ekstroemtj]

for shure you tested your fuel pumps already.

[lewisweller]

In fact no I haven't. I haven't the fittings and gauge to do the test, but I guess I need to asap. Anyone got a DIY for testing fuel pressure? IIRC 4 bar at idle and 4.5 bar at 2000rpm? Regulator Should hold in the line 3 bar after 60 seconds after switch off?

 

Also I'm thinking along the lines of tank vent or related component causing a fuel delivery issue when tank level is higher than 1/4 tank but that doesn't sound logical to me. 

[ekstroemtj]

this was a post from bigbuzuki.

 

i had similar problems like you.  That time i changed MAF, O2 Sensors and Fuel pumpsn on my old CS.

 

after run very nice.

 

This is from the publication Porsche Service Technik.

Regulation of Fuel Quantity.

As a result of the on-demand regulation of fuel flow capacity, fuel heating in the tank is reduced.

This is achieved by the DME control module switching on fuel pump 2 only as required, while fuel pump 1 is always activated.

With the following switching criteria, fuel pump 2 is additionally switched on to increase flow capacity.

At engine start and then 2 seconds run-on (with a hot start 5 seconds run-on).

With a calculated fuel consumption of > 50 liters/hour the pump is switched on, and at < 45 liters/hour switched off again.

With a fill level of < 10 liters in the tank, it is switched on and with > 15 liters it is switched off again.

[lewisweller]

Thanks Thomas interesting info, what a weird fuel pump setup Porsche have! Lol.

So what still begs the question, if one of my pumps (namely the secondary right side passenger no'2 pump) is not delivering flow and or pressure properly what does that have to do with fuel level in the tank? 

[ekstroemtj]

I dont know if this is right but i guess the secondary pump is not delivering fuel to our engines. It just pump the fuel to the other side of the tank. So i believe the fuel level matters. Second thing to remember. The pumps getting realy hot when working.  An almost empty tank doesnt cool the pumps enough and its shorten the fuel pumps life

[lewisweller]

This theory would indicate a low tank level would or could cause reduced pump performance and heating issues, but my problem is the reverse of that, given a lower tank level I have better running not worse. 

[ekstroemtj]

this was my story years ago. End of the story, the shop changed only ONE fuel pump. 

After they replaced both the problem was gone.

[lewisweller]

Interesting read thanks. 

Yesterday I pulled fuse 13 and 14 running on 1 pump left and right to see what was the difference. Summary idle the same fairly stable, revving was stuttering at 1k 2k 3k 4k approx rpm. So didn't really prove anything conclusive except obviously there is only 1 fuel filter and regulator so maybe one of these is the issue. 

No choice need to do fuel pressure test and pressure drop test. 

 

[ekstroemtj]

I believe the fuse trick only can show you if the fuel pumps are still alive. But in this way you cant find pit if they are working proper

[lewisweller]

Done some more reading and a strong possibility is the fuel pressure regulator. This being faulty would explain the stutter as when the regulator is failing it can't maintain the fuel line pressure through varying rev ranges as the injector firing frequency increases and this fluctuates the line pressure apparently, hence rich and weak mixtures. Still the tank level I don't see how it plays a role in this issue really. 

Being 114,000km I may change the filter and regulator now. The pumps are expensive so hope its not them at fault, as the fuse test didn't make any difference in running on either pump singularly I'm thinking it isn't a pump issue. 

Still need to do pressure  test .............to many jobs.........

[lewisweller]

The change over valve I ordered a week or so ago coma me in stock so have picked that up today and for the hell of it a new gas cap for $25. Will fit these tomorrow morning and see what happens. Not expecting miracles ........but you never know with weird illusive problems like this. 

[lewisweller]

Today I pulled out the fuel filter expecting to see the two part type which you can split and extract the paper element filter to replace or clean because I couldn't find one listed but it was the one piece type pn7L5919679 aka touareq filter. So I put it back in but before I popped off the hoses and the fuel that drained out was looking clean. 

Also of note was that both the carpets was cut so someone had obviously been in there before. 

I'm eagerly waiting to get down to 1/4 tank again. 1/2 tank left will drink that quickly tomorrow and we'll see wat's up. 

[lewisweller]

Sadly the 1/4 tank theory was a fluke, I ran it down today from 1/2 tank down to nearly empty and no dice. Same bull**** running jerky stutter. 

Tomorrow I'm going back to looking for vac leaks as I don't have the kit to test the fuel pressure yet. 

What a chore.........

[lewisweller]

Update for this I have just replaced two check valves found to be leaking numbers 13 and 25 on the diagram. 

Cleared the dtc codes (no actual codes but this clears the fuel trim learnt values) went for a quick drive 15km and it was definitely adapting to something. Not conclusive yet if this crack the problem so jury is still out, will put on some more km tomorrow and see what's up. 

[lewisweller]

Update on this......... the replaced check valves didn't fix the hesitation problem even though both check valves were faulty!! 

Alternator just failed so looking into the possibility that charging voltage stability may have something to do with the problem. 

New Alternator going on this week. 

[Brainz006]

It's stuttering under heavy acceleration / boost? My '06CTT does something similar - - annoying to me, but others might not notice. It's not rough when revving, though (thankfully). I wonder if it's aging MAFs and/or O2s? Laziness or miscalibration of either could cause issues under transient loads.

[lewisweller]

 

Under load yes but even if accelerating under half Throttle from low rpm up as the boost comes in it will kangaroo through the rev range, my wife feels it when she drives it and passengers notice it so its not like it subtle and in my OCD head even....... lol.

I'm excited about the new alternator and how a more stable electrical system in this very pernickity car will make improvements.

How sad, I'm actually excited.........

[lewisweller]

Found a suspicious looking sign of a leak where the PCV VENT pipe from each valve cover runs to the rear of the engine.

This is the turbo vent line T.

Maybe this is the cause of my stutter!!

 

Will investigate tomorrow.  

[lewisweller]

I Checked this turbo PCV vent pipe on the back of the engine no leaks but what was weird is I cant blow through it or suck any air. Its almost as if where it connects to at the turbo (item 6 & 7 on the parts diagram) is closed and somehow opens when required by oil pressure or something?

Anyone know how its works?

 

I also checked the very last check valve I didn't already check or replace and surprise surprise it is broken! Porsche really did on a number on me with this little pig............

New Audi Q5 3.0t in white was whispering to me in the showroom the other day............the cayenne is on its last warning

[lewisweller]

Best update this thread, the check valve above is not leaking. 

After new Alternator and new battery under warranty the car is running better, I will put some more km's on it before being sure as it's behaved well before and then became a stuttering **** again shortly after I announced a self healing miracle had happened. 

[lewisweller]

More driving and more interesting things, the engine definitely is more happy running, the short term fuel trims are -0.8% on both banks and long term fuel trim on bank 1 is -2.1% and bank 2 is -3.0% so pretty good! 

But, and here it comes......... if I idle it for some time do some traffic stop start and slow plodding around like today it really doesn't like it afterwards. You go to blast it out and it's a bit fumbled and hesitant, but if I'm blazing on the highways and good pace it's loving it. Very strange behaviour.

 

Anybody have any ideas what this is? 

[lewisweller]

So as not to leave this thread open ended for anyone reading in the future....  Don't you hate that!

 

I replaced the left side fuel pump and the running issues are 85-90% Fixed. Its drives much much better. Idle Dip alittle when coming to quick stop and very very slight roughness on acceleration I feel sometimes as the boost comes in. 

No more egg sulphur smell either. 

[ybe]

LOL well I told you it was the left fuel pump.. It was pretty easy to identify for me when reading your symptoms. But just keep in mind in time generally you want to replace the filter flange or at least clean the filter when you replace the left fuel pump because a dirty or clogged filter may have been an issue and can cause an issue with your new pump over time. And you will want to replace your right pump as well after a while because the fuel pressure is probably lower than normal or let's say not in sync with the newer pump...

[lewisweller]

33 minutes ago, ybe said:

LOL well I told you it was the left fuel pump.. It was pretty easy to identify for me when reading your symptoms. But just keep in mind in time generally you want to replace the filter flange or at least clean the filter when you replace the left fuel pump because a dirty or clogged filter may have been an issue and can cause an issue with your new pump over time. And you will want to replace your right pump as well after a while because the fuel pressure is probably lower than normal or let's say not in sync with the newer pump...

Bad news in fact the better running lasted for two days only and then it all came back like some bad nightmare. 

I don't know what happened but I have some theories that the right side pump non return valve inside the pump is leaking the pressure off from the left pump or I got a pipe somewhere in there split or I'm totally wrong and it's nothing to do with the fuel. 

 

For the sake of ruling out all fuel related issues now and for the foreseeable future I own the car I have already bought a new right side pump which I will install on Wednesday as I'm trying to run the tank back down to empty nearly. 

 

I have new filter and regulator with flange gaskets a few months back. 

 

I found a great diagram from someone else's post about fuel pump issues. 

Notice the red line from each pump that joins before going to the filter, my theory is if the right side pump is not running (ie usual part load driving and not too low fuel level) and the check valve built inside is leaking, the pressure and flow from the left pump is bleeding off and doesn't deliver correctly to the engine. 

Possibly when I did the left pump swap I inadvertently mad ethereal check valve work for a little while and then it started leaking again. 

It's a far fetched but possible theory I have to eliminate now. 

 

The only other thing I can think of is the turbo breather pipe at the rear is leaking for sure but that little bit of air can't do all this sporadic running problems and an ok idle and high rpm rev with a stumble at around 1000 and 2000 rpm when stationary parked holding the rpm is just not making any sense. 

 

My mind has run through so many possibilities and I've probably read over 2000 posts and technical informations about everything A-Z. I'm so sick of reading a long thread which ends with.... "and then I read the very helpful DTC code and replaced the coils and all was fixed" ahhhhhhggg if only it was that simple for me.