Bizarre change in engine performance with full gas tank?

posted this elsewhere but as I got no replies and I'm going a little paranoia crazy... I've been calling myself crazy so I won't mind if you guys say it too, but I can't explain it any other way. Put simply, I've now got 1189 miles and as long as I have a full tank to about 3/4 tank, the engine feels tight, throttle feels responsive, acceleration smooth and whatever "normal" is as far as pulling power. However, at 1/2 tank most noticeably, but seems to start a bit before then, car feels a lot less powerful, less responsive to the throttle, all in all a lot less performance feeling in all categories than when fuel tank is full. Maybe it's just that I'm turning off the engine while fueling up, but whatever it is, sometimes my car starts up in 'more powerful' mode versus 'less powerful' mode and the only consistent thing that I can concretely identify with this behavior is how the car feels right after I fill up the tank and when it's drained to around 1/2 tank. Bearing in mind that in the last 11 days I've put on nearly 1200 miles of driving, I've been stopping at and filling up at gas stations a lot more frequently so I've had plenty of opportunities to see/feel this behavior. Other than the 'you're just imagining it / you're crazy" replies which I'm not sure I wouldn't agree with, anybody have some vaguely rational reason why I notice or believe I notice this odd behavior?

continue to remain crazy. I spent the good part of today driving around burning off gas, now at just a little tick above 1/2 and my butt dynometer sure seems to indicate the slightly off performance again. Not to say it's some huge drop off, but something noticeable, say 10-15% different enough where it would be reasonable enough to say it's easily noticed. I heard the Porsche version of DME/OBD or whatever computer controls the car adapts itself to your driving style. Has no one had/heard anything like this? Driving me nuts.

I take it you have been using the same RON fuel. I don't know about the US but here in the UK, Shell serve up this fuel called Optimax and its the dogs bollox. Performance is noticeably faster using Optimax vs ordinary 95 Ron fuel. Just my two bobs worth.

Yep - same fuel. And after first noticing this I got futher paranoid and stuck to just one gas station. I'm using Chevron, and while it is an oxygenated gas (with methanol I think), it's not using the MBT or whatever the Porsche manual says you ought not to use.

Hmmm..... Any strange coloured smoke from the engine ( I hope not). Does it idle OK? Any difference in sound as you go thru the revs 1/2 tank or full? What sort of MPG do you get in either case? You say 1189 miles. Is that total mileage? it's a new car right? Dealer here

Nope - no smoke or any other negative signs. Idle is just under 1k, and is the same as before putting on PSE. And to clarify, it's not so much that I think it's "good" performance and then "bad" - it's more like "good" versus "not as good". Overall, it's still responsive to the throttle, etc. even when in this weird mode, but just 15% or so less than before. Brand new 2004 4S - usual new car miles when I picked it up, about 23. I'm now at about 1300 miles total. Yes - it is a different exhaust sound as I go through revs when tank is full versus 1/2 full. When 1/2 full and seeing this reduced performance, exhaust sounds louder and more of a sputter. (I Just got the PSE about 250 miles ago - there must be some break in happening because it's toned down a lot from the first observation of being very loud and sounding more of a riced up Honda than a beast like growl. When tank is full, exhaust sounds more of a good rumble and less sputter. According to my computer, since I've gotten the car my MPG is 14.2. In today's driving around town, 12.1 MPG I just filled up to full tank on the way home, and the last few miles of the drive was in the "good" mode after filling up. I swear, there's something going on here, but I just can't for the life of me figure it out. I drive the same, am using same gas each time, no real swings in how much I throttle when at full tank versus half, etc. I'm sure now it's not some crazy mental thing, like how some people think their car drives better just after cleaning it.

How is the temp? Any other symptoms? Try a tank full of Chevron w/ Techron or just add some on the next fill up.

Temp seems fine, just a peg to the right of the 180 mark after it gets warmed up. Oil level full. And except for one time, I always fill up with Chevron premium (w/ Techron) I'm back to normal mode right now, but taking a long drive tomorrow to see what happens once I burn off my full tank down to 1/2 again.

if you have already passed the break in period then try this. if you are babying your car during your break in period, the DME may have adapted to a less aggresive driving style. disconnect the battery and leave it for a few minutes to reset the DME then drive it hard for a few days/hundred miles. doing this may decrease the time the DME needs to re-adapt to a more aggressive driving style. i have read through the boards to do this whenever a mod is performed. it doesn't hurt, just keep your radio code handy.

admitedly, I have been babying the car through break in. I'm only on about 1300 miles, and I read in the manual not to exceed 4200 on the tac for the first 2000 miles. So I've been always shifting right before 4k on the tac, except when cold where I usually shift right around 3k.

just one comment-your mpg seems real low-it ought to be around 18-20 mpg. is this any relation to your problem?-dunno. why not just take it to your dealer?

truthfully, you can break that magical barrier and not damage your engine. i was told by the tech who replaced my boxster engine that yes i can go over 4200rpms and actually encouraged it. he said it would help to break the engine in. he did say not to stay in the higher rev ranges 5500~7000 for long and to vary the revs. this was echoed by another tech and my service advisor. to paraphrase the tech, just don't go crazy and rendline it at every stop.

It's all city driving. I'm in the LA area, and other than some drives up and down Malibu to deliberately put on miles to achieve my break in, most my driving is city streets, short highway stretches, or slow inching highway driving not inclined to give good gas milage. I thought a 911 was 18-20 MPG only on susained highway driving? As for taking it to a dealer, I'm afraid if I go in right now without some concrete 100% If...Then proof that can be tested and reproduced, I'll either just be laughed at, or even worse, be noted as a "it's only in his head" complainer. So when I go in for squeaks and stuff, I won't be taken seriously. But the major reason is I really don't want to be turning over my car for an authorized test drive of 1/4 to 1/2 of a tank to some dealer mechanic/rep. Like I said, it's not so much "good" vs "bad" performance, but simply "good" vs "not as good". I've already called a couple service departments anonymously to ask about this and no one had an answer, so if I went in, I'd have to let someone drive my car and experience it themselves. But like I said, only way for someone to experience it is to get both modes. Unless you're the owner and intimately familiar with your car, I don't think I can go in with a full tank, let someone take a short drive, then take my car back, go burn off 1/2 tank myself, then return, and let that same person take another test drive. I doubt their association and memory as to my car's performance would be good enough to distiniguish the two test drives unless they got in and did one long 1/2 tank test drive.

Intellectually I'm aware of this, and it's certainly not my belief that actual car techs can't be more aware of the cars they work on than the manufacturer's guidelines which I assume have to be done because they can't know what kind of idiot and all the varying conditions the car might be subjected to. However, I'm basically a manual slave. Whatever it says to do or not do, I stick by it. that's why I hate break in periods - I want it to be over as fast as possible. I only got the p-car 14 days ago and I'm now up to 1300 miles even though my M5 is my daily driver. When I got my M5, I put on the 6000 break in miles needed in just 4 weeks. I heard they already burn these engines in at the factory well beyond 4k rpms. True?

fuel pump?

Well, I guess the best statement I can give you is 'That's weird' At first I thought it may have been a bad batch of gas, but you've already re-fueled several times. Are you using 93 octane? Anything less and the car detunes itself. Maybe it resets to the higher performance when you fill up, then detunes when it realizes it's a lower octane. The only other thing I can come up with is the driver adaptive nature of the car. With the tank full, you're testing it so you're driving like Mario Andretti when you leave the gas station, and the car sets performance mode. When you are satisfied that it's "good", you start driving like Miz Daisy, go through city traffic, consequently with low gas mileage and low demand on the car. By the time it's half full, the car has adapted to the "driving Miz Daisy" lower performance mode. You notice it, then Mario Andretti gets in the car to drive to the gas station to fill it up. And the cycle starts over. How's that for imaginative thinking . PSE always sounds louder when cold, better tone when hot. It also changes tone and loudness as the exhaust system matures. "Sputerr" exhaust doesn't sound right, but if it is not accompanied by rough idle and rough revving, I'd say that was a biased description due to thinking something was wrong. Anyway, I'd say there is nothing wrong with the car. Just drive the car more often and take it to the canyon more often. Use 93 octane. Enjoy the car, break it in well. After you've "driven it like you stole it" a few times and the symptom still persists, then have the techs give it a once over.

How about taking it in with 1/2 tank, so they can drive a little and then top it off?

that's what i've been thinking all along here. if this isn't you being "crazy" , sounds like a problem in the fuel system. the fuel pickup is at the bottom of the tank, so the more gas you have in the tank the more pressure you'll get. normally this would be regulated by the fuel pump, but if the fuel pump is bad, it could explain this behavior. it could also be a bad fuel filter or a slightly pinched fuel hose. i'd put the car up on jack stands, get under it, and check for any constrictions in the fuel system. if you know how you could also pull the fuel filter and check that it's working ok. basically just open it up and make sure the inlet and outlet are clear and that the metal screen in it (the actual filter) is clear of debris. if those things look ok i'd have the dealer check out the fuel pump. since the car is under warranty so you can get it checked out for free.

hmmm...didn't think think it was related but I do have another odd fueling related condition that I was ignoring until I solved this one. Would this be another sign of a bad fuel pump or something with the fuel system? Whenever I stop to fuel up, usually when I'm at 1/3 to 1/2 tank, I've always had the fueling automatically cut off about 1/5 to 1/6 before a full tank. I get the auto cutoff and if I try to depress the pump handle again, it goes for just a second before it cuts off again. Basically, what you'd expect if you really had a full tank, but my gas gauge showed perhaps 4/5 full. So I end up just taking another couple minutes to add another 1-2 gallons or so via start/stop spurts. Still relatively cool in SoCal, so it's not a high temperature day causing pressure expansion in the gas tank, which I've had return false 'full tank' cutoffs at the pump before in my M5.

...I definitely think there's something wrong with your car. The mpg figures are WAY out. I have a '99 996 C4 and get 31/32 mpg on motorway journeys (sustained 90mph). In heavy traffic, I'll get about 21/22 mpg.

Moz, I have a 2003 C4S. 7,000 miles. It's getting around 19 MPG city and 24 or so long highway.

Moz, In some p-cars If run the car until the gas light came on then refueled your gas gauge would only show 3/4 full. It's a recall on come 911s and they should fix you up with no problem... Sorry I didn't see what model you have, but this was a problem is the C4s. Rob

the fuel sending units were recalled on some 911s as rob says. i had mine replaced 20K+ miles ago. if the gas pump is always cutting off before your gas gauge says the tank is full, then your gas gauge is wrong. the gas pumps have a very simple hydrodynamic shutoff in them to tell when the tank is full. if you try and top off, the vapor recovery system will simply start siphoning the fuel back into the hose to prevent it from spilling over. i would be that you're losing 1/2 of the fuel you're topping off with to the vapor recovery system. that's why gas nozzles say "don't top off" on them! jason latif, those MPG figures sound a little too good to be true! with my cruise control on at a sustained 80 MPH i've never been able to get over 26.4 MPG. and in the city i've never gotten over 20 MPG. how are you calculating your fuel mileage? back to the original topic of this message, if you can't find any constrictions in your fuel system then take it to the dealer and ask them to run diagnostics on the fuel pump.

I just drove as gently as I could in city driving of about 13 miles. Not too much stop and go as traffic was fairly light for the LA area. Basically stuck to 1st-3rd gears, and always shifted right around 3k to 3.3k. My trip MPG showed 14.9 MPG. I have a brand new 2004 4S. Are all of you getting much better street MPGs? (if you aren't driving like you're racing that is)

...nothing scientific - simply using the numbers on the trip computer. I'm not sure how accurate this is of course. Also, in the UK I use Shell Optimax fuel which has a very high RON rating.

given your gear selections as you note above, you are prolly in the right range with MPG. One knock on the tip is that it shifts up too soon, (and it is a fair knock for back country driving at under 40 mph). However because it does that, my MPG is an average of 23.5 for city/highway driving. Re the engine performance issue......only way to really diagnose it is to do a dyno comparison........that is a pretty expensive proposition. Another test may be to have the tech put the car on the computer after some driving in both conditions.....I am not sure the latter will show a performance difference, but if there is something untoward going on in the engine it may show up then..... good luck

Jason - Are your UK gallons the same as ours here in the US? I thought you might have imperial gallons which are larger and would account for the increased mpg (or is that mpig?)

Hence the wide difference in fuel consumption

Thanks for all the feedback so far - this thing is driving me crazy. First, I went into my dealer today - they ran my VIN and said nothing outstanding as far as recalls (on my 04 C4S). Made appointment to come in on Wed to get tested for the 3/4 tank full reading when putting in gas from empty, and the performance difference when at full tank versus 1/2 or less. This weekend, I filled up to full tank again, and for the hell of it figuring it couldn't hurt I bought some STP Fuel System cleaner and poured that in. I think these things are a load of crock for new cars, but again, what the hell - had nothing to lose other than $7. Drove it 20-25 miles and didn't really notice anything other than what I've already observed as "better/good" performance when tank is full and thought it was a $7 experiment confirming my belief these additives are just useless for new cars. However, aftering parking car overnight, I drove again this morning. Wow - it was night and day even versus what I had ranked as the "better/good" behavior with a full tank. So, maybe coincedence, but full tank plus STP seemed to do something even beyond just the raised level with full tank versus 1/2 tank. Throttle response was much better at all tac ranges, and now for the first time I actually notice some of the supposed strong torque the 911 marketing books talk about - especially in 1st and 2nd gear even starting at a lower tac. Could actually feel something at 2k going up to 3k, where before it was just a whine and not much power feeling until 4k, where I was always shifting due to break in. I know this p-car is not my M5, but it now had very acceptable pickup, pep, butt dynometer feeling of the car pulling well, whatever you want to call it. Again, not to say that it was weak before, but it sure seemed like there wasn't much until I had just hit 4k or so. So now I have three levels of relative performance - "best", which I only experienced once toay with full tank and STP poured in from day before with about 20-25 miles run on it. "full tank" mode which seems ok but an obvious level below "best", and then of course the "1/2 tank or lower" mode in which it just seems a signficant performance loss/difference. 15-20%, something easily noticeable and not some mental placebo effect. Anyway, this morning it felt so good in "best" mode that I just kept driving instead of going to the store, and drove down to Long Beach in a long 30-35 mile drive. Whole lot more pickup in each gear and seemed like the engine felt tight, maybe the better word is seemed like the compression was a lot better. Got down to dealer, explained my deal about under hood sticker and this performance mode thing driving me crazy, and made my appointment. Starting driving back and drove around in a long loop up to Hollywood and back to Manhattan Beach until I burned myself down to 1/2 tank. Just before 1/2 tank, after coming to a stop at a light, it switched from "best" mode to "1/2 tank or less mode" and compared to what it was just like, felt like crap. All of a sudden, almost no torque feeling until around 4k on tac, and a lot less throttle sensitive it seemed. Since my dealer appt is Wed, I didn't want to fill up as I want to burn it all the way down to reserve and make sure I duplicate the 3/4 full reading when filling up. I have no idea what's going on, but it's definitely tied to some full tank versus 1/2 tank thing. Not sure if the STP did anything - won't know until I fill up again and see what the full tank performance is like. I'm really going to ask them to check out the fuel pump, filters, etc.

Little experiment I did on my way home tonight, with car in what I call "1/2 tank or less" mode. I stopped at a light - all city driving, but completely flat streets, no hills or large slopes of any kind. Reset trip computer and then drove for 7.7 miles in a very robotic, excruciatingly careful manner to be as easy as possible (e.g. to see what max MPG I wouold get. So, my formula of driving was a) run through 1st to 3rd gears, b) gentle and slow as I can increase to throttle at all times - no sudden/fast throttle, c) no heel/toe or blipping throttle, d) once getting into 3rd gear, never exceeded 41 mph and always well under 3k on tac So, at end of trip, this is what I got on trip computer: trip time: 22 min distance: 7.7 miles avg speed: 22 MPG: 17.7 That's it - driving even lower than the gentlest slow-mo granny out there, I maxed out 17.7 MPG in flat city driving; good traffic flow so I think over those 7.7 miles I only stopped at a red light maybe 9-10 times. Sounds like a lot, but this is LA. Going to try this on highway tomorrow after getting up to 6th gear and see what I get with a 6th gear run for 7-10 miles with speed set to 55 mph.

...I'm not sure - but a UK gallon is 4.55 litres. What is it in the US?

ah, that explains the higher MPG. A US gallon is a little less than 4 litres! nice catch redcab!

sounds a little low to me. i've tried similar experiments in my C4, just because i was curious what kind of city MPG you get when you drive super conservatively, and i always get in the low 20s. basically i was just bored a couple of times and i wanted to see how high i could get the MPG to go. driving like this in areas with few or no lights, i get 24+ MPG consistently. i would think our cars should get very similar MPG figures, though yours is the 3.6 and a couple of years newer.

I get 14mpg pure city (small town) driving; lots of lights, lots of stop and go. 24mpg pure Interstate.

Fuel pickup/computer disagreeing with different data?

This will probably drive you crazy until you come to some resolution if there is a problem or not. Best thing to find resolution is to throw the car on a dyno. Do it with an 1/4 tank. Leave it strapped to the dyno and dump in another 3/4 tank. Run the car again. On a normally aspirated car, you'll see very small differences from run to run. If you see 10%+, then you know you have a problem.

Here are a couple of theories: 1) Maybe you've got brakes (caliper)dragging. You stop for fuel and cool down the brakes, so the drag goes away. After driving for a while the brakes get hot and drag. Any signs of hot rotors? 2) If you had debris in the fuel tank, then filling might redistribute it. After dirving for a while the debris could block the pump suction screen again, reducing fuel pressure and losing performance. 3) you've gone completely crazy and are imagining everything. The is an explaination for this, we just don't know what it is!

I didn't read all the replies but i had/have the exact same problem. Some times my car feels a lot faster than other days. First of all you'll be glad to know that once you have 4000 or so miles you will feel the car is much faster. Secondly I noticed that if I'm stuck in traffic or not moving more than 20 mph the car becomes slow when I try to accelerate quickly...I personaly think that there isn't enough cold air in/around the engine and you lose power, once you get on the highway and come to a stop again the car feels fine...I noticed that my car is sensative to outside temperature. On a cold day my back will sink into the seat and on a really hot day it will feel like i'm moving off in second gear when I'm in first. I'm just glad somebody else has experienced my problem as well and i'm not crazy.

Might be your fuel filter is clogged or dirty.. There is more presure pushing when you have a full tank. I remember on my supercharged Tacoma I had much better performance witha a full tank since my fuel pressure was better. It might sound crazy but it was the truth. Good luck explaining it to the dealer.

Palting, where can you find 93 octane gas in Los Angeles? I only see 92 everywhere, and it pisses me off.

Paul - where do you see octane 92? It's all Octane 91 at the pumps as far as I see (same as where I came from, NorCal). Other than straight from the pump (in other states), you can achieve Octane 93 by mixing Octane 100 (available at select Union 76 stations) with the normal premium Octane 91. The Union76 website used to have a station finder with a little search allowing you to filter on only race gas providers. But now you just get back one huge list and you have to randomly select each one to see if they have it or not. I go to the one on San Vicente, about 4 miles north-east of Marina Del Rey.

took my car to dealer today - got a couple other fixes while I was there for slightly cracked clear side marker and window velvet causing streaks when rolling windows down/up. anyway, dealer diagnosed my 3/4 tank full thing as the TB discussed here - so they sent away for a new sending pump which will be installed tomorrow. they're not sure yet about the switch in modes from normal to less peppy. when I get my fuel sending pump installed tomorrow, they're going to take a look at a range of stuff I think. crossing my fingers.