I just got my 99 A4 2.8. The first time I took it to the station for a fill up, I started by putting 87 in the tank (6 gal). Then, while I was pumping, I read the label, duh, and saw it needed 91. I switched over to 93 for the rest of the tank (10 gal). When I got back in the car and tried to start it, it wouldn't start. Cranked over fine, but wouldn't fire. after about 5 tries, it fired up, then ran fine. I assumed this was due to my mixing low octane gas with high. I loaned the car to a friend for his honeymoon, and when he came back he reported that it did exactly the same thing when he filled it up with straight 91 (twice). The third time, he turned the key on and let it sit for a few seconds before starting up, and it started fine. It seems like I have a fual pressure problem. I don't have a Bentley manual (yet), so I need to know:
1) How many fuel pumps are in the car (is there one in the gas tank, and one in the fuel line)
2) Where is the fuel filter, and how easy is it for a total moron to change
3) Has anyone seen this before, and is my diagnosis correct?

Thanks,

Dave

 

This is a common problem with the A4, not related to you putting mixed octane in the car.
When you put gas in the car, and fill it right up to the top, it allows for no room to create a vacuum.
Without this vacuum, little or no fuel gets sent to the combustion chamber until you crank it for a
while to build the vacuum. This is why it only happens when you fill up the tank.

A clogged filter would cause the car to stumble under load, not fail to start.

 

So, if I understand you correctly, there is (probably) nothing wrong with the car, and the easy fix is to stop before I fill it up the whole way?
On a related issue, can I fill it with 87 in a pinch?

Dave

 

In a pinch and you have nothing else to use, yes you can. The car will retard its timing to prevent detonation, however it can only do this as long as you drive normally. If you put in 87 or 89 and drive the car hard you will get some detonation, and we all know thats bad for the car. Your mileage will be pretty bad with anything less than 91 too.

Don't do anything less than 91 very often. It does have consequences. Fouled plugs, pitted chamber, plugged cat...

When you gas up, just don't top it off and you should be fine. If it continues even after not filling up all the way I would look into the EVAP system to see if you have a leak somewhere.

 

Thanks, will do. My scottish blood shows through every time I pull up to a gas pump, but I love this car too much to hurt it intentionally. Every once in a while, I go to a gas station that has only 87 and 93, and it just kills me to pay for two extra units of octane I don't really need... I'll probably assuage my conscience by mixing again... Let's see... (87*4+93*7.8)/11.8 = 90.966...

Man, am I cheap!

Dave

 

OMG...


Do you actually do that?

4 gal. @ $2.50 + 7.8 gal. @ $2.70 = $31.06

11.8 gal @ $2.70 = $31.86

Grand Savings = $.80

Sketchiness = Priceless

 

.80/11.8 = .067796 / gal
24 mpg
48 mi commute
.68 / week savings
$35.25 / year savings

In 900 years, I can buy a new Audi...

No, I don't, but my stomach clenches every time I pull up to the pump.

[:@]

 

wow - I put in 94 if I can find it.