Strange Issue
 

I have a 1988 S4. Over the last several months, I have been working on a variety of issues with the car but haven't done much over the last two months thinking I had fixed all but some minor issues. It had been about a month since I drove the car so I took it out for about 25 miles. The car drove normally for the first 10 miles or so.

After slowing down to turn around, I noticed the the car seem to have a hard time accelerating from a stop. It would accelerate, but it seemed like it was being held back or the parking brake was on. I continued to drive and shifted through all of the gears. The car drove, but still felt like it was being held back. At about 60 mph for a few minutes, it seemed to correct and it was like a turbo kicked in and was driving normally again.

At the next stop light, the car seemed to have the same challenges starting from 1st gear and continued through all the gears. It stayed that way until I got home which was another 10 miles. After making it home, I was backing into its parking space and the car cut off and didn't want to start again. The engine compartment was very hot but the temperature gauge was just a little above normal.

I decided to let it be for a while and try to figure out the problem.

For reference, some of the things I did a few months ago was changing the fuel pump and filter and fixed the shifter after it came loose on me and wasn't shiftable. I drove the car after both of those repairs and the car performed fine. The only other things I've done have been to the sunroof and interior.

I did push the AC button on to see if it would work since it has been a while since I tried it. The light on the button didn't come on and I didn't feel anything when I pushed it. I pushed it back off and nothing really seemed to change either way. I experienced the issue about 10 minutes after pushing it. I don't see an association but wanted to give as much info as possible. When the car had its issue, it felt like more than what it feels like when AC kicks in.

Any ideas or things I can do to troubleshoot the problem?

Did you check any of the wheels to see if they were exceedingly hot? Maybe you have a brake caliper that's stuck.

Makes sense that it could be something like that, but felt like something else. I will check that though.

I don't see where you have a solution to your issue. So, here goes. Those suggesting a dragging brake may be correct. This is not the same as blocked brake hoses, refusing to release fluid, or rusty calipers. This is a vacuum booster problem. The booster gets brake fluid inside. Yes, I know - but the master cylinder has no visible leak, nor loses fluid. A previous owner could have changed the MC. Anyway, the fluid causes a check valve to stick allowing the booster to build up pressure. Punching the brake pedal a couple of times will allow it to release. I have taken the booster apart and cleaned them out and they worked OK.

Don't know why this happens to 928's. I have had the problem on 3 cars of the '83, '84 genre. Of the hundreds of cars I have owned, including a dozen 928's, Mercedes, BMW, 924, 944, American, only the 928's have had this problem. Your symptoms sound similar.
Good Luck,
Ron

I wonder what the mileage is and what service has been done? This could be a timing issue or fuel problem.

W/exception of engine heat, I had the same problem regarding the master cylinder. Changed it out and it is fine.

It might be that the brake fluid has never been flushed. It's a good idea to replace the fluid as it collects moisture.