I have an 04 r53, that is slightly modded. In the past couple of months, I've noticed that my car is not accelerating like it use too. I only notice this hesitation when I go WOT, and it starts to stutter at the 4k rpm mark. When I am in 1st and 2nd gear, it is completely normal. However, once I'm in higher gears the hesitation starts to occur. When I am in 6th gear and I floor the pedal, the car struggles to stay at 4k rpm and it can't go higher.

I believe this all started after I changed my belt (Gates K060535), installed colder spark plugs (NGK BKR7EQUP), idler pulley (11288625983), oil pan gasket (11131487221) and valve cover gasket (11121485838KIT). Since then I have also replaced my o2 sensor (Bosch 13878) after the catalytic converter, and my camshaft sensor (12141485845) after my car threw the p0340 code.

I am at a loss to what is wrong with my car. The belt that I changed was smaller than the belt that came with the original dinan pulley. Would the smaller belt and the colder plugs have an effect on my original tune? Would my tune be causing the acceleration issues? The last time I was at the dealer they said I had a leak in my air intake tube, but after inspection I did not notice any cracks. Currently my car has no check engine lights on, and other than poor mpg, the car runs smoothly.

If anyone can help me figure out this problem that would be awesome. I don't want to continue blindly throw new parts on the car. Thanks in advance

Danny

 

Did you pull the plugs to see what they look like?

 

I did not pull them out, I figured they wouldn't be the problem since I just installed them recently. Do they tend to fail?

 

A visual inspection for intake leaks is not sufficient. While at idle, carefully spray starting fluid around the S/C boots and intake hose. If the rpm's increase, you have a leak. Also, hesitation at WOT around 4k would indicate the possibility of fuel starvation. You don't mention your odometer reading, but you may want to consider that the fuel filter needs changing or that you have some trash in the tank.

 

My car has over 100k, and I was looking into replacing the fuel filter soon. I'll pick up a bottle of some starting fluid and give it a test.

 

Update: So I installed an autometer boost gauge today, and it doesn't read any boost. At idle it reads -16 vacuum, but when I drive it hard, the needle never goes over 0. When I installed it, I did kink the tubing. Would that have an effect? I'm not sure if this is related to my current issue or not.