Diagnose performance issue

[bimmer7410]

Good afternoon

I need a bit of insight on the current issue ive chosen to tackle.
My 540i (03) quickly jumps off the line in the mornings. It is my second most loved feature of the car behind the driving dynamics.

Heres the problem: after the car has been driven about 15-20 minutes, it gets SLUGGISH. I am outpaced by every other car from a light without damn near gunning it.
Noticed the other day when attempting to merge into traffic that even when flooring it the car does not respond in kind.
MPG is complete ass as well. My pixels dont work well enough to see the car computed value, but i'd estimate about 13-15MPG on average. I get maybe 230-40 miles on the tank before getting the fuel light. This is without flooring it.
One thing I do notice is that I get the early detonation sound when accelerating when the car hasnt warmed up yet. BUT i am able to get good acceleration at that point, so it is likely unrelated.
I have no MIL light.
I'd like to figure out this issue so I can move on to others asap. Thanks in advance for any help provided.

[edjack]

You need to do some diagnostic work. How do the fuel trim numbers change with time?

Any stored codes?

Does the fuel pressure drop off? Your model has a Schrader valve on the fuel rail. Easy to measure.

How many miles on the cats? They may be clogged. Any history of major missing/fuel cutoff? Measure the back pressure by removing the front O2 sensors, one at a time, and look for a change after the time you state. More than 1-2 psi at speed is bad.

[geargrinder]

Agreed. Need to read the DME (ECU) codes, and, see what fuel trims are (check trims BEFORE you clear any codes as they'll get reset if you clear codes first).

Checking fuel pressure under load is bit hard for most civilians, as is measuring cat back pressure. But check the other stuff first.

What you describe as 'early detonation' is concerning.

Whats cars history? New to you? Old to you? Mileage on it? Any maintenance done on it ever?

[bimmer7410]

You need to do some diagnostic work. How do the fuel trim numbers change with time?

Any stored codes?

Does the fuel pressure drop off? Your model has a Schrader valve on the fuel rail. Easy to measure.

How many miles on the cats? They may be clogged. Any history of major missing/fuel cutoff? Measure the back pressure by removing the front O2 sensors, one at a time, and look for a change after the time you state. More than 1-2 psi at speed is bad.

Most of these things I'll likely have to get checked at an indy. I have no method for checking back pressure. What tool do you usually use to perform this check?

Agreed. Need to read the DME (ECU) codes, and, see what fuel trims are (check trims BEFORE you clear any codes as they'll get reset if you clear codes first).


Checking fuel pressure under load is bit hard for most civilians, as is measuring cat back pressure. But check the other stuff first.

What you describe as 'early detonation' is concerning.

Whats cars history? New to you? Old to you? Mileage on it? Any maintenance done on it ever?

So even with no CE light, there can be powertrain codes set on the car? Interesting.
Ill try and get them checked this weekend.

No idea on the car's maintenance history. Picked it up for 1500 not running. Water had flooded floor (vapor barrier) and rusted out the fuses, so I was able to get it for a good price. Had 107k miles and interior looks good.
Figured I'll be doing most the work on it. Love the car so far even if i have much maintenance to do ahead.
But yeah it definitely has a bit of detonation when pushing the gas before the car is all the way warm.

[geargrinder]

Yeah ok you could have a zillion codes. Yes even without the CEL. (SES).

Does the SES light even light up at start up? Evil crapbox sellers have been known to tape over or break/remove those.

If you're self resurrecting it, unless you're super computer averse/retarded, you should strongly consider getting a good set of diag software and cable and a laptop to be able to do it yourself.

If it's true detonation you shouldn't be driving it hard unless you really want to get to see the inside parts of your motor.

[bimmer7410]

Yeah ok you could have a zillion codes. Yes even without the CEL. (SES).

Does the SES light even light up at start up? Evil crapbox sellers have been known to tape over or break/remove those.

If you're self resurrecting it, unless you're super computer averse/retarded, you should strongly consider getting a good set of diag software and cable and a laptop to be able to do it yourself.

If it's true detonation you shouldn't be driving it hard unless you really want to get to see the inside parts of your motor.

Ill definitely be purchasing that very soon. In the meantime, the dealer is the only place I can get these codes read at, correct?
And yeah ill be taking it easy on it

[geargrinder]

Absolutely not. A good bmw Indy or a tuner or a local enthusiast with the right gear that'll do it for free/beer are all options.