Uppallnight:

What and when should that vacuum line be pulling vacuum? I had the vacuum gauge on it today (via a tee connector) but at idle and revving to 3000 rpm produced no vacuum on my gauge. Engine was at operating temp....


2003 X5 3.0i Sport MT
2004 M3 Coupe
1997 M3 Coupe

Since you are chasing a lean condition that line is after the MAF , so it can be a source of unmetered air. I would plug the line to see if you still have a vacuum leak, or lean condition with misfires.

Upallnight:

Did that last week - plugged the vacuum line going back the frame to the regulator - no change.

My question was more about when that line should be pulling vacuum. Plugged it changed nothing and the truck still threw misfire codes and entered limp mode. Put a vacuum gauge on it in my garage today, started engine, rev'ed to about 3000 rpm, and the gauge never fluctuated or registered any vacuum.


2003 X5 3.0i Sport MT
2004 M3 Coupe
1997 M3 Coupe

Rookie,

Be careful here, this is a very contentious subject as to how the fuel pressure on the M54 engines is controlled.

The M54 has a "hose" connected to the integral fuel pressure regulator in the fuel filter. Let's AGREE not to call this a "vacuum" hose, because as you have started to learn there is NO VACUUM present in this hose.

The M54 engine operates on a FIXED or STATIC fuel pressure, it is not controlled or influenced by engine manifold vacuum. If you pay attention to where the fuel pressure hose is connected under the hood, you will see the hose is connected FORWARD of the throttle plate. There is no "vacuum" forward of the throttle plate. The purpose of this hose it to provide a "filtered" atmospheric reference for the fuel pressure regulator. This keeps moisture and dirt from plugging the fuel pressure regulator and causing the fuel pressure to be adversely impacted by outside influences.

JFOJ:

You rock! Thanks for that info, and confirms my thoughts about what that line is actually used for. I need to have the vacuum gauge read vacuum between the throttle plate and the intake valves.

I am only aware of two vacuum ports on the rear of the intake manifold. One is used to pull vacuum for the secondary air system (electronic valve controls vacuum to secondary air valve off this port), the second one is of larger diameter and plugged on my truck. Double-checked the plug today to be sure it was intact and snug - found out it was not original and was in good shape.


2003 X5 3.0i Sport MT
2004 M3 Coupe
1997 M3 Coupe

Rookie,

Your problem is not likely going to be a vacuum leak based on the problems while cruising. During cruise the intake manifold vacuum drops and it lower than at idle so vacuum leaks impact the engine far less while cruising then while at idle.

Just watch your fuel economy needle on the dash, this is effectively a vacuum gauge, it mimics one for the most part, but it is not driven by vacuum. Think if this gauge as high vacuum for high MPG and lower vacuum for lower MPG. I think you will quickly get the point.

Hopefully later today I will have some time to look over the latest Log data. But there does not appear to be a good enough smoking gun form looking at the Log data so far. I need to look at the O2 sensor data a bit closer to see if I can figure out the "Chicken vs Egg, which one came first" situation. This may be a bit misleading and we need to be cautious about things.

You are at least gather Log data while doing other useful things like using the fuel pressure gauge. This problem is going to be a case of elimination of possible problems that will likely lead to the solution.

What I am a bit worried about at this point is there may be some strange/unusual electrical problem or even a possible flaky DME. But I am not ready to condemn the DME yet.

Can you confirm that your X5 has a manual transmission? I believe both your X and SarTech both have manual transmissions.

This may feed into part of the equation, but not 100% sure yet if this is a factor, but I have some suspicions.

JFOJ:

MT 5-speed confirmed. SarTech also has an MT.


2003 X5 3.0i Sport MT
2004 M3 Coupe
1997 M3 Coupe

From your log, the long term fuel trim starts at 0. The long term fuel trim starts to go positive as the engine is running. The car computer should retain the long term fuel trim from the previous engine running. Are you resetting the long term fuel trim before starting the log?

If you are resetting the trouble codes you need to drive the car long enough for all the drive cycles to show complete. Running the car for 10 minutes doesn't really give an accurate picture as to what the engine is doing.

Clearing the codes DOES NOT clear/reset the Fuel Trims.

Resetting/Clearing Adaptations clears or will zero Fuel Trims and standard/non Pro OBDII tools do not support Resetting/Clearing Adaptations.

How do you explain the long term fuel trim being at 0 at the start of the log?