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Discussion Starter #1 (Edited)
Hi there!

Iv got a problem which has gotten alot worse over time where my 328ci mpg has gotten almost half, only on motorways do I find it barely hovering at around 29, which even then sucks because it usually sticks around 34.

In the city iv been getting 16 mpg which is crazy! My low end torque is gone aswell.

Point is iv changed the lower intake boot, tested the DISA valve, oil change, filter changes, checked for leaks but I can't find anything. Checked the MAF and it's reading good values when monitoring.

I suspect it's a leak because I have noticed as I press hard on the throttle, there's a whistling, swooshing noise which is noticeable and it comes from
near the glovebox.

I don't think it's ccv related but could it be vcg? Maybe the vanos seals? I also noticed there being a bit of fuel on the DISA valve upon inspection. I am really stumped would really appreciate some ideas!

Thanks!
 

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Start by posting the engine coolant temperature as well as the Fuel Trims a warm idle and steady cruise.

Also keep in mind clogged catalytic converters as well as problematic cam sensors.

You claim all the readings are "good", but you need to provide actually data for feedback.

How many miles on this car?

Was any specific work performed just before all of this happened?
 

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Discussion Starter #3
I'll try to get some readings but my coolant is always either at half or just under. I think the thermostat is stuck open but should that make a difference?

The MAF is between 4 g/s and 40-50 g/s, I'll try to log it and show how it changes with throttle.

There has been a load of work done to the car but nothing engine related, just suspension work really. The mileage is 99k.

The only other thing I did was I opened the butterfly valve in the exhaust but that's it.
 

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I'll try to get some readings but my coolant is always either at half or just under. I think the thermostat is stuck open but should that make a difference?

The only other thing I did was I opened the butterfly valve in the exhaust but that's it.
Engine coolant temperature matters a LOT. You cannot rely on the dash gauge and if the temp gauge is anything but at 12 o'clock, you likely have a soft thermostat.

Was the exhaust butterfly stuck closed? This can impact performance and mileage. The butterfly should be OPEN when the engine is off.

Also what is your Average KPH on the OBC for each tank of fuel? Not the Average KPL? This is important to understand how much the car may actually be stopped and idling and/or how slow traffic may be in your area.

Is the 99k in Miles or kilometers?
 

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Discussion Starter #5
Thanks for the reply!

Right, as said I will change the thermostat as soon as I can. My OBD2 Cable arrived today with INPA, and ill try and see any other codes that are fired up! The valve was stuck closed so i've opened it with some brute force.

Being in the UK, its in MPH. My average MPH is usually around 30-40 depending on the journey's im doing. 99000 Miles on the OBC. The point is i've noticed both a signficant drop in lower end torque and this crash in MPG, even though i've been doing the same journeys. Im averaging 130-150 miles on half a tank, which is insane as it used to do 200 easily. I'v worked on all kinds of BMW's but older ones. I still have a strong suspicion that this is a vacuum related leak somewhere but we shall see!

Another observation, when stopping the car, the revs drop, drop below idle, and then it bumps back up to idle revs...Very frustrating.
 

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Your engine is running cold and I suspect that economy hit is very much related. By the time you see it running below center on the gauge it's running really low.

On a vintage I'd change the vanos seals for your low end punch.
 

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Discussion Starter #7
Engine coolant temperature matters a LOT. You cannot rely on the dash gauge and if the temp gauge is anything but at 12 o'clock, you likely have a soft thermostat.

Was the exhaust butterfly stuck closed? This can impact performance and mileage. The butterfly should be OPEN when the engine is off.

Also what is your Average KPH on the OBC for each tank of fuel? Not the Average KPL? This is important to understand how much the car may actually be stopped and idling and/or how slow traffic may be in your area.

Is the 99k in Miles or kilometers?
Hi there!

It's been a while but let's just say it's been a hectic time!

Iv realised a few more things I wanna point out...

I remember a few months ago before this problem arose, I would get the burning rubber/oil smell in the cabin when I drove the car hard...this doesn't seem to happen anymore.

I also noticed that when the car is below the quarter level (it's started snowing) the mpg is the way it should be and the torque delivery is spot on. The second the engine warms to the mid-point again I notice the rough idle, low torque and terrible mpg. I'm changing the thermostat anyway but I don't think it's related.

I also noticed that swooshing noise that would occur on heavy acceleration that seems to be coming from near the glovebox comes only when the engine is hot (I.e. the mpg is down)...which is again leading to the idea that there is still a vacuum leak involved.

What inlets and possible leaks are there at the back left side of the engine? I really think it's something there because everything is sort of adding up in a sense. VCG possibly?

Haven't had a chance to dig around and had the bonnet flip up on me whilst driving, along with a broken wing mirror and a cracked leaking rim, alot to take in 1 week lol!

Thanks alot man!
 

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Discussion Starter #8
Your engine is running cold and I suspect that economy hit is very much related. By the time you see it running below center on the gauge it's running really low.

On a vintage I'd change the vanos seals for your low end punch.
Running cold it seems to run alot better! And yeah I'll do the seals at the same time! Makes alot more sense to do it then!
 

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Replace VANOS seals but NOT OEM they are useless (wrong material). If cold replace thermostat. How long since fuel filter replaced it's well overdue if not done already? Same with spark plugs?
 

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Without data, there is no way to really tell what is going on.

If the engine runs better when cold, the problem is likely the IAT is getting high and reducing the timing. A bad DISA could also be a problem along with swooshing noise may even be a leaking power brake booster, however, you say it is near the glovebox, however, the location of the noise could be misleading.
 
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