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Registered
2004 325Ci Manual
Joined
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5 Posts
Discussion Starter · #1 ·
Hey Guys, new poster here!

Recently, I cleaned my IAC valve and throttle body and replaced a crumbling fuel pressure regulator hose connector; after starting up my car I found it had a rough cold start. I can often hear the IAC valve knocking very loudly while the revs bounce around for a couple of minutes before setting at roughly 550 rpm. Thinking the ECU must be adjusting to an unstuck IAC valve, I decided to take the car for a couple of test drives and noticed a severe reduction in power output while the fuel consumption gauge doesn't move for a few seconds when taking off from a stop but then proceeds to shoot to the max along with revs, despite there being no difference in acceleration.
I have noticed the issue seems to be getting worse in subsequent trips, with fuel being lost very quickly and am not too sure what could be causing these issues.

My mechanic was unsure about what could be causing these issues but suggested I check for hidden vacuum leaks and my OBD scanner isn't turning up any codes. I have been reading about similar issues in other forums and have seen a couple of suggestions about resetting the throttle body, should this have been done after the throttle body clean?

Would anybody happen to be familiar with these symptoms or know any ways to address them?
 

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Registered
2000 E46 323i, 3.0L and 2.0L Z3's
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980 Posts
It sounds like you have a vacuum leak. If the car ran OK before the work, it is likely something you did while doing the cleaning.
  • Did you seat the ICV back into the rubber grommet correctly? It can be hard to push it in completely. A badly fitted ICV could be the source of the noise you talked about.
  • Did you fit the lower rubber boot correctly onto the throttle body? There is a rubber tab on the bottom of the boot that locates between two webs on the throttle body. If it is not lined up correctly, it can cause the boot to fit incorrectly and cause a vacuum leak.

A smoke test is the best way to find a vacuum leak.

As you have a scanner, you can do a test to see if you have vacuum leaks to find.

The proportion of unmetered air from a vacuum leak to metered air is higher at idle and lower at revs. Do this test.

With a hot engine at idle, check the 4 fuel trims, 2 short-term and 2 long-term. If any are > 8% then you likely have a vacuum leak. Rev the engine to 3,000 rpm for 30 seconds and watch the short-term fuel trims. If they reduce significantly, might even go negative, then it is confirmed that you have a vacuum leak. Smoke test the engine to find the leaks.

If the short-term fuel trims do not reduce significantly, then it’s something else.
 

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Registered
2004 325Ci Manual
Joined
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5 Posts
Discussion Starter · #3 ·
Hey NZ00Z3,

Thanks for your reply. Prior to the IAC valve work, I had issues with the thermostat and expansion tank which made me take the car off the road months earlier but before that I didn't notice any issues with drivability. When reinstalling the ICV, I put a thin layer of vaseline on the outside of the junction which helped to push the ICV all the way into the grommet till it was touching the ridges on the valve but i did have some trouble reinstalling the lower boot and forgot to check the tab.

I'll take a look at the trims and lower boot connection and post an update if anything shows up.
 

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Super Moderator
2003 330cic, 2003 325iT
Joined
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6,879 Posts
First off never use Vaseline on a car. Bad for the rubber. Use dish soap if you need a simple lubricant.

Just like you had to refresh your cooling system, now you need to refresh the intake. See my sig.
 

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Registered
2004 325Ci Manual
Joined
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5 Posts
Discussion Starter · #5 ·
Cheers for the heads up, Archbid. I applied it in extremely thin amounts because I was iffy about using a lubricant on hoses which have to hold together, but I'll steer clear of the vaseline completely.

I've gone through the intake refresh guide and I'm halfway through everything, but that was extremely helpful with breaking down the system as a whole.
 

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Super Moderator
2003 330cic, 2003 325iT
Joined
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6,879 Posts
Cheers for the heads up, Archbid. I applied it in extremely thin amounts because I was iffy about using a lubricant on hoses which have to hold together, but I'll steer clear of the vaseline completely.

I've gone through the intake refresh guide and I'm halfway through everything, but that was extremely helpful with breaking down the system as a whole.
The rule of thumb is to lubricate with the substance that the connection is carrying - oil for oil, water for water, coolant for coolant. Silicon grease for o-rings if you can't get things to move - or if it is carrying air ;)

Buy a tube of this to have around: Ace Waterproof Grease - Ace Hardware

And BTW, when you replace the intake gasket, be careful of the little pieces that can fall out around the openings as you pull the seal. Just do it over a clean workbench away from the edge.
 
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