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2003 325i (M54B30 swap)
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Discussion Starter #1
Hey everyone!

I recently replaced the MAF on my b30 swapped 325i and encountered what is legitimately the weirdest thing I’ve ever seen. When the new MAF is connected, the engine sounds like it’s backfiring through the intake. It’s popping and banging when the new MAF is plugged in, but stops as soon as the MAF is unplugged. I’m slightly concerned to even run the engine to check INPA for the MAF reading because I fear the backfire might bang up the intake.

Is it even possible that a MAF would cause something like this, or is it likely that connecting the new MAF is illuminating another issue that’s causing this backfiring? It’s super weird and I found a couple threads where people talked about it a little but nothing that seemed resolute. I literally didn’t even know an engine could backfire through the exhaust!
 

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2003 325i (M54B30 swap)
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Discussion Starter #4
The MAF is used genuine from an auto recycler with a warranty. Does say Siemens on the side. I’m gonna return it and order new from FCPeuro I think to troubleshoot. I did also get a new MAF connector, but I’m pretty sure all the wires are in the right spot of the connector. I can upload a pic or test with a multimeter if anyone knows a good way to make sure it’s right.

Honestly, I’m not running it again with the MAF plugged in because I really don’t want to blow up my intake... I finally got this engine running again (thanks to some of your help :) ) and I don’t feel like sourcing another manifold lol
 

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2000 E46 323i, 3.0L and 2.0L Z3's
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390 Posts
Two possibilities come to mind:

1) The secondhand MAF is good. You have vacuum leaks to find, so smoke test the engine. Reasoning. With the MAF unplugged, the DME uses fueling tables in the DME. These are a little richer than normal. Running the engine with the MAF unplugged lets the engine mask the vacuum leaks (lean running) with the richer mixture. Plug the MAF in and the engine is experiencing a lean mixture and is back firing and missing.

2) The secondhand MAF is Bad. The engine is running lean and backfiring and missing. Change the MAF.
 

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2003 325i (M54B30 swap)
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Discussion Starter #6
Two possibilities come to mind:

1) The secondhand MAF is good. You have vacuum leaks to find, so smoke test the engine. Reasoning. With the MAF unplugged, the DME uses fueling tables in the DME. These are a little richer than normal. Running the engine with the MAF unplugged lets the engine mask the vacuum leaks (lean running) with the richer mixture. Plug the MAF in and the engine is experiencing a lean mixture and is back firing and missing.

2) The secondhand MAF is Bad. The engine is running lean and backfiring and missing. Change the MAF.
I agree with your reasoning entirely. I bit the bullet and bought a new MAF, new TB gasket(already done the intake manifold and DISA gaskets a month ago), new CCV and some vacuum line. Hopefully that cocktail will throw me a bone, but I figured I’m going to take the intake manifold off to check for damage from the backfiring so I may as well get the CCV done while I’m in there. If anybody wants to chime in with other intake leaks to check, I’ll list what I’ve got done or will do

1) Intake boots are good
2) Intake Manifold Gasket is new
3) DISA has new, very tight gasket and has been rebuilt
4) TB gasket incoming
5) CCV incoming
6) vacuum line incoming

I checked the brake booster hose and am getting good suction off the outlet into the brake booster. I’m sort of restoring this particular car so I don’t mind doing a little extra since I’ll be in there anyways.

Thanks guys :)
 

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In my experience with MAF disconnected DME would add about 20% of fuel masking even huge air leaks making you think MAF's the problem. And make you throw away lots of money for a new MAF.

I experienced loud intake bangs on cold startups that were caused by thorn lower intake boot at that elbow that goes to ICV. With MAF disconnected engine started and idled much better and there were no intake pops. It even caused MAF g/s readings to be too low and even air leaks between the throttle plate and MAF cause fuel trims to behave as if it was a bad MAF not a vacuum leak. So I went for a new MAF but didn't make any difference at all.

I see you think you're intake boots are good but I would triple check them. I was 100% sure mine were good too because I had just finished refurbishing and reinstalling the intake and knew they were intact as I installed them. But installation of upper boot probably put too much stress to the old rubber of the lower boot and it cracked at that ribbed elbow that goes to the ICV.

As for the pops they really sound awful like something is breaking in the engine but I don't think they're as harmful as they sound. It's probably unneccessary to remove and check the intake again. Just smoke test it. Smoke test will also confirm if the rubber boots are indeed in good condition.
 

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E30M3 Race F10 535 R1150Rt M Coupe
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Last time I saw that was a maf installed backwards. Just what I witnessed once.
 

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If you install the MAF backwards, does the engine try to run backwards?

t
;)
 

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2003 325i (M54B30 swap)
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Discussion Starter #10
Hah! Backwards MAF install makes reverse the forward gear and the engine intakes through the tailpipe!

Just as a heads up for you guys (who are rad for your input!) my parts should be here tomorrow or Wednesday and I’ll check back in. I do want to get the manifold off to replace the CCV though. I’ve done it before so I can get it off in 2 or so hours. I’ve got a lot of time this week and it makes it easier to get to some of those vacuum lines anyhow.
 

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2003 325i (M54B30 swap)
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Discussion Starter #11
Hi everyone!

Parts finally came in today and I’m getting to work on everything. I have a question, though, on the vacuum lines etc in the back of the intake manifold. I was watching 50skid’s videos as a general guide to the vacuum lines, and the back of my intake manifold seems to be missing some stuff from the video. When I check my vin on realoem, the schematic that looks like it should include that stuff shows no parts for my car, strangely. We’re there e46s both with and without the whole black and white little valve and the vacuum lines and stuff? I’m attaching photos for reference as I don’t think I’m verbalizing it extremely well.
Mine:
908872


50skid (all credit to him for his fantastic work!):
908873

RealOEM:
908874


Apologies if I’m being annoying, I just want to chase down every possible vacuum leak while I’ve got the intake off. Thanks again :)
 

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E30M3 Race F10 535 R1150Rt M Coupe
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Replacing the vacuum leak common offender bits is commendable. However there's no substitute for a smoke test.

Naturally I use a rather large Snap On one, but the can type mineral oil homebuilds seem to work well.

Should also be done after you've completed the work. In order to double check your work!
Assume nothing.
 

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2003 325i (M54B30 swap)
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Discussion Starter #13
Replacing the vacuum leak common offender bits is commendable. However there's no substitute for a smoke test.

Naturally I use a rather large Snap On one, but the can type mineral oil homebuilds seem to work well.

Should also be done after you've completed the work. In order to double check your work!
Assume nothing.
I plan on it! I’ve done a little research on some of the homemade stuff, and I think once I’ve got this whole thing put back together properly, I’m gonna run to Ace and see if I can’t give it a shot. I’m pretty much just hoping my brake booster isn’t leaking, since those are a bit pricy. All the other vacuum leak stuff is at least fairly inexpensive.

Just hoping to find out about those lines and such on the back of the manifold so I can throw everything back together. RealOEM is showing that from 03/2003 (I’m guessing MS45.1 cars?) the SAP changed and doesn’t actually connect to the back of the intake, so I guess it’s correct that there’s nothing back there?
 

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Yes- depending on the generation and drivetrain, the valve is vacuum operated or something else.

The early RWD cars and all 4wd cars look like 50's kid's video.

The later (and I think the changeover was mid 2003, with MS45, as you say) cars do that valve differently, without vacuum.

So yup, you're right.

t
 

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2003 325i (M54B30 swap)
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Discussion Starter #15
^Thanks :)

That helps me get going. I really didn’t want to stick the manifold back on without being 100% sure I didn’t have any lines back there I needed to add or anything. For anybody curious, I’m almost done getting everything back together (just the fuel rail and wiring harness for them back in) and will pop back in once I’ve got that finished up and the new MAF on. If I’m still leaking air after the following

-New Intake gaskets + bolts torqued to 11 ftlbs
-new (used but good condition) valve cover, VCG and bold grommets
-New TB gasket, torqued to spec
-New CCV and hoses
-new fuel injector o-rings
-new MAF

I’ll have to wait for my homemade smoke machine to be ready to test :)

thanks again guys
 

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2004 325i automagic
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If its not too late change the three vacuum port caps on the back of the intake that are not being used ... one 7mm cap and two 3.5mm caps ... not that its ever too late, but its a bit easier with the intake out of the car.

Those caps get loose and crack with age creating vacuum leaks. If one falls off completely you'll likely have over 20% fuel trims.
 

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M54b25 and m54b30 cars don't have the same maf. Which one are you using? You should have the b30 (larger) one. Did you update the DME to b30 software? Is the backfiring happening when you accelerate or at idle? Does it idle smoothly or rough? Have you checked fuel pressure?

Sent from my S61 using Tapatalk
 

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2003 325i (M54B30 swap)
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Discussion Starter #18
MAF is B30 MAF, all associated hardware including intake manifold, TB, fuel injectors, DISA, airbox and intake hoses have been swapped over. Only partS re-used are ICV and snorkel. DME is flashed to b30. Backfires occur only at idle, obviously not smooth. Fuel pressure good and fuel pump is new oem replaced 3 months ago. Did not try to give gas when the engine was backfiring through the intake, didn’t want to push my luck. I’ve just about finished going back through everything that deals with vacuum on my engine, including VCG, grommets, intake manifold gasket, TB gasket, CCV, Vacuum caps are good, DISA gasket, fuel injector o-rings and everything torqued to spec. The only things vacuum related left to my knowledge would be the brake booster sucking jet pump thing and the brake booster itself.

All this stuff is nice and cheap to be honest, and the work doesn’t bother me to do since I’ve got plenty of spare time for the next couple weeks. I see it as a win just to get the cheaper, high labor stuff out of the way so I don’t have to worry about it. Like I posted earlier, nearly there on getting everything back together, just gonna knock out the fuel rail and its harness in the morning and should be able to start her back up.

I think NZ00Z3’s post was spot on. Either the MAF I’d bought before was bad or I had a huge leak somewhere. New MAF should eliminate half of that possibility. Hopefully my gutting the intake covers the other half.
 

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Sounds like you are on the right track with the maf and you swapped everything necessary for the b25 to b30 change. Not sure if you did a smoke test before but if you did, it doesn't test the brake booster. The check valve blocks smoke from entering the booster. If your brake booster is leaking bad enough to cause your symptoms, you would definitely hear hissing from the brake pedal area that changes when you press the pedal. Is this the first time starting it since the swap? Does it have any codes in the DME? Are you sure that the disa is working properly? These cars run surprisingly horrible if it's broken.

Sent from my S61 using Tapatalk
 

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Discussion Starter #20
I’ve had Balidawg help me with checking the codes previously, but as of right now I don’t have anything but I’ll double check when I’m done. DISA has good seal with my finger over the little vacuum hole and new o-ring. I’ve run it since the swap a good amount.
 
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