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Discussion Starter #1
Hello All,

Thought I had resolved my occasional starting and engine idle issues but it now seems this is not the case!

I bought the car (1999 728i) and immediately scanned for codes- the only code was P0340 relating to the camshaft position sensor. I erased the code and took it to my local independent BMW mechanic. Upon rescanning the codes, P0340 had disappeared (perhaps a historic code?) but a few misfire codes popped up. Following further investigation, the PCV system was replaced and the car ran much better and no codes reappeared.

A few days ago, the car struggled a few times to start when the engine was warm. The car then went into limp mode but no check engine lights or other warning lights appeared. Yesterday, the outside temperature was relatively cold and although the car started first time, the engine stumbled very badly between 500 rpm and 1000 rpm. It eventually dropped to around 300 rpm and then finally stalled. After the engine was warm it performed perfectly for the rest of the day. Today, the same thing happened except the engine never stalled but just continued stumbling until I drove off. There is no issue when sitting in traffic where the engine idles around 500 rpm.

Does anyone have any idea what this could be please given the lack of codes? I have had a few suggestions from mechanic friends including; MAF sensor, crank sensor and VANOS seals. I am now a bit lost and really want to sort this problem out.

Thank you in advance for any suggestions!
 

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Could be a vacuum issue but a strange one. Any vacuum that serious should eventually trigger a trim issue and light.
Could be the MAF, they are known to cause issues without throwing codes.
Another possibility is fuel delivery: intermittent pump/pressure, clogging filter, or leaking/clogged injectors.
I would probably start with either:
1) fuel pressure given lack of codes, or
2) unplug MAF (engine off) and see if it starts easier and doesn’t have issues.

And actually, before all that, validate good battery (12.4+ charge after overnight) and alternator (13.5-14.8) while running.


2001 750iL DD74441
Stock

RIP: 2003 540iA Sport GS56111
H&R front springs, Ultimate Cup Holder, Euro Dash & Armrest, Grom, BavSound Stage1
 

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Discussion Starter #4
Thank you both. I will check the battery (looks old) first, the alternator has already been inspected and is working properly despite being quite noisy (think it's a bearing?). Good idea regarding vacuum leak and I will also try unplugging the maf sensor. Would be great to solve this one! Thanks again!

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Discussion Starter #5
Quick update in case anyone wishes to know for future reference. I have checked the battery and alternator (both good) and eliminated vaccum leaks as the culprit. I have also cleaned the throttle body (and butterfly) for carbon build up. However, the problem still remains! I'm now 99% sure it's the MAF sensor, everyone keeps telling me these don't tend to throw codes. I will clean the sensor and if that fails replace it. Hopefully, problem solved...
 
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