Weak start up

PLAYER07

MEMBER
2004 BMW Z4
2.5L
V6
2WD
AUTOMATIC
157,000 MILES

I have been trying to track this problem down and it’s driving me crazy. My car will randomly have a weak start or sluggish start. It’s been like this since I bought the car in used condition and then the car finally stopped running. Changed the starter for another one that was off another BMW and same problem. So I thought alright this time I’ll buy new. Well, I bought a brand new Bosch starter and same problem happens from time to time. AutoZone says battery and alternator checks out. I checked the engine ground cable and it looks fine. What am I missing?

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

When you say weak start, do you mean the starter is turning slowly? If you do, have you checked for excessive resistance in the ground wire to the engine block? Although the ground may appear to be physically good, the wire may have deteriorated internally.

I attached a pic below of the entire starting system wiring schematic. If you take a look at it, you'll see there really isn't a lot that can cause this issue other than a weak connection.

Once started, does the engine run properly? Is there anything else you can think of that may lead to this happening?

Let me know if that is what you are referring to as far as a weak start.

Take care,
Joe

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Sunday, October 4th, 2020 AT 7:05 PM

PLAYER07

MEMBER
2 POSTS

Thank you for replying so quickly!
So the black wire I thought was a ground for the starter isn’t a ground cable at all. Good to know, thank you.
This video is the closest representation I can find of what it’s similar to. You hear it a lot better the second time he starts it.
https://youtu.be/W-fpCj4YUbg

Once started the car runs perfectly fine no hiccups or anything. It’s just when starting up that it wants to throw a fit. I’ll be honest I didn’t test the ground wire, I just looked at it. I did however have some thick wire laying around from a old stereo upgrade on an old truck. What I did was bolt one side to the ground bolt that’s on top near wheel well inside engine bay and then the other side where you would put hooks or whatever it may be to pick up the engine. This didn’t seem to help , maybe my “ground” locations weren’t the greatest.
Thank you.

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Sunday, October 4th, 2020 AT 7:25 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

On the schematic, the red is 12v from the battery at all times. The black wire going to the starter is what triggers the starter to engage. It should only receive 12v when the key is in the start position.

Now, the negative terminal on the battery should go to the engine block and to the body near where the battery is located.

I did listen to the video. Is it the starter turning slowly that you are experiencing or a delayed start?

Joe

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Sunday, October 4th, 2020 AT 7:59 PM