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Engine idle fluctuates up and down a few times then shuts off

Tiny
PALOSBB40
  • MEMBER
  • 1991 BMW 735
  • 3.5L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 220,000 MILES
I press gas pedal and it's fine and if I try to drive it engine shuts off.
Sunday, June 7th, 2020 AT 2:15 PM

30 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
I suspect this is a vacuum leak causing this issue. Here are a few guides that help with that. Let's make sure we don't have a vacuum leak and go from there.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://youtu.be/4ktw1X4W_-k

If you find no leak, does the engine start right back up but stall anytime you put it in gear? If so, let's start running through this guide.

https://www.2carpros.com/articles/stall-at-idle

Thanks
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Sunday, June 7th, 2020 AT 2:53 PM
Tiny
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Hi, thank you. I just did a visual inspection of hoses and it looked fine, but did find a big crack on the Intake Boot - Air Flow Meter to Throttle Housing that connects to the EGR valve. I'm going to order the part and thanks again.
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Sunday, June 7th, 2020 AT 3:43 PM
Tiny
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Sounds great. This will clearly cause what you are seeing. However, if this doesn't repair it then I would get some parts cleaner or carburetor cleaner and spray it around the lines and intake to see if the RPM comes up as it says in that guide. If it does then you have a leak.

Thanks for the update.
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Sunday, June 7th, 2020 AT 5:36 PM
Tiny
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Oh yes, I will do that and thank you.
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Sunday, June 7th, 2020 AT 5:40 PM
Tiny
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You are welcome. Let us know what happens or if you need more help.

Thanks for using 2CarPros.
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Monday, June 8th, 2020 AT 3:31 AM
Tiny
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Hi, did replace air boot and I didn't find any other leaks using throttle body spray. Engine does start up and idle without a problem. The engine did rev up for the first time but did do a small backfire and when I put it in gear the engine does stall within a minute or two.
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Monday, June 8th, 2020 AT 6:08 AM
Tiny
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Update: it now takes three cranks to start up and stalls when I put it in gear whether the EGR valve is connected or not. I did also recently replaced fuel pump and fuel filter. I do have a bad battery and jump start each time.
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Monday, June 8th, 2020 AT 8:13 AM
Tiny
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Okay. That is interesting. So now that it stalls when going into gear, you are correct in disconnecting the EGR because that will cause this issue. However, so will insufficient fuel pressure. I understand that you just replaced the fuel pump but we need to monitor the fuel pressure and make sure it is not low when this happens. Also, do you have a scan tool to monitor the o2 sensor voltages?

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Tuesday, June 9th, 2020 AT 5:23 PM
Tiny
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Hi thanks. I don't have an obd 1 scanner and did try obd 2 scanner with adapter and it didn't work. I also disconnected the hose from the fuel pressure regulator with no difference in change when engine was running. I did put another new fuel pump I had as backup and now car just cranks and doesn't start.
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Tuesday, June 9th, 2020 AT 5:32 PM
Tiny
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Hi, the latest update is, I did put back the better quality fuel pump and started and drove around the parking lot, but still it has rough idle and either wants to stall or completely stalls when I put it in drive or reverse. I disconnected the hose from fuel pressure regulator and there was no difference the way engine. Ran. I also switched the fuel relay switch with no difference and did check for vacuum leaks again finding nothing.
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Wednesday, June 10th, 2020 AT 8:51 AM
Tiny
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Do you have a vacuum gauge? If so, can you attach it to the engine's vacuum system and get a video of the needle when the engine is running rough? Maybe we don't have a vacuum leak but maybe a valve sticking causing a similar issue.
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Wednesday, June 10th, 2020 AT 6:49 PM
Tiny
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I don't have a vacuum gauge and wouldn't know how to use it. I'm almost getting ready to give up and get rid of the car.
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Wednesday, June 10th, 2020 AT 10:13 PM
Tiny
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Thank you and I appreciate all your help.
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Thursday, June 11th, 2020 AT 4:48 AM
Tiny
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Okay. However, let's try one more thing before we throw in the towel. I was just reading through this and I don't see where you replaced the EGR valve itself.

I see the other issues with the hose and things but the most likely cause of an engine stalling when putting into gear is a sticking EGR valve.

Let me know what you think. Thanks
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Thursday, June 11th, 2020 AT 6:43 PM
Tiny
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Thank you, I never replaced it before but car wont start when EGR valve is disconnected. I also have a broken accelerator cable and been that way since I bought the car. I noticed when I wiggle it and adjust it the car will stay idling steadily and I think that might be the culprit and never been a problem until now.
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Thursday, June 11th, 2020 AT 7:01 PM
Tiny
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Well, that will definitely need to be addressed as that too will cause this. I would progress with your plan and replace that then see what you have. Let us know how you make out. Thanks
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Friday, June 12th, 2020 AT 6:22 PM
Tiny
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Thanks. Did have throttle body cable replaced and car went back to cranking and not starting. This time I removed the crankshaft position sensor and tested it with multi-meter and was at 530 ohms and according to the revised BMW repair manual I found on the internet says it should read 540 ohms. I hope this is the problem.
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Saturday, June 13th, 2020 AT 8:05 AM
Tiny
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I show the sensor should be 520 +/- 30 ohms so I am not so sure that is the issue.

However, if it is cranking but not starting, we need to stick to the basics to figure out what it is missing. We need to check spark and fuel. Fuel is the easiest, just spray some starter fluid in the intake and see if the engine fires. If not, pull a plug and connected it back to the ignition and hold it against the block to see if there is spark while cranking.

https://www.2carpros.com/articles/car-cranks-but-wont-start
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Saturday, June 13th, 2020 AT 7:43 PM
Tiny
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Hi, I cleaned the CSP, because it was very dirty and the wire connector is coming apart and I also swap out the fuel pressure regulator had a used one I had here and didn't tighten it all the way. The car did start did see fuel spray out of fuel pressure regulator. I tighten it back up and the car started again, the engine did want to stall unless I gave it gas. I'm posting pictures of the CSP.
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Sunday, June 14th, 2020 AT 12:02 PM
Tiny
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Were those pictures before or after you cleaned it? I would just replace it if it were me but I do know people who clean them.

Can you get a video of what it does when starting and stalling? Start the engine keep it running and then just let it idle so I can see what it does when it goes to stall. Thanks
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Sunday, June 14th, 2020 AT 7:06 PM

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