Car going down in revs and not responding to throttle input

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Ewan1
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Sun Feb 10, 2019 12:52 am

Hi,

I recently bought a 1990 316i with the m40b16 engine. When I bought the car there was a problem where that as the car was started while it was already warmed up it would barely be able to keep itself alive and sometimes stall. Helping it to stay alive with the throttle pedal didn’t work, there would be no response and after a couple it would pick up and then stop again. After a couple of minutes it would fix itself and you could drive it around without any problem. I drove it about 300km home.

Now when I start the car it is idling on very low revs and will sometimes stall. While driving it will also randomly cut up every few seconds. So as I’m on the throttle it will fall back to what feels like zero throttle input and the car will start to slow down. While this is happening there is no response to the throttle pedal at all. As long as there are gears/revs left it will stay running. After a couple of seconds the car will respond to the throttle again and rev up like it should. As the change can be quite quick it sometimes drives a little bit jerky.

When the car has been parked for some hours it will drive away fine but after a couple hundred meters to about 1 kilometer the problem will come back.

I’ve already cleaned the throttlebody and the idle control valve. I’ve also tested the throttle position sensor and it reads like it should.

As there are so many things that can be the problem my question is if there is anyone who recognises these things or knows what part of the car mostlikely has a fault.

Thanks on advance,

Ewan
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Brianmoooore
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Sun Feb 10, 2019 9:48 am

What does the tacho. needle do when the car misbehaves? Does it drop to zero, or track the actual revs?
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Ewan1
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Sun Feb 10, 2019 10:40 am

Brianmoooore wrote:
Sun Feb 10, 2019 9:48 am
What does the tacho. needle do when the car misbehaves? Does it drop to zero, or track the actual revs?
It does track the actual revs
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BenHar
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Sun Feb 10, 2019 10:59 am

Have you checked the rotor arm and distributor cap?

Ben
Ewan1
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Sun Feb 10, 2019 12:39 pm

BenHar wrote:
Sun Feb 10, 2019 10:59 am
Have you checked the rotor arm and distributor cap?

Ben
No I have not, how would I go about checking these? Do I have to replace them and then see if it’s fixed or can I inspect them some other way?
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BenHar
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Sun Feb 10, 2019 7:08 pm

Google it or take the car to a garage.

It's not difficult but it's also not easy to write a couple of lines on a forum to explain it.

Ben
Oseanosea
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Mon Feb 11, 2019 6:10 pm

Have you considered the fuel filter? I’ve experienced similar symptoms with a partially clogged fuel filter in 2 of my vehicles.
“Keep lowering your standards until you achieve your goal.”
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Brianmoooore
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Mon Feb 11, 2019 6:38 pm

Ewan1 wrote:
Sun Feb 10, 2019 10:40 am
Brianmoooore wrote:
Sun Feb 10, 2019 9:48 am
What does the tacho. needle do when the car misbehaves? Does it drop to zero, or track the actual revs?
It does track the actual revs
In that case, we can probably discount problems with most of the LV electrics.
The M40 engine is sensitive to the condition of the HV electrics, so a visual check of the distributor my be a good (and free) idea, as BenHar says. You're checking for any cracks, significant bits of the metal parts being eroded away, and the condition of the carbon button and its spring.
If all is OK there, I suggest you invest in a cheap (sub £20) ebay fuel pressure gauge kit, and fix that up so you can see it while you drive the car.
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Ewan1
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Tue Feb 12, 2019 12:59 pm

Figured out that the problem was the MAF sensor. Guessing it didn't read the incoming air correctly and in return wasn't putting in enough fuel making it run lean. Replaced the sensor and now it's working correctly again.

Thanks everyone for all the tips!
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Brianmoooore
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Tue Feb 12, 2019 2:11 pm

Great news :thumb: Your description of the fault was too general and vague to home in on anything specific without more info. and testing, but a duff AFM certainly fits your symptoms. We would have got to it eventually!
You haven't mentioned whether or not your car has a cat., but if it hasn't, and the AFM you have fitted is from a post 08/91 car, you may have a problem with CO levels when your next MOT test takes place, as the late AFM doesn't have an adjustment pot. fitted. Even with the earlier AFM, yu may run into problems if the pot. has been 'adjusted'.
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Ewan1
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Wed Feb 13, 2019 1:52 pm

Brianmoooore wrote:
Tue Feb 12, 2019 2:11 pm
Great news :thumb: Your description of the fault was too general and vague to home in on anything specific without more info. and testing, but a duff AFM certainly fits your symptoms. We would have got to it eventually!
You haven't mentioned whether or not your car has a cat., but if it hasn't, and the AFM you have fitted is from a post 08/91 car, you may have a problem with CO levels when your next MOT test takes place, as the late AFM doesn't have an adjustment pot. fitted. Even with the earlier AFM, yu may run into problems if the pot. has been 'adjusted'.
Hahaha yeah the problem itself was pretty vague to begin with and a lot of things that could be the source. Happy to have it running well again.
Not really sure if I have a cat or not, haven't checked yet. The new AFM was said to be fitted on an 88 car. It passed MOT (Dutch version of it) last November so it's still some time before the next one, will check these things before then.

Thanks again!
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