Throttle Surging & Pulsating

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Yojedi
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Throttle Surging & Pulsating

Post by Yojedi »

My fully rebuilt B35 (w/ Motronic 1.3 ECU) is exhibiting some strange behaviors. Engine is still fresh and I've been slowly breaking it in by doing longer and longer drives. There's been a few strange symptoms so far;

1. Every so often during just normal drive throttle would suddenly surge momentarily for a second or two.
2. Sometimes idle RMP would jump way up to about 1200+/- and held there.
3. Even worse was that throttle would suddenly pulsate between 1000+/- and 1700+/- (without my foot on the throttle), and held there.

Condition 1 could happen rather quickly during the drive. Conditions 2 & 3 so far had happened only like 15~20 minutes into the drive. The whole thing would go away once I shut down and restarted engine. After that I could drive home without anything acting up again. At first I suspected an air leak post MAF which caused engine to lean out and surge in RMP. But when 2 & 3 began to happen my suspicion begin to point to TPS or MAF itself. Unfortunately I do not have spares to swap out and try. What do you guys think?

Jerry
demetk
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Re: Throttle Surging & Pulsating

Post by demetk »

I would look at the TPS first. Does your TPS have the round plug?
Yojedi
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Re: Throttle Surging & Pulsating

Post by Yojedi »

demetk wrote: Nov 04, 2021 9:11 PM I would look at the TPS first. Does your TPS have the round plug?
Yes it does have the round plug.
Yojedi
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Re: Throttle Surging & Pulsating

Post by Yojedi »

Anyone?
Mike W.
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Re: Throttle Surging & Pulsating

Post by Mike W. »

I'm not that familiar with M1.3, but I think this applies. If it sees idle, as in the TPS adjusted properly, throttle closed and idle switch made, it will cut off fuel above ~1300 RPM. If you have false air, meaning a vacuum leak, Motronic will adjust fuel, within limits, for the air going in. So, if you have excess air, M1.3 will supply the correct amount of fuel, but then the revs get too high for it to be at idle (it thinks you're going down a hill for instance) and will cut off fuel until RPM is below ~1300 when it starts fuel again. And on and on.

Except, I just noticed you said MAF. If you really have a MAF, has it ever been fully sorted out? I hear they can be a real problem to get set up right. But if you have an AFM, the above should apply.
demetk
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Re: Throttle Surging & Pulsating

Post by demetk »

Manual or auto? Pull the plug and test pins 4&6 for continuity with the throttle closed. These types of throttle sensors are very hard to find and expensive. If you have a manual tranny I suggest you switch to the e28 TPS with the 3 pin plug.
Yojedi
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Re: Throttle Surging & Pulsating

Post by Yojedi »

Mike W. wrote: Nov 06, 2021 8:58 PM I'm not that familiar with M1.3, but I think this applies. If it sees idle, as in the TPS adjusted properly, throttle closed and idle switch made, it will cut off fuel above ~1300 RPM. If you have false air, meaning a vacuum leak, Motronic will adjust fuel, within limits, for the air going in. So, if you have excess air, M1.3 will supply the correct amount of fuel, but then the revs get too high for it to be at idle (it thinks you're going down a hill for instance) and will cut off fuel until RPM is below ~1300 when it starts fuel again. And on and on.

Except, I just noticed you said MAF. If you really have a MAF, has it ever been fully sorted out? I hear they can be a real problem to get set up right. But if you have an AFM, the above should apply.
Mike,

I mis-spoke. I actually have an AFM.

Just a few days ago I experienced a more significant trouble with respect to this whole thing. I drove out to the 2021 So. Cal BMW Vintage event with my 5. Fifty+ some miles of morning drive at 80mph all the way to Woodley Park and my 5 ran flawlessly. However on my return drive in the afternoon, about 10 minutes into it the engine started to act up again, pulsating and surging. I managed to drive the car home after three hours of gingerly shifting gears right under 3000 rpm and crawled slowly in high gears.

What I have found from this episode is; regardless in which gear whenever RPM pushes passed about 3000, throttle surging would for sure happen.

This leads me to question what specifically does Motronic do differently with respect to fuel injection and ignition when RPM reaches 3000 rpm?
Yojedi
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Re: Throttle Surging & Pulsating

Post by Yojedi »

demetk wrote: Nov 07, 2021 7:38 AM Manual or auto? Pull the plug and test pins 4&6 for continuity with the throttle closed. These types of throttle sensors are very hard to find and expensive. If you have a manual tranny I suggest you switch to the e28 TPS with the 3 pin plug.
Demetk,

Mine is manual. According to E24 ETM its TPS uses only pins 1 (ECU throttle closed input), 2 (ECU throttle open input) and 3 (GND). What are 4 & 6?
demetk
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Re: Throttle Surging & Pulsating

Post by demetk »

Yojedi wrote: Nov 16, 2021 8:42 PM
demetk wrote: Nov 07, 2021 7:38 AM Manual or auto? Pull the plug and test pins 4&6 for continuity with the throttle closed. These types of throttle sensors are very hard to find and expensive. If you have a manual tranny I suggest you switch to the e28 TPS with the 3 pin plug.
Demetk,

Mine is manual. According to E24 ETM its TPS uses only pins 1 (ECU throttle closed input), 2 (ECU throttle open input) and 3 (GND). What are 4 & 6?
Is the engine and motronic from an 89 e24?
Yojedi
Posts: 235
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Re: Throttle Surging & Pulsating

Post by Yojedi »

demetk wrote: Nov 17, 2021 10:22 AM
Yojedi wrote: Nov 16, 2021 8:42 PM
demetk wrote: Nov 07, 2021 7:38 AM Manual or auto? Pull the plug and test pins 4&6 for continuity with the throttle closed. These types of throttle sensors are very hard to find and expensive. If you have a manual tranny I suggest you switch to the e28 TPS with the 3 pin plug.
Demetk,

Mine is manual. According to E24 ETM its TPS uses only pins 1 (ECU throttle closed input), 2 (ECU throttle open input) and 3 (GND). What are 4 & 6?
Is the engine and motronic from an 89 e24?
I was told it’s a late 88 production date.
Mike W.
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Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Throttle Surging & Pulsating

Post by Mike W. »

Yojedi wrote: Nov 16, 2021 8:23 PM

This leads me to question what specifically does Motronic do differently with respect to fuel injection and ignition when RPM reaches 3000 rpm?
Nothing that I'm aware of. Seems like a lot of people have had ignition problems in recent years, especially cap and rotor causing odd problems, have you checked them? Fuel pressure/delivery also comes to mind with the symptoms, but a vacuum leak also seems plausible. Also, what does the tach do when it hits a wall at 3K? If it drops out or acts erratic, that points you towards things that determine spark.

Also, obviously just in the driveway idling, push and pull on wiring, especially around any connectors. I had a bizarre problem on an E24 long ago where it would cut out over half throttle. Load dependent, not speed. Turned out to be a broken wire inside a harness making intermittent contact, but when the torque exceeded a certain level, the engine would move on the rubber motor mounts just enough to cause it to cut out. Boy did that one drive me nuts.
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