355 Idle RPM dropping sometimes down to 650 for a sec or two while driving

Anybody had experienced the idle RPM dropping sometimes down to 650 for a sec or two while driving then raising back to 1100. Not every time but sometimes. When the car standing still the idle RPM is always 1100 on a cold or warmed up engine.

 

Solved. In my case it was none of the above but the first response from taz355 was in a right direction. I already had taken out and cleaned with gasoline the idle control valve and when shaking it the valve inside clearly was turning freely/not sticking to anything. With the cleaned valve the problem was still exactly the same. Added sealant to the thick rubber O-ring from the T-shape air channel in which the idle valve goes in: no change the problem was exactly the same. Didn't think of the MAF as if it's faulting it will be problems with engine loosing power and it wasn't my case. But I have a spare and I swapped it- no effect. Next I had read the Error Codes. I have a generic code reader and don't have a SD1/SD2 or Leonardo. So I the only code I had was P0153 02 Sensor Circuit Slow Response (Bank 2 Sensor 1). Replaced the front oxygen sensor on the driver side. The issue was still the same.

The solution: I kept asking around and the Service Manager from my local Porsche dealership after listening to the problem said: "It could be either a sticky idle control valve or the throttle position sensor. The computer doesn't know the correct throttle position." I said that I already had taken out and cleaned the idle control valve and the valve itself was moving clearly freely inside of the valve body. The Service Manager said: "Then it should be the throttle position sensor". I said: "I've been thinking about it!"

So bingo. The problem with my 355 was in the throttle position sensor as the guy had said. It is located on the Bank 1 (Passenger side) on the rear side of the engine. I took it out, cleaned with gasoline, put it back, took the 355 to a test drive, and: "Ta-da!" The magic moment. Everything is perfect. Smile on my face ;-) Driven it for 1hr.

Cleaning process:
The throttle position sensor is all sealed. It only has a window on the back where the throttle rod coming in. So I poured some gasoline in it, shook it a few times, twisted the plastic insert where the throttle rod goes in a few times left/right as far as it goes without applying any force. Turned the sensor over shook off all gasoline out. Repeated 3 times. Poured some gasoline to each connector, closed the connector by a finer, shook it a few times. Repeated with another connector.

Left the sensor belly up on a direct sunlight for a 1/2 hr to vapor all of the gasoline out. Assembled it back in to the car.

Thoughts:

Seems some oil vapor deposits/dust had accumulated inside of the sensor which the gasoline had cleaned out. Alcohol burns fat. So if you have problems with a fat, take some alcohol in and you will feel better ;-)

When putting the sensor back in I noticed it snapped in when setting in place. The engine block has a little O-ring around the throttle rod popping out. When I was taking it out, it had came out without snapping out meaning a stupid mechanic who was touching it before when assembling it didn't snap it in. But he had put the yellow paint over the holding screws all over marking his "warranty". I needed to scratch all of that paint off so I could put a flat screw driver to these bolts.

The cooling fans have nothing to do with such an issue, to my understanding.

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Yep! - for both points. Most of the time when going ton Neutral the engine was going to the proper idle RPM and I had observed the RPM was dropping relatively "softly" slowing the drop when getting closer to the 1100; and when it was the issue the RPMs were dropping fast down to 700 or 650; then to my understanding when the sensor finally was catching up after a sec or two it was telling to the ECU "I am on idle" and the ECU -> "open" to the idle control valve and the idle RPM were raising to 1100. I had it for a while dropping to 800, then to 700 so I didn't know of what to do. Now as I see it was getting worse over the time dropping it lower so the "Low Oil Pressure" light started to blink once or twice until the engine catches up with idle started making me looking into it.

$200 used on eBay. Ferrari 456 355 550 Throttle Position Sensor 158170. Not "cheap" but I'd say "cheaper vs. if you bring a car to a shop to fix it".

 

Good to know. I had an experience of dropping a drop of gasoline while cleaning something to a door strip rubber insert. That rubber strip is unforgiving. Nothing helped to cure the bubbles appeared on it. No heat gun helped. Those strips are really expensive for just a piece of rubber and nobody having those. Ended up getting it a brand new from Italy from the Factory.

 

You

If you don't agree, I am not forcing you. That's to my knowledge of how it works. But maybe there are other more health-related or well-being -oriented chat rooms you can discuss it. Good luck!

I solved the issue already, most likely my obd2 reader does have the option to read a throttle position, at this point I don't have a need to check. Hope it makes sense. But thanks, good to know.