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I just purchased an 89 prelude SI. near the end of abt a 25-30 mile drive, when i come out of gear the revs stay around 3 grand. i pedal the throttle and the motor chokes down, sometimes cutting off but sometimes i can gas it and keep the motor alive. It will crank right back up but still idle around 12-1500 rpm. When it happens i hear a sound like air escaping. I’m assuming a vacuum leak but it doesn’t happen all the time. My line of thinking is if i had a hole or popped hose, the problem would be continuous. Any ideas on where to start?
 

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What motor does it have? I checked and cleaned my fast idle thermal valve....iac valve...and I still heard the same air escaping sound (which was actually sucking too much air) and found it to be my intake gaskets.
 

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Noncompliant has a good idea. I wonder if your head is bolted down correctly. Maybe when the block warms up expansion of the metal (1) makes any problem with the intake gasket worse or (2) opens up an air passage independent of even a good gasket. Or maybe your gasket is just bad.

I believe the ECU operates in closed loop mode when it is cold, i.e., the mixture is determined according to preset values in the ECU without regard to sensor input. When the engine is cold, you're getting the factory air/gas mixture ratio and so smooth operation. However, when the engine reaches operating temperature the ECU starts using sensor input. If you've got an air leak somewhere the ECU will think the mixture is too lean and so adjust injectors to add more fuel. More fuel means more power. You might be able to see black smoke out of the exhaust which is an indication of a too-rich mixture.

I once forgot to reconnect the vacuum line to the cruise control control mechanism on my '89 Prelude Si. The effect was a pronounced surging not a steady, higher RPM. I couldn't hear any hissing sound either. Maybe different vacuum lines left unconnected or broken DO hiss and the effect is not a surge, I don't know. Your ears may be your best diagnostic tool at this point.
 

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Can spray carb cleaner thoroughly around intake gaskets where you may hear hissing sound. If you notice a change in what it's doing you have an air leak. Just be sure to do it section by section so that you may be able to exactly pinpoint where your issue is to determine what it is.
 

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Noncompliant has a good idea. I wonder if your head is bolted down correctly. Maybe when the block warms up expansion of the metal (1) makes any problem with the intake gasket worse or (2) opens up an air passage independent of even a good gasket. Or maybe your gasket is just bad.

I believe the ECU operates in closed loop mode when it is cold, i.e., the mixture is determined according to preset values in the ECU without regard to sensor input. When the engine is cold, you're getting the factory air/gas mixture ratio and so smooth operation. However, when the engine reaches operating temperature the ECU starts using sensor input. If you've got an air leak somewhere the ECU will think the mixture is too lean and so adjust injectors to add more fuel. More fuel means more power. You might be able to see black smoke out of the exhaust which is an indication of a too-rich mixture.

I once forgot to reconnect the vacuum line to the cruise control control mechanism on my '89 Prelude Si. The effect was a pronounced surging not a steady, higher RPM. I couldn't hear any hissing sound either. Maybe different vacuum lines left unconnected or broken DO hiss and the effect is not a surge, I don't know. Your ears may be your best diagnostic tool at this point.
I may have that backward. Closed loop means the ECU is working off of input from the sensors. Open loop means it's reading from "maps" (data tables?) embedded in the ECU to calculate engine responses. See here.
 

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Non_complaint's advice is the best in my opinion but my experience with carb cleaner is it will help only a little bit. Perhaps you will get better results.

Air intake control valves are famous for playing up on this car. Checkout how many second hand ones are on eBay, they are a sought after commodity. Welcome to the Prelude club. :)
 

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I just thought of an idea. There is a hatch in the carpet on the front passenger side where the girls feet go. With the car running, open up the hatch and see if the LED is flashing. If it flashes 16 times without stopping, then it is the air intake control valve in which case you will want to clean it. I have memorised the number because that's how much grief it has given me. Its a bitch of a job to clean so make sure you have an afternoon off and a pick set because you will need to take your injectors off to get to the bolts with the B20A engine. Enjoy. :)
 

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I just purchased an 89 prelude SI. near the end of abt a 25-30 mile drive, when i come out of gear the revs stay around 3 grand. i pedal the throttle and the motor chokes down, sometimes cutting off but sometimes i can gas it and keep the motor alive. It will crank right back up but still idle around 12-1500 rpm. When it happens i hear a sound like air escaping. I’m assuming a vacuum leak but it doesn’t happen all the time. My line of thinking is if i had a hole or popped hose, the problem would be continuous. Any ideas on where to start?
FAQ (Frequently Asked Questions), Common Repairs, etc

Read up.
 
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