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Discussion Starter · #21 · (Edited)
So i replaced the hose and the problem still persists.

I read up on common causes to my symptoms and a bad temperature sensor can be one of them. Given I have intermittent temperature readings where the needle goes up on a cold start, and given this problem only happens after the engine has warmed up, I'm going to guess its a bad temperature sensor and order a new one.

I'm not totally convinced its the temperature sensor. The coolant temperature gauge sender goes straight to the gauge so I don't see how the PGM-FI ECU could use data from that sensor. Perhaps it uses data from the temperature sensor at the bottom of the radiator? I'm not on the computer ATM so can't check wiring diagrams as I haven't gotten around to installing a PDF viewer app on my new phone.
 

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Discussion Starter · #22 · (Edited)
So I just checked the 1990 shop manual for my 1991 Prelude and found there is a coolant temperature (TW) sensor right next to the coolant temperature gauge sender which runs to the PGM-FI ECU.

I'm not getting an ECU code 6 but I could perhaps warm up the engine and check for 200-400 ohms at the sensor. However, that would mean waiting until tomorrow which is not acceptable so I going to go ahead an order a new sensor now.

I don't see a torque specification for the sensor so I will install it hand-tight.
 

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Maybe I should wait and measure the resistance. A bad coolant temperature sensor should not be causing the gauge to go up on a cold start, right?
There are two sensors. The single wire sensor is for the gauge only, and does not communicate with the computer. The two wire sensor is for the computer, and is unrelated to the gauge.

In other words, the two sensors are independent of each other. So if you are having strange issues with the gauge, its either the gauge itself, the single wire sensor is faulty, or you have air pockets in the coolant system.

Im sure you burped your coolant system after replacing the hoses, and seeing as this is an issue on cold start, id suggest replacing the single wire sensor to fix your gauge. This is going to be unrelated to your idle issue though.
 

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Discussion Starter · #25 ·
In that case, I am going to say I have air pockets in the cooling system because a bad coolant temperature gauge sender would not affect the engine by causing it to ruff idle and stall when accelerating after it has warmed up.

Yes, I burped out my cooling system again tonight in accordance with the shop manual.

I took some photo's of the air breathing system in case somebody can see something wrong that I can't see.

Vehicle Motor vehicle Automotive air manifold Auto part Automotive exterior


Motor vehicle Car Automotive design Automotive fuel system Vehicle


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Discussion Starter · #27 ·
I have ECU code 14 (EACV) that has been in the ECU's memory for a few weeks. I knew about this but dismissed it because I think it happened before this problem started. Tomorrow I will clear the ECU's memory and see if it comes back when the car stalls.
 

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Discussion Starter · #30 ·
If you watch the bouncy idle in the first video in my first post with the sound up loud (the its really quiet), its quite random and may be an indication of a bad hose. Do you have any advice on finding a bad hose without any equipment?
 

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Then I would replace the idle control valve. It sounds to me that the motor in the eacv cant keep up with the engine, getting stuck or moving slow.

It didnt sound to me like the idle was surging. But if you want to check for vacuum leaks, get a can of brake cleaner and spray around your hoses, throttle body and intake gaskets, injectors, and all around the intake manifold. Vehicle must be warmed up, and do not spray around your distributor or spark plug wires unless you want to set your engine on fire.

If you have a vacuum leak, the engine will idle up because you are introducing fuel through the leak. Then its just a matter of finding it.
 

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Discussion Starter · #40 ·
When it hasn't warmed up I can absolutely thrash it. Twenty minutes later once its warmed up, the problems start and its not drivable- the engine kicks when idling like in the first video in the first post and then the RPM's drop when accelerating or the RPM struggles to rise, like in the second video in my first post.

The volume needs to be turned up really loud when playing the videos.

This problem started after I removed the pipes goings into the throttle body and the vacuum hoses when I removed my engine wire harness. I've removed the pipes going to the throttle body many times over the years, never had a problem but this was the first time I have ever removed the vacuum hoses, but I have triple checked that I have put the hoses back correctly.
 
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