1st start of the day, idle never higher than 750-800

Joined
1 July 2024
Messages
97
Location
Prosper, TX
Now that "colder" weather is here in Texas I noticed that the idle never really raises regardless of the ambient temperature outside. Per the manual (I-66) if I have issues with cold start (which I don't really) or if the idle is out of spec 1000-2000. It says to check the fast idle control valve.

I-69 states however if the check engine light is not on to do a series of checks.

I am going to start with page I-69 but is the fast idle valve typically a point of failure?

Granted most people don't have a ton of miles on theirs I am just curious if this is typically a point of failure and if I should order from amayama.
 
After sitting for about 7 hours initial start was between 900-1000 no load, at engine temp 700. It does jump a bit prior to getting to normal operating temp.
 
First off, give consideration to the fact that on a 1991 your tach is probably suffering from calibration drift. If you think you have a problem I would be confirming the actual idle speed using an external tach and the tach test loop.

Your warm idle of 700 RPM is on the high end of acceptable (650+/- 50). Again, I would confirm what the actual value is with an external tach before doing anything. The drop from 900 - 1000 to 700 RPM would be relatively consistent with what the 1991 service manual describes on page 11-68 for warm up idle increase which is between 150 and 300 RPM. It is not obvious to me that you have a problem. On page 11-68 of the 1991 manual there is a graph which shows the engine RPM versus coolant temperature. Unfortunately, the .pdf I have has no units on the RPM scale.

I think there may be a problem with the 1991 service manual on page 11-68 because I think that reference should be to the Fast Idle Valve, not the EACV. The description of the operation of the FIV on page 11-83 which describes its responsibility in controlling warm up idle speed is what I would expect. The EACV is responsible for the Idle up associated with things like the alternator and air conditioning and idle control when you hit the brakes and release the clutch.

Full disclosure, I have a 2000 with drive by wire so I have neither an EACV or FIV plus I have the secondary air injection which further elevates cold start RPM so I have no direct NSX experience.
 
First off, give consideration to the fact that on a 1991 your tach is probably suffering from calibration drift. If you think you have a problem I would be confirming the actual idle speed using an external tach and the tach test loop.

Your warm idle of 700 RPM is on the high end of acceptable (650+/- 50). Again, I would confirm what the actual value is with an external tach before doing anything. The drop from 900 - 1000 to 700 RPM would be relatively consistent with what the 1991 service manual describes on page 11-68 for warm up idle increase which is between 150 and 300 RPM. It is not obvious to me that you have a problem. On page 11-68 of the 1991 manual there is a graph which shows the engine RPM versus coolant temperature. Unfortunately, the .pdf I have has no units on the RPM scale.

I think there may be a problem with the 1991 service manual on page 11-68 because I think that reference should be to the Fast Idle Valve, not the EACV. The description of the operation of the FIV on page 11-83 which describes its responsibility in controlling warm up idle speed is what I would expect. The EACV is responsible for the Idle up associated with things like the alternator and air conditioning and idle control when you hit the brakes and release the clutch.

Full disclosure, I have a 2000 with drive by wire so I have neither an EACV or FIV plus I have the secondary air injection which further elevates cold start RPM so I have no direct NSX experience.
Your input is always appreciated. I will get an external tachometer reading to verify or at least confirm if the reading on the tach is correct or not before moving forward.

After reading more into the service manual I was also considering that nothing may be wrong, I guess I am so used to other vehicles that cold start idles are 1200-1500 so I thought maybe the fast idle was not working as intended.
 
For a "norm", my stock 92 holds a slightly higher idle when starting cold, but it goes down to normal VERY quickly (say 1-2 minutes) which is well below "up to normal" water temp range. No matter how cold or how much later i think a second start up later in the day would have a normal low idle from the start. I haven't owned an external tach for many moons.
 
For a "norm", my stock 92 holds a slightly higher idle when starting cold, but it goes down to normal VERY quickly (say 1-2 minutes) which is well below "up to normal" water temp range. No matter how cold or how much later i think a second start up later in the day would have a normal low idle from the start. I haven't owned an external tach for many moons.
Yeah that's how mine has been since I owned it, but recently thought maybe it's supposed to idle higher at first start / colder temps. Good to know mine isn't the only one who does this so Old Guy is likely right and nothing is wrong. I'll at least verify the rpms so I can confirm my tach is working correctly.
 
For 91 - 94 models (and 95 Type-R only) that are non-DBW, the fast idle target is around 1,500rpm depending on the coolant temperature.

Once fully warmed up, the target idle rpm is about 800rpm for MT.

At lower rpm range, it is fairly normal to have around 100rpm offset on our gauge cluster due to the calibration done through the pot.
It’s cross coil vector based with needle having its own weight so depending on which way the needle approached, there is always some reading difference.
Also, there is no temperature compensation circuit.

If you have very stable idle rpm after fully warmed up, just use that position as the 800rpm (MT) instead of the indicator lines on the dial face.

With the cold engine, you should see somewhere around 1,300 – 1,500+ rpm on the [ 1st ] fireup of the day unless you live in extremely hot tropical weather all year round.


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The Fast Idle Valve (FIV) is not electronically controlled.
Instead, it’s just mechanical relying on the expansion/contraction of the wax element.
The wax element is held inside the bottom chamber where I’m holding it with my finger.
It’s submerged in the coolant.


AP1GczPm1owmaQxEOL_wvF541NGzciY_gHqkd-ETKRZtUuzlsyB4XlCQZRwxS_Fc1t3zog3BUnWGHJOcgj3HQfVi0xTCcUJR-Ekum8w5B2zzk3GeQ6vTIb6gasIV4e2JBY4eDlZU_aXnziTlJe5YFBpeXHYdTw=w1080-h608-s-no

At the very top, if you remove the brass coloured flat plate/cap (should replace o-ring if opened), you will see white outer sleeve with the brass plunger inside.

In this photo, you can see the oval shaped opening at the 12 o’clock position.
(Better view in the 1st photo.)

This opening is connected to the extra air channel located at the TH body.
Same channel used for the EACV as well as the base idle air adjust screw.

If you have ever cleaned the TH body, you will notice small circle opening at about 11 o’clock position when looked at the TH body from the air box side.
That’s the entrance of the extra air channel and the air is pulled by the vacuum of the intake manifold.


Back to the FIV.

When the engine is cold, there is a small gap (just like in the 2nd photo) between the brass plunger and the white outer sleeve. Thus, the extra air from the oval opening can enter the intake manifold (via the circule opening - 1st photo) through this gap under vacuum.

As the coolant gets warmer, the wax element expands and pushes up the plunger towards the white sleeve.
Eventually, the plunger meets the internal wall of the white sleeve resulting in good seal.
No more extra air sucked into the manifold through the FIV and the EACV is the main controller for this air channel.


[ [ You are not supposed to disassemble the FIV any further. ] ]
I did it at my own risk.

The white sleeve is actually threaded into the FIV metal body.

So, by rotating it, you can adjust the initial amount of the ‘gap’ against the plunger under the cold wax element.
Thus, this will change the target fast idle rpm.

You can’t test the gap adjustment if the coolant is even slightly warm so most likely, forced to wait after parking overnight before trying the new adjustment.

So, again, don’t disassemble FIV unless you know what you are doing.
The workshop manual states DO NOT disassemble.

Unlike EACV, once the engine is warmed up, there is no air flow inside the FIV so fairly clean even after so many mileage.
However, seen some failure with it in the past.
Though, normally, it’s the other way round and the idle rpm stayed extremely high even after fully warmed up.


Instead of taking any risk, if you are happy to replace the o-ring, just remove the top brass flat plate from the FIV while the engine is cold and look for the tiny gap between the white sleeve and the centre brass plunger.

If you start the engine under this condition, you should be able to feel the vacuum air flow by covering the gap with your finger.

If no vacuum, that means the plunger got stuck closed (very unlikely as it’s against the spring load) or the wax element failed.

Kaz
 
Once fully warmed up, the target idle rpm is about 800rpm for MT.

Yes, the idle speed setting in the 1991 service manual is slightly confusing. Page 11-84 says for a manual transmission set to 650+/-50 RPM with the EACV disconnected. Turn engine off and connect the EACV and restart and for a manual you should be at 800 +/- 50 RPM so the EACV must be by-passing a small amount of control air at hot idle speed.

So, is the EACV actively controlling by-pass air during the warm up process? That would seem like a strange over lap of purpose? If the EACV contributes air during warm up why bother with an FIV at all. The limited number of Bosch pre DBW port injection systems that I am familiar with use a single electrically controlled idle air valve for all start up air, warm up and idle up functions. No need for a separate valve for warm up air.
 
Yes, the EACV plays a fairly large role in cold start and cold idle control. There are several tables defining the baseline coolant temp vs. EACV duty cycle and it is pretty far open during cold idle.

I've actually deleted the FITV completely since I have ITBs and my car relies fully on the EACV for cold starting, and it does okay down to 20-25F. I'm not sure why Honda went with both but its a fairly common configuration on their other 90s cars as well. Maybe for redundancy?
 
For 91 - 94 models (and 95 Type-R only) that are non-DBW, the fast idle target is around 1,500rpm depending on the coolant temperature.

Once fully warmed up, the target idle rpm is about 800rpm for MT.

At lower rpm range, it is fairly normal to have around 100rpm offset on our gauge cluster due to the calibration done through the pot.
It’s cross coil vector based with needle having its own weight so depending on which way the needle approached, there is always some reading difference.
Also, there is no temperature compensation circuit.

If you have very stable idle rpm after fully warmed up, just use that position as the 800rpm (MT) instead of the indicator lines on the dial face.

With the cold engine, you should see somewhere around 1,300 – 1,500+ rpm on the [ 1st ] fireup of the day unless you live in extremely hot tropical weather all year round.


AP1GczMoZpwBxatSMxu_J_r7kB69jxerj-P9aMQmg_HOuwbGieyFHnTF4CSfKD-kzIfDoE-Qr-bOrA1e-TfobV4ZEsI1aFLrXfkhDReR5DBoCsXOcTLoZdBWTR9ufO1o4nH2enlM58C96RMpvHAC2c-n0lE1Mg=w1080-h608-s-no

The Fast Idle Valve (FIV) is not electronically controlled.
Instead, it’s just mechanical relying on the expansion/contraction of the wax element.
The wax element is held inside the bottom chamber where I’m holding it with my finger.
It’s submerged in the coolant.


AP1GczPm1owmaQxEOL_wvF541NGzciY_gHqkd-ETKRZtUuzlsyB4XlCQZRwxS_Fc1t3zog3BUnWGHJOcgj3HQfVi0xTCcUJR-Ekum8w5B2zzk3GeQ6vTIb6gasIV4e2JBY4eDlZU_aXnziTlJe5YFBpeXHYdTw=w1080-h608-s-no

At the very top, if you remove the brass coloured flat plate/cap (should replace o-ring if opened), you will see white outer sleeve with the brass plunger inside.

In this photo, you can see the oval shaped opening at the 12 o’clock position.
(Better view in the 1st photo.)

This opening is connected to the extra air channel located at the TH body.
Same channel used for the EACV as well as the base idle air adjust screw.

If you have ever cleaned the TH body, you will notice small circle opening at about 11 o’clock position when looked at the TH body from the air box side.
That’s the entrance of the extra air channel and the air is pulled by the vacuum of the intake manifold.


Back to the FIV.

When the engine is cold, there is a small gap (just like in the 2nd photo) between the brass plunger and the white outer sleeve. Thus, the extra air from the oval opening can enter the intake manifold (via the circule opening - 1st photo) through this gap under vacuum.

As the coolant gets warmer, the wax element expands and pushes up the plunger towards the white sleeve.
Eventually, the plunger meets the internal wall of the white sleeve resulting in good seal.
No more extra air sucked into the manifold through the FIV and the EACV is the main controller for this air channel.


[ [ You are not supposed to disassemble the FIV any further. ] ]
I did it at my own risk.

The white sleeve is actually threaded into the FIV metal body.

So, by rotating it, you can adjust the initial amount of the ‘gap’ against the plunger under the cold wax element.
Thus, this will change the target fast idle rpm.

You can’t test the gap adjustment if the coolant is even slightly warm so most likely, forced to wait after parking overnight before trying the new adjustment.

So, again, don’t disassemble FIV unless you know what you are doing.
The workshop manual states DO NOT disassemble.

Unlike EACV, once the engine is warmed up, there is no air flow inside the FIV so fairly clean even after so many mileage.
However, seen some failure with it in the past.
Though, normally, it’s the other way round and the idle rpm stayed extremely high even after fully warmed up.


Instead of taking any risk, if you are happy to replace the o-ring, just remove the top brass flat plate from the FIV while the engine is cold and look for the tiny gap between the white sleeve and the centre brass plunger.

If you start the engine under this condition, you should be able to feel the vacuum air flow by covering the gap with your finger.

If no vacuum, that means the plunger got stuck closed (very unlikely as it’s against the spring load) or the wax element failed.

Kaz
Thank you for taking the time for all this info kaz! My vehicle has just about 237k miles now so this not working does not suprise me. If Anything I will order one from amayama which will allow me to disassemble the old one and see how it works.

Is there any event that the throttle body is causing the FIV to not function as intended? maybe some build up? I have not had the time to take apart of the throttle body to clean it just yet.

Thanks again in advance!
 
If the passages to the FIV or the EACV inside the throttle body are dirty this will impede air flow through the two valves and reduce the high idle RPM. In addition to doing the simple test in the SM to confirm that the FIV is doing something, cleaning the passages might be useful before deciding to order a new FIV. No point in installing a new FIV and finding out nothing has changed.
 
amazing engineering solution.......wax.....
 
If the passages to the FIV or the EACV inside the throttle body are dirty this will impede air flow through the two valves and reduce the high idle RPM. In addition to doing the simple test in the SM to confirm that the FIV is doing something, cleaning the passages might be useful before deciding to order a new FIV. No point in installing a new FIV and finding out nothing has changed.
Yeah, I've been meaning to clean it anyways so i'll start there. The FIV isn't insanely expensive, it's about 58 dollars through amayama and I have a big order to make already anyway.
 
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