Difficult to takeoff from a stop - idle is stable

[trymes]

My ‘94 has become pretty difficult to get going in first gear. It has a manual transmission, and basically, it has become difficult to get rolling from a stop. If you give it enough gas, it will go, but feels like you’re slipping the clutch too much, but if you don’t, it seems far too easy to stall. If you just let off the clutch pedal without touching the gas, it will almost immediately stumble down to 500RPM as soon as the clutch starts engaging. If you immediately press the clutch back in, you can catch it and it’ll idle back up.

The car idles fine Normally, and with the A/C on, steady as can be, but it feels like it should be more forgiving. In other words, lately, I constantly feel like I’m still learning how to drive standard when in the NSX - I’m “nervous” when starting off from a stop, while other cars don’t give me the same problem, and neither did the NSX.

I’ve seen various threads about IACV and such, but those tend to be cars with issues idling, or revs dropping when stepping on the clutch, and so on. I’m also just having tough time coming up with a good phrase to search for with this problem, so if anyone can point me in the right direction, I’d be thrilled.

 

[goldNSX]

Hard to say. Does the clutch shudder while starting to take off from a full stop?
A manual NSX does requiry a little throttle compared to new cars which are more forgiving.

 

[trymes]

Yeah, I don't expect it to be the same as my Accord or my MG, but it's really, really touchy. The engine doesn't just lug down a little if you let the clutch out just enough to drag, it immediately seems to want to stall when it starts engaging. Otherwise, it's solid. It doesn't dip down when depressing the clutch pedal, it doesn't hang up when coming to a stop, it's just difficult to get a smooth takeoff unless you really get after it (and then it's not smooth).

I need to get in the car again and see if having the A/C on makes a difference in the behavior. Also, does the fast idle valve actually increase the idle when the A/C is on? If so, where should I expect it to be when the A/C is on?

Tom

 

[Honcho]

Yeah, I don't expect it to be the same as my Accord or my MG, but it's really, really touchy. The engine doesn't just lug down a little if you let the clutch out just enough to drag, it immediately seems to want to stall when it starts engaging. Otherwise, it's solid. It doesn't dip down when depressing the clutch pedal, it doesn't hang up when coming to a stop, it's just difficult to get a smooth takeoff unless you really get after it (and then it's not smooth).

I need to get in the car again and see if having the A/C on makes a difference in the behavior. Also, does the fast idle valve actually increase the idle when the A/C is on? If so, where should I expect it to be when the A/C is on?

Tom

Tom-

From what you're describing it sounds like a lightweight flywheel. Do you know if you have one in the car? The ECU adds fuel when you begin to accelerate to avoid stalling, but I'm not sure that's it. The fast idle valve does not increase idle during A/C operation- it is only driven by coolant temperature. The two systems are separate. However, the IACV does change idle when the A/C kicks on. It could be that your IACV is sticking closed but, looking at the ECU, the IACV does not start to open until 3,000 rpm, which is higher than most takeoff rpm.

I think it might be a clutch engagement issue where the clutch either is sticking or is not engaging smoothly as you let out the pedal. Several possible causes of that. When was the last time you had the clutch slave cylinder replaced?

 

[Old Guy]

I going to pick up a bit on what goldNSX started and ask about the clutch. First, how many miles do you have on the car and is it still the original clutch? Have you checked the engagement point on the clutch / pedal free play? I have a 2000 with the later design clutch; but, my observation is that the NSX seems to have a clutch mechanism design which provides a fairly high release point and a fairly narrow friction point (certainly compared to old school vehicles like your MY and my 1971 Volvo). This does seem to be a bit of a Honda thing on their sporty cars because my son's RSX displays the same characteristics although not as bad as on the NSX. The narrow and high engagement does make stop and go traffic a bit of a concentration exercise to be smooth unless you adopt the rev it like crazy and slide the clutch - which is definitely going to shorten clutch life. If you think that the problem represents a change from the past then check the free play and engagement point first because a pedal that is engaging at a higher point will make smooth driving harder.

I have the later DBW system so I have no direct experience with the IACV on the early cars. However, from my experience with other cars with idle air by-pass valves they have limited ability to by-pass enough air to make significant engine power. They can by-pass enough air to manage the increased load associated with the air conditioning and electrical loads; but, not a lot more. None of those cars had enough idle air by-pass capability to do a normal clutch engagement without stalling unless I apply some throttle. That said, if your IACV is dirty it could be making the transition from idle to throttle operation a little more tricky. However, all the other parameters suggest that it seems to be operating OK. If you are inclined to explore, you could remove the IACV and clean it out to see if that makes an improvement (gave replacement seals / gaskets on hand).

 

[goldNSX]

A dual clutch system can be a little bit tricky esp. if it gets older. When I bought my 91 the clutch had a serious shudder which would stall any engine. With a new clutch it was way better but not as easy as a single disc clutch and I've mainly driven MTs so far. In my FK8R, I can maneuver inch by inch by just letting the clutch grab a little bit without any decrease in idle rpm but won't be able to do so in the NSX. That's why I've asked the clutch behaviour.
The A/C does not raise the revs at idle.

 

[Jerry@Mobile]

When was the last time the throttle body was cleaned? Jerry

 

[RYU]

It sounds like whatever setup you have (stock or modified) has not changed and this "takeoff" issue just started happening right?

In other words, it's NOT like you just swapped into a lightweight flywheel and learning how to drive that, right?

It sounds like this is a fuel control issue that started happening with nothing else changing in your setup.

If you can confirm those things... I can weigh in further (FWIW)

 

[Old Guy]

Just because I was interested, when I was out doing some errands this morning I tried a 'no throttle engage the clutch' start. A bit to my surprise with a very slow clutch release the car started moving, the engine did not stall and the RPM stayed pretty steady around 900 RPM. It was off that fast idle cat warm-up sequence that the 2000+ cars do; but, the AC was running so the ECU was getting an throttle increase signal which may have helped with the start. Not something I am going to try regularly since it is a slow take off and I don't want to have an F150 Super Duty embedded in my trunk by some dude in a hurry.

Short answer, perhaps the IACV does contribute to a smooth idle to power transition so it might be worth checking to make sure it is clean.

 

[RYU]

i've always wondered... is the IACV an on/off plunger activated solenoid? As in, it's not a progressive bleed of vacuum that meters the air at idle. I've always thought it was an either open or either closed.. not a little open or a lot open, etc. IIRC it's a 3 wire connector so i'm thinking it's a bleed but i've also read it was an on-off switch in the old Hondas.. not like the new 5 wire new IACVs used in the K motors and S2000s

 

[Honcho]

i've always wondered... is the IACV an on/off plunger activated solenoid? As in, it's not a progressive bleed of vacuum that meters the air at idle. I've always thought it was an either open or either closed.. not a little open or a lot open, etc. IIRC it's a 3 wire connector so i'm thinking it's a bleed but i've also read it was an on-off switch in the old Hondas.. not like the new 5 wire new IACVs used in the K motors and S2000s

It's a progressive opening solenoid. When the car is moving over 3 mph, it starts to open at 3000 rpm and is fully open at 5000 rpm. It also opens and closes a specific amount based on lots of other events (A/C kicking on, cold temps, high altitude).

 

[furinax]

I would also ask about the lightweight flywheel. I had one and you had to be super sensitive with slipping the clutch. I kept stalling until I swapped it out with my Exedy.

That being said, how long have you been driving the NSX? Do you daily it? I definitely think if you daily it eventually you'll learn the friction zone but if it's just a weekend car you will probably not get used to it unless you drive it more. I am 5 years into my ownership and still stall every once in a while (usually in reverse).

If you're not new to the NSX, maybe a throttle body and EACV cleaning will do the trick. It's something you can easily do yourself and can significantly improve throttle response on non DBW cars. My IACV and TB needs to be cleaned once every few thousand miles because it gets gunked up. The car will stall if the A/C turns on and the idle has dropped below 1000 rpm.

 

[RYU]

It's a progressive opening solenoid. When the car is moving over 3 mph, it starts to open at 3000 rpm and is fully open at 5000 rpm. It also opens and closes a specific amount based on lots of other events (A/C kicking on, cold temps, high altitude).

at the risk of sidetracking this thread.. you're talking about the IACV not the EACV, right?

 

[Old Guy]

at the risk of sidetracking this thread.. you're talking about the IACV not the EACV, right?

In Honda / NSX speak, there is no IACV. On pre DBW cars there is an EACV (Electronic Air Control Valve) which is the ECU controlled solenoid operated valve Honcho refers to. I believe some people refer to this as an IACV (Idle Air Control Valve) which is a term used by some other vehicle manufactures for approximately the same function as the EACV on the NSX. There is a separate idle air control valve on the pre DBW cars called the fast idle valve which admits additional idle air during warm up and is a purely mechanical valve controlled by the coolant temperature which operates much in the same fashion as the thermostat.

I have no idea why Honda has a separate mechanical idle air valve in addition to the EACV. Most manufacturers of this vintage get by with a single ECU controlled valve for both idle control and fast idle during warm up.

 

[Honcho]

at the risk of sidetracking this thread.. you're talking about the IACV not the EACV, right?

In Honda / NSX speak, there is no IACV. On pre DBW cars there is an EACV (Electronic Air Control Valve) which is the ECU controlled solenoid operated valve Honcho refers to. I believe some people refer to this as an IACV (Idle Air Control Valve) which is a term used by some other vehicle manufactures for approximately the same function as the EACV on the NSX. There is a separate idle air control valve on the pre DBW cars called the fast idle valve which admits additional idle air during warm up and is a purely mechanical valve controlled by the coolant temperature which operates much in the same fashion as the thermostat.

I have no idea why Honda has a separate mechanical idle air valve in addition to the EACV. Most manufacturers of this vintage get by with a single ECU controlled valve for both idle control and fast idle during warm up.

Beat me to it.

 

[RYU]

Darn it! I keep getting the acronyms mixed up! How long have I owned this car............... seriously...

 

[docjohn]

Darn it! I keep getting the acronyms mixed up! How long have I owned this car............... seriously...

.but you have some very good vlogs:biggrin: