Performance and idle speed question

[No Name]

I have a question about performance and idle speed.  I have a 78 280z, runs great, just replaced the one part it seemed to need to enhance its performance which was the lower pcv hose, the old one was crap.  Boy what a difference the new one made...

However, once I replaced it the idle went up from 800 to about 1000rpm.  And I noticed the car's throttle switch didn't kick in to send it to full enrichment for full throttle application.  So I lowered the idle to about 750 or 800rpm with the engine up to temperature of course..  Then I took it for a spin and it blew me away, I mean it threw me back in my seat, the performance was 1000 times what it was at the previous idle setting.  

My question is, how does the idle screw adjustment effect the idle?  Is it adjusting airflow or does it control fuel flow somehow.  I assume it's air...  

Any input from you guys would be great.  Would love to know more about what's happening here.... 


-- Jeff..

 

Edited November 11, 2016 by JFRocks
simplifying question

[Mark Maras]

  I'm not familiar with the term (part) called a throttle switch. What type of fuel delivery system does the car have? Fuel injection (factory?), or carburetor(s), description and quantity. I'm curious what color the plugs were before the improvement.

[Zed Head]

No offense, but it's probably just a coincidence.  The proper way to tell if the "full enrichment" was happening would have been with a meter, at the ECU connector.

From an experimental perspective, if you want to verify that the 1/2 turn of the screw was the cause, you would turn the screw back to where it was.  The problem should return.  The idle adjustment screw just controls an air passage that lets air past the throttle blade.  It's the one with the big head and the spring underneath the head.

Could be that your distributor breaker plate is sticking, and it broke free when you were testing.  Just one possibility.

[No Name]

The throttle switch is the three prong switch on a 280z throttle body that adjusts the throttle as you depress the gas pedal.  The car runs lean until a certain contact point and under full throttle application it puts the car under full enrichment for acceleration.  

 

[No Name]

Thanks I appreciate the answer.  I just wanted to know if it controlled airflow in some way.  The fact that it does would affect the throttle switch's operation.  This explains why every mechanic I've had has told me not to open that idle screw too much.  It does effect performance.   

Thanks for the info.  The distributor is fine, it's all brand new.  It's just a matter now of getting everything dialed in.  Definitely no coincidence.  The throttle switch is counter controlled by two sub-systems in the car.  When the car is cold and being enriched by the air regulator being open, it detects that and doesn't operate the same as it does when the car is hot and not under enrichment.  That said, if the idle screw adjusts air in the system, then it would definitely effect the throttle switch's operation.  No doubt..  

Thanks for the responses..  

 

Edited November 11, 2016 by JFRocks

[Zed Head]

17 minutes ago, JFRocks said:

Thanks I appreciate the answer.  I just wanted to know if it controlled airflow in some way.  The fact that it does would affect the throttle switch's operation.  This explains why every mechanic I've had has told me not to open that idle screw too much.  It does effect performance.   

Thanks for the info.  The distributor is fine, it's all brand new.  It's just a matter now of getting everything dialed in.  Definitely no coincidence.  The throttle switch is counter controlled by two sub-systems in the car.  When the car is cold and being enriched by the air regulator being open, it detects that and doesn't operate the same as it does when the car is hot and not under enrichment.  That said, if the idle screw adjusts air in the system, then it would definitely effect the throttle switch's operation.  No doubt..  

Thanks for the responses..  

 

Sorry, and again, no offense, but until you turn that screw 1/2 turn and go out and show that it causes the engine to fall on its face again, you're just guessing and believing "mechanics" who probably don't really understand the engines.

This - "When the car is cold and being enriched by the air regulator being open, it detects that and doesn't operate the same as it does when the car is hot and not under enrichment." and the "no doubt" cause a lot of doubt out here.  Your conclusions and understanding are not right.  The AAR is on its own circuit, the ECU has no idea what its doing.

As I said, no offense intended.  Thanks for sharing your observation.  Go turn that screw 1/2 turn, you can do it by hand, and see what happens.  The idle speed will increase slightly and that's all.

[Zed Head]

p.s. only going in to detail because your first post suggested that you wanted understand how the EFI system worked.  You're n the right place, there are many members out here who understand the EFI system in great depth.  Your conclusions have potential, but are not right.

[No Name]

I appreciate the no offense comment..

Dude I already have done a trial and error by putting it back and it did go back to not functioning properly.  I'm far from stupid and have had this Z for 20 years, I know all its quirks at this point.  My question was simply what does the idle screw control, is it air or does it electronically effect fuel flow.  The answer is air, so it's easily figured out from there.  When the idle is high, the kick in point of the throttle switch changes within the gas pedal itself.  You have to push further for it to ground out and kick into a full enrichment state.  When the idle is lower you have to push less on the pedal.  So with all the new air flow improvements I've made recently, the higher setting was just never kicking in.  

The problem has been deduced effectively.  And of course i put it back and tested it a few time.  In fact I tried 3 settings, and found the one where it worked perfectly and the car held 800 rpm.  The car will old 600 but I don't want to or need to go that low.  

There just wasn't any documentation I could find anywhere that explained what the idle screw does.  i.e. how it controls the idle not just that it does.  

When the car is cold in the morning the air regulator is obviously open.  That little plug that's in the front of it sends a signal to the throttle valve switch that basically disables it.  When the air regulator opens, Then the throttle valve switch is active.  It has 3 prongs inside it, as you press the gas pedal they come closer together and eventually if you press enough ground out, which puts the car into what's called full enrichment.  Upon normal gas pedal operation i.e. driving at 40mph or whatever it's not under full enrichment.  The throttle switch is effected by the idle screw in that the further open the idle screw is the further you have to push the pedal to get it to kick in to give you acceleration.  So there is a correlation,

I've already been through this 10 years ago with CA smog check.  The car was always coming back rich back then.  Turns out it was the throttle switch.  Back in the old days kids would bend the prongs inside to keep the switch grounded at all times.  This kept the car under full enrichment (hence the rich smog test).  The reason someone would do that is for off the line speed.  The cars are faster off the line if you're racing around with it.  Since my car was once owned by a Hollywood stuntman, that made sense to both me and the mechanic...  So we replaced the switch, been passing smogs every year since.  No more CO issues. 

All of the above said, I'm not a novice with these cars, but it was always unclear to me how the idle screw did its job.  

Car's running great, idles great.. and yes if I open that idle screw again it'll impede the operation of the throttle valve switch once more... 

Thanks for the help, and no offence taken..  

Regards,
Jeff

[Zed Head]

9 minutes ago, JFRocks said:

  When the idle is high, the kick in point of the throttle switch changes within the gas pedal itself.  

Dude.  The above statement is wrong.  Sorry.  It's all described in the Factory Service Manual, in detail, with pictures.

[No Name]

Yes I have that manual as well....   My statement is not incorrect, but I think I phrased it wrong.  I should say, it's a lighter / milder kick in when the idle is set higher.  That's because of the idle screw effecting the air flow through the system. 

However it will vary from car to car depending on the modifications you've made.  My engine has been significantly modified from stock.  But there are still some stock items in place, such as the throttle body etc..  So perhaps again I should have been more clear... 

I really don't want to get into a pissing contest with you.  I came here to ask a simple question and granted I didn't go into much detail about my specific car, but all I'm getting is flak and some guy who doesn't even know what a throttle switch is..  

You answered my question.   Got it, the idle screw controls air.  Done... I'm good.  Thanks for the help.   

[Zed Head]

p.s. most mechanics today will assume that the ECU has sensors for every component and use an IACV and/or ignition timing to control idle speed.  The early EFI system is very primitive and hard to comprehend for today's code readers.  The components mostly work independently with the engineers assuming that each individual component is adjusted to a certain specification.  Very little adjusting to conditions, besides the coolant temperature sensor.  They're very close to being electronic carburetors.

 

[Mike]

On 11/11/2016 at 10:10 AM, No Name said:

I appreciate the no offense comment..

Dude I already have done a trial and error by putting it back and it did go back to not functioning properly.  I'm far from stupid and have had this Z for 20 years, I know all its quirks at this point.  My question was simply what does the idle screw control, is it air or does it electronically effect fuel flow.  The answer is air, so it's easily figured out from there.  When the idle is high, the kick in point of the throttle switch changes within the gas pedal itself.  You have to push further for it to ground out and kick into a full enrichment state.  When the idle is lower you have to push less on the pedal.  So with all the new air flow improvements I've made recently, the higher setting was just never kicking in.  

The problem has been deduced effectively.  And of course i put it back and tested it a few time.  In fact I tried 3 settings, and found the one where it worked perfectly and the car held 800 rpm.  The car will old 600 but I don't want to or need to go that low.  

There just wasn't any documentation I could find anywhere that explained what the idle screw does.  i.e. how it controls the idle not just that it does.  

When the car is cold in the morning the air regulator is obviously open.  That little plug that's in the front of it sends a signal to the throttle valve switch that basically disables it.  When the air regulator opens, Then the throttle valve switch is active.  It has 3 prongs inside it, as you press the gas pedal they come closer together and eventually if you press enough ground out, which puts the car into what's called full enrichment.  Upon normal gas pedal operation i.e. driving at 40mph or whatever it's not under full enrichment.  The throttle switch is effected by the idle screw in that the further open the idle screw is the further you have to push the pedal to get it to kick in to give you acceleration.  So there is a correlation,

I've already been through this 10 years ago with CA smog check.  The car was always coming back rich back then.  Turns out it was the throttle switch.  Back in the old days kids would bend the prongs inside to keep the switch grounded at all times.  This kept the car under full enrichment (hence the rich smog test).  The reason someone would do that is for off the line speed.  The cars are faster off the line if you're racing around with it.  Since my car was once owned by a Hollywood stuntman, that made sense to both me and the mechanic...  So we replaced the switch, been passing smogs every year since.  No more CO issues. 

All of the above said, I'm not a novice with these cars, but it was always unclear to me how the idle screw did its job.  

Car's running great, idles great.. and yes if I open that idle screw again it'll impede the operation of the throttle valve switch once more... 

Thanks for the help, and no offence taken..  

Regards,
Jeff

Jeff, nice job fixing the problem.  Contrary to other opinions, I appreciate you following up with your findings. It may help someone else in the future.