High RPM problem on new 300ZX
Alireza Vali
One thing that's strange is that when I take car out of gear after coming
to a stop, the RPM seems to get "stuck" somewhere between 1,200 and 1,500
RPM for about 3 to 7 seconds after which it drops to roughly 900 with the
AC on, and around 700 with the AC off. I took it back to the dealer so
they can take a look at it. When I went back to pick up the car, they
told me that this is normal and the RPM is computer controlled. The guy
said that it revs higher for a few seconds to "burn off excess emissions
and to keep the injectors clean." What? What? I didn't understand what
the heck he was talking about! I just took the car and decided to check
into what he's talking about. Is he just feeding me some nonesense?
I have just two questions:
1) Do any other 300ZX owners observe this type of engine behavior?
2) What the heck is this guy talking about? (if the momentary high RPM
is by design)
Thanks for any help.
--
Ali R. Vali - re...@magellan.csr.utexas.edu
Andrew A. Spencer
i dunno about the BS he fed you for the reason(could be the reason, sounds
like BS to me!) but i think this effect is just part of having computer
controlled fuel injection. while 3-7 sec sounds like a long time to rev
that high, a slight hesitation when coming off throttle seems to be normal
from my experiance(and i hate it!!!, especially between shifts!). I've
always wondered, what kind of chips do they use? how fast, how many bits,etc.?
Seems to me they need to use some quicker ones!
DREW
--
Did you ever dance with the Devil in the pale moon light?
Jim Carr
>
>Hi. I recently bought a new 93 Nissan 300ZX. It is a non-turbo coupe.
>One thing that's strange is that when I take car out of gear after coming
>to a stop, the RPM seems to get "stuck" somewhere between 1,200 and 1,500
>RPM for about 3 to 7 seconds after which it drops to roughly 900 with the
>AC on, and around 700 with the AC off. I took it back to the dealer so
>they can take a look at it. When I went back to pick up the car, they
>told me that this is normal and the RPM is computer controlled.
It does not surprise me that it is computer controlled. I have no idea
if this is normal or not, this should be the place to find out. The
other option is to ask an independent mechanic who specializes in
Nissan sports cars. My cousin-in-law used to be a racing mechanic
for them, and runs a small shop in the Austin area if you are really
located at UTexas. Garan would give you a straight answer.
> The guy
>said that it revs higher for a few seconds to "burn off excess emissions
>and to keep the injectors clean." What? What?
Not so implausible. US EPA regulations put a lot of emphasis on emissions
when the car is idling, so there are all sorts of strategies employed to
deal with this. One is to recycle some of the exhaust back in with the
intake air. It could be that that particular engine needs a bit higher
rpm to deal with this junk and the computer does this automatically.
--
J. A. Carr <j...@scri.fsu.edu> | "The New Frontier of which I
Florida State University B-186 | speak is not a set of promises
Supercomputer Computations Research Institute | -- it is a set of challenges."
Tallahassee, FL 32306-4052 | John F. Kennedy (15 July 60)
Carl Peterson
I think what is happening is that there is a mechnicism(sp) that is part
of the throttle which keeps the engine from dropping to a normal idle when the throttle is suddenly released. It is part of the setup that comes into play
when the A.C is turned on. I think this is normal, as it happens on my
old Plymouth Reliant( except mine sticks on the high idle).
Alireza Vali (re...@magellan.csr.utexas.edu) wrote:
: Hi. I recently bought a new 93 Nissan 300ZX. It is a non-turbo coupe.
Giannhs (John) Alexiou
>Thanks for any help.
The fact is that inside the engine's combustion chambers carbon always
deposits and destroys the normal flow of the mixtrure. Also the spark
doesn't propagate right , so a short REV up will normally burn up the
carbon. It is good practice to rev-up your car before parking it after
driving in trafic. Usually cars like the 300ZX are made to operate
at high rpm's while driving , but when stuck in traffic carbon
accumulates like crazy and so you need to clean up your engine.
The solution is to use better gas quality (Not sold in the US there
is a 98 octane Unleaded fuel available in Shells in europe) or
some additives might help.
--
John Alexiou, Georgia Institute of Technology , Mechanical Engineering dept
Internet: gt4...@prism.gatech.edu
Alireza Vali
>Alireza Vali (re...@magellan.csr.utexas.edu) wrote:
>: One thing that's strange is that when I take car out of gear after coming
>: to a stop, the RPM seems to get "stuck" somewhere between 1,200 and 1,500
>: RPM for about 3 to 7 seconds after which it drops to roughly 900 with the
>: AC on, and around 700 with the AC off.
>
>of the throttle which keeps the engine from dropping to a normal idle when
>the throttle is suddenly released. It is part of the setup that comes into
>play when the A.C is turned on. I think this is normal, as it happens on my
>old Plymouth Reliant( except mine sticks on the high idle).
One thing to keep in mind is that the RPM "sticks" around 1,200 and 1,500
RPM for a short duration regardless of whether the AC is on or off. A
couple of 300ZX owners (1991 and 1990 models) have responded that they
have not observed this behavior in their cars. My next move is too seek
an independent 300ZX mechanic's input on this.
Martin Chung
>Thanks for any help.
First of all, the fuel injection system on the 300ZX is computer controlled-
it's a sequential multi-point system with a dual plenum. Sequential
meaning that the fuel is injected at each cylinder just before the intake
valve for that cylinder opens, multipoint meaning that there is one
injector per cylinder. Basically this system is about as advanced as they
get these days. Nissan's system is based on the Bosch LH-Jetronic/Motronic
systems, using a hot-wire air mass sensor.
One of the subsystems on this system is the idle speed stabilizer or
idle speed control device (ISCD). Its job is to admit extra air around
the throttle plate and control the idle speed under different conditions.
The computer controls the idle speed via the ISCD by monitoring various
inputs such as the throttle position, engine temperature, battery voltage,
rpm, and so on.
When the air conditioner is on, the computer signals the ISCD to raise
the idle rpm so that the compressor can function efficiently and so
that the 'drag' caused by the compressor doesn't cause the engine
to idle too low. Typically, the rpms will stay about the same or rise
100-200 rpm. This should explain the 900 rpm idle with a/c on and
700 rpm idle with a/c off.
The second situation appears to be the computer's doing again. The
system has an injector cutoff programmed into it -- this is called
coasting-cutoff. Basically, if the throttle is closed and the rpm is
high enough, the computer will shut off the injectors. Doing this
helps to save gas and reduce emissions when you engine brake down a
long hill, for example. At a certain rpm, the computer resumes
fuel injection again to prevent stalling. What you may be observing
is the resumption of injection at about 1200-1500 rpm, at which point
the ISCD starts functioning to bring the idle down. It is this
transition point when the injection comes back on which may take a few
seconds to settle down back to idle. Typically, the transition back
to injection is smoothed by retarding the ignition timing.
You can try an experiment: rev the warm engine in neutral to say 3000 rpm,
then take your foot off the throttle. The computer should signal for
injector cutoff; the revs will drop quickly, then hesitate about 500+ rpm
above the normal idle and drop at a controlled rate down to the idle rpm.
You can also try this by pushing in the clutch while engine braking down a
long hill that gets the rpms up. The cutoff and resumption rpms vary
according to temperature. When the engine is colder, injection resumes
at a higher rpm to prevent stalling.
Hope this helps! The hesitation at 1200-1500 doesn't have much to do with
keeping the injectors clean or deposits down -- fuel injection doesn't
cause much carbon depositing, etc. -- it's that pretty violent transition
where the engine starts burning gas again which the computer is trying
to soften.
MC
--
Martin Chung (ch...@sol.UVic.CA)
Programmer/Analyst & full-time student!