Curing Erratic Idle

7/28/2019 Curing Erratic Idle

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CURING ERRATIC IDLE (Part 1)

eneral:

ratic idle is one of the most common

oblems with our cars. This is caused by the

umber of engine components that are

fecting it, such as: IAA (Idle Air Adjusting

nit), Air Regulator, Throttle Bodies, Injectors,

AS, fuel delivery, vacuum piping...

his article will cover the three components

entioned first.

AA-unit:

his unit consists mainly of two valves: FICD

d AAC. The FICD valve compensates for

PM drop when AC compressor is activated.

he symptoms of the FICD failing are idle

op and vibrations when switching on the air

ndition.

he AAC (Auxiliary Air Control) valve is

ntrolled by the ECU (PWM-control) to sete idle speed to a pre-programmed level. The

CU switches the AAC on and off very fast

d thereby is able obtain a gradual opening of

e valve. The AAC opening ratio can be

bserved on a Techtom MDM-100. If the valve

completely closed at idle it could mean that

e idle screw is not adjusted correctly (too

uch air enters through it) or there is a vacuum

ak (could be dirty TB's not closing all theay).

ery often is the IAA unit (pic 1.) diagnosed

ulty and replaced. Since it costs ~$200 it's

orth to take the time and fix it yourself. Just

long as the solenoids are electrically intact

ere's not much that can go wrong with this

1.

2.



nit.

rst off, check the solenoid resistance (don't

rget to clean the connectors too).

CD => ~22 Ohm

AC => ~10 Ohm

ou can also check the solenoid operation by

plying 12V from the battery to its connectors

ou should hear a click).the resistance is far off then you really need

new IAA-unit, if not then read on...

emove the IAA from the plenum. It's attached

it with 4 screws. There is also a gasket

tween the IAA and the plenum. I reused the

d one since it wasn't broken. Disassemble the

lenoids as in picture 2. Check if the spring

side them is not broken.ow clean the solenoids and the IAA-unit

sing with WD-40 or Brake Cleaner or

milar.

eassemble and it's as good as new.

ir Regulator:

he purpose of the air regulator is to raise the

le during engine warm-up by letting

ditional air to enter the intake.he symptoms of the air regulator going bad is

ther low unstable idle during warm up or

nstantly high idle.

he regulator consists of a bimetal-activated

utter that closes after ~5 min of engine

peration and cuts off the airflow.

ote: It's normal that the shutter is only

rtially open at room temperature.

heck the resistance between the pins in the

nnector. It should be ~75 Ohm. If not good

en replace the whole regulator.

nother problem could be that the regulator is

ogged, causing the shutter to stick.

he air regulator is located on the rear of the

enum and is attached to it with 2 screws.

here are 4 screws keeping the housing

3.

4.

http://home.swipnet.se/e-solutions/Contactclean.html




gether. Remove them and clean the regulator

ternals (pic 3).

hrottle Bodies (TB's):

irty throttle bodies can be the cause of

nstable idle and hesitation.

emove the rubber hoses attached to them. It's

siest when the hoses are warm as it makesem softer. Twist them and pull (push them

rther on to the intercooler pipe too). Detach

so the small hoses beneath the TB's.

se an appropriate detergent to clean the TB

ternals. I've found Brake Cleaner and WD-40

work well. Use a toothbrush to scrub on the

rface.

ry and reassemble.

ome people reported that after cleaning the

B's they had to reset the ECU to get the idle

eed down. I would suggest disconnecting the

gative battery terminal before you start

eaning the TB's. It takes ~30 minutes for the

CU to forget what it has learned and that is

ow long it will take (at least...) to clean the

B's.

fter cleaning the TB's the idle may need to be

adjusted as the amount of air entering the

enum may have changed (depending on how

rty the TB's were). Adjust the Throttle

osition Sensor first (see Part 2) and then

just the idle as described below.

le adjustment:

ormally the engine should idle at ~800RPM.it's to high then you should try to adjust it.

et the engine warm up to normal operating

mperature. Put the transmission in neutral

ar.

isconnect the connector on the AAC valve

ic 2).

ith the idle screw (pic 1) adjust the idle

http://home.swipnet.se/e-solutions/IdleTech2.html

http://home.swipnet.se/e-solutions/IdleTech2.html



cording to the values listed for the different

models:

A AT = 720 RPM

A MT => 650 RPM

T => 700 RPM

econnect the AAC and you should hear the

gine rev up ~50RPM. That means that AACworking correctly and the ECU has control

ver the idle speed now.

MPORTANT! The purpose of the idle screw

to set the bottom value of the idle and let the

CU control it. If you adjust the screw for

gher idle than the above then the ECU is no

nger in charge of the idle speed.

you've got a Techtom MDM-100 then the

le adjustment is easier.

et the engine warm up fully and watch the

AC opening on the MDM while you're

justing the idle screw. Adjust it so that the

AC is 15%-20% open.

you're not able to reach the 15% and the

DM shows a value below that even with the

le screw fully in then you've got a bigcuum leak somewhere (see how to find it

ERE).

CURING ERRATIC IDLE (Part 2)

http://home.swipnet.se/e-solutions/Vacuumleak.html




eneral:

his part describes further investigation on

nstable idle problems.

ain topics are Coolant Temp sensor, TPS,

ower balance test, Ignition Coils, Spark plugs,

jectors, CAS, MAS, Fuel delivery, PCVs,

cuum leaks and compression test.

oolant temperature sensor:

he ECU coolant temp sensor is a very

mmon cause of high idle, hesitation and

fety boost mode.

s located in front of the engine on the upper

uminum coolant pipe (see pic 1).

ost often the source of the problem is bad

nnection caused by corrosion on its contactee Connector Cleaning document for the

eaning procedure).

ou can also check the resistance of the sensor

y measuring it acrossits terminals.

he resistance should be:

1-2.9 kOhm at 20C (68F)

68-1.0 kOhm at 50C (122F)

30-0.33 kOhm at 80C (176F)

PS - Throttle Position Sensor:

oorly adjusted TPS can cause jumping or high

le. The TPS is located on the left throttle

ody, pic 2.

o measure its output voltage (if you haven't

ot a AVC-R or AFC or a Techtom MDM-

00) you have to insert a thin wire in its

rness contact (pic 2), since it has to be

nnected while measuring.easure between the middle pin and the

assis or battery ground.

he voltage should be between 0.4V and

45V (the spec says 0.5V but that's to high

om my experience).

o adjust the voltage, turn ignition switch ON,

osen the screws holding the TPS and turn it

ghtly (CW => -) until the voltage is within

1.

2.

3.





ec. Tighten the screws and recheck voltage.

pen the throttle fully and check that the

utput voltage is ~4V.

mportant: If you haven't cleaned the throttle

odies, do it first (seePart1) and then adjust the

PS.

worn out TPS can cause problems like:

mping RPM while driving and shifting

oblems with auto transmission.

new TPS costs ~$29. Check the part number

ERE starting at page 2-A-8, section Throttle

hamber.

ower Balance test:

the engine is missing at idle, try to determine

the problem is connected to one of the

linders.

art the engine and proceed to disconnect each

the coil connectors (pic 3), one at a time.

he idle should drop when disconnecting a

il. If the idle doesn't change at all or changes

nly slightly compared to other cylinders then

ou've narrowed the problem to a single

linder.

nition Coils:

asiest way of checking a coil is to swap it

ith another good one.

you discovered one faulty cylinder in the

ower balance test then swap the suspected

il with a coil from a cylinder that was firing

rrectly. Perform the power balance test again

d see if the problem moves together with the

il. If yes then replace the coil.

ou can also check the resistance between the

iddle and the left pin (looking from the side

the engine) of the coil connector. It should

~0.7 Ohm.

4.

5.

6.

http://home.swipnet.se/e-solutions/IdleTech.html

http://home.swipnet.se/e-solutions/Partno.html

http://home.swipnet.se/e-solutions/Partno.html

http://home.swipnet.se/e-solutions/IdleTech.html



heck also the coil connectors for corrosion

d overall condition. The locking pin tends to

ake and the connector can come off by itself.

ee Contact cleaning document for more info

n repairing the harness.

park plugs:

xamine all the spark plugs carefully causeey've got a lot to "tell".

e sure to know which one comes from which

linder. If you installed spark plugs that are to

ld then they could foul and contribute to an

nstable idle. Use the colder PFR6B-11B plugs

n a modded Z. Going one grade colder to

FR7B-11 is only necessary when pushing

ERIOUS power while racing. The standard

p is 1.1mm. The gap should be lowered onodified engines. I'm running on ~0.8mm gap.

ompare all the spark plugs to see if you've got

problem with one of the cylinders. If one

ark plug is abnormally fouled that could

ean that one injector is leaking. If there is an

l leak in the system (e.g. from a turbo) then

ou would see some burned oil residue on the

ug.

normal plug will be clean with white or tansulator. See www.ngksparkplugs.comfor

ore info on plugs.

jectors:

jectors themselves are hard to diagnose. Like

entioned above, a leaking injector could be

agnosed by looking at the spark plug. There

also one more hint that I've heard of. If you

t the car sit for a couple of hours and thenmove the plug then you would smell a strong

el smell from the cylinder that has a leaking

jector and the plug would probably be wet

ith fuel.

ou can also examine if the injector is

cking" if you let the car idle and put a long

rewdriver on the top of the injector while

utting your ear to the handle. You should hear

7.

8.

9.


http://www.ngksparkplugs.com/

http://www.ngksparkplugs.com/




oud clicking. Compare the sound from all

e injectors. If an injector is not clicking then

s dead.

ou can check the resistance of the injectors as

ell by measuring it across its terminals. It

ould be between 10 and 14 Ohm.

ost often though, the problem is caused byrroded injector connectors. They are

cessible from the top of the plenum (pic 4).

they are broken or in a really bad shape then

would recommend that you replace them.

ou can use two sets of injector connectors

N: 24079-25P26 (~$20).They are meant for a

from the 80's but they'll fit, well almost...

ou will have

knock off a plastic pin on the injectornnector, not to difficult. See the Connector

eaning document for more info on this.

AS:

he Crank Angle Sensor informs the ECU

out the current position of the crank shaft

ic 5).

he most common reason for it affecting the

le is the connector getting dirty, although if e CAS connection is completely broken the

r won't start at all. The best way to examine

AS is to pull it out completely. Be sure to

ark its position so you can reinstall it like it

as before use millimeter precision here).

ean the connector if required. Check also

at the sensor is spinning freely without the

arings making to much noise and vibrations.

heck that the half circle pin inside its shaft istact. Some early model Zs had wrongly

achined heads causing the pin to wear out.

he ECU will not always report an error on the

AS. I had to put the ECU in test mode and

en crank the engine before the CAS-fault

de showed up.

10.







AS:

ass Airflow Sensor (pic 6) tells the ECU how

uch air is entering the engine which in turn

ves the information about how much fuel

eds to be supplied.

heck the MAS by looking through it to see if

e hot-wire is damaged or clogged by dirt.

ou can gently clean the hot-wire by spraying

with a contact cleaner that doesn't leave a

otective film. I cleaned mine with brake

eaner since it evaporates completely but it

uld be a little harsh for the vulnerable hot

ire..

ever remove the mash screen! It serves both

a protection and has its effect on the airflow.

ou can check the MAS voltage at the ECUee ECU contact diagram) with a volt meter.

should read ~0.8V with ignition on (some

ople get 0.08V on the Techtom) and ~1.5V

idle, unless you've got a vacuum leak

mewhere...

acuum leaks:

asically examine all of the hoses for leaks. If

ere is a vacuum leak somewhere then air

at's not registered by MAS will enter the

stem causing the engine to run to lean.

ee the Finding Vacuum Leaks article for more

fo.

uel delivery:

heck the fuel pressure after the fuel filter.epressurize the fuel system by disconnecting

e fuel pump under the spare tire or

sconnecting the

el pump relay (see pic 7). Start the engine

d let it stall. Crank it again a couple of times

let more fuel out.

here's always a bit of pressure left in the

oses so have some rags handy.

http://home.swipnet.se/e-solutions/ecuharness.html



http://home.swipnet.se/e-solutions/ecuharness.html



isconnect the fuel filter hose from the metal

pe on the plenum. Insert a T-pipe there (pic

. Attach a pressure gauge to the T and tighten

l clamps.

ow reconnect the fuel pump harness and

essurize the system by turning the ignition

y 3 times ON and OFF (the fuel pump

perates for a second or two after ignition on,ou should be able to hear it).

heck for leaks and start the engine.

he fuel pressure at idle should be

proximately 2.5bar (36 psi)

he fuel pressure should rise with the pressure

the intake manifold.

isconnect the vacuum line from the pressure

gulator (pic 9) and the fuel pressure should

se to ~3 bar (~43.4 psi)

CV-valves:

he Positive Crankcase Ventilation valves are

cated under the plenum on its right and left

de (see pic 10)

aulty PCVs can cause crank case air to be

cked in to the plenum at times when it's not

pposed to.

hey can also get stuck closed so thatcessive pressure is built-up inside the crank

se causing the engine oil to leak

rough the rear main seal and sometimes the

l dipstick to pop out.

here is also one more symptom that happened

me. The spring inside the PCVs got weak so

ey opened wile braking and

at caused the engine to stall after a fairly hard

aking.would recommend everyone with an older Z

replace them. They cost~$3.4 so it's not a

g deal (PN 11810-40P00).

e sure to get new rubber hoses since the old

nes are probably hard as a rock and you have

cut them.

he part numbers are 11826-30P11 for the left

ne and 11826-30P01 for the right one (~$23



tal).

emove the battery while replacing the PCV

n passenger side.

o not over torque them when installing, you

n brake the plenum!!

ompression test:

lways a good way to see the condition of thegine internals, especially if you find that

ere is a problem with a

rticular cylinder when looking at its spark

ug.

elow is the procedure as described in the

ervice Manual (except for the fusible link).

1. Warm up the engine

2. Turn ignition switch off 3. Disconnect the fusible link (by the

battery) named "Fuel pump, Inj."

4. Remove all spark plugs

5. Attach compression tester to no. 1

cylinder

6. Depress accelerator pedal fully to keep

the throttle bodies wide open.

7. Crank engine and record the highest

gauge indication8. Repeat the measurement for each

cylinder.

lways use a fully charged battery to obtain

e specified engine revolution (~300RPM)

hile cranking.

ompression pressure:

q Standard: 12,85 bar (186 psi) for NA ;11,79 bar (171 psi) for TT

q Minimum: 9,81 bar (142 psi) for NA ;

8,83 bar (128 psi) for TT

q Maximum difference between

cylinders: 0,98 bar (14 psi)

cylinder pressure in one or more cylinders is



w, pour a small amount of engine oil into it

rough the spark plug hole and retest

mpression.

q If adding oil helps compression, piston

rings may be worn or damaged. Replace

piston rings after checking the piston

for damage.q If pressure stays low, a valve may be

sticking or seating improperly. Inspect

and repair the valves and their seats.

q If compression in any two adjacent

cylinders is low and if adding oil does

not help, there may be a leakage past

head gasket surface.

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Curing Erratic Idle

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