Curing Erratic Idle

Wikis > Curing Erratic Idle


Erratic idle is one of the most common problems with our cars. This is caused by the number of engine components that are affecting it, such as: IAA (Idle Air Adjusting unit), Air Regulator, Throttle Bodies, Injectors, CAS, fuel delivery, vacuum piping…
This article will cover the three components mentioned first.
This unit consists mainly of two valves: FICD and AAC. The FICD valve compensates for RPM drop when AC compressor is activated. The symptoms of the FICD failing are idle drop and vibrations when switching on the air condition.
The AAC (Auxiliary Air Control) valve is controlled by the ECU (PWM-control) to set the idle speed to a pre-programmed level. The ECU switches the AAC on and off very fast and thereby is able obtain a gradual opening of the valve. The AAC opening ratio can be observed on a Techtom MDM-100. If the valve is completely closed at idle it could mean that the idle screw is not adjusted correctly (too much air enters through it) or there is a vacuum leak (could be dirty TB’s not closing all the way).

Very often is the IAA unit (pic 1.) diagnosed faulty and replaced. Since it costs ~$200 it’s worth to take the time and fix it yourself. Just as long as the solenoids are electrically intact there’s not much that can go wrong with this unit.
First off, check the solenoid resistance (don’t forget to clean the connectors too).
FICD => ~22 Ohm
AAC => ~10 Ohm
You can also check the solenoid operation by applying 12V from the battery to its connectors (you should hear a click).
If the resistance is far off then you really need a new IAA-unit, if not then read on…

Remove the IAA from the plenum. It’s attached to it with 4 screws. There is also a gasket between the IAA and the plenum. I reused the old one since it wasn’t broken. Disassemble the solenoids as in picture 2. Check if the spring inside them is not broken.
Now clean the solenoids and the IAA-unit casing with WD-40 or Brake Cleaner or similar.
Reassemble and it’s as good as new.

Air Regulator:
The purpose of the air regulator is to raise the idle during engine warm-up by letting additional air to enter the intake.
The symptoms of the air regulator going bad is either low unstable idle during warm up or constantly high idle.
The regulator consists of a bimetal-activated shutter that closes after ~5 min of engine operation and cuts off the airflow.
Note: It’s normal that the shutter is only partially open at room temperature.

Check the resistance between the pins in the connector. It should be ~75 Ohm. If not good then replace the whole regulator.
Another problem could be that the regulator is clogged, causing the shutter to stick.
The air regulator is located on the rear of the plenum and is attached to it with 2 screws.
There are 4 screws keeping the housing together. Remove them and clean the regulator internals (pic 3).

Throttle Bodies (TB’s):
Dirty throttle bodies can be the cause of unstable idle and hesitation.
Remove the rubber hoses attached to them. It’s easiest when the hoses are warm as it makes them softer. Twist them and pull (push them further on to the intercooler pipe too). Detach also the small hoses beneath the TB’s.
Use an appropriate detergent to clean the TB internals. I’ve found Brake Cleaner and WD-40 to work well. Use a toothbrush to scrub on the surface.
Dry and reassemble.

Some people reported that after cleaning the TB’s they had to reset the ECU to get the idle speed down. I would suggest disconnecting the negative battery terminal before you start cleaning the TB’s. It takes ~30 minutes for the ECU to forget what it has learned and that is how long it will take (at least…) to clean the TB’s.

After cleaning the TB’s the idle may need to be readjusted as the amount of air entering the plenum may have changed (depending on how dirty the TB’s were). Adjust the Throttle Position Sensor first (see Part 2) and then adjust the idle as described below.

Idle adjustment:
Normally the engine should idle at ~800RPM. If it’s to high then you should try to adjust it.
Let the engine warm up to normal operating temperature. Put the transmission in neutral gear.
Disconnect the connector on the AAC valve (pic 2).
With the idle screw (pic 1) adjust the idle according to the values listed for the different Z models:
NA AT = 720 RPM
NA MT => 650 RPM
TT => 700 RPM

Reconnect the AAC and you should hear the engine rev up ~50RPM. That means that AAC is working correctly and the ECU has control over the idle speed now.

IMPORTANT! The purpose of the idle screw is to set the bottom value of the idle and let the ECU control it. If you adjust the screw for higher idle than the above then the ECU is no longer in charge of the idle speed.

If you’ve got a Techtom MDM-100 then the idle adjustment is easier.
Let the engine warm up fully and watch the AAC opening on the MDM while you’re adjusting the idle screw. Adjust it so that the AAC is 15%-20% open.
If you’re not able to reach the 15% and the MDM shows a value below that even with the idle screw fully in then you’ve got a big vacuum leak somewhere (see how to find it HERE).

1.IAA unit & Air Regulator
2.FICD & AAC valves

3.Air Regulator

4.Throttle Body



This part describes further investigation on unstable idle problems.
Main topics are Coolant Temp sensor, TPS, Power balance test, IgnitionCoils, Spark plugs, Injectors, CAS, MAS, Fuel delivery, PCVs, vacuum leaksand compression test.
Coolant temperature sensor:
The ECU coolant temp sensor is a very common cause of high idle, hesitation and safety boost mode.
It’s located in front of the engine on the upper aluminium coolantpipe (see pic 1).
Most often the source of the problem is bad connection caused by corrosion on its contact (see Connector Cleaning document for the cleaning procedure).
You can also check the resistance of the sensor by measuring it acrossits terminals.
The resistance should be:
2.1-2.9 kOhm at 20C (68F)
0.68-1.0 kOhm at 50C (122F)
0.30-0.33 kOhm at 80C (176F)

TPS – Throttle Position Sensor:
Poorly adjusted TPS can cause jumping or high idle. The TPS is locatedon the left throttle body, pic 2.
To measure its output voltage (if you haven’t got a AVC-R or AFC or a Techtom MDM-100) you have to insert a thin wire in its harness contact(pic 2), since it has to be connected while measuring.
Measure between the middle pin and the chassis or battery ground.
The voltage should be between 0.4V and 0.45V (the spec says 0.5V but that’s to high from my experience).
To adjust the voltage, turn ignition switch ON, loosen the screws holding the TPS and turn it slightly (CW => -) until the voltage is within spec. Tighten the screws and recheck voltage.
Open the throttle fully and check that the output voltage is ~4V.

Important: If you haven’t cleaned the throttle bodies, do it first (see Part1) and then adjust the TPS.

A worn out TPS can cause problems like: jumping RPM while driving and shifting problems with auto transmission.
A new TPS costs ~$29. Check the part number HERE starting at page 2-A-8, section Throttle Chamber.

Power Balance test:
If the engine is missing at idle, try to determine if the problem isconnected to one of the cylinders.
Start the engine and proceed to disconnect each of the coil connectors(pic 3), one at a time. The idle should drop when disconnecting a coil.If the idle doesn’t change at all or changes only slightly compared to other cylinders then you’ve narrowed the problem to a single cylinder.

Ignition Coils:
Easiest way of checking a coil is to swap it with another good one.
If you discovered one faulty cylinder in the power balance test then swap the suspected coil with a coil from a cylinder that was firing correctly. Perform the power balance test again and see if the problem moves together with the coil. If yes then replace the coil.

You can also check the resistance between the middle and the left pin(looking from the side of the engine) of the coil connector. It should be ~0.7 Ohm.

Check also the coil connectors for corrosion and overall condition. The locking pin tends to brake and the connector can come off by itself.
See Contact cleaning document for more info on repairing the harness.

Spark plugs:
Examine all the spark plugs carefully cause they’ve got a lot to “tell”.
Be sure to know which one comes from which cylinder. If you installed spark plugs that are to cold then they could foul and contribute to a nunstable idle. Use the colder PFR6B-11B plugs on a modded Z. Going one grade colder to PFR7B-11 is only necessary when pushing SERIOUS power while racing. The standard gap is 1.1mm. The gap should be lowered on modified engines. I’m running on ~0.8mm gap.
Compare all the spark plugs to see if you’ve got a problem with one of the cylinders. If one spark plug is abnormally fouled that could mean that one injector is leaking. If there is an oil leak in the system (e.g.from a turbo) then you would see some burned oil residue on the plug.
A normal plug will be clean with white or tan insulator. See for more info on plugs.

Injectors themselves are hard to diagnose. Like mentioned above, a leaking injector could be diagnosed by looking at the spark plug. There is also one more hint that I’ve heard of. If you let the car sit for a couple of hours and then remove the plug then you would smell a strong fuel smell from the cylinder that has a leaking injector and the plug would probably be wet with fuel.
You can also examine if the injector is “ticking” if you let the car idle and put a long screwdriver on the top of the injector while putting your ear to the handle. You should hear a loud clicking. Compare the sound from all the injectors. If an injector is not clicking then it’s dead.
You can check the resistance of the injectors as well by measuring it across its terminals. It should be between 10 and 14 Ohm.

Most often though, the problem is caused by corroded injector connectors. They are accessible from the top of the plenum (pic 4). If they are broken or in a really bad shape then I would recommend that you replace them.
You can use two sets of injector connectors PN: 24079-25P26 (~$20).They are meant for a Z from the 80’s but they’ll fit, well almost… You will have
to knock off a plastic pin on the injector connector, not to difficult.See the Connector Cleaning document for more info on this.

The Crank Angle Sensor informs the ECU about the current position of the crank shaft (pic 5).
The most common reason for it affecting the idle is the connector getting dirty, although if the CAS connection is completely broken the car won’t start at all. The best way to examine CAS is to pull it out completely.Be sure to mark its position so you can reinstall it like it was before(use millimetre precision here).
Clean the connector if required. Check also that the sensor is spinning freely without the
bearings making to much noise and vibrations.
Check that the half circle pin inside its shaft is intact. Some early model Zs had wrongly machined heads causing the pin to wear out.

The ECU will not always report an error on the CAS. I had to put the ECU in test mode and then crank the engine before the CAS-fault code showed up.

Mass Airflow Sensor (pic 6) tells the ECU how much air is entering the engine which in turn gives the information about how much fuel needs to be supplied.

Check the MAS by looking through it to see if the hot-wire is damaged or clogged by dirt. You can gently clean the hot-wire by spraying it with a contact cleaner that doesn’t leave a protective film. I cleaned mine with brake cleaner since it evaporates completely but it could be a little harsh for the vulnerable hot wire..
Never remove the mash screen! It serves both as a protection and hasits effect on the airflow.
You can check the MAS voltage at the ECU (see ECUcontact diagram) with a volt meter.
It should read ~0.8V with ignition on (some people get 0.08V on theTechtom) and ~1.5V at idle, unless you’ve got a vacuum leak somewhere…

Vacuum leaks:
Basically examine all of the hoses for leaks. If there is a vacuumleak somewhere then air that’s not registered by MAS will enter the systemcausing the engine to run to lean.
See the Finding Vacuum Leaks article for more info.

Fuel delivery:
Check the fuel pressure after the fuel filter.
Depressurize the fuel system by disconnecting the fuel pump under thespare tire or disconnecting the
fuel pump relay (see pic 7). Start the engine and let it stall.Crank it again a couple of times to let more fuel out.
There’s always a bit of pressure left in the hoses so have some ragshandy.
Disconnect the fuel filter hose from the metal pipe on the plenum.Insert a T-pipe there (pic . Attach a pressure gauge to the T and tightenall clamps.
Now reconnect the fuel pump harness and pressurize the system by turningthe ignition key 3 times ON and OFF (the fuel pump operates for a secondor two after ignition on, you should be able to hear it).
Check for leaks and start the engine.
The fuel pressure at idle should be approximately 2.5bar (36 psi)
The fuel pressure should rise with the pressure in the intake manifold.
Disconnect the vacuum line from the pressure regulator (pic 9) andthe fuel pressure should rise to ~3 bar (~43.4 psi)

The Positive Crankcase Ventilation valves are located under the plenumon its right and left side (see pic 10)
Faulty PCVs can cause crank case air to be sucked in to the plenumat times when it’s not supposed to.
They can also get stuck closed so that excessive pressure is builtup inside the crank case causing the engine oil to leak
through the rear main seal and sometimes the oil dipstick to pop out.
There is also one more symptom that happened to me. The spring insidethe PCVs got weak so they opened wile braking and
that caused the engine to stall after a fairly hard braking.
I would recommend everyone with an older Z to replace them. They cost~$3.4 so it’s not a big deal (PN 11810-40P00).
Be sure to get new rubber hoses since the old ones are probably hardas a rock and you have to cut them.
The part numbers are 11826-30P11 for the left one and 11826-30P01 forthe right one (~$23 total).
Remove the battery while replacing the PCV on passenger side.
Do not over torque them when installing, you can brake the plenum!!

Compression test:
Always a good way to see the condition of the engine internals, especially if you find that there is a problem with a
particular cylinder when looking at its spark plug.
Below is the procedure as described in the Service Manual (except forthe fusible link).

Warm up the engine
Turn ignition switch off
Disconnect the fusible link (by the battery) named “Fuel pump, Inj.”
Remove all spark plugs
Attach compression tester to no. 1 cylinder
Depress accelerator pedal fully to keep the throttle bodies wide open.
Crank engine and record the highest gauge indication
Repeat the measurement for each cylinder.
Always use a fully charged battery to obtain the specified engine revolution (~300RPM) while cranking.
Compression pressure:

Standard: 12,85 bar (186 psi) for NA ; 11,79 bar (171 psi) for TT
Minimum: 9,81 bar (142 psi) for NA ; 8,83 bar (128 psi) for TT
Maximum difference between cylinders: 0,98 bar (14 psi)
If cylinder pressure in one or more cylinders is low, pour a small amount of engine oil into it through the spark plug hole and retest compression.

If adding oil helps compression, piston rings may be worn or damaged. Replacepiston rings after checking the piston for damage.
If pressure stays low, a valve may be sticking or seating improperly. Inspectand repair the valves and their seats.
If compression in any two adjacent cylinders is low and if adding oil doesnot help, there may be a leakage past head gasket surface.