This article describes further investigation on unstable idle problems. Main topics will include the coolant temp sensor, TPS, power balance test, ignition coils, spark plugs, injectors, CAS, MAS, fuel delivery, PCVs, vacuum leaks and a 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 aluminum coolant pipe.
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 across its 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 located on the left throttle body.
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, since it has to be connected while it is 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 too high from my experience).
To adjust the voltage, turn ignition switch ON, loosen the screws holding the TPS and turn it slightly CLOCKWISE 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 – Go back to Part 1 and then adjust the TPS.
A worn out TPS can cause problems like: jumping RPM while driving and shifting problems with auto transmission. A replacement TPS is not expensive and can save you a ton of hassle.
Power Balance test:
If the engine is missing at idle, try to determine if the problem is connected to one of the cylinders. Start the engine and proceed to disconnect each of the coil connectors 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 down to a single cylinder.
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 it does, then replace THAT coil.
You can also check the resistance between the middle and the left pin (looking from the engine viewpoint) 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 break and the connector can come off by itself.
See Contact cleaning document for more info on repairing the harness.
Examine all the spark plugs carefully because you can learn a LOT by reading your plugs. Be sure to remember which one comes from which cylinder (I use an old shoe box, with six holes oriented the same as the engine – don’t forget to mark ‘FRONT’ on the box). If you installed spark plugs that are too cold then they could foul and contribute to an unstable idle. Use the colder PFR6B-11B plugs only on a heavily modded Z. Going one grade colder to PFR7B-11 is only necessary when pushing SERIOUS power and for racing. The standard gap is 1.1mm. The gap should be lowered on modified engines. For example, 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 www.ngksparkplugs.com for more info on plugs.
Injectors themselves are hard to diagnose. 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. 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. They are meant for a Z from the 80’s but they’ll fit if you knock off a plastic pin on the injector connector, not to difficult.
The Crank Angle Sensor informs the ECU about the current position ofthe crank shaft.
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 (this needs to be precise – use an awl to scratch a mark on the CAS and the housing to line it back up perfectly). Clean the connector if required. Check also that the sensor is spinning freely without the bearings making noise and vibration.
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 (also referred to as MAF) 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. Electronics cleaner works well – DO NOT use brake cleaner or carb cleaner!
Also, never remove the mesh screen! It serves both as a protection and on the airflow measurement. You can check the MAS voltage at the ECU with a volt meter. Here’s your pinout diagram:
It should read ~0.8V with ignition on (some people get 0.08V on the Techtom) and ~1.5V at idle, unless you’ve got a vacuum leak somewhere.
Basically examine all of the hoses for leaks. If there is a vacuum leak somewhere then air that’s not registered by MAS will enter the system causing the engine to run to lean. See the Finding Vacuum Leaks article for more info.
Check the fuel pressure after the fuel filter. Depressurize the fuel system by disconnecting the fuel pump under the spare tire or disconnecting the
fuel pump relay.
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 rags handy. Disconnect the fuel filter hose from the metal pipe on the plenum. Insert a T-pipe there. Attach a pressure gauge to the T and tighten all clamps.
Now reconnect the fuel pump harness and pressurize the system by turning the ignition key 3 times ON and OFF (the fuel pump operates for a second or 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 and the fuel pressure should rise to ~3 bar (~43.4 psi)
The Positive Crankcase Ventilation valves are located under the plenum on its right and left side.
Faulty PCVs can cause crankcase air to be sucked in to the plenum at times when it’s not supposed to. They can also get stuck closed so that excessive pressure is built up inside the crankcase, causing the engine oil to leak
through the rear main seal and sometimes the oil dipstick to pop out.
I would recommend everyone with an older Z to replace them (PN 11810-40P00) as they don’t cost much at all. Be sure to get new rubber hoses since the old ones are probably brittle. The part numbers are 11826-30P11 for the left one and 11826-30P01 for the right one. Remove the battery while replacing the PCV on passenger side. Do not over torque them when installing, you can break the plenum!
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.
- Warm up the engine
- Turn ignition switch off
- Remove the fuel pump fuse
- 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.
- 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. Replace piston rings after checking the piston for damage.
- If pressure stays low, a valve may be sticking or seating improperly. Inspect and repair the valves and their seats.
- 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.