If you haven't already done so, check the A.A.C. valve as described previously. I guess the theory here is that if this valve is open, the initial fuel/ air mixture will be too lean, giving rise to a stall. If the valve is jammed open, your car may still be idling correctly if some one has already adjusted the idle screw to compensate.

Next is what the manual calls the power balance test. This is where you run the engine and try and determine if one of the injectors is faulty by disconnecting the injector connector harness from each injector in turn to see if there is a "momentary engine speed drop". If there isn't there could be a problem with that cylinder' s injector or spark. The manual uses the word momentary because as soon as the ECU detects an idle speed drop, it attempts to compensate for it. I found it easier to listen for an additional roughness in engine idle, which is quite noticeable. This power balance test is easy if you have a Consult (Nissan's diagnostic computer), because the consult can command the ECU to drop power off each injector in turn. It's a bit of a hassle to actually get at the injector connector, especially injector number 5, so what I did was pull off the spark coil connector instead. I figured it wouldn't do to much harm if I did it just for a few seconds.

Crank Angle Sensor (near LHS Throttle Valve):


If there looks like there's a problem here, mark the crank angle sensor so you don't have to reset the timing, then remove it (3 bolts). Turn the ignition on, don't start the car, turn the crank angle shaft and check that each injector makes a clicking sound.

If there's a problem, check the circuits, bearing in mind that the crank angle sensor triggers the ECU, and the ECU in turn pulses the injector.

I never dismantled my crank angle sensor (made by Mitsubishi), but a description of it is on page EF & EC of the manual. It works as follows: The crank angle shaft rotates a disc with slits in it. There 2 sets of slits. According to the manual, the outer set on the disc contains 360 slits, the inner set 6 slits. There are 2 LEDs on one side of the disc, and 2 photo diodes on the other side of the disc. As the disc rotates, it interrupts the light beams between the LEDs & photo diodes, producing a pulse stream, which is fed to the ECU via 2 separate wires as 1 degree signal and 120 degree signal. There's also some electronics in the crank angle senor which squares up the pulses from the photo diode for the ECU.

I'm not quite sure why there aren't 720 slits on the disc to get the 1 degree signal (maybe there are) because when the engine crankshaft rotates 360 degrees, the crank angle sensor, because it's driven off the left hand exhaust camshaft rotates only 180 degrees. Perhaps some one can enlighten me on that.

I have heard of several instances where hesitation was traced to the crank angle sensor or it's connector. Russell Siebert reports:

"Date: Wed, 13 Dec 1995
From: roptix+ix.netcom.com (Russell Siebert)
Subject: 300zx TT Engine Missing

I was having problems with my engine missing on my '90 300TT a few months ago. I took it to the dealer at least 5 times for it. The last time they still could not find the problem, just costing me a load of $. Any way, I asked to talk with one of the techs there. I explained what was happening, he opened the hood with engine running wiggled a connection which killed the engine. He proclaims "Well, there's your problem."

Apparently, the connection to the crank angle sensor can get loose and corroded, compromising the connection and causing an irregular miss. He cleaned and crimped the connections and no problem ever since. The crank angle sensor connection is on the front right side as you are facing the engine a little under a large air hose. Be careful not to get your hand caught in the fan if you try and check out this possible problem.

Russell 'Silky Smooth' Siebert
'90 300TT"

Next on the hit list is the ignition system: timing and spark check.

Ignition timing

 I went and bought a timing light, and made up an adaptor so that I could connect the timing light sensor. To make the adaptor, I went to the automotive shop and bought some high tension wire and the end bits to make up in effect an extension cord. To make the chord stiff enough to push down into the hole and onto the spark plug, I sleeved the high tension cord with an aluminum tube.

Remove the number 1 coil and attach the adaptor & timing light.

The engine timing should be 15 plus or minus 2 degrees. If I would have done this test first, I would have found my hesitation problem immediately, but I wouldn't have learned so much.

Spark test

I tried to do this test outside in the bright sunshine, but found I couldn't see the spark very well (no smog here). So either do it at night or in the garage with the door closed, and you'll not only see it clearer but you'll see any spark leakage, i.e. where the spark is jumping from places other than the plug tip.

• Remove each of the 6 coils in turn, put a known good plug in, grounding the plug, and crank the engine and confirm there's a good spark.
• If all is OK you might want to pull out each of the spark plugs in turn and check their spark performance.

Russell Siebert :

Date: Tue, 16 Jan 1996
From: roptix@ix.netcom.com (Russell Siebert)
Subject: <300> Hesitation

For those having problems with hesitation, try Tim's suggestion of checking the spark plug assemblies. The #5 was loose on my 90 300TT just like Tim's. It would also be worth checking the crank angle sensor connection which is located (as you are in front of the car) at the "2 o-clock" position on the front of the engine behind the fan. Mine was loose and corroded and causing a hard to trace hesitation. A little cleaning and crimping of connectors has cured that problem.

Russell
90 300TT
378 Hp

Power Transistor Unit (behind wires) :

• If all is not OK, check the transistor ignition unit connectors, or swap the unit for a known good one, which is what I did. This unit can be a source of intermittent operation, so if this is happening, a swap to a good one would be preferable to confirm the problem.

Various IZCC Internet Z Car Club members have mentioned that this unit can "soft fail", (not sure of the details) i.e. you can still operate the car, but it won't run well.

In the USA, this unit is a recall item. In other countries (Switzerland, Australia, New Zealand) it isn't.

If your fuel delivery system is not up to scratch, then when you demand power, you may get hesitation, cutting out or complete stall. I fluffed around at first checking the fuel damper, fuel pump connections, filter etc, but if I were doing this again, the first thing I would do is put a pressure gauge on the fuel system.

I went out and bought an oil pressure gauge which could read to 80 psi. I think if you're going to do maintenance on these cars yourself, it pays to have an oil pressure gauge, because the oil pressure sender unit on the 90+ is notorious for reading intermittently or not at all.

The fuel pressure in the system will be in the range 35- 50 psi so an oil pressure gauge makes an ideal instrument for this check. Here's what I did:

• go down to the local gas (as in methane) shop/ engineering shop/ hardware and get a 5/16 brass T connector suitable for rubber hose connections. Down here it's called a hose tail T connector.
• The local automotive supply shop will have some 5/16 in (internal diameter) rubber hose suitable for 100 psi or so.
• Get the necessary lines, hose clips and adaptors to connect the gauge, so you can observe the gauge while you're testing or driving.
• Remove the fuel filter. Make sure the filter is not clogged by blowing through it- It should be quite easy to do so.
• I plumbed the T connector in on the input side of the filter in case there were any foreign particles- rubber from cutting the tube etc- in the additional piping. I also had a dry powder fire extinguisher handy.
• I started the engine and checked all the connections, including joints in the gauge about 3 times. This sort of thing always makes me nervous.
• At idle the fuel pressure should be around 37 psi. Jump on the gas pedal and the pressure should rise to about 44 psi. Go for a test drive and check that the pressure is within these limits under all operating conditions.

 

Fuel Damper (RHS) :	Fuel Pressure Regulator (LHS) :
Fuel Regulator Solenoid Location (LHS) :	Fuel Temperature Sensor (LHS) :

 

• Check that the pressure regulator is effective by temporarily disconnecting the vacuum line from it. With the regulator vacuum line disconnected from the pressure regulator, you should see the effect on the gauge- the fuel pressure now peaks to about 50 psi. The pressure regulator vacuum is fed via solenoid which is under the control of the ECU. The purpose of this is to provide a "pressure up" for improved starting when the engine is hot. The fuel temperature sensor helps the ECU decide when to operate this solenoid.

I didn't notice any effect on engine performance when I disconnected the fuel damper or when I disconnected the pressure regulator, thus slightly increasing the fuel supply, confirming Jeremey's experience:

"From: techsup@a-t-m.com (J. Sibert)
Date: Wed, 10 Sep 1997
Subject:<90+> Fuel Pressure Regulator

Ok guys, well, was under the hood last night cleaning things up and noticed that the vac line to the FPR was off.. I noticed no performance difference what so ever, checked the plugs, wasn't running too rich or lean..... Is this a problem? Shouldn't that have affected SOMETHING? I was thinking maybe by default the FPR was open or something...

J. Sibert
Clearwater, FL
91 300ZX TT Stage IV"

Perhaps when all is working well, the closed feedback loop involving the exhaust gas senor enables the ECU to compensate for regulator malfunction...

Warning: wiring harness color coding may be different for different models. These colors are as stated in the English version of the manual. I found color coding different for the Fairlady Z model.

The throttle position sensor consists of two parts: a potentiometer, and a combined idle/full throttle switch. If your car is an auto, the throttle position sensor is also wired to the auto transmission (A/T) computer. The idle switch is connected to the ECU, and the full throttle switch to the A/T computer. The bottom in line connector is for the sensor, the top one for the switch. Its a straight forward procedure to check these items. Turn the ignition on, put a voltmeter between ground and the throttle sensor orange/ blue wire. Should be 5 volts. This power source comes from the ECU. Next the white wire. With the throttle valve fully closed, about 0.4-0.5 volts. Slowly open the throttle valve and observe the voltage increase to about 4 volts, in proportion to throttle opening. Next the black wire. This is ground (grounded vi the ECU), so there should be 0 volts there under all circumstances. To check the ground circuit, turn off the ignition ( I would recommend disconnecting the battery), pull off the ECU connector, and confirm there are negligible ohms between the black wire and pin 30 on the ECU connector.

The ECU uses the throttle position sensor to determine the idle position of the throttle valve, but if it detects a sensor malfunction, it makes use of the idle switch information instead. The idle position info is used for things like "fuel cut" action, when you take your foot off the gas pedal. To check, turn the ignition on and put a voltmeter between ground and the green/red wire on the switch connector. Should read about 12 volts. Now put the voltmeter on the light green /red wire. With the gas pedal fully released, voltage should be 9-10 volts. Now push the gas pedal down a bit. Should go to 0 volts.

To get at these wires, I bared some of the insulation on the wires so I could put the meter probe on, then afterward taped them up. As I plan to keep the car, I decided I probably would want to make this measurement again in the future. An alternative would be to make up connector interface adaptor.

After chasing idle problems for about 3 months, Dave Quennell finally got to the bottom of things:

From: "Dave Quennell" d.quennell@xtra.co.nz
Subject: Idle?
Date: Mon, 13 Apr 1998 10:54:39 +1200

Yep, its FIXED!

Thanks Steve, for that draft of the dissertation on how to fix idle/stumble. It turned out to be the throttle sensor connector, managed to remove some almost chrome flake looking stuff off the make connectors! Tightened it all back up, and threw it on. All fixed, the car idles better than it EVER has! AND, no more off throttle surging etc as you come up to intersections, very annoying. Its behaving itself again!

Again thanks Steve, Tom and Henri have supplied me a lot of help on what this could be as well, all very useful stuff.

Who said these cars were complicated (ME!)

Dave Quennell
Christchurch
New Zealand
IZCC#3239

No fault found so far? Try this: Make sure the ignition is off, access the ECU, undo the 3 bolts that hold it in, remove the plastic cover that go over the wires, and undo the connector. Use a 10mm spanner to undo the connector. Make sure it comes out equally far at both ends. If you don't you may bust it. Ditto when screwing it back on. Spray some WD 40 onto both parts of the connector and reassemble. There's a tab that shows when the connector is fully mated.

A posting from Greg Pacifico (edited):

Subject: <280zx> Unplugging ECU

My '83 was running lousy for the first 5 minutes or so after start up (warm or cold). It would hesitate and feel like there was no power. It would suddenly go away after a few minutes like someone threw a switch, a timed thing, not related to temperature, speed or anything else I could figure out. I've spent a few evenings studying the service manual trying to figure it out, but to no avail. Yesterday I unplugged and replugged the ECU and just like magic, the problem's gone!

Greg Pacifico
'83 280zx
IZCC #815

I actually exposed my ECU early on in the testing so I could make voltage measurements directly at the ECU connector.

If the car is still hesitating, and you haven't found the fault, you need to check the ECU power supply and grounding (page EF & EC 105 in the manual. If all this checks OK, swapping the ECU with a known good one will reveal all.