If the problem happens when the engine is cold, but tends to disappear once the car has warmed up, then it may be related to faulty components that affect cold/hot operation behavior- the engine or air is cold but the computer doesn't know about it because of sensor circuit failure, or the computer knows its cold, but the various actuators or circuits aren't functioning correctly to change the engine inputs: fuel & air.

Keep in the back of your mind, though, that the reverse can happen- the engine is actually hot, but the ECU thinks its cold!

Hesitation when the engine is cold (Page EF & EC 78 in the manual)
 
 

PCV valve on LHS :	PCV valve on RHS :

After checking the spark plugs as described in the preliminary check section, check the PCV (positive crankcase ventilation) valves and hoses. There's one on each side of the engine. Use a pair of pliers to crimp the hose by the valve to shut off the air supply to the inlet manifold. If the engine speed goes up, or becomes more stable, then you've located the problem.

To check operation of the valve, pull off the hose from the valve (this will require a bit of patience and grunting). If the valve is working properly, you should hear a hissing noise from the valve with the engine running at idle. Put your finger over the valve and you should feel a strong vacuum.

If that's OK, then you're onto the air flow meter. The air flow meter is a hot wire type. As more air passes over the hot wire, more current is required to maintain the same hot wire temperature. If the air is colder, again more heat will be lost from the hot wire, so there's no need for a separate air temperature sensor. The current is dropped across a resistance and measured by the ECU as a voltage, which is interpreted as an air flow.

• Remove the panel that covers the air box. You'll then see the air flow meter (about 3 inches square) with the cable going up to it.
• Fold back the connector rubber to expose the connector.
• Put a voltmeter between ground and the white wire that's second in from the end.
• Start the engine & warm it up.
• At idle the voltage should be between 0.8 and 1.5 volts.
• You have to remove the air flow meter to check the hot wire for damage or dust.

 

Air Flow Meter :

 

If the ECU detects that the air flow meter or circuit is not working, it estimates the air flow from the throttle position sensor. You can still start and drive the car, but ECU will prevent the engine revs from exceeding 2,400 rpm.

Air turbulence in and in front of the air flow meter has been blamed for many hesitation problems. The theory is that the air flow meter will give a false output to the ECU, as the flow over the hot wire will not be representative of the flow across it's section. Here are some past postings on this subject (edited):

"From: Amit Kumar  akumar@cs.tamu.edu
Date: Fri, 1 Dec 1995 11:48:27 -0600 (CST)
Subject: <300> Engine cutting out under braking

I've had this problem ever since I bought the car ('91 TT) 3 months ago. The engine cuts out under hard braking (normally from 55+ mph) with the clutch pedal fully depressed. ( I guess the clutch not getting fully disengaged could cause the RPMs to drop too low - but I don't think this is the case.) If I engage the clutch again or turn the key, it will start right up. It's pretty scary when this happens while going around a curve, since it causes the power steering to cut out as well. Doesn't do too much for my confidence! Somehow this problem never occurred during the (fairly long) test drive, but started to happen 2-3 hours after I took possession. (Actually, it may have once, but at that I think I may've thought (!) it was something I'd done wrong.) The dealership that sold me the car said they were unable to duplicate the problem since they were unable to take the car over 55 mph. (At that time the problem occurred only at above 70 mph.) They did agree to let a service guy ride with me while I tried to duplicate the problem, but we never got around to it since I moved out of town the next day. The service guy at the next dealership I took it to immediately told me it was a problem with a transistor that was causing this to happen, and that it would be fixed under a recall. Well, the work they did seems to have done something, since it doesn't happen as frequently anymore. However, now that I'm not able to duplicate the problem at will, it's harder to show them what's happening. Has anybody had (or is having) a similar problem with their late-model Z? I'm planning to take it to a different dealer, but it'd be nice if I knew what the problem was, so I could tell them exactly what I need done.

Muchos gracias.

Amit
'91 TT

From: MaxBoost@aol.com
Date: Sun, 3 Dec 1995 15:53:11 -0500
Subject: <300> Re: Engine cutting out under braking

Seeing that it is a used car, it may have been in a front end collision. When the front end is repaired the plastic inner fender may be loose or missing. If this is the case, the airflow from the front tires at highway speeds disturbs the airflow into the engine inlet ducts. These ducts are located in front of Both tires and lead to the air flow meter in the center of the car. If the airflow through the airflow meter is disturbed, it can cause an engine stall with the clutch depressed. You should check the inner fender panels or install a pop charger and gain some HP and the problem.

MaxBoost
85 300ZXT

From: vickrey@cts.com (Donn Vickrey)
Date: Sat, 9 Dec 95 09:48 PST
Subject: <300> Re: Engine cutting out under braking

Amit -
The missing fender theory is a likely cause of your problem. The Z airflow meters are very susceptible to turbulence and that can cause the idle to dip abnormally low and can even cause stalling. If you do switch to a POP charger filter, make sure that you reinstall the plastic tray that fits under the stock air intake box. Otherwise, the POP charger can be susceptible to the same problems.

Donn Vickrey,br> '78 280Z,'90 300ZXTT
IZCC #443, ZCSD #56

From: Amit Kumar  akumar@cs.tamu.edu
Date: Mon, 11 Dec 1995 10:41:34 -0600 (CST)
Subject: <300> Re: Engine cutting out under braking

I spoke with Clark at Wolf Tech and somebody at Stillen last week about this. While neither of them could think of any reasons, they both agreed that it sounded like a plausible explanation. Meanwhile, I've ordered the fender skirt (or whatever it's called) at the local dealer. Hopefully, putting that in will solve the problem. I've also decided to go with the pop-charger to make sure my engine doesn't take a drink of stagnant water anytime soon.

Amit
'91 TT"
 

Throttle Valve Area :	Remove the hose and you see the throttle valve directly :

Next are the Throttle valve deposits. For this you will need some carburettor cleaner. Over time, a black sludge builds up behind each of the two main throttle valves. I suspect this comes from the EGR (emission gas recirculation) input which is fed in through a hole just behind the valves. These deposits block air for small throttle openings, hence causing a momentary hesitation as the main valves open.

• Undo and remove the 2 hoses connecting the throttle valve opening. I put a rag underneath the throttle valve opening to catch the carburettor cleaning fluid that will flow out
• Open the butterfly valves
• Squirt carburettor cleaning fluid in to dissolve the deposits.

In my TT, I noticed that whenever you stepped the accelerator lightly (with no engine load) , the engine rpm dropped about 50 rpm for about 20 milliseconds, before climbing. As soon as I removed the throttle valve deposits, this characteristic disappeared altogether.

Another source of problem not mentioned in the manual under hesitation when cold is the temperature sensor & connectors.

Coolant Temperature Sensor/Connector :

 

There is description for a procedure on how to clean the coolant temperature connector.  Click here for procedure.
 

The left hand sensor senses for the ECU, the right hand one operates the temperature gauge. Although both connectors are prone to corrosion, the sensor connector for the ECU is much more prone despite the fancy construction. To check, roll back the rubber around the connector and put a volt meter between ground and the yellow/green wire. Start the engine from cold, and you should see the voltage rise progressively from 5 volts to almost 0 volts.

If the temperature sensor circuit is open or shorted, the ECU assumes the engine temperature is 20 deg C or 68 deg F while starting, then when running, assumes the engine temperature is 80 deg C or 176 deg F. Here's a report from Peter Garcia (edited):

Subject: <300> Hesitation solved!
Hello everyone,
First of all thank you very much for your help in trying to remedy my situation. I am sorry to say though that although I tried everyone's various suggestions, the reason for my hesitation was due to something different. Today, I turned on the car and warmed it up as usual, and then drove it around the perimeter of the campus. The car was still hesitant and it was REALLY starting to get on my nerves. Anyway, I parked the car and did as someone suggested and checked the spark plug assemblies to see if they were loose or something. Weirdly enough, one of them did kind of click into place after I pushed on it. So I thought hey, that could have done it! WRONG!!! As a result, the car started to drive very, very, very badly. It was missing, and the car wouldn't want to move. Big time loss of power! The engine sounded like crap! It scared the heck out of me. Anyway, I immediately called Nissan to see if I could take it in to the shop and they said that they couldn't see it until Friday. I explained to them that I needed the car for work and that I didn't want to drive the car like it was for fear that I would damage something else. (By the way, the guy knows me by now from other maintenance checks, and because there aren't many 300ZXTTs in David Koresh Town, U.S.A.) Anyway to make the long story shorter, he fitted me into his tight schedule and actually found the problem! (That's pretty good for a rinky dinky town like this one) The part you've been waiting for: the computer coolant temperature control sensor (as he called it) was telling the car that the coolant temperature was -2 degrees. Therefore it made the car run very rich. The sensor cost a whole $23, but the labor, and another spark plug which was ruined because of the problem tallied it up to $103 and change. After driving it around town today and tonight, I must say that the Z's ready to kill once again!
I hope that this helps everyone who has been having similar problems. If you have forgotten, this goes for my car which is a 1990 TT. Once again, thanks for listening and helping out! I REALLY appreciate it!
Yours truly,
The one who's car every boy on campus wants,
Peter Garcia
Baylor University
IZCC #1065
1990 300ZXTT, 1985 300ZXT, and
'79 280ZX