For almost a year now we have been experiencing an intermittent cold start problem on our N-reg Nissan QX (3.0-litre, European edition, 85,000 miles). The engine starts instantly, but immediately after that will not fast idle and dies. It has to be started and "caught" manually with the accelerator pedal which is difficult to do and usually pushes far too much petrol through the motor, usually flooding it. The problem is only evident when the car has been left over ten hours without running - if it is left for a shorter time it will start and cold idle perfectly.
Efforts by our dealer to cure the problem have involved resetting the cold start stepper motor (the problem goes away for a bit but always returns), stripping, cleaning, reassembling and testing stepper (ditto) and replacing the airflow meter (which showed repeated failure on the diagnostic log). The dealer was confident that the latter would solve the problem but it has returned only five days after the new part was fitted. Both the stepper motor and (brand new) flow meter now appear to be working correctly, and no further errors are showing up on the dealer's diagnostic system.
Is there anything else anyone could suggest that might be causing this problem? All this messing about has involved repeated dealer visits with overnight tests due to the long cooldown period that seems to be necessary to make the fault appear, not to mention shedloads of money (QX ancillary parts are not cheap!). The car is otherwise reliable and runs well but this fault is driving us crazy and the dealer now freely admits to being baffled.
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Possible clues:
www.autofan.com/forum_thread.asp?message=4979
townhall-talk.edmunds.com/direct/view/.ef22785.eea...0
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What\'s for you won\'t pass you by
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I'm baffled that they should have replaced the MAF sensor, since I can't see how it would have caused this problem.
The QX (like certain other Japanese cars - some Honda, Toyota and other Nissans) uses a 'waxstat' type element to hold the throttle slighlty open on a cold start. The waxstat element is mounted under the throttle housing and retained by a C-clip with a single screw in it. When the engine is properly cold the element's plunger is extended against a cam on the throttle assy and holds the butterfly slightly open. As the coolant heats up the wax melts, the plunger retracts and the engine comes off fast idle and then the stepper takes over idle speed control.
First off, make sure that the throttle disc and inside of the housing are spotlessly clean (use some carb cleaner, a toothbrush and kitchen tissue to get it all really clean) - the throttle is only held open a 'crack' and any muck will serously reduce the fast idle speed.
When its all clean and with the engine totally cold, check the waxstat plunger protrusion and ensure it is to spec. (the dealer will have the spec., or I can e-mail it to you).
If the plunger is not protruding enough then you will have to replace the waxstat element (a 10 minute job).
I think the above will cure your problem.
The only other thing I can think of is problems with the CTS - but I would think the dealer has measured hot and cold resistances.
I don't think the idle stepper is your problem. You would be likely to notice stalling at junctions if the idle stepper was bunged up. BTW - don't meddle with the adjusting screws on the stepper housing - that will just make matters worse.
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PS - I'm assuming the engine behaves normally once started and running - i.e. not down on powers, hesitating etc etc? If its OK otherwise then I think you can discount other engine sensors, inlet manifold leaks etc.
The VQ-series engine and its electronic management (Nissan ECCS) is a terrifically robust and reliable unit, easily good for 250k miles. If it doesn't run right then look for the simple things first without diving in and replacing expensive components on a 'poke and hope' basis.
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Tim
Did you ever get this issue resolved
I have the same problem with my Nreg 3.0L QX and two differnet Nissan dealers have been unable to Fix
I love this car but it will have to go if a can't get this fixed
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Have you checked that the throttle body is clean? Is the waxstat working OK (this is easy to check)? The other thing to check would be the crank sensor output. If it runs OK once started then its nothing too serious.
I would try an independent rather than Nissan dealer.
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