| Erratic rough hunting fluctuating idle vacuum leak A widely fluctuating idle is a classic symptom of an intake manifold leak, vacuum line leak or leaking component linked to a vacuum line. This can also show up on a vacuum test gauge as a very low reading 5 – 10 at idle. With the Z, a vacuum leak should be suspect if the symptoms are exhibited but the ECU is not throwing off a defect codes.
I had a very erratic idle that I tried everything to find the problem with. It turned out to be severe enough to actually occasionally stall the car. One day while coming down a hill coming up on a stopped truck at a red light I pushed in the clutch and the brake to stop. The car stalled, and when it did, I lost the power steering and power brakes. In my few WTF seconds trying to figure out what had happened, I judged that I did not have enough distance to pop the clutch restart the car and regain vacuum to the brakes and steering in time to stop before crashing into the truck. I was able to tug on the steering wheel enough to steer the car to the side of the truck. Luckily there was enough room without hitting anything. Yes, I know in hind sight I should have thrown on the emergency brake or stomped with all my weight on the brakes or something else but in the few seconds available, this seemed the best alternative. I mention all of this just to highlight the fact that a vacuum leak can be more than a nuisance, it can be dangerous as well, and so should be attended to as soon as possible. I did not originally realize that I was looking for a vacuum leak. I looked on the web and everyone had different opinions on what the problem with the idle could be – fuel pressure, injectors, timing, rings, valves, etc. etc. I got some software and hooked up my laptop to the car, it showed a bad driver’s side O2 sensor. I hoped that was it. I had replaced the passenger side sensor some time back as it is easy to access. The driver’s side though is much harder and Nissan dealers use a special tool to get to it. The service input tech. told me that they had a mechanic who was “an expert with these cars”. I said fine and left it. I came back and the sensor had not been replaced. The tech said the service guy said the engine was totally shot and did not want to work on it because it would end up costing me thousands and thousands of dollars – much more than the car was worth and so did not do it. This prompted a little chat between me and the service manager. I emphasized that I did not want the engine rebuilt or any major work - just the O2 sensor replaced please. They went ahead and did it. The problem with the idle was not solved but I did not mind because A) someone else got to skin their knuckles (the real reason I suspect the mechanic did not want to do it) on the job instead of me and B) it needed to be done anyway.
It did not fix the idle though. I next hooked up a vacuum gauge. Here I will just mention what an absolutely valuable tool a vacuum gauge is. I am talking about a diagnostic test gauge, not the kind you install in the car. There is no excuse for not owning one. They are cheap – about $15.00. They can be hooked up with a plastic “T” connector in any vacuum line off the intake manifold and they give all kinds of valuable information and are easy to read with the engine running. I have listed a site that show how to read the gage. You can download a pdf file for printing. [ http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm ] A search on you tube will provide plenty of videos as well.
My very low reading indicated an intake manifold (or vacuum problem). A search of the 300zx net produced all kinds of leak detection schemes, some involved sending smoke through the intake and some sending pressured air through with a warning that sending more than 6lbs pressure could blow out a seal. I did not have the equipment to do either of these. My technique was much simpler. Actually, two techniques. One was to bypassed one vacuum system at a time, plugging the access point on the intake manifold to see if the problem shown on the gauge would go away or the idle would smooth out. The second was even easier, since the first technique would not work very well if there was more than one leak. One might mask the other. The vacuum lines either go directly to a valve (which is normally shut), are routed through a solenoid to a valve (also normally closed with power off or are routed to a booster (power brakes) or an air storage device. Some vacuum lines ( AIV, EGR) receive vacuum through the intake but are also connected to a suction tube through a solenoid, but none of the lines vent to atmosphere, which would be the same as a leak. If you unhook the line from the intake and simply blow into it – you should not be able to. So if you can blow into it, it means that the lines are venting to atmosphere and either the line is disconnected, has a crack in it or the valve itself is leaking. In addition, since you are doing this with the engine off, you should be able to hear the escaping air and be able to pin point the source of the leak. It was in this way that I found my leak. I blew on the hose leading from the intake manifold to the clutch booster and there was no resistance in addition I could hear the leak, it was coming from inside the driver’s side fender. There is a lot of vacuum stuff going on inside that fender. You have the line to the AIV valve (1992 and older), lines to the PRVR tank and in the case of twin turbo cars, you have the second of the two air tanks for the vacuum power clutch system, with a vacuum hose running to it. This last item that was leaking on my car. Once this hose was fixed, the car idle became rock steady and the gauge indicated a normal vacuum reading at idle. The thing is, even if I had paid the Nissan dealer the thousands and thousands of dollars their mechanic thought it would take to rebuild the engine or replace every part on the engine, it still would not have fixed the idle problem. It just shows that trouble shooting techniques of some of the “experts on these cars” is to simply replace one expensive component after another hoping to fix the problem, instead of actually taking time to eliminate systems by trouble shooting them and this in spite of their thousands of dollars in test equipment. If I had used the smoke injection method or air pressure method it might have told me the problem was in the fender but not which system. I did find one more leak from a visual inspection. The “T” connector to the wastegate contains a small metal bullet like structure with a hole in it. It looks just like the small air restrictors (probably the exact same) that some people use to boost turbo pressure beyond 9lbs. It precisely measures vacuum bleed off that goes to the wastegate actuator. It is a nightmare to get to. This little metal part fits into the small vacuum line off the solenoid and as might be expected, stretches out the line slightly where it is installed. My line had a break at one of the two ends where it was installed. This make sense as all these hoses (and plastic parts) lose their elasticity and become brittle as they age and are subjected to temperature extremes and vibration and tend to break where they are stretched, typically at the edge where there are attached to fittings. I found and fixed this break before I found the major leak. I hope this write up has been useful to someone.
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15:34:02 07/16/12
