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Nasty idle issue. (random)


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Hey guys.

I'm hoping that someone can shed light on my issue. Since about September my Z (1976 280z (with all emissions crap)) has had an off and on issue with rough idle occasionally upon restart. This is what happens pretty much every time and happened to me...oh twenty minutes ago. I went to the local store to pic up some goods. Drove about thirty minutes there and parked the Z. Turned off the fuel pump (I have a kill switch) and went on my way. Came out about twenty minutes later and placed all the crud I bought in the trunk (or whatever the rear section is called) turned on said fuel pump and went to start the car. Car runs like crap. Idle varies from 500 to 750 and bounces. When I accelerate I get nasty backfiring (or prefiring as it sounds like its in the manifold). I wait about two minutes with the car at about 2k and it evens itself out and drives like new. This past Saturday I went and met with Z Train (Bill) who attempted to diagnose said issue. But as would be expected my Z refused to reproduce said issue so i couldn't really show him what it does. But just for the hell of it we replaced the crappy Champion plugs with the proper NGK plugs and did the air gap in the dizzy.


Does anyone have any ideas? Any at all? This problem is driving me nuts! Could there be vapor lock or something? My first thought was that the kill switch on the fuel pump was causing it so I left the pump on all day but it still did it to me. Anyone have an idea?


Thanks alot guys




Yes Bill it did it to me AGAIN of course and was annoying as would be expected. I'm thinking its fuel related somehow as ignition is out of the question in my point of view as we have new plugs. I'm going to do the cap/rotor and wires next weekend JUST in case.

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I would do a fuel pressure test when its exhibiting the problem. At max pre start pressure, idle, and load.


Also try shaking the afm, see if there is a idle change. Check the fuel filter as well.

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Thanks for the reply. The fuel filter is new. I replaced it Nov 2009 as I also thought that this would cause the problem. I'm planning on installing an inline fuel gauge right past the fuel filter here in the near future so I can tell you the pressure fairly soon. I'm a bit confused on how the fuel pressure could cause this issue as the car drives fine (prefectly fine no issues at all) before the incident and a few minutes after. Could pressure change so drastically so soon? I've only owned this Z since August and it is my first one so by all means I could be wrong. How exactly would shaking the AFM cause the idle to change?


Thanks for your time


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I am not 100 % sure of this, but my understanding is that there is not feed back to the engines computer for the fuel pressure, or the fuel/air ratio. (no O2 sensor)

If this is the case, it is extremely important that you know what the fuel pressure is when you are having drive ability issues.

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I'm not saying it is a fuel issue but if you have proper timing and spark thats where I'd go. Sometimes when pumps are on the way out they will partially fail, then work ok, then partially fail, etc... Sometimes only under heavy load, sometimes at rapid rpm gain yada yada. Like Daniel said though its imp to know what the pressure is doing. Could be something else fuel related, pressure test will confirm or rule that out.


Sometimes a failing afm (and maf) will change idle when you shake it. It also rules out a loose connection there. It could be fuel but it could also be electrical, like something having bad contact some of the time. I would break out the multi meter and start checking then check again when its actually happening.


You have to just chart it out and go step by step then by process of elimination you'll lock it down.

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Ok guys thanks. I'm planning on installing an inline fuel gauge right past the filter to see what the pressure does. I was actually thinking the same thing with the fuel pump. That it could be on its way out and be partially not working or something. I'll let you all know what happens. Thanks



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If you have a factory service manual, the place to check it is where the FSM tells you to.

If you do not have one, I would suggest that if you do an permanent fuel pressure gauge, you check it at the rail, or pipe that is directly connected to the injectors themselves.

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Hey guys

A bit of an update. This evening I started the Z as I had to move it away from the rain. In the process I decided to do some poking around under the hood to see what could cause the idle to change drastically as it is. I shook the AFM as someone suggested but that didn't do a thing to the idle. Secondly I saw three cables going to a connector that is connected to the throttle valve switch. I grabbed all three wires and lightly tugged them towards the passengers side. BAH idle decreased a bit and car started running a bit rough. (it wasn't fully warmed up yet so it was sitting at about 1.5k but it most changed the speed of the engine). I released said cables and the idle returned to normal. Did the same thing again and idle changed and returned to normal when released. Could this be causing the idle issue I'm having? If so what should be my course of action?


I do have a FSM. According to it I should test harness continuity, injector sound, water temp sensor, air temp sensor, valve switch test, cold start valve and FP. Of these the water temp sensor is brand new, I do have injector sound. The other tests for what I can read in the FSM is to do continuity tests (other then Feul Pressure). Has anyone ever done these and how hard are they? Do I require more than just a mulitmeter? Anyone give me any hints or anything on these tests.


I'm taking it in to the mechanic this Thursday to get my new rims installed so I'm going to ask them to do a FP test for me and I'll post what it is here when I get it.


Thanks again guys


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That'll be the throttle position sensor. IIRC It tells the ECU if the throttle is closed (idle) and if wide open. (full throttle) At any other position other than idle and full throttle the ECU has a stored fuel map of how much gas to mix with a certain amount of air to give the cleanest and most economical running. Full throttle tells it to ignore this and run rich for acellerasion and performance. Idle position has it's own special settings. If you are idling and the sensor isn't closed like it should be then the ECU will assume you are driving and will get confused.

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You really should have checked at load and max prime as well. Could have also checked the fpr while the gauge was on there.


Break out the multimeter yet?


No I have not broken out the DMM. I'm a bit afraid to mess with electrical things on such an old car. Plus I've never done it and I'm a bit timid. Anyhow the FP was nice and steady for a good ten minutes. Did not rev the engine but I'm going to install that inline one pretty soon so I'll give you guys a nice reading of ranges here in a week or so.



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