240Z, 260Z, 280Z Performance / Technical Discussions related to performance motor enhancements, upgrades.

Strange miss.....

Old 07-18-2005, 05:32 AM
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Strange miss.....

Trying to work the bugs out of my recently aquired 78 280Z, and for the most part have it running great. The problem I am experiencing is an inconsistent, stubborn miss while driving the car, and occasionally when idling the car.

The car will run great (driving the car) when suddenly it will start to miss. Any attempt at hitting the throttle while this is happening results in a miss, and in any gear (5 speed) the miss is the same and will not allow the car to accelerate. Stopping the car, the car will continue to run roughly at approximately 500 rpm- giving any throttle and the car will still miss in neutral. Putting the pedal to the floor, the car will stumble and die. Sometimes it will clear up and the car will run normally again....Now here is the strange thing- I have discovered that when this happens, turning the key off and then back on (whether in neutral and restarting or while still in gear and just turning the key back on) fixes the problem immediately, until it happens again, usually within 3-5 miles..Other than this, the car runs great.
Regardless of whether I'm driving or idling in neutral, the key off and back on approach fixes the problem temporarily.

Any suggestions? My suspect areas are mass air meter, coil, air temp sensor or possibly ECM...? I'm new to these cars but am learning. Any suggestions would be appreciated. Thanks!!
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Old 07-18-2005, 08:20 PM
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My guess, and I'm not as good as lot of the other guys around here, are going to be your air flow meter or your distributer. Check out some of my posts describing some of the problems I've been having and see if any of it sounds familiar and then.
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Old 07-19-2005, 05:08 AM
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Originally Posted by stcrispin
My guess, and I'm not as good as lot of the other guys around here, are going to be your air flow meter or your distributer. Check out some of my posts describing some of the problems I've been having and see if any of it sounds familiar and then.

Yeah, I think the prime suspect would be the meter as well. I'll check out some other posts, thanks for the input!
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Old 07-21-2005, 10:05 AM
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Grrrrrr

Checked the AFM, potentiometer is good and all of the contacts are within factory spec's. Air temp sensor is working as well within spec's.....time to go to the ignition I guess...
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Old 07-25-2005, 02:43 PM
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Throttle position switch contacts (on the the throttle body) are a possibility, and perhaps the water temp sensor (also check that your coolant is topped up enough, too, or the sensor won't be immersed). Also possibly your alternator (there are some other posts on the details of the alternator/fuel pump link, somewhere around here)
good luck,
Dave
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Old 07-26-2005, 05:32 AM
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Originally Posted by BoulderZ
Throttle position switch contacts (on the the throttle body) are a possibility, and perhaps the water temp sensor (also check that your coolant is topped up enough, too, or the sensor won't be immersed). Also possibly your alternator (there are some other posts on the details of the alternator/fuel pump link, somewhere around here)
good luck,
Dave

Actually, I believe the mass air meter is the culprit. I checked this again and discovered there is a problem with the potentiometer in the MAF sensor. Operating the flap, the reading is 6V @ start. Moving the flap immediately takes the voltage to around 2V, then the voltage drop is gradual. The jump from 6V to 2 volts is definitely a problem. I don't know what the voltage should be with the flap closed, but the voltage drop should be gradual, with no sudden voltage drops. If this doesn't fix the problem I'm going to try the throttle switch. Water temp sensor is new and functioning correctly, as is cold start valve...Thanks for the input!
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Old 07-26-2005, 09:14 AM
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Glad it sounds like you've got it isolated. Sorry it turned out to be the really expensive part.
Dave
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Old 07-26-2005, 09:38 AM
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Originally Posted by BoulderZ
Glad it sounds like you've got it isolated. Sorry it turned out to be the really expensive part.
Dave
Does the aftermarket make a good performance air meter that will work with a basically stock engine? I don't like the design of the meter at all- Besides the fact that it is huge, the flap looks like it could become a problem if it ever binds..A sampling tube or newer Pro-m style would seem to be a much better design....
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Old 07-27-2005, 11:48 AM
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I've not seen one personally, but hopefully someone here has. I vaguely remember somebody kind of building their own a year or two ago, using pieces off of other cars (240SX or Maxima, maybe?). All you really need to do is replicate that voltage curve from the potentiometer. It'd be some tinkering but probably quite do-able. Sorry I can't remember where I saw that info/article... maybe someone else here does?
Dave
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Old 07-27-2005, 12:53 PM
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I seem to be having a similar problem with my g/f car....she has a 76 280z. we recently replaced the afm with a remanufactured one...nothing seemed to be a problem with it....but we put it in and went for a test drive and we shift into 2nd gear and get the pedal about half way to the floor and it feels like no fuel is getting to the engine for combustion! could anyone help me out with this? would a fuel pump be the problem? or new air filter or ?? not to familiar with this area of the car....any help would be greatly appreciated!
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Old 07-28-2005, 05:15 AM
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Originally Posted by BoulderZ
I've not seen one personally, but hopefully someone here has. I vaguely remember somebody kind of building their own a year or two ago, using pieces off of other cars (240SX or Maxima, maybe?). All you really need to do is replicate that voltage curve from the potentiometer. It'd be some tinkering but probably quite do-able. Sorry I can't remember where I saw that info/article... maybe someone else here does?
Dave

I thought of this too, but the problem I see with this is compatibility between plugs from the harness. I have looked at several meters, most have 3-5 contacts, I believe the 78 is around 10 contacts. I guess if you know what each contact is for you could do this, but would it be more trouble than it is worth? The replacement meters aren't cheap though. The cheapest I could get one was $161 (with core).
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Old 07-28-2005, 05:23 AM
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Hmmm

Originally Posted by diflictedminds
I seem to be having a similar problem with my g/f car....she has a 76 280z. we recently replaced the afm with a remanufactured one...nothing seemed to be a problem with it....but we put it in and went for a test drive and we shift into 2nd gear and get the pedal about half way to the floor and it feels like no fuel is getting to the engine for combustion! could anyone help me out with this? would a fuel pump be the problem? or new air filter or ?? not to familiar with this area of the car....any help would be greatly appreciated!
I wouldn't confuse a dead pedal with a lack of fuel. It's either lack of fuel or lack of spark. I would suggest this- Drive the car in a parking lot and shift into second to duplicate the miss. Turn the car off, coast to a stop and immediately pull a plug. If it's ignition related, the plugs will be wet from unburned fuel and you will know this is an ignition problem. Usually, a bad pump will not allow the car to start....Try this and see where your problem is. Once you have isolated the problem the troubleshooting will be easier.
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