Rough idle, then dies. Cylinders not firing? - ZDriver.com


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Old 11-25-2014, 07:54 PM   #1
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1978 280z. Rough idle, then dies. Cylinders not firing?

Hello! After a few weeks of no issues with the car, Im coming home today and start noticing my idle getting rougher and rougher. As I pull into the garage, its getting really bad, all i smell is unburnt gas from my exhaust and i see clouds behind me.

The car now barely starts, if I rev it it goes till I let off the gas, then dies as the rpms fall. I have good fuel pressure, so thats not it.

I replaced the ignition coil, dist cap and rotor about 3 weeks ago, have the plugs, but still need to replace the plugs and wires. Tomorrow after work Ill be picking up a spark plug socket and will take care of those, but other than my plugs being bad, what other ideas do you guys have?

I got used to the Z running reliably again, its going to break my heart to have to drive the truck again.

Last edited by r2doesinc; 11-25-2014 at 08:57 PM.
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Old 11-25-2014, 08:10 PM   #2
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The only other thing that I can see that seems strange is my temp gauge. The needle is about 1/4 of the way across the gauge, I think normally it reads half?

One other user mentioned something about their water temp sensor being the culprit, does this seem likely?

Ive also check my AFM and all seems to be fine there. I dont believe it was ever tampered with, and its moving as it should - i think - when i crank the engine.
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Old 11-25-2014, 08:31 PM   #3
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Well, nobody can tell exactly what model you're asking about. Please do everyone a favor. Sign in, go to User CP menu (upper left), click Edit Signature and add your Z model, year, shifter, turbo or NA, etc. Saves you having to add it to every post. (Guys here go crazy if you don't lock your Q to your specific Z every time you post...They just ignore questions without enough info) Cool?

When you have switched out the plugs and wires - and the problem is not solved - check your sensors. Your equivalent of the Z31 cylinder head temp sensor (CHTS) could be guilty. Sounds like it to me. Also check your air regulator solenoids - assuming your model has them. When they malfunction, ordinary running is totally screwed up.

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Old 11-25-2014, 08:55 PM   #4
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Done. Thought I had actually done that already when I updated my avatar. My bad :P

1978 280z

Will get the plugs and wires in ASAP tomorrow - may even make the run on my lunch break - and will report back.

I think my equivalent would be the temp sensor I mentioned earlier.

http://www.atlanticz.ca/zclub/techti...tempsensor.jpg

Just went back out after I figure the car would have cooled down a bit, and now its not starting at all.
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Old 11-25-2014, 09:33 PM   #5
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I was just reading over the electrical schematics for your '78. It's so different from the Z31s that I can't be of much help to you. Looks so much simpler!

When you have gone through your sensors and it looks like they are all working right, your next best solution may be to start tracing down your harness circuits. Because your problems came on so fast after a period of good running, it also sounds like a signal/connection issue. On older cars, connectors get corroded and grimy over the years, sometimes just decide to stop conducting one day, just like that, no warning. So, going through the harness one connector at a time is a good idea anyway, even if you don't have an issue now, because it can help you avoid future problems. All you need is a circuit tester, a can of spray cleaner and some time.

Would be good to hear from our 280Z/ZX guys about now. They really know their beans here.

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Old 11-26-2014, 07:38 AM   #6
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Im pretty sure the sensor is the culprit. This morning it started right up and idled in the garage for about 15 minutes while I had some coffee.

Will get back when I have the new parts in.
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Old 11-26-2014, 12:52 PM   #7
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Good result of your work so far. I like your can-do approach. Be sure to report back with what you find. We like (need) the feedback!
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Old 11-26-2014, 05:32 PM   #8
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New plugs and wires in today, as well as the new temp sensor. Unfortunately, no change.

Everything just seems to be very very slow, its idle is around 800 - down from a normal ~1100. The voltage meter in the car is ticking around a little erratically as well, I imagine because of the alternator turning so slowly?

1. Fuel - check.
2. Spark - check.
3. Air - unproven

I messed with the AFM a bit to see what would happen, lightly turning it while the car was running. Turning it even a little in either direction was enough to kill the car.

Anyways, at this point im out of easy ideas. I dont suspect any electrical issues this time around, I think the shop pretty much handled all the efi wiring last time i took it in.
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Old 11-26-2014, 06:37 PM   #9
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Download a copy of the Fuel Injection Manual at XenonS130. It covers the 75-83 fuel injection systems. Read through it, especially the troubleshooting sections.

What did your plugs look like?
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Old 11-26-2014, 07:09 PM   #10
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Ive gone through most of that with previous issues. Ending up replacing the ECU and the FI relays, as well as doing the fusible link > maxifuse upgrade.

I will need to go through and test all the AFM stuff first. I remember seeing the process for checking the pins but i dont remember which ones off hand.

The plugs looked....not too terrible but super black. The car always has always smelled like its running a bit rich, this was just confirmation of that.

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Old 11-26-2014, 10:08 PM   #11
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You replaced the water temp sensor - so where does your temp gauge read now?
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Old 11-26-2014, 10:12 PM   #12
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Well it doesnt run long enough to warm up, so it reads the lowest it will show
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Old 11-26-2014, 10:52 PM   #13
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Quote:
Originally Posted by r2doesinc View Post
Im pretty sure the sensor is the culprit. This morning it started right up and idled in the garage for about 15 minutes while I had some coffee.
So you switched the sensor, your wires and plugs - but it will not even idle now? Sorry if I misunderstand - but is that where you are now?
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Old 11-26-2014, 10:53 PM   #14
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Its at the same point it was before, super rough idle and will eventually die after a few minutes. I havent let it run long because the whole garage fills with exhaust.
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Old 11-27-2014, 09:33 AM   #15
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This morning I started it up, nice strong idle.

Over the next 10 minutes i watched the idle slow little by little.

By 8 minutes, I was down to 6-800rpm. By the time it died at 9, it was just barely over 500rpms.

I noticed that unplugging the water temp sensor actually causes the car to die if its running, or not start.
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Old 11-27-2014, 09:36 AM   #16
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Ive seen a lot about the EGR valve, but im not seeing how to physically check that it is open.

Does anyone have a process to go through to physically check the egr valve? What should I be loking for there?
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Old 11-27-2014, 01:08 PM   #17
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EGR sounds like a good guess at this point. It's in Section EC of your service manual - with a procedure for checking it out, like around EC-12 etc.. You may be looking for vacuum hose issues now. Have you checked/replaced your hoses? Engine performance relies on them so they need to be working right. The origs can't hack the engine environment for very long, not for 30+ years anyway. Splits and cracks are inevitable. Not a big deal to replace them - with the correct diameters (be sure to match the diams). Just a guess at this point. If you do that, trace and replace one at a time so you don't screw them up.

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Old 06-02-2015, 08:13 AM   #18
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I had a similar issue, I jumpered the connector at the CHTS and the car stopped running rich.

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Old 03-14-2016, 07:04 PM   #19
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For anyone interested, I ended up selling the car around the last posting. Bought it back from the guy a few months ago now. He got it running beautifully. Apparently it was the ECU, which I had already replaced twice lol.
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