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Discussion Starter #1
I'm a rookie to this so be kind. Just looking for advice on possible solutions for my 1978 Datsun 280Z (California model) running rough after it gets to running temp. Starts great and runs smooth on all 6. As soon at it gets to running temp it starts missing (I'm told its missing on cylinders 1-2).
1. does anyone know of any good Datsun mechanics within 100 miles of southwest Michigan? I can't seem to find anyone who has interest in working on it?
2. just looking for suggestions for what this could be. I've seen that these tend to have problems when they get really warm, but not sure what it means when it runs rough when just getting to running temp.


thanks
 

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Sounds like you may have a faulty coolant temperature sensor. Does it blow Black smoke when it is running rough?
 

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Here, in order of cheapness/freeness, are a few suggestions:
1: Go through and clean every electrical connection you can find under the hood. If you can't physically clean them with a wire brush or spraying electrical cleaner, make-unmake the connections several times to 'freshen' them up. These cars are very sensitive to good electrical connections and many of those get diry/corroded and build up resistance over the years. And I'm talking EVERY connection - injectors, sensors, battery terminals and even grounds.
2. When the car starts running roughly, pull off either the spark plugs or the injector connectors one at a time (injectors are easier and won't shock you). What should happen is the car will run worse when you do this. If there is NO change for one or more cylinders, those ones aren't firing or getting fuel for some reason.
3. Do a vacuum test. These cars are also very sensitive to having a tight vacuum system. This test works best if you have a vacuum gauge, but you can also pull off one vacuum line at a time and see if there is any difference in the way the car runs, which may lead you to the problem. YOu can also spray carb cleaner or WD40 or starter fluid or something similar around sensors and manifolds. It the rough running suddenly smooths out when you do this in a certain area, it could mean you have a leaky gasket or seal. The air going into a Z engine is metered and any extra leaking in will make it run bad. FWIW, I have found it to be common on the Zs I've bought to have a bad intake manifold gasket. They're a pain to swap out, but if all else fails, you might consider trying that.

4. Pull off the fuel inlet line under the hood, put one end in a jar or bucket. Remove the small wire going to the starter (so the car won't try to start). Have someone turn the key as if starting. See if fuel pumps out INTO THE JAR OR CONTAINER, NOT ON THE GROUND. No need to measure it at this point, just see if it flows out in a nice strong stream. Then put it back together. Obviously, be VERY CAREFUL around raw fuel.
5. Make 100% sure your battery is good and putting out max volts. If in doubt, grab a new one. Also, corroded battery terminals/wires can sap a lot of power. If yours look bad, replace with aftermaket ones.

6. Check the timing and make sure your vacuum advance is working. Requires timing light.

7. Is your fuel fresh? If you're trying to bring the car back after sitting a long time, the fuel could have spoiled. Drain it and put new gas in.

8. Compression test. This will require borrowing or buying a compression gauge.

9. Buy a factory service manual, digital or the actual paper-and-ink book. They are mandatory for anyone doing their own work on these cars.


Bottom line - the early Zs and their simple fuel injection systems (compared to today's cars) are easy to understand if you take the time. The engines are bulletproof and problems like yours are hardly ever mechanical in nature. Most problems I've come across in 25+ years owning these cars are electrical or vacuum related.



Good luck and let us know what you find!
 

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Discussion Starter #7
Thanks everyone for your help. I’ll start checking into your suggestions in the next week and get back.
 

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Discussion Starter #8
Response to Gr8gary.
It does not blow black smoke but it does back fire a little and you can tell all 6 are not firing. Had someone adjust valves, timing and work on the fuel mixture. Will have to start checking all the electrical connections and vacuum. Does the fact that it starts and runs great for the first 10 mins.
 

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Response to Gr8gary.
It does not blow black smoke but it does back fire a little and you can tell all 6 are not firing. Had someone adjust valves, timing and work on the fuel mixture. Will have to start checking all the electrical connections and vacuum. Does the fact that it starts and runs great for the first 10 mins.
how do you know all 6 arent firing? are all 6 getting spark? use the cigarette w/hand pump into the brake booster's intake manifold inlet to find and patch the leaks
 

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Discussion Starter #10 (Edited)
Saturday i refreshed all the electrical connections and grounds. Went through all the vacuum hoses and replaced all that I could. Checked vacuum at the brake booster and it was steady at 15 during idle. I think that may be low but it’s 41 years old and has 109k miles so I would guess my rings and valves are a little worn. Engine began running rough again after it got to running temp. Removed all spark plugs wires one at a time. Ran worse when 6,5,4,3 were removed. Ran same when 2,1 were removed? Think I have spark since as I started to replace wire I could hear if sparking.


Wednesday Evening: Started again cold and it ran well. Thought I would run to get 93 octane gas. Topped off with 7.5 gallons. Then it continued to run well and I could not get it to run bad even after I drove if for 20 mins. Does a full tank or removing the gas cap provide clues to why it starts running bad? Maybe I have an incorrect gas cap or have a fuel tank problem?


Thursday am: threw caution to the wind and thought I would drive it 30 miles to work. Did not start immediately like normal and heard a minor back fire as it started? Ran great the first 15 miles. Had to sit in construction for 5 mins. Noticed the idle was high but it was running well. Temperature stayed and did not increase. As I was sitting there it was like someone threw a switch and all of a sudden it started running bad again (on 4 cylinders). Idle dropped from 1,400 to 600 and had to give it gas to keep it going. Once I started moving it ran a little better but you could still tell it was not running well. stop and topped it off with gas. Ran a little better but something is still not correct? Should I look at the coolant temperature sensor next? thanks
 

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Discussion Starter #13
Need help with testing results - what do I do next?

7-1-19Update
  • 1978 280Z (California model) Car starts and runs well for 15 mins, as soon as it gets to running temp it starts running really bad and stops running after 35 mins.
Did several tests based on the “Engine Fuel” service manual.
The following check points all passed – Throttle valve switch (idle & full), Thermotime Switch, Ground Circuit, Altitude Switch, Revolution trigger signal, Powerline circuit and Injector and resistors (all 6 cylinders)

I need helpon the following:



Check Item #1-(3) Air Flow Meter
  • Terminals 6 & 8: Got reading of 182.2, but I did not get a Continuity audible beep like I did on other terminal checks. Is this ok?
  • Terminals 7 & 8: Got readingof 183.6, but I did not get a Continuity audible beep. Is this ok?
  • Terminals 8 & 9: Got readingof 101.5, but I did not get a Continuity audible beep. Is this ok?
Check Item #1-(4) Air Temperature Sensor
  • Terminals 6 & 27: No reading and no Continuity audible beep.
Check Item #1-(5) Water Temperature Sensor
  • Terminals 13 & Body Medal: No reading and no Continuity audible beep.
Check Item #1-(6) Air Regulator
  • Terminals 34 & Body Metal: Got reading of 63.0, but no audible beep. Is this ok?
Parts I checked are:
  • Air flow meter – everything checked out ok including the “Air temp sensor”
  • Water Temp Sensor – tested ok, but still replaced with a new one I had just purchased (new unit also tested ok)
  • Thermotime Switch – passed butI had a challenge getting a 56.2 ohm reading.
So, FSM says if the continuity does not exist to trace the circuit. Sorry I’m not sure what that means. Do they want me to follow the wire from the EF control unit to the sensor? If the sensor is not the problem, then what am I looking for in the wiring?

Thanks
 

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7-1-19Update
  • 1978 280Z (California model) Car starts and runs well for 15 mins, as soon as it gets to running temp it starts running really bad and stops running after 35 mins.
Check Item #1-(5) Water Temperature Sensor
  • Terminals 13 & Body Medal: No reading and no Continuity audible beep.






Thanks

what was the resistance reading in ohms at the 36pin connector pin 13 and chassis ground. What was the temp of the engine.
 

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Discussion Starter #15
For 13 and ground I could not get a reading. The engine had not ran for a couple days at the time so I assume the motor temp was about 80 deg F
 

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if you have no resistance that is the problem. You will need check at the temp sensor next. take a pic so we know you are testing the right one. The problem will have to be the sensor or the wiring to it. No resistance is telling the ECU you live in the artic circle.
 

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Discussion Starter #17
checking no continuity in water temperature sensor

Dave - thanks for all your help so far. Sorry I lack automotive knowledge but I'm slowly learning and I will also have this car for the rest of my life.


Ok I know the new water temperature sensor is good. So I must have a problem with the wire or the electrical connection.
- Any tips on how I go about testing the wire so I can figure where the problem is?
- can I assume I have a similar problem with the air temp sensor since I also did not get a reading at the 6 & 27 terminals?
 

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simple wire tracing. Start by making sure you multimeter is working, some that are not auto ranging can lead you astray. Get the known good one out of the T stat and start checking it, BTW NEVER assume a new part is good, check before installing.


Once you are sure you know the new part is testing good, then its time to inspect the wire harness. Here continuity testing with a tone function of the multi meter is important. You know one side of the sensor connector is ground so test that. One lead of the meter to the chassis ground then the other lead to the sensor connector. ONE of them must have continuity, if neither do then follow the sensor harness to the 1st break point, a bullet connector test there. Use logic to find the break in the circuit.


Do all this testing with the 36 pin connector disconnected and the battery disconnected.
 

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also make sure you have the right sensor plugged into the right sensor plug and the right bullet connectors hooked up to the right bullet connectors (sub harness to main harness)
 

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I just check mine, the bullet connector sub harness is marked with some red tap wrapped near the bullets. that is the THERMOTIME harness, the other sub harness wth NO RED TAPE is the CTS. You can just disconnect there leave the sensor plug attached to the sensor, and get a reading off the bullets (the side that is plugged in) you should see somewhere around 2k-1.2k (2000 to 1200) ohms, depending on local temps (cold motor). Once it has warmed up you should see around .3k or 300 ohms. that would be around 180f water temps. Trace the wire harness from the CTS and follow it back toward the firewall along the top of the engine. You should be able to find the bullet connectors. Make sure you don't break the wires when you disconnect it, they will prob be stuck. again you should see a RED piece of tape wrapped around the harness near the bullets, that is the harness for the thermotime, it matches the main harness also marked with red tape. You want the NON red tape for the CTS wires.
 
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