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Help! 280zx turbo is sick


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#1 Heroez

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Posted 15 January 2012 - 06:26 AM

Hello Z people. I have what seems to be a problem I just cant solve. I am working on a 1983 280zx turbo, 5 speed, federal model. It is clean and looks well taken care of, all original. The car is new to me, and has a problem. While driving it will suddenly lose power, stumble, and backfire. It will only do this at light throttle pressure or cruising in 2-3,000 RPM. If I press the throttle harder or completely let off, the problem is gone and the car acts normal. I have read all kinds of posts from a dozen or more websites. I tried all the usual things and I will try to list all I can remember. Vacuum pressure at idle is 20, so no big leaks. Hoses look new and feel supple. Fuel pressure numbers checked out and considered good. Went throuh a tank of Chevron 93 octane gas. Cleaned electrical connections under hood and at ECU. New fuel filter, battery, alternator. Removed clogged catalytic converter. Swapped in a spare TPS. I went for help to an elder at a Z car shop. He sent the ECU to get rebuilt, tuned the car, and made a test pipe which the welds didnt hold so my muffler swings around. The tuner was dumbfounded and was going to swap in an AFM as a guess. He did offer some advice as I was leaving to never come back: Just dont drive in that RPM range. Great huh? So now a large chunk of funds are gone with the wind. So by now I have cleaned connections 3-4 times. Went forward with a tune up. New NGK plugs, cap, rotor, PCV valve. Did the one by one pull on the injector wires and all cylinders are firing. Desperate now I pull the cover off the afm and move the carbon trace board around and try several locations. Nothing. The problem persists. So I fall into depression and just walk away for a bit. Eventually I snap out of it and buy a multimeter. I pull up the 81 turbo supplement FSM and do the AFM check. First at the ECU pins. Range should be 240-400 ohms. I get 199, a fail. Next it says test at the AFM. 199, fails again. It says replace AFM. So excited now, I buy an AFM from the classifieds. I test it. 201 ohms, fail. Tried it anyway. Problem still there. Now... it may seem like I may know what I am doing, but I am in the dark, way out of my element. I need some advice on what to do next. I am lost. Please help.

Edited by Heroez, 15 January 2012 - 06:57 AM.

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#2 Swervey McZCar

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Posted 15 January 2012 - 07:33 AM

how did you replace the TPS without a multimeter? you should check it's adjustment. Also recheck the connectors at the ECU. They are problematic and cause weird behavior. Try wiggling them as you drive. I replaced mine with the plugs from a z31. I removed each pin and relocated it to a bare z31 connector. Not too much trouble.
Where's the timing set with the engine at operating temp?

Edited by Swervey McZCar, 15 January 2012 - 07:45 AM.


#3 Heroez

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Posted 15 January 2012 - 09:05 AM

how did you replace the TPS without a multimeter? you should check it's adjustment. Also recheck the connectors at the ECU. They are problematic and cause weird behavior. Try wiggling them as you drive. I replaced mine with the plugs from a z31. I removed each pin and relocated it to a bare z31 connector. Not too much trouble.
Where's the timing set with the engine at operating temp?

I had a spare TPS from a 1981, which I just used as a tester, but removed it. Its just a 2 wire on/off style on the turbo. At the time I tested it with key on, and pressing the linkage with a test light. For fun I adjusted it to different settings to look for a change. I may as well do whatever the supplement says to test it. Its been high on my suspect list. ECU connection too. Is that the only way to test them? I can try a wiggle test again, could not hurt. I am interested in how the wire terminals can be released from the connector housing. Im not sure what to use. A needle, cat whisker? I would try the 300zx ECU plugs at this point. The timing...the tuner said it was fine. I dont know much about it on the turbo. I dont have a timing light. Something to read about for sure. I forgot to mention my skill level is middle low under the hood. What do you think?
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#4 SHO-Z

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Posted 15 January 2012 - 11:01 AM

No expert on this but sounds like a pluged fuel filter to me.

#5 Heroez

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Posted 15 January 2012 - 11:20 AM

Wouldnt that have shown up at fuel pressure readings?
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#6 Heroez

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Posted 15 January 2012 - 01:22 PM

Went ahead with the TPS. It passed the 2 tests at the ECU, and the part itself. It then said to go to the TVS section and follow directions for adjustment. I adjusted it with the multimeter method. Turning it untill it was in the off position where there is no reading. Let the car get up to temperature and the drive was the same. Accelerates/deccelerates fine but still stumbles and backfires while trying to cruise with light throttle pressure in the 2-3k rpm range. I did wiggle the ECU plugs while it was sputtering and noticed no change. The problem is still there. Any suggestions folks?

Edited by Heroez, 15 January 2012 - 01:27 PM.

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#7 Gollum

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Posted 15 January 2012 - 08:55 PM

Try driving without the knock and o2 sensor? Amazing what problems they can cause.

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#8 Tony D

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Posted 15 January 2012 - 09:32 PM

Everybod here is missing the fact he used the wrong FSM to test the AFM, but both he had in the car tested bad, and the place he will never go back to suggested AFM as well.

If your replacement tests bad, do you really expect it to act any different from the one already in the car testing identically?

What was that definition about trying the same thing repeatedly and expecting a different result?
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#9 Tony D

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Posted 15 January 2012 - 09:33 PM

Rested with proper FSM, if it tests bad, either replace it, or don't drive in that rpm range.
Misanthropic Anthroparion Class 5 Hoarder, aspiring to posthumous fame as my containers are cut open and the market floods with crap I've squirrelled away over the years! I endeavour to persevere...

#10 Heroez

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Posted 16 January 2012 - 05:01 AM

I do not understand how I used the wrong FSM. If I use the 1983 FSM, the AFM test is for the ECU with one big connector, not the ECCS type I have. Also the value range for the first test is different. 200-400. I thought that the turbo supplement for 81 was all there was. Is there an 83 turbo manual? Is it in the 83 FSM and I missed it?
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#11 Heroez

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Posted 16 January 2012 - 06:55 AM

Try driving without the knock and o2 sensor? Amazing what problems they can cause.

I did a test run without the oxygen sensor. It puts the computer in fail-safe mode. I had not touched the knock sensor. I will look into that. Thanks for the suggestion.
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#12 ctc

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Posted 16 January 2012 - 06:56 AM

1983 FSM has both systems in it (EF & EC page 61). ECCS is trouble shooting for the turbo ECU.

1983 FSM Link (Xenon s130)

EF & EC Page 100 starts the AFM testing.

I have read on another thread here, can't find it, of similar problems as you are describing. In that case it was the ECU connectors.

Edited by ctc, 16 January 2012 - 06:58 AM.


#13 Heroez

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Posted 16 January 2012 - 11:39 AM

Ok. I missed that in the 83 FSM. I will have to check that out. The ECU plugs have been suspect to me as well. Any suggestions for replacing or repairing ECU connectors? I know the 300zx used that style also and they are rumored to be a better fit. How do you release the wire terminals from the connector?
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#14 Josh280z

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Posted 16 January 2012 - 12:14 PM

That sounds like a fuel pump issue to me. Whenever I put a load on my old fuel pump going down the road the engine would stutter and stumble in gear. Whenever I took it out of gear it ran fine.
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#15 macambra

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Posted 16 January 2012 - 02:21 PM

Ok. I missed that in the 83 FSM. I will have to check that out. The ECU plugs have been suspect to me as well. Any suggestions for replacing or repairing ECU connectors? I know the 300zx used that style also and they are rumored to be a better fit. How do you release the wire terminals from the connector?


Go to the autoparts store and buy the tool to release the pins. I have seen them in my local NAPA store.
83 ZXT 5Sp T-top
Enjoy the Shift...

#16 macambra

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Posted 16 January 2012 - 02:42 PM

Ok. I missed that in the 83 FSM. I will have to check that out. The ECU plugs have been suspect to me as well. Any suggestions for replacing or repairing ECU connectors? I know the 300zx used that style also and they are rumored to be a better fit. How do you release the wire terminals from the connector?


Go to the autoparts store and buy the tool to release the pins. I have seen them in my local NAPA store.
83 ZXT 5Sp T-top
Enjoy the Shift...

#17 stravi757

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Posted 16 January 2012 - 06:45 PM

Still sounds like your problem is the AFM. had the exact same problem before. If memory serves correct, its called a dead spot in the AFM.

#18 Swervey McZCar

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Posted 16 January 2012 - 08:14 PM

I use a tiny flat head screwdriver to release the pins. There are small holes next to each pin to stick it into. A needle, pin or tiny paperclip will work also. If the wiggle while driving test doesnt change anything, this is not likely the culprit.

#19 Heroez

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Posted 17 January 2012 - 07:12 AM

After using the 83 tests, I believe my AFM is bad. I got a reading of 120.4 and the number grew as I slowly opened the flap. Then after a the flap was open maybe a quarter inch I got an infinity reading and the numbers became .239, .283, .314, .321, etc. A couple of wiser folks agreed it was bad. Working on getting a remanufactured unit. I hope that is the end of the stumble at cruise.

Edited by Heroez, 17 January 2012 - 02:12 PM.

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#20 RB240zDET

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Posted 17 January 2012 - 07:31 AM

I hope it fixes your problem Hero. If you ever need mechanical help let me know. I would gladly lend a hand.




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