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RE: early '77 280Z

Ever seen that theatrical party skit where some sober soul (let's call him Mr.Fairface) is hypnootized for a lark, so that at the utterance of a "trigger" word, he is immediately transformed into Mr.Uglyface, a roaring drunk? Then, with the trigger word, Mr.Fairface is back, etc.

This, gentle reader, is the apt illustration of my Z problem.

Mr.Happyface is a happy, sweet running engine with a strong, clear idle, a sweet sensitivity to a gentle tip-in of the throttle, and a strong, sure pull from almost any rpm. A true baseline of operation that would do any mechanic proud.

Mr.Uglyface is a cranky, ill mannered motor that refuses to idle, won't tolerate throttle, and which bucks and lurches up to about 2K rpm, if you can get it that high. On the way, while you're trying to clear its throat, it farts light grey mist from the tail pipe. A plug read in the wake of Mr.Uglyface screams "way too rich", which pretty much eliminates fuel pump issues.

But here's the punchline: the change from either face to the other happens like someone spoke the trigger word, or threw a switch (in electrical terms). And the transitions are independent of coolant temperature. And it still happens if I unplug the CSV. Oh, and the CTS is new, and given the natural action of a thermocouple, I would rule out its ability to produce instantaneous changes. I tend to also exclude the AAR for the same reason. Also, the TPS has been carefully calibrated from the start.

In use, despite my fiddlings with the AFM, the idle mix, timing, checking vacuums, etc., the periods of Mr.Uglyface are becoming more frequent and are lasting longer.

Crazy, huh?

Any thoughts?


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Sounds reminiscent of 70's vintage control module failures.  All is well, and then it isn't.  Let it cool and all is well again.

If you found no problem with timing, vac, etc., it might suggest erratic spark behavior brought on by the ECM acting old.

You'd have to connect a timing light to it and keep the gun in the cockpit.  When it starts acting up, check the spark.

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Interesting thought, one I hadn't considered. It does address the sudden change of condition symptom tho.

I admit being stuck on the idea that it is a fuel delivery issue, but hey. When in the throes of spasm, I don't see/hear consequence from improper combustion, but I'm willing to believe I  could be wrong; It wouldn't be the first time.

While I'm not optimistic about the prospects of detecting temporary insanity, I'll review the FSM section.

Anyone else second this motion?

Still open to all thoughts...

While not applicable to your theorem, I should like to provide some background I neglected in the first post:

Just to round out the picture, the Z is in its ShakeDown phase after a rather comprehensive preservation effort that included an engine rebuild. Engine assembly included a full diagnostic on all electrical/EFI components as they were installed. Also, the wiring harness is all original, intact, and fully functional.


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9 hours ago, ensys said:

Still open to all thoughts...

Not really something to ad, just check for all clean contacts in wiring and also check if the new part(s) are working..  new does not mean working good!  (something i got people very angry with but again.. something you have changed in a new part, does NOT mean it's working now.. check check  doublecheck ? ...   

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9 hours ago, dutchzcarguy said:

Not really something to ad, just check for all clean contacts in wiring and also check if the new part(s) are working..  new does not mean working good!  (something i got people very angry with but again.. something you have changed in a new part, does NOT mean it's working now.. check check  doublecheck ? ...   

I agree.  The concept of "infant mortality" in new parts is a hard one to accept.

Perhaps confirming all the variables we've read about, rather than trusting them as new, or not likely, or even impossible might be prudent.  But, it sure sounds like a failing solid state device to me.


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I wouldn't have this post drop into obscurity without noting my appreciation to you and Mr.guy for doing me the courtesy of addressing my problem in the true spirit of this site.


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Well, since you asked...

I installed an aftermarket ICM (Standard Motors LX511), and so far, the results are encouraging.

It's early in the Trials period, but most of the symptoms have subsided (the idle remains unsettled when cold, but I believe that's a different story).

I'll know more after more use, a plug read, and a fiddle or two, but I think this direction is the right one.

Thanks for your interest.

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Second Update or Deja Vu All Over Again:

The change of ICM is a bust.

In 5 operation cycles (all in town), the first two were Heaven; it ran just as it should. The third, toward the end, was marked by a brief hiccup, but okee, dokee otherwise. The fourth (toward the end) had a hiccup and a stall.

The fifth tho, was all-Hell-all-the-time. Not even brief moments of sanity. This last time out (maybe a 6-7mi. trip which was unavoidable) was a nitemare. Had to re-start several times after every stop and a few in between. A good time for a strong battery.The car is undrivable.

So the Trusty Z is sidelined again.

Frankly, I am befuddled (even more than usual).

Current clues:

- The primary spasms occur between 500 and 2K rpm; this is the window in which it will not run, usually on the heels of deceleration or blipping to keep the revs up at rest. Between 2K and 3K, deceleration has a noticeable effect.

- The spasms are not accompanied by the expected misfire histrionics; that is, no coughs, no wheezes, no back or front fires, no farts or burps. Just on and off.

- During these spasms, and in the lowest rpm range, one can, with judicial foot work, get some control of the throttle with the clutch out, but the very moment any load is introduced (like engaging the clutch) below say, 3K rpm, the engine will cut out. I found revving to over 3K (which wasn't easy at rest) and slip/dump the clutch the only way to get rolling from a stop. I bet I put 10K mi. on the clutch in less than an hour. Above 3K, the motor sings like a bird.

- If in gear, sometimes pumping the accelerator vigorously will restore life suddenly when it quits, but not always. And again, all without any unseemly noises that would indicate distress.

- I have recently discovered that if in the throws of spasm with a suddenly dead engine, life is instantly restored by turning the key to off and back to on, or more interesting still, by turning the key from on to touch start for the briefest of moments (the pinion barely kissing the edge of the ring gear, but not engaged). However, I think there is no reason to believe its malfunction could cause these symptoms. More likely it is a matter of sending a jolt that resets the true culprit.

- The effects of these measures are very temporary.

- The problem is constant thru the full engine temp cycle.

-Tach functions as normal thruout.


So, I reckon it's back to basics of assessing  electrical components.

I suppose it's possible that the new ICM could fail in the exact same way over maybe 25mi., but I see that as a pretty long shot.

I still don't think it's a fuel pump malfunction, but I guess at this point, anything is possible. This will be hard to test because I know it runs and can provide for high revs, and I've not quite figured out how, during duty cycle testing, to separate its own function from that triggered by other circuits.

Also, I doubt these conditions indicate coil (which was recently tested) or distributor (freshly re-built) issues, but hey.

I'm thinking to re-visit the TPS. Maybe the F.I. relay needs a closer look (another tricky test), as it is the junction of several run systems.

I suppose the AFM remains a potential suspect, but its only switch (fuel pump cut off) isn't active when the flap is any degree of open, and I don't see the sweeper suddenly changing its character of operation.

And I don't discount the possibility that the ECU has lost its mind, so maybe another full spectrum continuity test. But I am really hoping to find the culprit before I come to this one; replacing one old ECU with another old ECU is too much like trading a known headache for an unfamiliar upset stomach.

Am I overlooking something?

Anybody out there feel like helping a fellow Z driver to wrestle a challenging puzzle?

Or am I just too dumb to see some obvious answer? If so, be a sport and point it out to me, please.


Edited by ensys
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That's a lot of very good information, and of course, it changes the search path.

You wrote the problems lives in 500-2000rpm range.  Perhaps the AFM windings are damaged in that range since that may well be where the wiper has spent most of its time.

If there is any way you could plumb in a fuel pressure gauge to read dynamic fuel pressure it would be very helpful.  That might help separate fuel system from ignition system diagnosis.

As for ignition,  connect a timing light to any plug wire, put the light in the cabin, and go for a drive. If it acts up, turn on the light and point it at anything.  If the light flashes without interruption, it would suggest the problem is not ignition. 

It's a bit of Dr. Frankenstein's monster I know, but perhaps worth the effort.  I've solved more than a few problems this way.

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My kudos for your apparently rare sense of persistent helpfulness. I thank you, Sir.

I too, have considered the fuel pump function, especially in light of, if the FSM is to be believed, the fact that the F.I. relay has just passed the muster of diagnostic exam  (thus debunking my current reckon of root cause).

As I continue to hope the fault lies away from the ECU, I am drawn to the remaining suspects, TPS, or (dread the thought) the AFM.

But still, I remain flummoxed by the symptoms: pumping the throttle sometimes restores function, and how ign. sw. play does the same more reliably..

Add to this the fact that I still cannot decide whether the fault lies with fuel delivery or ignition.The plugs still present sooty, but this still only confirms poorly combusted fuel, but fuel none the less.

It does occur to me to monitor fuel pump function as the reading might be clue-like. The problem here, is that the "clue" might well be second hand to the root issue.

The idea of verifying ignition at the plugs has some merit, but when the engine stops, I have no doubt that the ignition has ceased to function. The issue remains, "why?".

At this point, as suspects go, there is the TPS, or a time to re-visit the AFM. Or perhaps the intolerance to load points to the distributor/timing functions.

With the pass of the F.I. relay, I'm running out of suspects.

I can't escape the sense that I am missing something very obvious.

- Edit -

The problem with the AFM theory is that it does not possess the ability to change from sweet to nasty instantaneously. A bad sweep can't turn good and back again at the drop of a hat.

The fact that there is no more "sweet" leads me to believe that whatever the cause, it has failed permanently.


Does this point to a distributor function?




Edited by ensys
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On 8/22/2020 at 10:28 PM, ensys said:

Mr.Uglyface is a cranky, ill mannered motor that refuses to idle, won't tolerate throttle, and which bucks and lurches up to about 2K rpm,

I "cleaned"my 280zx engine last week  (and failed.. it needs a brush and polish..)  but after i cleaned it with water from the hose i had some water in the connector on that little box which sits on the throttle shaft and i had an engine that did what yours is doing.. it idled well but i could not get it over 1500-1700 rpm's..  maybe your little box on the throttle shaft is "kaputt"  

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