To: (z-car list)
Date: 9 Jul 91 02:41:13 EDT (Tue)
From: John De Armond
>I have a 1979 280ZX and I think I am having a fuel
How many miles? That may be important.
>The main symptom is excessive CO emissions at
>idle. The car runs fine, but I won't be able to renew my
>registration until I get this fixed. The standard is 0.5% and I am
>running at around 3-4% CO. This is not a trivial amount and tends to
>indicate an overly rich mixture.
How about physical symptoms? Does it run well? Rough idle? Missing
on any cylinders? Any plugs black or different than the others? Did
the problem just pop up, in other words, did it pass last year?
Does that model have a catalytic converter? Those numbers indicate
some pretty gross problem so it should not be too hard to diagnose.
>1. With the ignition on, but the engine not running, the pressure
>regulator mounted on the fuel rail assembly for the injectors makes
>a hissing noise. Is this normal? Could it indicate a bad pressure
>regulator or a leaking injector?
this is correct. the pressure regulator bypasses excess flow back to the
>2. Is there an easy way to determine whether an injector is leaking?
>Removing injectors is a pain (I have done it before).
yep. Follow the procedure in the manual and detach the hose from
the cold enrichment valve and insert a pressure gauge. With the ignition
on and the engine stopped, manually operate the air meter vane to make
the pump run and read the gauge. Compare it to the book. Release the
vane and let the pump stop. IF the pressure does not hold, you have
Before ripping out injectors, check the checkvalve on the
outlet of the fuel pump located in front of the gas tank. This valve
holds static pressure in the system and sometimes leaks.
Secondly, check the pressure regulator for leaks. It regulates pressure
to a fixed value relative to intake manifold pressure so there is a
vacuum line leading to it. If the diaphram leaks, there will be raw
gas flowing into the manifold through this sensing line. Simply
pull the sensing line off and look for drips with fuel pressure in the
system. Failing that, you probably have a leaking injector if the
fuel pressure won't hold for several minutes.
>I am really hoping the high CO level is caused by one of the above,
>since I don't want to troubleshoot the EFI control module, without
Checking out the electronics is pretty easy with no more information
than available in the manual. All you do is look at the injector firing
pulse width using either the factor tester or a 'scope. The millisecond
values for idle under various temperature conditions are not in the
standard service manual but are in the diagnostic instrument manual and
are in the FI training suplement available from Datsun. See the
monthly posting for title. I'd be glad to quote numbers for you except
that my books are all at the book hospital recovering from
smoke inhalation :-(
I've never seen a partial failure in the module. Either it fails
catastrophically or it works. I HAVE routinely seen air meter drift
and large throttle switch timing changes caused by wear. Check the
throttle switch timing per the manual. Make sure the off-idle contacts
are de-energized at idle. And make sure that the throttle stop is set
per the manual and that idle speed is adjusted using the idle bypass valve.
There is a mixture adjustment on the air meter. You have to pry out a
sealed cap and/or chip out some epoxee but it is there. If that
adjustment cannot bring it back in, then you may have to open the air
meter and readjust the potentiometer. The wiper can be adjusted
on the vane shaft. The potentiometer is very nonlinear and is the
primary determinant of mixture strength so a little adjustment goes a
long way. you really need the use of an exhaust gas analyzer to
do this adjustment for emission purposes.
If you take these steps and if you know that your cat converter is
working (assuming there is one), then we'll discuss some more
drastic steps. One common one I use is to bracket the air temperature
thermister with a pot which allows you to set the mixture over a fairly
wide range. You GOTTA have an exhaust analyzer for this step.
One last note. If the car is high mileage, check the cam timing. There
are several holes in the cam sproket that allow you to adjust for chain
wear. You might need to compensate the cam a bit.
Subject: Z-car problem
Date: 9 Jul 91 22:37:54 EDT (Tue)
From: John De Armond
>The car runs quite well. The idle is fine. I can not detect
>any missing cylinders.
>>this is correct. the pressure regulator bypasses excess flow back to the
>Could you elaborate a little here? I am not sure whether you
>mean that hissing is normal or that it indicates a bad pressure
>regulator or leaking injector.
The way the fuel system works is that a positive displacement pump near
the tank pumps a more or less fixed volume of fuel. This fuel flow
through the injector manifold and then to the pressure regulator. This
regulator is a back pressure regulator that bypasses any flow not used
by the injectors back to the tank. Since the pump volume is vastly
more than the fuel injectors can flow under any condition, there will
always be some bypass flow through the regulator.
>With the ignition on and the engine stopped, I am quite sure that the
>fuel pump on the car is running, as I can plainly hear it. I don't
>need to do anything to the air meter for the pump to run. Is
>this a problem or just a difference in models? I haven't specifically
>looked in the manual yet regarding this. I will be hunting down a
>suitable pressure guage soon.
Now we're getting somewhere. The fuel pump is controlled by an aux
contact on the air flow meter. No air flow no fuel pump operation.
This is a safety fuction to prevent fuel from being pumped when the
engine is stopped and perhaps a line is broken as in after an accident.
The ignition switch "start" position bypasses this switch so that the
pump will run during cranking.
Has the cover been removed from the air flow meter? This cover is normally
sealed in place with RTV. It is typical that when someone tinkers with
the air flow potentiometer setting, they screw up the fuel pump timing
so that the pump runs all the time. If the pot arm has been moved forward
to enrichen the mix, it will move the fuel pump actuation arm so that the
contacts are made.
If the cover has not been removed, it is possible that the air flow flapper
is sticking such that it does not return to rest properly. this could be
due to wear or debris or backfire damage. IN any event, the apparent flow
signal would be higher than normal so the mix would be enrichened.
I'd remove the cover and inspect things. It is not uncommon to have the
flapper damaged or broken by intake backfires.
>I have tried leaning out the mixture by adjusting the idle mixture
>screw located under the sealed cap you mention. It seemed to have
>very little effect as determined by repeated attempts to pass
>the emission test after trying progressively leaner settings.
>So far I have tried 3 turns leaner. Would the potentiometer adjustment
>you refer to affect an emmission problem at idle?
Yes, see above. Your best bet is to gain access to an exhaust meter
and adjust the air flow meter for proper emission. This is, of course,
after you make sure there are no mechanical problems with the meter.
Here's one other little trick that might get you past the emission check.
Pull out the dipstick. This causes the PCV flow to enter through the
dipstick rather than from the intake boot. This flow bypasses the
air flow meter. This means that the meter will receive a lower flow signal
which should lean the mix - assuming the meter is working which I suspect
it is not.
[[ The following correction was emailed to me. -- Norman ]]
Date: Sat, 26 Apr 2008 00:13:37 -0700 (PDT)
From: TERRY DOYLE <email@example.com>
On 1979 280zx there is no fuel pump switch in the AFM, and no
wires for it in the harness. I have this car and speak from first
hand knowledge (5 spd manual).
Also I do not see the switch on this model at the junk yard. The
female plug on the harness is the same, but no contacts and no
wires, and the AFM does not have the pins for the switch.
Go ahead and use name and email address. Z car in the subject
will keep it from being tagged as spam.
Something people might like to know is that Saab (900 or 9000 I
think it was) injector connectors fit Z cars. It also has one
that fits the throttle switch. These have a spring clip that you
squeeze to release. Easy on easy off. Most Z cars have corrosion
on these connectors and need to be replaced.
Date: Thu Jul 11 07:50:27 1991
To: (z-car list)
Subject: Re: Fuel Injection Problem/Questions
>I understand that you can put an ordinary oscilloscope on the
>injector solenoid wires and get a pretty good idea if they
>are working right - it seems especially plausible that you
>could spot _one_ bad one this way.
That is correct. My "high-zoot" fuel injection troubleshooting
kit consists of an o'scope and a pulse generator to trigger
the ignition. if I'm having to recurve the airflow meter, I use
a depth micrometer to position the flapper at known intervals.
I should note that the Z fuel injector computer fires all injectors
at once. Only one output transistor. There are individual ballast
resistors for each injector but they still all fire at once. It is,
however, helpful to examine the current drawn by each injector.
Simply monitor across the ballast resistor. You can spot a sticking
injector because the dI/dT curve will have anomalies in it.
>From what my mechanic friends tell me, there are at least some
>shops that have very complete computer-based analysis tools that
>can take a number of readings at various points in the fuel
>system electrics and diagnose problems. Have you asked your
>local friendly dealer? It might be easier to spend a few bucks
>this way than hours screwing around with it yourself sans
>information on how it works.
Possibly. I have the manual on the Datsun factory unit. It's main
function is to simulate sensor signals. One can do the same thing with a
known good set of sensors. The other thing the unit does is display on
a digital readout, the firing duration of the injectors. A bit easier
to use than a scope but no more useful. The biggest problem in going
to a dealer will be the Zippy on the controls.
Subject: Re: Hesitation problem with EFI system
Date: Friday, Oct 16 91 03:01:53
From: John De Armond
>I have finally gotten my FI system tuned in for descent performance.
>I cannot tune out a SEVERE lean hesitation that occurs when either power
>or high RPM launches. If I pull the R's up to say 2K and dump the clutch she
>falls on her face, the same condition occurs when shifting at anything above
>3-4K. Damnably embarassing, not to mention hazardous to my engines health!
>Could this be a problem inherent with the MAS?
>The sensor is located about 18" away from the hat, would relocating it closer
>resolve the problem?
>Probably the same problem, but, she doesn't acclerate as well as expected.
>The acceleration is somewhat ?wavey?. No jerking, just a funny fluctuation
>as I start to fly.
>Millam E. Tackitt firstname.lastname@example.org
Millam, I don't recall which EFI you're working with so this is kinda general.
Those two problems sure do indicate unstable airflow across the MAF sensor.
If stratification or turbulence is occuring at some throttle opening
such that the hot wire does not see the flow, leanness would result.
Could the fluctuations you feel be thought of as related to what
you'd imagine turbulence to feel like?
Best way to diagnose this is to record the MAF output signal and look
at the trend. Do you have access to a storage scope? Or a stripchart
recorder? An analog voltmeter with a fast movement might even
show it. You could get a VU meter from Radio shack and scale
the signal with a pot. The VU meter is very fast so it should show
You might also look at the pulse width to the injectors. A scope
is again handy. If you don't have that, a laptop PC or other portable's
parallel port and a touch of code, with the injector signal, isolated
through a resistor, applied to one bit works well. I might even
whip out the code if you're interested. My fuel injector testing
software with tiny mods would do it.
I'm assuming that you've checked all the obvious stuff like fuel pressure.
Could it be something really kinky like a loose wire in the fuel
pump circuit momentarily coming loose under acceleration? That wouldn't
explain the surging but it might account for the acceleration leanness.
From: John De Armond
X-Source: The Hotrod Mailing list
Date: Jul 1992
Subject: Re: Cougar ramblings
>looks like my MAS is going south on me, started when i nailed the gas and
>only got a seriously lean condition instead of the usual blast of acceleration
>then this morning i pull up to a stop sign and suddenly the idle is lean
>and i CANNOT leave the stop. finnaly got her to hit at about 3000 so i let
>her loose and went sideways around the corner and down the street (glad the
>cops were nowhere near). she mellowed back out and ran just fine till i got
>home and tried to put her into the garage. same lean condition and no power
>spun the tires when she finnaly caught. new black streaks :)
>no information in the "tech" information section.
>Anybody know how to check the condition of one of these things without
Sure. I'm pretty sure that unit outputs a voltage proportional to
flow so just hang a voltmeter on it and see if the output goes up
when you accelerate. There should be power, ground, and output leads
so finding the right one is not hard. GM is fond of the variable
frequency output so if they used that type, the troubleshooting is
even easier. Here all you have to do is hook a walkman headphone up
through a 100 ohm or so resistor (or use a Radio Shack amplified speaker)
to the output and listen for the pitch to change as you rev the engine.
Date: Fri Jul 23 18:44:20 1993
From: rsiatl!emory!EBay.Sun.COM!Dennis.Loyer (Dennis Loyer)
Subject: Re: help! car stalled.
> The stall is sudden so me thinks it must be electrical. cranks over
> after stall so battery is connected ok. Me thinks it's the coil. What
> do you all think??
> I seem to have the xhitties luck after a car event!
> I got my weekend planned. :(
> [Mine did that for a couple of years before I tracked down the problem -
> a cold solder joint on the PCB inside the ignition module. JGD]
Heh heh.... This reminds of a problem I had with an old MG, Everytime I came
off a dead stop hard, the radio fuse would blow, but... only when there was a
passenger with me. Turned out that a speaker wire with a bare spot ran under
the passenger seat, said passenger seat had a broken mount, and everytime I
stomped on it, it shifted enough to press the bare wire against one of the seat
mounts (if I remember correctly) shorting out the radio.
I know it doesn't have anything to do with my Z, but this story brought back an
old, if frustrating memory :)
Dennis Loyer (77 280z)
[This problem drove me bat-shit. I was too stuborn to just buy a new
module. The car would not start when the temp was below freezing.
For a while I had a Q-beam spot lite in the car that I'd shine on the
box for a few minutes before attempting the first crank. One pass with a
soldering iron over the PCB didn't fix it. I finally found the problem
by measuring voltage drops between the PCB traces and component leads.
A joint that looked perfect was actually loose around the lead.
Desoldering, cleaning and resoldering solved the problem. I came as close
as a big guy like me ever will to doing a double-back flip. JGD]
From: John De Armond
Subject: Re: Automotive Brain Teaser (can you solve it, WATSON?)
Date: Thu, 26 Aug 1999 04:19:06 EDT
Mark Aarabi <email@example.com> wrote in message
> > Howdy Group,
> > From time to time, I will be posting some interesting and neat
> > driveability problems that I have encountered in the line of duty.
> > Your mission, should you choose to accept it, is to use the information
> > and clues that I have supplied to take an educated guess at solving the
> > problem. OK Watson, here's the first of our puzzles:
> > *VEHICLE: 1988 GMC G15 (1/2 ton van) 5.0L Throttle Body Injection A/T
> > *COMPLAINT: No-Start or Hard-Start problem particularly when the engine
> > is cold.
> > *SYMPTOMS: If a small amount of starting fluid was sprayed into the
> > throttle body, the engine would start and run normally. Once started,
> > the van would run great. There were no other problems or complaints.
> > *HISTORY: First shop replaced the fuel pump, screen, and filter. Second
> > shop replaced plugs, wires, cap, rotor. Third shop punted.
> > *OUR TESTING/OBSERVATIONS:
> > - I noticed that while cranking the engine, the fuel injectors would
> > spray ONCE then stop spraying altogether for several engine
> > revolutions. Then as I am still cranking the engine, the injectors
> > would finally start spraying fuel again and shortly after that the
> > engine would start.
> > -I checked fuel pressure: 15 psi (within specs. for TBI).
> > -I checked fuel volume: 1 pint every 30 seconds (very good).
> > -I checked ECM for codes: Code 32 present- EGR valve malfunction
> > (unrelated to our problem)
> > -I checked ECM parameters: Coolant temp sensor, Map Sensor, TPS, and
> > all other values were normal and within the expected range.
> > -I checked for Injector Pulse from the ECM with a NOID light: Got a good
> > bright flash when cranking the engine(both injectors).
> > -I checked injector resistance: both at 2.0 ohms (within specs.)
> > -I used an injector tester to manually pulse both injectors: Both had
> > the proper amount of fuel flow when commanded.
> > -Using a DSO, I checked the pick-up coil inside the distributor. It was
> > OK and was sending the proper signal to the ECM.
> > What caused the no-start or hard-start problem? Remember, a small shot
> > of fuel in the throttle body and the engine would start and run normally
> > without any other problems.
> > What do you think, my dear Watson?
I'll take a shot.
Hmm. Well let's see. The 2 ohm injector impedance indicates it's
probably peak-hold driven. may be important. I'm going to assume
What you didn't tell us:
* Battery voltage during the no-start period of cranking.
* Scope waveform of the injector when failing to start.
* Whether the injectors were firing but not discharging fuel during
* Fuel pressure during the no-start interval. (I know you said it
was OK but I want more detail).
* Whether you had spark during the no-start interval.
Assuming the injectors were NOT firing during the no-start period,
sounds to me like a voltage threshold problem. Either the peak
current isn't firing or else there is a low voltage cutoff for some
reason. perhaps a corroded power lead to the ECM or a corroded or
partially blown fuse. Sounds like the initial injector firing might
be from energy stored in the ECM filter cap. The battery voltage
drops below the threshold of ECM operation when the starter hits.
As the engine comes up to cranking speed and the starter amp draw
drops, the battery voltage rises to the point the ECU will fire
again. The NOID light should flash on just the hold current and
thus would miss the missing peak pulse. This would fit with the
problem being worse when cold because the starter current draw is at
If a normal pulse is on the injectors during the no-crank interval,
then I'd look at high fuel pressure, perhaps a sticking regulator.
I know that high fuel pressure can hold these large injectors
closed. I'd also look for momentary no fuel pressure. Had a fuel
pump outlet check valve stick in a car once. It would reliably
stick after hot soak and deadhead the fuel pump for a few seconds.
Would get one or 2 firings from the injectors and then nothing until
the little ball blew out.
If that's not it, then I'd need to poke around with my own DSO for
I did one time have a similar problem on a 75 Datsun 280z. The
problem was a cold solder joint in the ignition module that would
open when cold and close when the coil ballast resistor warmed the
module a bit. Crank with no start for several minutes and then hit
right off and act like nothing was wrong.
From: John De Armond
Subject: Re: The Solution to the Auto Brain Teaser
Date: Sun, 29 Aug 1999 05:53:27 EDT
Mark Aarabi wrote:
> Also, as NEON JOHN suggested (very well done by the way), there was also
> a problem with the PEAK section of the PEAK&HOLD injector waveform (not
> shown in the above snapshot) during start-up.
> Here is the kicker: once the engine was running, injector voltage and
> current waveforms looked nearly perfect. So why did the injector
> drivers only faltered during start-up? Well, I can assure you that it
> was not due to low system voltage... Even with a battery charger
> attached and ample starting power, the hard-start problem still
> persisted. I have wondered if Newton's 1st Theory would apply to this
> "an injector at rest tends to stay at rest; while an injector in motion
> tends to stay in motion..."
I'm not familiar with that particular ECU but it is not uncommon for
the drive to the switching transistors to be supplied differently
during the cranking regime. This is to provide more drive to the
injectors when the battery voltage can be on the margins. Rule of
thumb: if the starter can turn the engine, the ECU must fire the
injectors. So when the starter is going
WahWahWahWahClickClickClick, the injectors must still fire. Sounds
like this circuit was malfunctioning.
> *THE PITFALL:
> In my HINT last night I mentioned that many automotive instructors
> (particularly those teaching driveability/diagnostics) would probably
> tell/caution their students about this pitfall. While having a
> super-duper $5,000 digital lab scope at my disposal, I opted for the
> "quick & easy" route and checked injector pulse with a $5 NOID light
> (INSERT YOUR FLAMES HERE!). The problem is that the filament inside a
> NOID light bulb does not behave in the exact same manner as the coil
> windings inside an injector. The ground signal from the injector driver
> was more than sufficient to flash the noid light brightly but too poor
> to open a cold injector.
I have a nifty little homemade gadget I built way back in the days
of analog scopes and ECUs. It is nothing more than a small cast BUD
box containing a 15 ohm, 20 watt rheostat, a 1 ohm, 10 watt fixed
resistor, a homemade universal harness connector and a short cord
with a BNC on the other end. The harness connector consists of a
pair of small stainless welding rods peened flat and filed to the
shape of injector pins and then silver soldered to test lead wires,
followed by heat shrink tubing. These can be stuck into almost any
ordinary injector connector. Probably have to make another one for
the round TBI injector pins. Never had to work on one. Inside the
box, one rheostat end and the 1 ohm resistor are hooked in series.
The injector leads go to the rheostat wiper and the 1 ohm resistor.
The fixed resistor is there to keep one from accidentally putting a
full short on the ECU by turning the knob all the way down. The
scope leads are hooked in parallel to the injector leads. A
homemade dial printed on the laser printer is fixed to the outside
and calibrated in ohms.
In use, simply push the connectors into the injector connector, dial
in the injector resistance, hook the BNC to the scope and fire
away. I have a FlukeScope 97 which has the ability to store
configurations. I have the configuration to read this box stored in
a memory location. So all I have to do is turn the scope on, press
the recall button and it is ready to go.
This thing is so easy to use that I've never had the need to use a
> Once I finally hooked up my DSO to the injectors, the problem stood out
> like a sore thumb. Although I wasted several hours of diagnostic time,
> the customer was only charged for 1 hour of diagnostic time... after
> all, it was not their fault I was too lazy to utilize my cool toys from
> the very beginning. Lesson well learned (hope you did too).
Hmm, how does that old saying go, "when you have a hammer,
everything looks like a nail". I thought the modern version was
"when you have a DSO, every problems looks like a waveform just
begging to be scoped". :-)
hey, don't feel bad. I'm so biased against ECUs being bad that I'd
have probably spent another hour or so looking for some other cause
before finally punting and changing the box. We all have our
biases, I guess :-)