Idle fluctuation

[mike 90 RR]

I had this sorted 1 year ago .... The problem back then was the main + post on the fire wall was rusted and the whole car was starved of 12v spark feed

Now it is slowly coming back to haunt me again ;)

Symptoms .... When warmed it can occasionally struggle to idle ... 650 to stall .. stall back to 750 /// up, down, up, down ///
I can control the idle and make it stable 700rpm ... via applying & holding (just a touch) some gas pedal // "in effect bypassing the stepper motor and holding idle with the gas pedal"

I have done all the "feed ohm's test" on the ECU cable ... and nothing is out of the ordinary (as per Haynes repair manual)

It does it regardless of A/C on or off
I have cleaned the stepper motor & check for vac leaks
I have cleaned the MAF
It only happens when warmed after a good drive (half hour later)
It can miss behave at idle ... drive further ... then behave itself (problem fluctuates)
If I pull the plug on the MAF ... the motor stalls straight up

I have noticed the speedo stall a couple of times in the past 2 weeks // this is also when this motor idle drama has also started /// However ... The speedo is not stalling & then I get the problem straight up (no obvious connection)


Q... I replaced the electric speedo sender unit 2 years ago ... I have tested it via the Ohms test on the ECU side ... came up trumps (cold motor)

Any other tests that can be done?? ... (other than buy new speedo sender unit and see how it goes) .... such as Ohm testing the MAF directly .. as it could have a "cold joint" internal??

Mike

:)

[LOVEMYRANGIE]

Mikey, have had the same thing in the past. Need to set the MAF and check the idle screw in the plenum right near the butterfly.
Give me a call and I can show you how to do it over the weekend, but not Sunday as I wont be home. Saturday or Monday should be fine if you want to swing by.

[Hobbes]

Hi Mike,
Jumped through the same hoops many times on my 90RRC , and same probs on a TVR, so going over old ground here...but this is what worked for me.

Apologies for the lengthy post - possibly too much info on the ECU but it may help.

ECU - the 14CUX was a fairly sophisticated unit for its time . It's clever in that it will adjust to many parameters such as change of air filter automatically. It's data comes from the following a) fuel & water temp thermistors b) road speed (also drives speedo c) MAF d) engine rpm (coil) e) thottle position f) o2 from lamda equiped (CAT) sensors.

However it's main weakness is idle control. It's a check/respond/wait system - in other words the ECU will always try to set idle at 800. There's two main problems; firstly the stepper has 180 points and when ignition is switched off the ECU returns the valve to a "home " position ie fully open. However if it sticks the ECU gets confused as too what exactly is the home position as there's no feedback circuit; secondly, lets say we slam on the A/C.. the ECU sees a drop in rpm and mass air flow and signals the stepper motor to let more air into the plenum, however it's not that seamless - like 3 secs reaction time- and further it tends to over react - which is why we can get the idle dropping to stall and then soaring to 1500 before settling.

The important thing to remember is that at idle speed all the parameters that feed the ECU are exponentially more likely to cause things to go pear shaped.

1) Battery/electrics - the 14CUX is sensitive to a stable power supply(as you discovered with the naff ground) Although the battery may be servicable it's the easiest place to start. Parallel up a good battery or jump start unit - if the idle smooths and the stepper motor reacts quicker to load changes(A/C) that may be the issue - if not, and of course check all electrical connections are good, time to move on..

2) Base measurements: check the resistance of the water temp and fuel temp thermistors (documented in service manual). It'll be obvious if you've got a duff sender(open circuit or grounded)

3) MAF base measurements - connect a multimeter between red/black and blue/green with ignition on but not running. You should get between .2V>.7V
Fire it up and at idle you should see about 1.5-1.7V. This pretty much tells you the MAF is in order. You can aso check this by blowing on the MAF wires - you'll see the voltage rise sharply. As a final check go across Red/Black and blue red with the ig on - you should see 1-1.5V - the base CO value for a non-cat. If it's not a gas analyser check is a good move.

4) Check ignition timing is good & within limits.

5) Base idle - with the motor at temp Isolate the stepper motor by plugging the air bleed hose from the plenum next to the throttle body. I used a bit of steel bar and a jiffy clamp on the hose and a bit of fuel hose with a bolt in it on the Throttle body side. Clamp the vac advance hose.
Disconnect and reconnect the battery to clear any ECU fault codes.
You should be able to start the motor and adjust the base idle to 800 if it needs it.

6) Stepper motor - usual advice is to whip it out and give it a clean with carby spray. However I think this is possibly the better way(?)
a) unscrew it from the plenum and reconnect the multi pin. put it into a clear plastic bag with an elastic band around the cable
b) stuff a large rag into the hole in the plenum ie one that won't get sucked in. Or get wrapped around pulleys - take care.
c) Start the engine. Revs will soar to around 2000. If it doesn't start adjust the rag to limit airflow. The ECU will activate the stepper motor and the plunger will be driven out of the housing. It may take a couple of start stops but the plunger and spring should fall into the bag - a gentle wiggle to assist may help.
d) carefully clean the threads, plunger and key way and relube. Don't use anything heavy like copper grease or lithium. I used a bicycle silicone chain lube called "ICE" and it's worked spot on for ages.
e) screw the thread back into the worm - trick bit as you need to align the keyway - it's a feel thing. flip the ignition on and off a few time (no start) to confirm ok, then put it all back together.

Having done all that you should have a happy base system!!!!!! or not!

The road speed sensor issue/clue - as far as I know this sensor provides triggers to turn off idle control when moving (to maintain engine braking) and fuel cut off on throttle lift. Obviously check wiring etc but if all else fails it will show up as a fault code 68 on checker at your local friendly specialist.

Sorry that's long - hope it helps:)
Hobbes

[mike 90 RR]

and check the idle screw in the plenum right near the butterfly.


Ta for that
.... Will swing by .... The previous owner / mechanic has shagged with that hex screw + also the one at the MAF .... I can tell cause the "Anti tamper plugs" have been removed

I haven't touched them ... cause I don't have a O2 meter .... was going to leave this till I get to the EFI specialist in Balcatta ... wanted to get & install some new Gen 111 injectors first (Fleebay special from USA)

[mike 90 RR]

Apologies for the lengthy post - possibly too much info on the ECU but it may help.
Hobbes

I thank you kindly for your posting .... that was a mighty effort of writing ... It was a easy read .... and the cleaning / lubing method on the stepper will be done

I have no Lambada sensors on this setup (original system) .. non cat

The ECU injection system is just one of those drama queens when they play up

Between yourself and PhillpA .... The system makes more sense to work out the faults ... rather than ... Hit & miss part swapping = no real understanding of what went wrong.

Mike

:)

[PhilipA]

I would only add that a gradual deterioration may be caused by the oil mist from the ventilator coating the throttle body,and affecting base idle.

Clean the throttle blade and the hose input and where the throttle sits with carb cleaner, before checking base idle.

You can add a little to base idle also to say 600RPM. This stops it stalling so much as the stepper drops out.
Regards Philip A

[LOVEMYRANGIE]

Ta for that
.... Will swing by .... The previous owner / mechanic has shagged with that hex screw + also the one at the MAF .... I can tell cause the "Anti tamper plugs" have been removed

I haven't touched them ... cause I don't have a O2 meter .... was going to leave this till I get to the EFI specialist in Balcatta ... wanted to get & install some new Gen 111 injectors first (Fleebay special from USA)

Dont go with the injectors yet and GenIII (Holden / Chev???) are probably not the best choice.
Flow rate needs to be at 19lbs/hr not too much more than that.

The Ford C302 yellowtops are the most widely suggested replacement injector or the Bosch Pink Top. Providing they are a multi port nozzle and not the hose nozzle type like the Lucas units, injection focus and atomisation is far far better as the Lucas units spray the side of the manifold runners.

But do the base tuning first and work from there.

[Nomad9]

Hi Hobbes,
This wouldn't be similar to an 86 3.5 would it? A lot of what you talk about sound familiar.

[400HPONGAS]

Unlucky Nomad9 , your old Flapper system has basically nothing in common with the 14CUX system . Theres nothing interchangeable , (I lie , the Temp switch !)

[Nomad9]

Hi 400HP on gas,
Sort of guessed that might be the case, I thought maybe being made out of metal might be the only thing in common, temp switch.............bonus.

[Hobbes]

Hi Hobbes,
This wouldn't be similar to an 86 3.5 would it? A lot of what you talk about sound familiar.

Looks similar under the hood but very different.
On the early system idle is set by:
Setting correct advance at 600rpm with vac tube disconnected.
Reconnect tube and set idle using the air bypass screw on the plenum to 700>800rpm.
Adjust idle mixture to 1% CO2 or less using mixture screw next to afm.
Bit of juggling often required re last two steps.

To maintain idle under various conditions there's an extra air valve thats controlled by coolant temp and air temp from the afm, and additionally on air con vehicles a solenoid valve that opens to bump up idle when the a/c is switched on. Plus the ecu will also fire the cold start injector if required.

Apart from the obvious mechanical difference of flapper versus hotwire the later 14CUX processes various sensor inputs and then controls idle (above base) using the idle bypass valve (which is a stepper motor) and by lengthening the injector pulse width under cold start conditions.

(And just to correct myself! - base idle on the later systems should be set according to manual at 450>550rpm - not 800 but as noted by PhillipA 600 seems better and on my manual trans I set at 650 as its a bit less prone to stall)