Hallo. A few days ago, my S6 '99 NA Auto started to idle roughly / misfire at idle. It's OK while driving but when off throttle (junctions, stopped etc) it idles roughly and feels like it's only the ECU keeping it going. If left to idle when parked, it runs smooth then about every 20-30 secs or so, it misses a few beats then runs smooth again. That happens about 10s into the below video. I managed to get ConZult and OBD ScanTech running but not sure what to look for (listen, I can help you build a new fibre network but as for my Z, I lift the bonnet, go "hmm- hmmm" and reach for an expert - which is you lot :) ).

 

I've looked through a number of the rough idle threads here and have noted to check: good connections on each coil pack, plug, injector, vacuum leaks, AAIV connectors, O2 sensors, MAF.

 

It's all completely stock. Plugs last changed Mar 15 only done 32,000m since. No error codes. In case I need to replace any vacuum tubes, what dia are they and will any type do?

 

 

Here's the pic of the auto balance check:

 

And here's the bar display from OBD Scantech:

 

And here's the vid - hiccup about 10s in:

[video=youtube_share;ul9EhNqiJsQ]

 

Any suggestions for further tests gratefully received. Thanks.

when u disconnect the maf the car will be in limp mode. I did this to diagnose a dead maf sensor back when I had idle problems in 2015. with the maf disconnected my car idled perfectly which pinpointed my problem straight away. i replaced the sensor with a new oem one and i havent had an idle problem since.

Spent hours on this yesterday with Simon. Short version, TPS set to 0.48v (any lower and it won’t idle). Hard idle switch checked.

This is the only actual change that’s been done – before it was at 0.54v. Now it occasionally stalls at low revs e.g. during parking. So if anything, this adjust has made it a tad worse.

 

Fuel filter OK. Fuel pressure OK. Plugs look OK, lean, beige.

 

They’re Bosch FR7KPP33U

 

Smoke test OK.

 

IACV operates 0%-20%-80% when tested.

 

O2 sensors both operate – one is lazy and switches 1 sec (the blue one – LH bank, bank 2), the RH/red switches every 2s. Manual says more than 5 times in 10s so both within spec.

Unplugged the O2 sensors in turn – no change.

 

At idle, unplugged the MAF, it stalled.

 

Going from idle to WOT, MAF peaks around 3.4V (green), blue is LH O2, red is RH O2

 

compression test OK

scoped all injectors – all the same

 

This vid shows the IACV valve reacting to the stumble and getting the revs back. Then it goes down again until the next stumble.

[video=youtube_share;AM-qSguKPto]

This vid shows more IACV top right, revs top left. Simon pointed out the timing (on the right 6 down) isn’t as stable as he’d expect, nor is the CAS reference (middle, bottom).

[video=youtube_share;uEATuevD-6I]

 

There are more vids on this

.

 

The only change was adjusting the TPS from 0.54V to 0.48V and that’s made it a tad worse!

 

We’re stumped. Have we missed something obvious? If not, next steps could be to change out the O2 sensors, put some NGK PFR6B-11 in. If that doesn’t work, pull the injectors and take a look – even though they scope OK, they could be dirty, not spraying correctly.

 

I’m going to run some injector cleaner on the grounds it can’t hurt.

 

All words of wisdom gratefully received especially if you’ve had this “15 sec stumble at idle” and cured it.

 

Cheers!

Edited March 10, 20196 yr by Gio

I would like to suggest unplugging the mag sensor first then start the car as per normal. this is way I did when I was diagnosing my dead maf problem. it could change nothing mind you but I would also suggest changing the Bosch plugs for ngk plugs. there's a YouTube video I watched years ago of a US mechanic fixing a 300zx who stated in the video you must use ngk spark plugs on in our engines only anything else and the engine won't function within factory spec.

I found the video and its related to hesitation in the 300zx but might be useful information to help diagnose your problem.

 

but I would also suggest changing the Bosch plugs for ngk plugs. .

Other have suggested that - but I haven't got round to it yet because it's been fine on those plugs since 2014. And I don't understand why that would cause the over-lean and high IACV. Got a set on order now, in any case :)

Plenty going on there to comment about, will just do some frozen screen...hang on

 

Jeff

Ok here we go first with the screen shots>>

 

 

 

 

 

 

 

 

 

So first lets deal with the wandering / twitching ignition timing, the earlier cas units did suffer from early location pin wear and could cause drifting, as could a faulty cas itself, however the later one have an issue with interference from bad shielding on the cas pick up cable, watching via the Conzult ports always often shows a twitch, the later cable looms solved this, so in essence disregard that as a red herring.

 

 

The video does start with a low water temperature, I have no idea how long it had been running prior to the start of the video so could of still been in its warm up phase, if not however the temp was low and could point to an issue with the stat not closing fully ( common failure, many of my tech threads show this)

 

 

The injector dwell is too high at the start even with the lower temperature and would normally point to an issue with the afm, however we will see later this is not the case.

 

The 02 sensors are both toast, one worse than the other, the external structure is likely intact but the internal pre-heat circuits which are used as a reference calibration have failed / failing completely in one and the other is on its way, this can be seen as it comes to life when the throttle is stabbed hard and it warms up, disconnecting them is usually better than leaving them connected, the results may not be seen though whilst other faults exist as we will see.

 

The screen shot unfortunately does not show the tps hard close switch, this is essentially is the main way the ecu knows the throttle is closed fully, any issue with this can cause timing issues and stumbling, but as I cannot see it I cannot comments on it.

 

So the throttle position sensor you mention is at its lowest, any lower and the engine cuts too easily, so, you need to check the throttle balance, sounds like one throttle is slightly ahead of the other and that been the one the tps is connected too, I did a tech thread on how to check and adjust the throttle balance, this I cannot stress is of vital importance through the whole rev range to be correct, and they do change over time as the landing pad for the throttle stops wears, and of course after any adjustments to the the throttles the tps will need resetting.

 

 

See my throttle balance tech for this here >>

 

https://www.300zx.co.uk/forums/showthread.php?151304-Throttle-balance-tech&highlight=throttle+balance

 

 

So to the very obvious fault of the auxillary air valve is wide open 90% of the time, this is as the name suggests a valve that meters changing amounts of air passed in addition to the throttle butterfly`s, its primary role is to stabilise the engine when the the throttle is snapped shut, when braking, when coming to a stop and when the car is put into D on auto`s and engine drag occurs, this works in conjunction with the tps and the ignition timing.

 

So to explain, the engine tick over speed is also regulated by the aiv, when the engine is fully warmed up, with no air leaks and timing correct you should have something like this>>>>

 

Tick over speed around 800 rpm ( not the 750+minus 50 as in the manual, that was for a brand new engine )

 

Coolant temp 80c

Timing 15 deg

Engine speed 800 rpm

Aux valve 15 %

TPS 0.46V

Inj time 2.2 / 2.8

Maf 1.5 to 1.8

NGK spark plugs fitted

 

So your clearly not achieving these figures and the problem seems to be centred around incorrect air balance, and the afm issue is the first that needs addressing.

 

The aav been wide open is the ecu attempt to balance the engine, however the unit is failing to operate as it would be racing away if it was really open 90 %

 

The know issues with the unit is bad connector connection and also to get jammed, in lesser cases it fails and needs replacing.

 

To start with, check the top yellow connector, it often goes green and will need cleaning, next and to prove the point to see if it is jammed, whilst ticking over and observing the afm value, strike the unit firmly with the wooden handle of a hammer or similar, ( yeh really ) make a good few blows and see if the value changes as do the rpm.

 

If nether of those work, disconnect the connector and apply a 12volts supply to the unit, it should of course open wide and the revs rise to a fast race.

 

Finally if nothing, then it would be fair to assume the unit has failed.

 

 

Note: on some very odd occasions it has been observed the output from the ecu to the aav has failed at board level in the ecu and this will need replacing.

 

 

So moving on, lets assume you now have a working aav, good throttle balance setting and good tps setting,new 02 sensors ( or old ones disconnected ..same thing for testing) now the whole procedure needs doing again to see what figures your getting.

 

 

Jeff

 

 

Boost / air leak testing

 

https://www.300zx.co.uk/forums/showthread.php?154761-Odd-boost-leak-tech&highlight=AAV

 

Useful external links

 

http://twinturbo.net/nissan/300zx/tech/iaa/iaa.html

 

 

Edited March 11, 20196 yr by JeffTT

So your clearly not achieving these figures and the problem seems to be centred around incorrect air balance, and the afm issue is the first that needs addressing.

 

The afm been wide open is the ecu attempt to balance the engine, however the unit is failing to operate as it would be racing away if it was really open 90 %

 

Jeff, that's really helpful, thanks. I'll go through this in detail with Simon when I've had a chance to read thoroughly. In the mean time, you say afm issue and afm being wide open - do you mean the IACV / AAV?

 

Many thanks - Mike

Finally if nothing, then it would be fair to assume the unit has failed.

:D as I read through your advice and look stuff up, I found you replaced the IACV on my previous (blue NA S3) Z in 2008!

Cheers

Is there a how to replace MAF guide anywhere? I've tried search here, Twinturbo.net and the Aus forum but haven't found one. Yesterday, she ran worse than ever (OK the day before apart from the idle) and wouldn't go over 40. MAF voltage appeared to be 0.010V so it or the connector on the way out, I guess. Being useless at tech, I daren't try anything without a guide! Thanks.

Is there a how to replace MAF guide anywhere? I've tried search here, Twinturbo.net and the Aus forum but haven't found one. Yesterday, she ran worse than ever (OK the day before apart from the idle) and wouldn't go over 40. MAF voltage appeared to be 0.010V so it or the connector on the way out, I guess. Being useless at tech, I daren't try anything without a guide! Thanks.

 

I assume you still have the OE air box fitted Mike? This guide I just found for the panel filters seems to just about cover it. You would just need to remove the 4 bolts that hold the MAF onto the box lid once removed to replace it.

 

I assume you still have the OE air box fitted Mike? This guide I just found for the panel filters seems to just about cover it. You would just need to remove the 4 bolts that hold the MAF onto the box lid once removed to replace it.

 

 

Yep, Captain Stock, that's me :) Thanks for that guide. So the MAF is bolted to the airbox cover- follow that guide until the airbox cover removal, then unbolt the MAF from the cover - is that right?

 

I just checked and have now got Code 12 Air Flow Sensor which I've never had before. Oh well...

Looks like the MAF is now u/s as of yesterday. Thanks to Joely P, I summoned the courage to approach the Z with a socket. (I am now short of a 10mm socket and extension as I dropped it into the engine bay – which is why I should never be allowed anywhere near an engine bay).

 

Apologies to Jimmer and DJ Party Favor as I misunderstood their suggestions on MAF test. I’ve done that now and it will start and idle with the MAF unplugged. It will start but then die with the MAF connected.

 

MAF connected: start then die.

 

MAF unconnected, idles:

 

The error codes also point to this. I cleared them and restarted to check and Code 12 Mass Air Flow Sensor reappeared.

 

I’ve waved WD40 at the connector and plugged / unplugged a few times to clean. No change. I take it the arrowed brown bit on the harness doesn’t disconnect from the arrowed black bit?

 

That’ll be a dead MAF, I take it (unless a harness issue somewhere). I wonder if that was the cause all along: MAF sensor on the way out, causes idle issues, then fails completely. I guess the only way to test that is replace the MAF sensor and look again.

Start with MAF connected:

[video=youtube_share;LvLa42LUiS4]

 

Start without MAF connected:

[video=youtube_share;-bxaXdlhr7k]

 

Other things I noticed were cable ties instead of fixing the hose clamp, some missing bolts and worn grommets on the nose panel – also cracked scuttle where the RAC replaced the battery. Humph. Just more things on the list which every Z owner knows only too well!

 

Thanks to everyone who's helped - if I get there in the end, it will only be thanks to you.

I had a feeling it was the maf. I had the same thing happen to me as I mentioned earlier took me a few weeks to find info on the and video showing the exact same symptoms as I was having. their suggestion was replace the maf and it worked for me.

Today I changed the MAF and, short answer, she now starts and idles and the stumble has gone. It might be this whole thing was a MAF on the way out. Many things learnt thanks to the help on this thread. 2 things stand out for me on MAF:

 

1. the test isn’t unplug the MAF sensor while the engine’s running. The test is stop the engine, unplug the MAF and see if it will start / idle. If I’d done that days ago, could have spotted dodgy MAF earlier!
   
2. the voltage on the new MAF seems in the same range (more stable) as on the dodgy one at idle before it failed completely. So it looks like voltage isn’t the only diagnostic for a MAF on the way out.
   

 

First start up after replacing MAF

[video=youtube_share;0PbLyQUg5aE]

 

After 20 min test drive

[video=youtube_share;T29CXcxmPr0]

 

I still need to do a full ConZult and check against JeffTT’s figures to be on the safe side. But you can see the AAC is a lot better than it was.

 

Thanks to Joely P for pointing me in the right direction for airbox stuff. Might even replace my own air filters next time…

 

Had some trouble fitting the O-ring before I noticed the new MAF already came with one fitted d'oh.

 

Old and new MAFs. Someone’s clearly been a bit heavy with levering the hose off – and I wonder if debris from that might have damaged the MAF.

 

New Bosch 13091 O2 sensors to be fitted next week. Also proper NGK PFR6B-11

 

Given this is the first time I’ve ever taken a spanner to my Z for anything mechanical, I’m somewhat relieved the only casualty is a 10mm socket somewhere down on the engine tray where I can’t see it. Thanks to you all for your help and advice – wouldn’t have had the courage otherwise.

Good news Mike :thumbup1:

Great news Mike - and where did you source your new MAF from....?

 

Not that I need one yet (well, the mesh on mine does have a little ding in it), but Ebay is full of Chinese copies, so with OEM supplies apparently dwindling, all knowledge of potential outlets is useful..!!!

Z1 Richard sell an OEM Maf for $248 the list price is $448 so it is uber cheap.

Z1 Richard sell an OEM Maf for $248 the list price is $448 so it is uber cheap.

 

Cheers Jim....:cool:

PS if anyone wants the dud MAF to experiment on - or the unused O-ring - just pm me. Cheers.

Working through the bits of advice given here (thanks again), I’ve replaced the fuel filter. The MAF change seemed to be the fix but since I got the fuel filter, plugs and O2 sensors as recommended, might as well use them – particularly if the old O2 sensors are, as pointed out, borked (no heater).

 

Here’s what she sounds like when warming up.

[video=youtube_share;vD6gPuOR9rE]

 

Here’s the scantech bar display after a blat round the country lanes. I think you can see the O2 Sensors doing their switching thing and the IACV settles at 27 (manual says 15-40). Ignition also stable at idle at 15° which is what it should be.

[video=youtube_share;3zzyMjti_RY]

 

I did JeffTT’s throttle balance thing and it was OK, no adjustment needed.

 

I’ll do the idle adjust after the O2s & plugs are done. At the moment, I think the idle is a bit low. As you can hear/see the ConZult idle adj indeed drops the revs by 50rpm or so (disconnects the AAV I believe) which comes back afterwards so I hope just the idle screw needs adjusting.

[video=youtube_share;yLjTe-UxPZc]

[video=youtube_share;g3-hZd4zfls]

 

The idle adjust on Z32 wikispace given in other threads isn’t there any more. I found this pic of where the screw is

but can’t see that on mine – is it this one?

 

Will get back to this after the plugs and sensors are done. Cheers all and thanks for all the advice.

can’t see that on mine – is it this one?.

 

Yes, FSM drawing and part below.

 

 

 

Edited March 25, 20196 yr by AndrewG

Yes, FSM drawing and part below.

 

Thanks AndrewG, I'll have a go after the O2 sensors and plugs have been done. Cheers