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.

Evening Gio, there is plenty going on there, and the figures are showing the ecu is trying stabilise things. The injector idle is high, the aav is high, the tps is high and the temp is low. If this is a warmed up engine then you have a faulty thermostat and the engine is running in cold mode, this potentially will cause low boost also.

 

So first you need to address the running temp and confirm the temp when fully warmed up. Get the tps adjusted closue to 0.46v and at that mileage given the low price would replace the spark plugs for nice fresh ones.

 

Jeff

Thanks Jeff, hope all's well with you guys out there. The OBD bar display was grabbed first and 154 F is about 68 C then after a few minutes, I got ConZult working and temp was up to 81 C so that seems about right. The video was taken after a 5 mile drive and about another 10 mins at idle but I didn't check the displayed temp (will do next time). What would you say was fully warmed up?

 

There's no boost issue (NA). I'll check the plug type / records tomorrow.

 

If anyone can advise the vacuum tube dia, that would help save me a trip across the island.

 

Cheers, Mike

clean every connector you can get to, sometimes it really is as simply as that :)

Spotted one vacuum leak so better check the rest. Also found several threads eg https://www.300zx.co.uk/forums/showthread.php?151629-Hoses-needed-for-300zx which says I need 4mm and 6mm - but that is for turbo. Found the manual lists 14 bits of vacuum hose but doesn't give the size. Does an NA need both 4mm and 6mm? Thanks.

When warmed up, she idles smoothly for about 20s, then skips a couple of beats, the ECU catches it and runs smoothly again for about 20s, then skips, and so on. This is without throttle. Once throttle is open, runs smoothly.

 

So it's not rough all the time. When driving its fine, except when no throttle. One suggestion is something to due with fuel pressure but don't have the detail until tomorrow.

 

Any bright ideas? Ta, Mike

Did you adjust the tps and get the idle value into the correct range as mentioned by Jef?

Hi Daz, not specifically just yet. The TPS (without any foot-on-pedal) is currently reading 0.52 / 0.54 v which is not too far from the 0.46v mentioned by JeffTT and the 0.4 - 0.5v range I've seen elsewhere. Would this reading cause the intermittent stumble?

 

This is not "rough idle all the time" but "idle is stable for 20s, then something happens, it skips a couple of beats, then something happens and it goes stable again until the next 20s skip". So I think JeffTT's comment " the ecu is trying stabilise things." is on the money. The engine diagnostic bloke who is looking at it had a similar comment.

 

There hasn't been any work on the engine that might have triggered this.

 

I'm guessing that if it was an injector or other generic issue, it would idle rough all the time. As it is, seems like something the ECU can adjust for a bit, then it goes hiccup.

It’s not just as simple as setting the TPS to 0.46v, the TPS needs to be between the 0.4-0.5v range AND engaging the hard idle switch/idle control valve. Without that engaged, the idle control valve isn’t activated and isn’t controlling the idle.

AND engaging the hard idle switch/idle control valve. Without that engaged, the idle control valve isn’t activated and isn’t controlling the idle.

 

Thanks nickz32 - Consult first said that IACV-AAV was at 0%, then some time later, OBD Scantech said (pics earlier in thread) Idle Air Valve 59%. Is that the valve you mean?

Yes buddy, idle air valve is the Idle Air Control Valve (IACV)

 

Get the multimeter out to verify what consult/DataScan is seeing

 

https://conceptzperformance.com/wiki/index.php/Throttle_Position_Sensor

Update. Tech bloke says his diagnostic thing doesn't show any issues higher up the rev range so, as mentioned here, IACV is next on the list to check. I've asked him to recheck TPS/hard idle as well.

 

What he saw on Sat is that, at idle, the mixture goes lean (more on bank 1), the revs drop, the ECU increases pulse width to compensate, the O2 sensors see the mixture going richer, revs rise back to 750 or so, idles smoothly then mixture goes off again and round it goes again. More tests later today.

 

Cheers.

update 2.

Further digging around shows:

1. there's no EGR control valve
   
2. there's no AIV control valve solenoid
   
3. the TPS seems to be good
   
4. the IACV/AAC is at 56% so that's too high.
   
5. 
   

Simon reckons this is trying to compensate for a vacuum leak somewhere and wants to take the manifold off to inspect and test the IACV/AAC to see it can operate through full range. Before that will check / clean throttle bodies.

 

Any other pointers?

 

BTW I noticed in the 1996 manual (US market) that doesn't show AIV solenoid but does show the EGR. Does anyone know if the 99 NA should / shouldn't have these?

 

Thanks

I’m pretty sure they did away with the EGR from Series 4 onwards. If you can see the underside of your Plenum just in front of the PCV valves, you’ll see where the EGR would plumb in, it’s blanked off on the later plenums.

 

Out of interest, how does it idle with the MAF unplugged?

update 3: what's this unknown tube?

Looking around for damaged vacuum hose, Simon found this tube under the RH inlet manifold which exits at rear of manifold. Appears to be redundant. Is it?

PS thanks nickz32, will pass that on and ask.

Haven’t got a clue...... Andrew, Si, Joel any ideas?

update 4: The "stumble every 20s" happens higher up the rev range as well not just at idle, noticed this when warming up for tests. TPS / hard switch set and checked. MAF reads up to 3.5 at rpm. IACV valve operates OK when checked directly. O2 sensors switch rich/lean about every 3s (believe it should be every 2?). The only thing that seems to cure is when he sets higher fuel pressure with his scan tool. He's going to stick the scope directly on various things and also measure voltages directly - MAF, O2, injectors.

 

Any suggestions? He thinks - as JeffTT does, it's ECU compensating for something, the revs recover, then the ECU returns to base and the problem recurs. Then round we go again.. Thanks.

Not overly sure how he’s adjusting the fuel pressure electronically or infact adjusting it at all? The OE FPR has zero adjustability. The only thing I can think he’d be adjusting is the voltage to the fuel pump to increase the fuel pump duty..... but im not aware of a way to do that with the OE ecu.

 

A quick way to increase the fuel pressure up to 43.5psi would be to disconnect the vacuum reference pipe off the FPR and block off the pipe to stop the vac leak.

 

I’d physically check your fuel pressure on a gauge. Should be around 36psi at idle (43.5psi base minus idle vacuum pressure).

 

When was the fuel filter last changed

Any sign of any vac leaks on the FPR vacuum line?

Voltage to fuel pump? (Should be 9v at idle if the FPCU I working properly)

Edited March 7, 20196 yr by nickz32

Sounds like a fair shout, a fuel filter clogging could well be struggling to let fuel through at the lower idle pressure and the stumble is allowing the pressure to build back up again and run for another 20 seconds. When you’re driving, the increased fuel pressure is enough to tone it down.

Although having just watched the video of the “hiccup” it doesn’t sound like a fuel starvation struggle. Can you record it for longer so that we can see/hear the cycle of the hiccup?

Not overly sure how he’s adjusting the fuel pressure electronically or infact adjusting it at all? The OE FPR has zero adjustability. The only thing I can think he’d be adjusting is the voltage to the fuel pump to increase the fuel pump duty..... but im not aware of a way to do that with the OE ecu.

 

I’d physically check your fuel pressure on a gauge. Should be around 36psi at idle (43.5psi base minus idle vacuum pressure).

 

When was the fuel filter last changed

Any sign of any vac leaks on the FPR vacuum line?

Voltage to fuel pump? (Should be 9v at idle if the FPCU I working properly)

 

Thanks nickz32 - he said he did it with his kit but I'll ask the q when we speak later and I pass on your tips. There was a leak on the fuel damper > balance pipe tube but fixed that.

 

@ Joely P thanks, will try to do that when I go up there.

update 5: at idle, lambda is 1.4 (v lean) and fuel pressure 35psi. Then, as nickz32 mentions, took the hose off and it went to about 43. Then used a pump to add a bit of overpressure and got it to 45psi with a lambda of 1.089.

 

Not overly sure how he’s adjusting the fuel pressure electronically or infact adjusting it at all? The OE FPR has zero adjustability. The only thing I can think he’d be adjusting is the voltage to the fuel pump to increase the fuel pump duty..... but im not aware of a way to do that with the OE ecu.

 

A quick way to increase the fuel pressure up to 43.5psi would be to disconnect the vacuum reference pipe off the FPR and block off the pipe to stop the vac leak.

 

I’d physically check your fuel pressure on a gauge. Should be around 36psi at idle (43.5psi base minus idle vacuum pressure).

 

When was the fuel filter last changed

Any sign of any vac leaks on the FPR vacuum line?

Voltage to fuel pump? (Should be 9v at idle if the FPCU I working properly)

 

Fuel filter changed May 18. No vac leaks that he can see / hear. Voltage not yet checked.

The diagnostic thing is a Launch X431. This has tested the IACV valve through 0% - 20% - 80% and then back to 0%.

Coolant temp 89C

MAF voltage at 1,900 1.4V, at wide open 3.5V

When the revs stumble at idle, lambda goes high (lean), O2 sensors react, ECU widens injector pulse from 2.5ms to 3ms, revs recover, after a few secs goes lean and stumbles and round we go.

One of the tests says Bank 1 is lean, another says LH bank has better emissions (i.e. better mixture) than RH bank. Is Bank 1 the RH bank? I'm told the usual thing it that Bank 1 is the one with cylinder 1 which, since Cyl 1 is on the RH (facing forward) would mean Bank 1 is RH bank.

 

One of the O2 sensors switches at half the rate of the other. I've asked which is which and at what speed.

 

Simon is going to go through all the data he's found over the past few days and I'll post the key points later. I'll go up there at the weekend and see if I can get my ConZult working and compare notes. So any pointers as to specific tests or readings or thing to look for will be gratefully received. I'll try to take some vids as well.

 

Thanks for your help, guys - being on an island, it's not easy to drop by one of "our" known specialists so all this is very helpful. Ta :thumbup:

Bank 1 is drivers side, bank 2 is passenger side.

 

It’s starting to sound more like a general fuelling issue rather than an electrical glitch, however it also looks like you may have a lazy o2 sensor

 

I’d check the ohmage of your injectors to make sure you’ve not got a misbehaving injector. I’d also change your fuel filter to eliminate any potential restriction in the volume of fuel being sent to the engine. I’d also lest the fuel pump and measure how much it’s flowing.... from memory the TT pump flows circa 265lph, however I’m not sure what the flow rating of the NA pump.

Thanks nickz32, will pass that on.

Gio, has you guy tried running the engine with the O2 sensors disconnected ? see if there is an improvement..and going back to what I suggested earlier in this thread, get some new plugs fitted, not that expensive.

 

Jeff

Gio, has you guy tried running the engine with the O2 sensors disconnected ? see if there is an improvement..and going back to what I suggested earlier in this thread, get some new plugs fitted, not that expensive.

 

Jeff

 

Thanks. Going to try that today. And the MAF (also suggested by Jimmer).

 

PS do you know if the earlier O2 sensors (Bosch 13091) will work with a late car / ECU? https://www.300zx.co.uk/forums/showthread.php?187997-Replacement-O2-Sensors&p=1959352&viewfull=1#post1959352

Edited March 9, 20196 yr by Gio