5

I have a 99 Nissan Almera 1.6L MTX ( nearly identical to the Sentra sold in the US ) which has developed a problem with low idle, occasional stalling at stop lights, what seems to be misfires and occasionally getting the hiccups, where the rpms will start bouncing between 400 - 900 rpm until it goes away on it's own after a while. This seems to happen only after the engine has reached operating temperature.

At first I was sure that it must be an air leak or IAC problem, but when I shorted the 14 pin Nissan consult to flash any codes, it came up with a Camshaft Position Sensor error. I cleared the code a few times and it keeps coming back. The CSPS is in the distributor, so I opened it up and saw that oil is leaking from the internal O-ring ( not my video ). I'm guessing that maybe the internal bearing might be worn out also, causing it to occasionally stick at low rpm resulting in misfires.

So, while there's clearly a problem with the distributor, I'm still a bit hesitant to drop the money to replace it. The reason is because when the engine starts getting these hiccups and idle problems, if I turn on the A/C that causes the idle to stabilize around 900 rpms. However, if I then give it some gas it goes back to either a low idle or hiccupping, until I turn the A/C off then on again.

EDIT 29/12/15

So I got a scan tool and some new info:

enter image description here

This capture is at about 788 rpm I think with no loads. The first thing is that it reads lean at idle most of the time, but at 2k rpm it oscillates between rich and lean together with the O2 signal.

Something that stood out to me was the IAC percentage open which seems really high at 78%. If I turn on the headlights, it maxes out at 100% open:

enter image description here

If I turn the lights back off, it drops back immediately to 88%, then slowly returns to 78%.

I turned the lights back on, and once the temp hits 97*C and the radiator fan comes on the idle will drop to the 600's, and start bouncing. If I start turning the wheel back and forth to activate the power steering pump I can force the engine to stall. Which seems odd to me since there is a separate PS idle up switch, same as for the air conditioner. Yet when I turn on the AC, even with the lights and radiator fan already on, the idle up works and it goes up to 850rpm and everything stabilizes:

enter image description here

You can also notice that the injector pulse jumps 50% and the duty cycle almost doubles, even though there's been only a small increase in RPM.

Sooooooo, here's what I'm thinking. This car actually has what seems to be two IAC type devices sitting one on top of the other, one called the Auxiliary air control and the other called the Fast idle control device. I figure the maxed out one is the primary IAC, and maybe it's partially clogged and that's why the ECU is opening it so much. Maybe the radiator fan and power steering idle-up switch are connected to this one, which is already maxed out and that's why problems start when they come on, while the AC idle up is maybe connected to the second one.

I'm wondering if this could account for what seems to be a lean condition most of the time, or if that could be a separate problem of it's own? The thing is that this problem started gradually and it would seem highly coincidental to me to develop both a clogged IAC and an intake leak at the same time.

EDIT 1/1/2016

I opened the idle air screw 3/4 of a turn and now IAC is showing 10% open at idle with no loads, idle stable at 750 rpm, and I can put on the lights, defroster, fan and radiator fan, and swing steering wheel back and forth as much as I like without any problem. I can't force a stall like I could before.

This is idle with no loads:

enter image description here

And this is with everything going, lights, defrost, rad fan and cabin fan:

enter image description here

EDIT2 1/1/2016

So I just found in a GA16DE manual pg 59 that IACV-AAC/V at idle should be between 0%-40%. ( There's a list of engine control reference values starting on page 58. )

Here's a diagram:

enter image description here

On some of these engine's there is also a Fast Idle Cam ( pg 55 ) which I guess can open the throttle plate, I'll have to go check mine later and see if it has one:

enter image description here

Robert S. Barnes
  • 15,708
  • 69
  • 148
  • 239
  • No OBDII port to plug a scanner into? – Zaid Nov 29 '15 at 09:08
  • @Zaid No, it has the old 14 pin Nissan Consult interface. I actually ordered an adaptor for it, but it probably won't be here for a few weeks. – Robert S. Barnes Nov 29 '15 at 11:21
  • When you turn on the A/C, the engine revs will drop slightly, so the engine ECU will open the throttle bypass valve to bring the revs back to normal. Have you cleaned / checked and lubricated the butterfly valve on the throttle? – HandyHowie Nov 29 '15 at 14:53
  • @HandyHowie I got a scan tool and added some hard data to my question. – Robert S. Barnes Dec 30 '15 at 11:07
  • All the air flowing through these valves will be metered, so I can't see how this would cause a lean mixture. The throttle valve will have a screw on it for setting the minimum throttle. If you open it up slightly, the IAC should be able to close more and won't have to go to 100%. – HandyHowie Dec 30 '15 at 12:30
  • @HandyHowie So I should be able to test my theory about a clogged IAC by opening up the idle air screw? Sound right. – Robert S. Barnes Dec 30 '15 at 13:26
  • It's worth a try. It may just be tha through wear, the butterfly is able to close more than it is meant to, so the ECM is opening the IAC more to compensate. – HandyHowie Dec 30 '15 at 14:58
  • @HandyHowie I opened the idle air screw 3/4 of a turn and now IAC is showing 10% open at idle with no loads, idle stable at 750 rpm, and I can put on the lights, defroster, fan and radiator fan, and swing steering wheel back and forth as much as I like without any problem. – Robert S. Barnes Dec 31 '15 at 15:42
  • Should I make that into an answer? – HandyHowie Dec 31 '15 at 15:50
  • @HandyHowie Sure, I'll vote you up. – Robert S. Barnes Dec 31 '15 at 20:39
  • One thing that's puzzling me is the throttle position voltage. Is it normal for it to not change at idle? What happens at something like 4k RPM? – Zaid Jan 01 '16 at 07:21
  • @Zaid Why would the TPS voltage change if I'm not pushing down on the gas pedal? – Robert S. Barnes Jan 01 '16 at 09:16
  • That's why I want you to check the voltage at a higher speed. I don't know if the throttle plate is supposed to move on your Almera when you add loads at idle such as the AC, or whether the IACV is sufficient to cover the load requirement. 0.46 V looks like a default voltage to me and it would be good to rule out lack of TPS signal before blaming a clogged IACV – Zaid Jan 01 '16 at 09:47
  • @Zaid Take a look at my 29/12 edit. There are two IAC type devices on this car one on top of the other. I think the idle up for the AC is hooked up to the second one, because when the IAC was already maxed out w/ idle@762 and I turned on the AC idle jumped to 850 without any change in TPSv. – Robert S. Barnes Jan 01 '16 at 11:00
  • @Zaid OK, I'm going to have to go see if I've got a Fast Idle Cam connected to my TB. – Robert S. Barnes Jan 01 '16 at 11:35
  • Regarding the second IAC, if you're referring to AACV, that'll be the auxiliary air control valve which is not related to the intake side. – Zaid Jan 01 '16 at 13:12
  • @Zaid On pg 12 of the EC manual it lists both the IACV-AAC valve and IACV-FICD solenoid valve as part of the idle air control system. Why wouldn't it be part of the intake side, especially since they're both mounted right on the TB? – Robert S. Barnes Jan 01 '16 at 13:37
  • You should ask that as a separate question – Zaid Jan 01 '16 at 14:27
  • Just noticed @Zaid question about the TPS changing under load. The IAC would do all the adjustment at idle, so I wouldn't expect the TPS to change unless the throttle was pressed – HandyHowie Jan 02 '16 at 16:41
  • @RobertS.Barnes The manual clarifies the purpose of the AACV - I must admit that the term "auxiliary" threw me off. I thought it was related to the secondary air system which is why I was saying it didn't have anything to do with the intake side. – Zaid Jan 03 '16 at 07:57
  • @HandyHowie Based on the service manual, I believe the FICD is supposed to open the throttle plate when the AC is on. That should register a slight change in throttle position sensor voltage. – Zaid Jan 03 '16 at 08:30
  • Robert, I'm still suspicious of this 0.46 V. The service manual says that the TPS voltage should be 4.0 V with the throttle plate fully open (EC-GA-58). Could you confirm this with your CONSULT tool? – Zaid Jan 03 '16 at 08:36
  • Seems like your TPS is working from your other question. That would leave a clogged AACV as a distinct front-runner for your idling problems – Zaid Jan 03 '16 at 15:28

2 Answers2

4

Your CPS code won't explain the hunting idle; the symptoms point to a lean air-fuel mixture:

  • it doesn't stutter at low idle with the engine cold because the fuel management is designed to enrich the AFR while the engine is warming up (aka cold-start enrichment)

  • turning the AC on helps because it induces an additional load on the engine, which makes it demand more air flow that the fuel trims can then compensate for. Without the additional air flow the amount of unmetered air is hard for the fuel trims to compensate, resulting in hunting idle

You're going to be flying blind without a scan tool but the top suspects in my book would be an intake leak, malfunctioning idle-air control or an underreading MAF. It seems from your description that the O2 sensor responds well enough to changes in load so I'm inclined to think that it is fine.

Fixing the distributor should help smoothen out the idle quality. I'm not convinced that it will solve the hunting idle.

Zaid
  • 39,111
  • 48
  • 142
  • 290
  • The thing is, that the problem is not constant - I would expect the low idle to be constant once the engine is warm if it was the result of an air leak. But I guess the IAC or MAF being on the fritz could cause an intermittent problem. – Robert S. Barnes Nov 30 '15 at 14:52
  • 1
    I got a scan tool and added some hard data to my question. – Robert S. Barnes Dec 29 '15 at 18:46
4

When the throttle body becomes a little worn, it is possible for the butterfly valve to close too much for a steady idle engine speed. The ECM then tries to compensate by opening the IAC more than it is designed to under normal conditions. This can cause stalling and unsteady idle. There will be a screw on the throttle body that allows adjustment of minimum throttle. Try adjusting the screw to open the butterfly valve a little.

HandyHowie
  • 25,151
  • 3
  • 38
  • 75
  • So you think this is the problem and not that the IAC is getting dirty / clogged? – Robert S. Barnes Jan 01 '16 at 07:18
  • I believe so. A dirty IAC may add to the problem, and it wouldn't do any harm cleaning it, but you shouldn't need your IAC to maintain minimum idle revs. From my experience, that is when you start getting the surging problem where the ECM wants to back off the IAC because it thinks it is too far open, but when it does then the revs drop too far. – HandyHowie Jan 01 '16 at 10:24
  • So IAC should ideally be at 0% open at idle? I'd actually like to ask a separate question regarding setting optimum idle air screw position and it's relation to IAC percentage open at idle. – Robert S. Barnes Jan 01 '16 at 11:03
  • It would be difficult to say what it should be due to load requirements from things like the alternator at the specific time. I would be happy if the IAC % was low at normal idle. It would probably be a good question to ask. No doubt there will be engine specific specs for this. – HandyHowie Jan 01 '16 at 11:26
  • I just linked to an engine control manual which says IAC should be between 0% to 40% at idle with no loads. Mine was at 72% before adjusting the idle screw. – Robert S. Barnes Jan 01 '16 at 11:37
  • What you could do is switch everything possible on an then check the IAC %. If the % is no where near 100% then the IAC can't be too dirty. – HandyHowie Jan 01 '16 at 15:13
  • But why would that necessarily indicate it's not dirty? Maybe I've just got the screw open so much that the IAC doesn't need to open very much. At no load rpms where 750, IAC 10% and MAF 1.6 volts, while with lights, defrost, rad fan and cabin fan on rpms where at 762, IAC 62% and MAF 1.71 volts. It seems like going from 10% to 62% only resulted in a fairly small increase in air flow, although injector pulse went from 2.38ms to 3.09 which is about a 25% increase. – Robert S. Barnes Jan 02 '16 at 15:54
  • 62% sounds good. If it went to 100% IAC and the revs dropped a little, then that would indicate that the IAC was partially blocked and so wasn't able to allow the air required to maintain constant revs under load. The injector pulse will surely just be adjusted to match the air flow. – HandyHowie Jan 02 '16 at 16:34
  • It may well just be a dirty throttle valve. I have also seen this on a perfectly clean valve that was just closing a little too much. At least the answer will point someone in the direction of a simple fix. – HandyHowie Jan 03 '16 at 08:37