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1999 Altima misses off idle

Hi group,

I've got a 1999 Altima with 100,000 kms. Ever since I got it almost 2 years ago, it's had a slightly too high idle (varies, but around 850-900 rpm) and it also has a stumble off idle. The idle itself is more or less smooth, although if I listen at the end of the exhaust it does seem slightly irregular, but not by too much.

If I punch the throttle while it's idling, especially when it's just been started (warm or cold) or before it's warmed up first thing in the morning, it'll stumble for a second or so (the whole engine shakes), and then rev up as expected.

When I do this, as soon as I open the throttle I can hear a sucking sound from the end of the intake, and I can hear the injectors speed up quite a bit, but it's not until almost a second has gone by that the engine actually revs.

If I am less abrupt with the throttle, it'll rev just fine, and if I punch the gas when it's up around 1500-2000 RPM, there's no noticeable hesitation.

After the car's warmed up, the severity and delay decrease a fair bit, but they're still noticeable.

I've always suspected a vacuum leak, but today I went out and disconnected the intake tube from the MAF sensor, sealed it off with a plastic bag, and then tried blowing into the tube that runs off to the idle control valve. My plastic bag inflated slightly, but there was no obvious leakage from anywhere except a very slight and very slow hissing from around the EGR valve area that I wouldn't expect would affect anything.

I then tried unplugging the coolant temp sensor. This caused the idle to be even higher, and amplified the hesitation off idle quite a bit. I know this is no way to make a diagnosis, but it seems suspicious to me.

I had it to the dealer last fall, and everything checked out okay -- the TPS was within spec and the idle control motor was at 30% which apparently is normal.

In the past I've checked the throttle position sensor and MAF sensor ground, and both were fine, and I've replaced the plugs, wires, dist. cap and rotor within the last few years. I have tested the coolant temp sensor a few times and it did seem to change from hot to cold, but it's hard to get at with my multimeter leads and I wonder if it may be out of spec just enough to cause my particular problem but nothing more severe than that.

Can anyone suggest what else I should look at? I hear the coolant sensors are pretty common to go bad, and not too expensive, maybe I'll price one and swap it out just to cross one more thing off the list.

Thanks in advance for any help.1

Reply to
JM
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1st check for trouble codes! My guess is either an EGR valve malfunction or perhaps egr valve ports are clogged with carbon!
Reply to
Backbone

Forgot to mention, no codes set, and I cleaned the EGR and ports when I got the car (had the problem then also). Also, if I pull the vacuum line to the EGR valve, effectively disabling it, there's no change. It does feel like that kind of a problem though :(

Reply to
JM

If I had it I would look at fuel trim with a scantool, under many different driving conditions. If fuel trim was too far off I would suspect dirty MAF sensor elements first. I suppose that you have checked the integrity of the large intake hose (splits, cracks, fit)?

Toyota MDT in MO

Reply to
Comboverfish

Yes, I've been over that hose and the various ones that branch off of it a few times, and everything seems in order. I don't have a tool that will let me see the trim values but I'll keep that in mind in case I do take it somewhere for another look.

Thanks.

Reply to
JM

I was out today and noticed there is a distinct hissing sound when I rev the engine coming from somewhere around the EGR valve or possibly the intake manifold. I know this particular year Altima was known for the intake manifold gasket breaking down and leaking, but this sounds like it's closer to the firewall than that.

I went over all the vacuum lines I could see and they all look good, so I'm a little puzzled where this sound is coming from. I have tried shooting throttle body cleaner where the manifold bolts to the head, with no change in idle, and I also sprayed around the various metal bits of vacuum line, but this had no effect.

I'm half hoping I'm on to something though...

Reply to
JM

You may just be paying more attention to the sound of the idle speed control than normal. As it opens up (revving the engine up and down will cause this) more air will rush through the intake tube and cause a sound like you describe. Unplug the ISC connector and try again. See if this is the noise you are chasing.

Toyota MDT in MO

Reply to
Comboverfish

Actually I had the TPS unplugged at the time, which stops the ECU from opening the idle control valve, so it wasn't coming into play. I'm familiar with the hissing sound the IACV (Nissan's terminology) makes though, and this is different. It was a short, sharp hiss that corresponded with the suck sound from the end of the intake. I wondered if I was just hearing air rushing inside the manifold, but when I put my rubber listening hose up against the manifold, the sound was less audible.

I did later listen for the sound again with the TPS connected and the IACV operating normally, and it was still there, but much less apparent. I suspect this is because the IACV was opening and moderating the demand for so much air all at once.

I just wish I could pinpoint where the sound is coming from a little better.

Reply to
JM

For $15, you can get a mechanic's stethoscope at Sears (or most any auto parts), which I find to be better than a screwdriver.

Reply to
mst

I use a very long screwdriver. Use the handle as the listening device, the tip as the probe. Listen as you touch the probe to various locations - It'll be apparent as to the sounds exact location.

Reply to
Backbone

Which is why I added, "or most any auto parts" :)

Reply to
mst

what's a sears?

Reply to
Backbone

never mind i.e. I just goggled - is a store!! The place where you can get craftsman's tools. We don't have a Sear here!

Reply to
Backbone

You'll find that for air leaks (what the OP thinks he has) a length of tubing works better than a solid rod -like a screwdriver- for pinpointing the noise. It works quite well, actually.

Toyota MDT in MO

Reply to
Comboverfish

I'll have to pick up a length of tube for this -- I was using a straight hollow plastic tube which was better than the naked ear but hard to work around corners :). I'll post back if I learn anything interesting!

Thanks for all of the replies so far.

Reply to
JM

Ok, so I've had some more time to play under the hood, and I'm relatively sure now that I do not have a vacuum leak. I couldn't find anything by listening with my rubber hose, and I had a vaccum gauge connected today that showed just past the "normal" mark for vacuum at idle.

I tested the timing, and it's correct at idle, 20 deg BTDC. The plugs, wires, dist. cap and rotor are all new within the last year or so. The O2 sensor is original, but as I understand it, they are just for fine tuning the mixture and wouldn't likely cause this kind of problem. There are no trouble codes recorded.

I had the idea that maybe the fuel pressure regulator was bad, but it does seem to be at least doing something since the revs will increase if I pull its vacuum line off.

I thought maybe there was some EGR leaking into the intake manifold when I goosed the throttle, but I realized that would probably cause a very rough idle as well, which I really don't have. (The symptom is still present with the vacuum line that actuates the EGR valve unplugged)

I have noticed in the past that as the engine revs down to idle from around

3000 RPM or so, there's a little "bump" as it comes past 2000 or 1500 RPM or so, where the engine shakes slightly agains the motor mounts. Today I accidentally revved it up to around 3000 RPM when the engine was cool, and it actually popped (backfired?) lightly out the exhaust as it was coming down to idle, and did it a second time when I revved it again.

One thing I'm not sure I mentioned in my original post is that the idle is too high, and has been since I got the car. The idle screw is seized and the head pretty much rounded off, so I've always wondered if just adjusting that screw would set things straight. Right now I've managed to get it to a normal idle with the help of a restrictor I fitted into the idle air duct that branches off the intake hose.

Any thoughts on what else I should look at regarding the hesitation?

Reply to
JM

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