Altima hesistation off idle, revisited again!

The O2 sensor has nothing to do with fast changes like acceleration, and a 1999 Nissan should have a function to check the duty cycle of the O2 sensor while the engine runs.

Look at the throttle position sensor, MAP sensor, and fuel delivery rate and pressures (pump and pressure regulator), and see how the pressures hold up when the pump is turned off, to check for leakage of check valves. Don't replace parts unless they test out bad, and don't use a low-impedance analog meter (that is, one that doesn't need a battery to measure volts or amps (FET-VOMs are OK though), directly on an O2 sensor or you could damage it.

What if the ECU has richened the mixture enough (or leaned it enough) to affect off-idle performance? The test function shows about 10 crossovers (if that's the right term) in 10 seconds, but at idle, maybe 1 in 10 seconds. I was doing some reading that indicated a weak heater could cause bad response at idle when the exhaust stream isn't heating the sensor optimally so I half wondered it this might be something worth considering. This also would explain why the problem is more noticeable after a semi-warm restart since the heater might not work at idle speeds. A lot of supposition, yes, but I don't have much else to go on.

Already checked the TPS, and fuel pressure (not sure if that = delivery rate or how to test rate), and the pressure does hold after shutdown. The MAP sensor I haven't looked at yet, but I'll make a note to check it out if I can.

I know it's a bad idea to start replacing parts in an uneducated attempt to fix a problem, but with 100,000 km on this sensor I figure it can't hurt. I'm not about to start replacing every part under the hood... yet.

Thanks for your reply :)

I'd look at the O2 sensor directly, either with a digital meter that includes a bar graph display (responds faster than the digits) or an oscilloscope.

Most heaters are run all the time.

How did you check it? It takes only one bad spot to cause poor response. Turn it very slowly and check for smoothness of resistance change. Better yet, inject a weak signal into it and look for noise on the scope as you turn it.

Pressure and delivery are different. You measure pressure with a guage, delivery rate by running the pump for 30 seconds to see how much gas it pours into a coffee can.

How long? An hour?

I hope you have a factory or Mitchell book because for fuel system problems they're far better than a Chilton's Hayne's. I'd also check

and One of those has factory-trained mechanics.

Unfortunately I don't have access to that kind of equipment... maybe my analog meter would still give a good idea of how often the voltage is swinging back and forth though.

Sorry, that was wasn't worded quite right, what I meant was, if the heater was not working 100%, it might explain the increased hesitation after a warm restart because the engine hasn't run enough to warm the sensor back up to normal temp.

Again, I don't have a scope at my disposal, but I have checked the resistance while opening the throttle, and got a smooth increase or decrease in resistance (forget which way it goes). This was with my digital meter though, so maybe I should try with the analog in case it does a better job of reporting any momentary dropouts.

Ok, I did come across something like that in the service manual, but they want you to do it by pulling the fuel rail and putting a bucket under each injector. That's a bit further than I want to go in my driveway, but maybe your method would be possible. I would expect though if there was a volume problem I would have issues at WOT, which I don't.

Not sure about how long, but at least 30 minutes... I just know from having had a fuel pressure gauge connected recently and having left it connected after I had shut the engine off for at least that long. Also I can leave the car sitting all day and it will start on the first 1/4 crank most of the time, so if it is losing pressure it can't be too much.

I do, I've got the factory service manual.

Thanks, I've posted on a few forums before, but not those two I don't think. I'll give them a try.

"Hesitation off idle" is all I need to see to point you towards the most likely cause of your problem.

Your 2.4L has an EGR valve that feeds exaust gas to all four cylinders individually via cast runners inside the intake manifold. When some but not all of these runners clog up, the remaining open runner gives all of the exaust gas to it's cylinder. This results in a small misfire or stumble only during EGR operation. The EGR valve doesn't open at idle; it starts to operate off idle from approx 1500 RPMs and higher during light to moderate load conditions. If those conditions match your car's hesitation enabling conditions, then we're on to something.

To see if this is your problem, simply remove the vacuum hose from the EGR valve and test drive the car to see if the problem has disappeared.

To repair this condition, you will have to remove the intake manifold and find and clean these runners with some sort of flexible pipe cleaner and carburetor cleaner. If your manifold is a two piece design you may be able to remove just the plenum and get to the passages that way.

As far as the high idle goes, it's most likely a vacuum leak such as the PCV hose, brake booster, etc.

Toyota MDT in MO

Well, it happens right at idle, and above 1500 RPM I can't really notice it at all.

Sorry, I actually already tried this, and no change. :( The one thing I've been wondering is if there's any way the exhaust gas might be getting sucked into the intake in the space between the EGR valve and where it mounts, but there should be less vacuum there when I open the throttle I would expect, and if at idle it was pulling exhaust in there it would likely run very badly.

I suppose it's possible the EGR valve is leaking enough to allow the exhaust in even without the valve opening, but I've had it off and cleaned it, and tried blowing into it, without any leaking. I wanted to disconnect the pipe that feeds the exhaust into the valve, but it seems to be quite well fused into the exhaust manifold.

It is a two part setup; I'll take a look to see what might be involved in getting it apart, although it doesn't seem like this would be the problem in my case. I wasn't aware there were separate passages for the EGR system though, that's good to know.

I've looked, but haven't been able to find any leaks. I pressurized the throttle body/intake manifold by sealing off the intake hose and blowing into the idle bypass hose, and there was no leakage. Right now I've got the idle "adjusted" with a rubber o-ring in the idle bypass tube that restricts the idle air. And no, it makes no difference to the off-idle hesitation.

Thanks for the reply!

Is there a ridge of carbon in the throttle body around the closed butterfly position? If there is, it's like closing the throttle by rotating the butterfly a tad. A lot of play in the butterfly spindle can cause wierd symptoms as well.

There shouldn't be, I've cleaned it a few times since I got the car, but I'll double check next time I'm playing with it. The car has a cold idle cam that actually opens the throttle as the engine cools, and the symptom is present when cold, so any carbon buildup likely wouldn't come into play in that case. I will ensure there is no play in the spindle or mechanism overall though.

Thanks for the reply.

One thing I forgot to mention in all this is that when I have a vacuum gauge connected, the reading has a "vibration" to it. The vacuum is good and strong (22 inches), but it swings back and forth maybe +/- 1 inch (as per the scale on the gauge, not an actual inch) and does it about 6-8 times per second. I just realized as I wrote the above that the tach has always had a slight vibration to it at about the same speed as the vacuum gauge does. Once I rev the engine off idle, the vibration goes away.

For all I know this is totally normal, but why it would only appear at idle is a bit of a mystery to me.

On Sat, 15 Apr 2006 20:00:29 GMT, "JM" graced this newsgroup with:

less air, poor air/fuel mixture. At idle it's just more noticeable. When was the last time you cleaned the throttle body intake?

Today, although admittedly it was only a quick job as it was threatening to rain and I was almost out of throttle body cleaner. I have tried giving it a thorough cleaning before using copious amount of cleaner and not had any affect on things. Woudln't a (for example) leaking intake manifold gasket on one cylinder cause this? Off idle or with the throttle open there would be less vacuum behind the throttle plate so that might explain why it's less apparent then.

But, there's no other evidence of such a leak...

On Sat, 15 Apr 2006 20:56:12 GMT, "JM" graced this newsgroup with:

it's certainly possible that you've got an intake leak, one way to find out is to use a spray bottle and spray a fine line of water around the intake gaskets and see if the engine drops in idle.

I suspect though that it may be something like a dirty MAF or, the throttle body is still dirty. The symptoms you mentioned still point to that.

I've tried that, only with TB cleaner, and had no change.

Well, the MAF does respond to airflow according to my meter, so I think that's doing its job; can you suggest any other ways of cleaning the TB that might get better results? In the past I've cleaned the ridge of carbon off around the throttle plate, and sprayed TB cleaner into the open intake, which usually results in some black smoke out the exhaust once the car starts, but other than that and spraying directly into a vacuum line, I don't know what else to attack.

I'd like to get the idle control motor out and clean its seat and pintle but it's just about impossible to get to. I have disconnected its air supply tube from the intake hose though and shot some cleaner in there.

On Sat, 15 Apr 2006 22:08:23 GMT, "JM" graced this newsgroup with:

hmmm...

..the most cost efficient way is that way you've cleaned it. The only other way is a complete tear down and that's WAY more of a PITA than you'd want to get into.

If the control motor was bad (or going bad), it probably wouldn't hold an idle at all. Another possibility is that you've got either a bad injector or hopefully, just dirty injectors. Have you tried a botlle of BG-44K? It just may need a good injector cleaning.

There's nothing worse than hunt and peck and that's what it sounds like you're doing now. If the BG-44K doesn't help, then I'd have some diagnostics run on it. You may have an ignition problem.

I don't think the throttle body has much to do with the flutter in the vacuum, if that is what you are thinking. The flutter is probably due to a sticking or burnt valve. It disappears at higher RPM simply because your gauge can't move fast enough to see it at anything above an idle.

-jim

On Sat, 15 Apr 2006 17:37:32 -0500, jim graced this newsgroup with:

go point. It could be ignition related as well. Personally, if some quick, cheap fixes don't correct the problem, IMHO, I'd stop wasting time and money and get it professionally diagnosed.

What about a valve out of adjustment? It would have to be rediculously out I suppose? The car has always had a tapping at startup on warmer days that I've always suspected was a valve, but the dealer's opinion is that it's probably a hairline crack in the exhaust manifold since these valves rarely need adjusting and it's apprently a lot of work to adjust them. The engine uses shim adjusted, directly actuated valves; is it still possible one is sticking or not closing fully?

If a valve was stuck fully open I'd expect to have a dead cylinder with no compression -- would a cylinder with a burnt valve, etc. show lower compression?

Would you expect a burnt/sticking/misadjusted valve to cause the hesitation also?

My spark plug hole seals are starting to leak now, so I may just have to pull the valve cover to replace them in the near future. Maybe I'll also see what I can see as far as the valves go at the same time.

Thanks for the suggestions,

====================Classification: HA98-005

Reference: NTB99-006

Date: March 15, 1999

1998-99 ALTIMA, MIL "ON" AND/OR POOR DRIVEABILITY DUE TO A/C EVAPORATOR CONDENSATION LEAKING ON ECM

APPLIED VEHICLES: All 1998-99 Altima (L30)

SERVICE INFORMATION

If a 1998-99 Altima exhibits a MIL "ON" and/or poor driveability such as engine hesitation or no starting, the cause may be air conditioning (A/C) evaporator condensation leaking onto the engine control module (ECM). Air conditioning condensation can accumulate and leak onto the ECM if the A/C evaporator drain tube becomes blocked by debris such as leaves, insect nests, etc.

just a thought.