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#1
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Altima hesistation off idle, revisited again!
Ok, so I'm still trying to figure out my hesitation off idle in my 1999
Altima. I've checked for vacuum leaks and I'm relatively sure there are none. I've checked the fuel pressure, and it's within spec. The regulator behaves as described in the service manual when I disconnect the vacuum line. I tested the MAF sensor and its output voltage rises smoothly with any increase in throttle opening. I tested the TPS and it operates smoothly and is within spec. The coolant temp sensor and intake air temp sensor both are about right according to the service manual. It has clean plugs, newish rotor, cap, and wires, and is not burning oil or coolant. There are no trouble codes stored, and aside from this problem and having a slightly high idle, the car runs smoothly and has good power at all other revs. The problem manifests itself as a drop in revs/stumble when I tap the throttle off idle. It's most obvious with small abrupt throttle openings, the kind you would use to take off from a stop (it's a manual trans). It's more severe when the car has been started between 5 and 10 minutes of having been shut of and is still warm. With a shorter or longer stop, it doesn't tend to be any worse than normal. I tested the O2 sensor as well as I could with the tools I have; my analog meter's lowest range is 10V, so it's hard to accurately monitor the sensor with it, and my digital meter I suspect is a bit slow to show the voltage changes from the sensor. But, with the digtal meter, the voltage seemed to settle around .2 volts at idle, and would spike up to around .7 if I revved the engine. I measured the resistance of pins 1 and 3 (the heater element I suspect) of the sensor, as per the service manual's instruction, and it reads almost 8 ohms hot, and 4.6 ohms or so at around 5 degrees C. The service manual says it should fall between 2.3 and 4.3 ohms at 25 deg. C, so I assume the idea is the resistance increases with temperature. If that's the case, then at 25C, I would expect the resistance to be more than 4.6 ohms as I recorded at 5 degrees, so that is somewhat suspect. I priced a new sensor at around $100 CDN today from the dealer, so I'm half tempted to install one. If it doesn't fix the problem at least it will be new and might help my fuel economy a little although it isn't poor to begin with. Any thoughts/suggestions as to what else I might look at? |
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#2
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Altima hesistation off idle, revisited again!
JM wrote: > I tested the O2 sensor 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. |
#3
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Altima hesistation off idle, revisited again!
> wrote in message oups.com... > > JM wrote: > >> I tested the O2 sensor > > 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. 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. > 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 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. > 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. 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 |
#4
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Altima hesistation off idle, revisited again!
JM wrote: > > wrote in message > oups.com... > > 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. > > 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. 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. > 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. Most heaters are run all the time. > Already checked the TPS, 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. > and fuel pressure (not sure if that = delivery rate or how to test rate), 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. > and the pressure does hold after shutdown. 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 www.nissanforums.com and www.nicoclub.com. One of those has factory-trained mechanics. |
#5
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Altima hesistation off idle, revisited again!
> wrote in message
oups.com... > 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. 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. >> 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. > Most heaters are run all the time. 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. >> Already checked the TPS, > 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. 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. >> and fuel pressure (not sure if that = delivery rate or how to test rate), > 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. 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. >> and the pressure does hold after shutdown. > How long? An hour? 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 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 I do, I've got the factory service manual. > www.nissanforums.com and www.nicoclub.com. One of those has > factory-trained mechanics. Thanks, I've posted on a few forums before, but not those two I don't think. I'll give them a try. |
#6
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Altima hesistation off idle, revisited again!
JM wrote: > Ok, so I'm still trying to figure out my hesitation off idle in my 1999 > Altima. > > I've checked for vacuum leaks and I'm relatively sure there are none. > I've checked the fuel pressure, and it's within spec. The regulator behaves > as described in the service manual when I disconnect the vacuum line. > I tested the MAF sensor and its output voltage rises smoothly with any > increase in throttle opening. > I tested the TPS and it operates smoothly and is within spec. > The coolant temp sensor and intake air temp sensor both are about right > according to the service manual. > It has clean plugs, newish rotor, cap, and wires, and is not burning oil or > coolant. > > There are no trouble codes stored, and aside from this problem and having a > slightly high idle, the car runs smoothly and has good power at all other > revs. > > The problem manifests itself as a drop in revs/stumble when I tap the > throttle off idle. It's most obvious with small abrupt throttle openings, > the kind you would use to take off from a stop (it's a manual trans). > It's more severe when the car has been started between 5 and 10 minutes of > having been shut of and is still warm. With a shorter or longer stop, it > doesn't tend to be any worse than normal. > > I tested the O2 sensor as well as I could with the tools I have; my analog > meter's lowest range is 10V, so it's hard to accurately monitor the sensor > with it, and my digital meter I suspect is a bit slow to show the voltage > changes from the sensor. But, with the digtal meter, the voltage seemed to > settle around .2 volts at idle, and would spike up to around .7 if I revved > the engine. I measured the resistance of pins 1 and 3 (the heater element I > suspect) of the sensor, as per the service manual's instruction, and it > reads almost 8 ohms hot, and 4.6 ohms or so at around 5 degrees C. > > The service manual says it should fall between 2.3 and 4.3 ohms at 25 deg. > C, so I assume the idea is the resistance increases with temperature. If > that's the case, then at 25C, I would expect the resistance to be more than > 4.6 ohms as I recorded at 5 degrees, so that is somewhat suspect. > > I priced a new sensor at around $100 CDN today from the dealer, so I'm half > tempted to install one. If it doesn't fix the problem at least it will be > new and might help my fuel economy a little although it isn't poor to begin > with. > > Any thoughts/suggestions as to what else I might look at? "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 |
#7
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Altima hesistation off idle, revisited again!
"Comboverfish" > wrote in message oups.com... > 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. Well, it happens right at idle, and above 1500 RPM I can't really notice it at all. > 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. 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. > 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. 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. > As far as the high idle goes, it's most likely a vacuum leak such as > the PCV hose, brake booster, etc. 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! |
#8
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Altima hesistation off idle, revisited again!
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. |
#9
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Altima hesistation off idle, revisited again!
Sticky EGR perhaps?
SteveB wrote: > 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. > > |
#10
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Altima hesistation off idle, revisited again!
"SteveB" <sbrads@nildramDOTcoDOTuk> wrote in message
... > 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. |
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