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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / February 2007Long vs short term fuel trim & hesitation?
Hi all, I'm still trying to figure out the cause of the slight hesitation off idle in my 1999 Altima. I'll skip the history and the details of what I've tried so far, but suffice it to say that all the regular tuneup items have been replaced, EGR valve and passages have been cleaned, coolant temp sensor is new, etc etc. I've posted in detail before if anyone wants to know more background. Anyway, I've got an Autotap interface for my laptop now so I'm able to monitor a few of the OBD paramaters (when the software isn't crashing). Everything looks as I would expect as far as sensor readings; the TPS, MAP, MAF, coolant and intake air temp sensors all seem good. I have a question about the short term fuel trim though. When I first started the car with the scanner connected (it was warm, and had only been off for maybe 5 minutes) the short term trim was reading around 0%, and the hesitation was at its worst (as it always is when the car is just started). When I blipped the throttle slightly, which is when the hesitation manifests itself, the short term trim dropped to around -10% after the engine recovered from the miss, and then after returning to idle, the short term trim was back to about 0%. After a few minutes, the short term trim worked its way down to around -8% at idle, and would still drop a little lower when I blipped the throttle. The hesitation at this stage was less apparent though, as it normally is after a few minutes of running. So, I'm wondering if the ECU is compensating for an over-rich mixture, and that's over-richness is what is causing the hesitation. Interestingly, the long term trim is actually close to + 10%. So is the short term trim just a fine tuning either side of that long term value, in which case maybe there is no problem here? Or is there something fishy going on with the two values working in opposite directions? Thanks in advance for any insight! does the slight hesitation result in a miss-or more importantly a 'ses' light to illuminate? For what it's worth-suspicious of a fuel injection issue-does your equipment show any specific cylinder missing? Wonder if an injector cleaning of some magnitude is in order. jerry@peru-motors > Hi all, > [quoted text clipped - 33 lines] > > Thanks in advance for any insight! > does the slight hesitation result in a miss-or more importantly a > 'ses' light to illuminate? For what it's worth-suspicious of a fuel > injection issue-does your equipment show any specific cylinder > missing? Wonder if an injector cleaning of some magnitude is in order. > jerry@peru-motors Unfortunately, no, there's no CEL or any stored codes.. that might make the diagnosis a little easier. > Hi all, > [quoted text clipped - 33 lines] > > Thanks in advance for any insight! Indeed, a rich mixture at idle can cause the hesitation. Check the behavior of the oxygen sensor at idle vice at speed e.g. 2000-2500 RPM. If the O2 sensor is getting tired it may switch rapidly, which is normal, at speed when it is hot but then switch slowly at idle when it is cooler. That slow switching rate in turn causes the mixture to be wrong at idle and results in a hesitation when you initially rev up the engine from idle. Good luck. Al > Indeed, a rich mixture at idle can cause the hesitation. Check the > behavior of the oxygen sensor at idle vice at speed e.g. 2000-2500 [quoted text clipped - 3 lines] > be wrong at idle and results in a hesitation when you initially rev up > the engine from idle. Good luck. Al Well, it is a new sensor, but I did notice that the sensor seems to be switching slowly at idle vs. at 2000 rpm or so. I seem to remember reading that the system is only designed to work above that speed anyway though, so maybe that's normal. Or maybe there's an issue with the O2 sensor heater causing the sensor to cool at idle speeds? One thing I forgot to mention that may fit with the rich mixture theory is the fact that the car seems to be slow to warm up, compared the '94 Altima I had before this one. In the winter I can be driving for 30 minutes or more before the needle reaches the normal point on the gauge, or before it even comes off the cold mark on really cold days (-20 C). The rad hose to the thermostat on these days is usually barely warm. I replaced the thermostat last summer, but saw no real change. Maybe this is normal though. Thanks for the input. >>Indeed, a rich mixture at idle can cause the hesitation. Check the >>behavior of the oxygen sensor at idle vice at speed e.g. 2000-2500 [quoted text clipped - 19 lines] > > Thanks for the input. You did the coolant temp sensor but have you cleaned your throttle body? It does wonders for low rpm smoothness if your TB is very coked up. And, by the way, use throttle body cleaner that's safe for fuel injected motors. Regular carburator cleaner will clean the throttle body, but it will also strip away the fine film on the inside of the intake manifold which helps to discourage carbon buildup. CD > You did the coolant temp sensor but have you cleaned your throttle body? > It does wonders for low rpm smoothness if your TB is very coked up. I have cleaned the TB, not for a while now, probably not too much more than a year ago. The hesitation was present then, and didn't seem to improve after the cleaning, unfortunately. I did use throttle body specific cleaner. Thanks for your input though! >>You did the coolant temp sensor but have you cleaned your throttle body? >>It does wonders for low rpm smoothness if your TB is very coked up. [quoted text clipped - 5 lines] > > Thanks for your input though! Since you're in very cold area, why not try blocking the radiator like truckers do on their rigs? Put a big piece of cardboard in front of the radiator to kill the airflow. Maybe the car wasn't designed optimally for extreme cold, or however cold it is by you. CD > Since you're in very cold area, why not try blocking the radiator like > truckers do on their rigs? Put a big piece of cardboard in front of the > radiator to kill the airflow. Maybe the car wasn't designed optimally for > extreme cold, or however cold it is by you. I did think of that, but haven't tried it yet... The average temp this time of year is probably around -6C so it's not super cold all the time, but we have had a few mornings where it dropped to -20C and on those mornings the car barely warmed up at all before I got to work, which is about a 20 minute drive, mostly city driving. > > Indeed, a rich mixture at idle can cause the hesitation. Check the > > behavior of the oxygen sensor at idle vice at speed e.g. 2000-2500 [quoted text clipped - 19 lines] > > Thanks for the input. If it's got a new O2 sensor, then I suppose that's not the problem. The O2 sensor will drive the fuel mixture at idle and at higher RPMs as long as the system is in closed loop. It should go into closed loop soon after warmup. If it is not warming up, then you're right it is likely running rich and thus causing the hesitation. I presume you've checked the obvious things e.g. thermostat is the right temperature, cooling fans don't come on too soon or too long etc. If all that is OK, then maybe somehow coolant flow is getting past the thermostat. If you can get hold of one of those infrared laser pointing thermometers it might help to assess heat flow during warmup. They're pretty cheap nowadays. You've in a very cold climate where there's not much margin for error in coolant flow. I wish I could be of more help but you're in for some detective work here. Good luck. Al Just to make sure: Is it a hesitation or a miss? Is it rough for a while or momentarily off? Did you check timing? Vacuum leaks? > Hi all, > [quoted text clipped - 33 lines] > > Thanks in advance for any insight! > Just to make sure: > Is it a hesitation or a miss? Is it rough for a while or momentarily off? > Did you check timing? Vacuum leaks? Well, it's more of a hesitation. I shouldn't have used the term "miss" I suppose. What happens is there's about a 1/2 - 3/4 second pause when I step into gas where the tach actually drops maybe one or two needle-widths (highly precise measurement there) and then it responds. It *feels* like it's starving for fuel when I open the throttle for a split second, but the fuel pressure is ok, and I can hear the injectors respond when I snap the throttle from under the hood... I have a video I recorded to try and capture the effect that might help; I'll upload it and post a URL if it might help anyone get a feel for what's happening. The timing is 20 deg BTDC at idle, on spec, and I haven't been able to find any vacuum leaks.. the idle is very smooth and steady. When I first got the car, the idle was high, close to 1000 rpm, and I had a seized idle adjust screw, but since I replaced that part I was able to almost bring the idle to around 750 rpm, and over time since then it seems to have dropped to about the proper speed. Thanks for your reply! It's a little hard to see/hear it from the video (taken from my phone) but here's a Quicktime recording of it: http://www3.ns.sympatico.ca/pmason/altima_hes.3g2 I tried to use my finger over the bottom part of the lens to show when I was pressing and releasing the throttle. |
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