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Car Forum / Honda Cars / September 2005idle sensor
1993 Honda Accord EX idle sensor constantly surges. Recently replaced with a used sensor, could this be the problem (needs brand new one) or could it be an electric problem. > 1993 Honda Accord EX idle sensor constantly surges. Recently replaced with > a used sensor, could this be the problem (needs brand new one) or could it > be an electric problem. -------------------- If your rad has any air in it, (some of) the temp sensors will not be emmersed when when the thermostat opens. Fill the reservoir at least to the MAX line, and top the rad up to the top. Air will often burp out when engine next cools and things will start working correctly. Don't put tap water in your Honda (Hondacide). Honda premix coolant would be best. 'Curly' Classic symptom of a vacuum leak. >> 1993 Honda Accord EX idle sensor constantly surges. Recently >> replaced with a used sensor, could this be the problem (needs brand >> new one) or could it be an electric problem. > > Classic symptom of a vacuum leak. Yes, if the idle surges ABOVE normal speed and does not drop to normal or below while surging. A slight air leak will cause an elevated idle; a gross leak will cause surging well above idle. Low coolant in the head will fool the ECM into thinking the engine is cold, since the temperature switch is exposed to air. The ECM then increases the idle, then discovers things are still not right, so it cycles back and forth unable to find the correct setting. A thermostat stuck open can also cause a surging idle, for the same reason as low coolant.  SignatureTeGGeR® The Unofficial Honda/Acura FAQ www.tegger.com/hondafaq/ I suppose it depends on the car. My other-car Bosch L-Jetronic has a fuel cutoff at high idle w/ throttle switch closed. (Safety feature. Probably universal.) A gross air leak sends RPMs up, fuel is stopped, RPMs drop, fuel is turned back on, RPMs rise, etc. As for your low coolant theory, I feel any temp sensor in a running engine is hot whether immersed or not, being efficiently heated by radiative transfer inside a closed cavity. Like an oven. I'll wager if you disconnect your coolant temp sensor, assuming an NTC thermistor, so infinitely high resistance simulates low temperature, your engine will not surge. Just my uninformed opinion. > I suppose it depends on the car. My other-car Bosch L-Jetronic has a > fuel cutoff at high idle w/ throttle switch closed. (Safety feature. > Probably universal.) A gross air leak sends RPMs up, fuel is stopped, > RPMs drop, fuel is turned back on, RPMs rise, etc. Sounds exactly like what happens to both the cars I tried a gross air leak on. I don't know enough about the Honda PGM-FI to know if it has a similar cutoff, but I suspect it does, otherwise you'd run the risk of a throttle- closed over-rev. > As for your low coolant theory, I feel any temp sensor in a running > engine is hot whether immersed or not, being efficiently heated by > radiative transfer inside a closed cavity. Like an oven. Could be. I'll admit to parroting what I've consistently read, which may not necessarily be true. Parrots know not of empirics... > I'll wager if you disconnect your coolant temp sensor, assuming an NTC > thermistor, so infinitely high resistance simulates low temperature, > your engine will not surge. Just my uninformed opinion. Worth a try, that is. Mine's right below the distributor, so it's even easy to unplug. I've got calls to make tomorrow, so I'll have her nice and hot. Be back to you tomorrow with the exciting results! SignatureTeGGeR® The Unofficial Honda/Acura FAQ www.tegger.com/hondafaq/ >>I suppose it depends on the car. My other-car Bosch L-Jetronic has a >>fuel cutoff at high idle w/ throttle switch closed. (Safety feature. [quoted text clipped - 12 lines] > Could be. I'll admit to parroting what I've consistently read, which may > not necessarily be true. Parrots know not of empirics... no, it's definitely the real deal. i've worked on that issue a number of times. the people that have the most problem with it are the ones that think the sensor "should" transmit the right signal with the logic shown above. >>I'll wager if you disconnect your coolant temp sensor, assuming an NTC >>thermistor, so infinitely high resistance simulates low temperature, [quoted text clipped - 6 lines] > I've got calls to make tomorrow, so I'll have her nice and hot. Be back to > you tomorrow with the exciting results! >>>As for your low coolant theory, I feel any temp sensor in a running >>>engine is hot whether immersed or not, being efficiently heated by [quoted text clipped - 7 lines] > that think the sensor "should" transmit the right signal with the > logic shown above. Well, I just tried disconnecting the TW sensor the way misterbeets said. The idle did not surge. I made sure I was unplugging the correct one too. However, in addition to the Check Engine light coming on, the idle increased from 750rpm to about 950. Then it began a slow, unsteady hunting between 850 and 950. It would go down to about 850rpm, then unsteadily increase to about 950, then down to 850. Total tach needle movement during the hunting was less than 1/8". It took about ten seconds or so for it to hunt up between 850 and 950, and another ten to go back down to 850. If you did not have a tach and weren't really paying attention to the engine, you'd easily miss the cycling. The wild, violent swings I saw when unplugging the PCV valve were another story entirely. When I plugged the sensor back in, the car seemed to ignore it completely, with the hunting idle and Check Engine light remaining on. Once I shut the car off then restarted it, it was back to normal, at a steady 750rpm. >>>I'll wager if you disconnect your coolant temp sensor, assuming an >>>NTC thermistor, so infinitely high resistance simulates low >>>temperature, your engine will not surge. Just my uninformed opinion. SignatureTeGGeR® The Unofficial Honda/Acura FAQ www.tegger.com/hondafaq/ A commendable effort. On second thought, though, I'm not sure it proves anything. If the ECU is turning on the Check Engine Light, it's probably ignoring the sensor. The true test would be to progressively reduce the coolant level and see whether surging commences. But, as I mentioned, there's no reason to think it would. First because--I still maintain--the sensor output is nearly the same whether it's heated by liquid or hot gas, or by conduction via its threaded mount in the cylinder head, or radiative transfer, or all three. Certainly it's not cold enough to trigger fuel enrichment And second because, even if this were not true, there is no feedback mechanism I am aware of to bring the RPMs down if fuel enrichment were to raise them. > A commendable effort. On second thought, though, I'm not sure it proves > anything. If the ECU is turning on the Check Engine Light, it's > probably ignoring the sensor. > The true test would be to progressively > reduce the coolant level and see whether surging commences. ...which I am not prepared to do on my car, having paid a substantial chunk of change to replace a failed head gasket a few years ago. It would be nice if someone with an end-of-life vehicle that was being junked or rebuilt could perform this test to confirm your theory. > But, as I > mentioned, there's no reason to think it would. [quoted text clipped - 3 lines] > threaded mount in the cylinder head, or radiative transfer, or all > three. Certainly it's not cold enough to trigger fuel enrichment It sounds plausible to me. Confirmation by experimentation would be nice. > And second because, even if this were not true, there is no feedback > mechanism I am aware of to bring the RPMs down if fuel enrichment were > to raise them. Based on what I've seen, this may be true. An air leak RAISES the idle in a feedback EFI system. A carbureted car would see the idle DROP. The two cars (one OBD-I and one OBD-II) I induced air leaks on were unable to maintain correct idle speed with even small air leaks. SignatureTeGGeR® The Unofficial Honda/Acura FAQ www.tegger.com/hondafaq/ > And second because, even if this were not true, there is no feedback > mechanism I am aware of to bring the RPMs down if fuel enrichment were > to raise them Stupid me. Of course there is: the RPM sensor which normally keeps the idle stable. But why would a fuel enriched idle be unconttrollable by this mechanism, and cause surging? But I'm still pretty sure any temp sensor screwed into the cylinder head but not in direct contact with coolant is simply not cold enough to cause fuel enrichment. >>And second because, even if this were not true, there is no feedback >>mechanism I am aware of to bring the RPMs down if fuel enrichment were [quoted text clipped - 5 lines] > > But I'm still pretty sure look at the sensor dude, it's not reading from the thread conduction, [steel is a lousy conductor anyway], it's reading from the central insert. that needs to be bathed in fluid, and dense fluid too, not aerated foam. if you don't believe it makes a difference, drain half your coolant. the motor will run fine, the gauge will not show overheating, but the mixture will be /way/ rich, you'll get intermittent code 1's [obdc0], significant loss of power and gross fuel consumption. and this effect is not unique to hondas. > any temp sensor screwed into the cylinder > head but not in direct contact with coolant is simply not cold enough > to cause fuel enrichment. > A commendable effort. On second thought, though, I'm not sure it proves > anything. If the ECU is turning on the Check Engine Light, it's > probably ignoring the sensor. Could be. But if so, why did I experience the elevated idle and slow hunting? SignatureTeGGeR® The Unofficial Honda/Acura FAQ www.tegger.com/hondafaq/ |
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