Edmunds.com | Inside Line | Auto Observer Your Account | Help | Directory
You are here:
![[F]](http://a332.g.akamai.net/f/332/936/12h/www.edmunds.com/media/townhall/webxicons/site.gif){kind=small}
Forums
Hatchbacks Chevrolet Metro/Geo Metro Chevrolet/Geo Metro
1696 messages, Last post on Dec 06, 2009 at 11:26 PM
You are in the Chevrolet Metro/Geo Metro Forum. Your Host is pf_flyer
|
|
|---|---|
|
1996 1.0 5 spd Been on this one for months- starts fine but can't give any throttle before it warms up, or it floods out. Checked the voltage at the Coolant Temperature Sensor by backprobing. It has three wires: Light Green/Black, which is common and goes to the MAP, TPS, and IAT. Gray/White, which is the signal to the ECM. Yellow/White, which is not shown on wiring diagram, but I assume is the input from the ECM. LtGr/Bk is ground- no voltage. Gy/Wt reads 2.34V when cold, decreases with rising engine temp and finally goes to 0.5V when hot, at which point the cooling fan kicks on and takes it back up to around 0.65V. (With two different CTS units- same within .2V) At anything over 1.0V, the engine will flood and stall if the throttle is opened even slightly. These values seem reasonable if the temperature was around zero- but it's 80. What does not seem reasonable is the voltage on the Yellow/White wire which I believe should be the standard 5V input from the ECM, but reads a steady 9.2V, regardless of temperature and even when not running with the ignition on. If the input voltage is nearly twice what it should be, then the CTS could have exactly the correct impedence, but the output signal would be much higher than the ECM expects, and it would compensate with a mixture richer than Bill Gates. Is it possible that something is creating a field that the wire is having additional current induced in it somehow? My DMM read nearly a volt with the positive probe held in the air near (2 inches) the sensor wires. I hope this is the gremlin I have been looking for since April, but I don't have a clue how it could get 9.2V. Maybe something a little over 12v, yes- but not 9. Anyone see this before? Hints? My crystal ball keeps saying to try again later....
|
|
|
Replying to: shaggyman1 (Aug 17, 2009 10:20 pm) But there is one other factor here which proves this point: The CTS resistance is highest when it is cold, and drops as the engine warms up. This means that the voltage signal the computer "sees" from the sensor is LOWEST when the engine is cold; and becomes higher as the engine warms up. So if your supply voltage was too high, the computer would get a signal that the engine was hotter than it really is. And under that condition, the engine would not go rich when throttle was applied. Instead, it would go too lean. But you claim the engine is flooding. Actually, I believe you are confusing the behavior of a flooding engine with one that is going too lean and starving for fuel. An engine which is too rich is typically insensitive to small changes in throttle opening; and will easily accept sudden openings in throttle position. A rich engine will also run much better when it is cold than it does when it is warmed up. On the other hand; an engine which is too lean will stall out if the throttle is opened when cold; but will run much better after the engine temperature comes up to normal. So I believe this is what your engine has been doing. Hopefully this will give you a better sense of where to look for the source of the problem. Here's a simple test you can do to prove this: Disconnect the plug to the IAT sensor, and then try running the motor. I bet you'll find that the cold stalling is gone. The IAT sensor is just like the CTS. Its resistance is highest when the air temperature is cold. So when you disconnect the IAT plug; the computer thinks the air temperature is below zero; and richens up the mixture.
|
|
|
Replying to: jessegeo (Aug 16, 2009 1:45 pm) |
|
|
|
|
Replying to: zaken1 (Aug 17, 2009 11:39 pm) The voltage most definately decreases with increasing temperature, which means the resistance is increasing. WTF? How could both my sensors work backwards? And the fan comes on when the voltage hits half a volt- shouldn't it be coming on with some increased voltage? If I disconnect the CTS the fan comes on, which implies that a zero volts signal received triggers the fan circuit. Tried disconnecting the IAT- no change. (It is receiving an index of 4.95V from the ECM) Tried measuring the voltages accross the signal and common ground wires, rather than grounding to the battery- same. It read 2.65V after sitting all night, and decreased as the engine warmed to a minimum of 0.50V, at which point the cooling fan came on. Maybe I drew the wrong conclusion from results, in that the signal of 0.5V is interpreted as zero, the same as if the sensor was disconnected, and the ECM thinks it's a malfunctioning sensor and defaults to cooling fan ON? I'll go and get another sensor today and see what it does. I could swear that when I tested the impedence on that unit it decreased with heat. I am disturbed that the wiring diagrams only show two wires, rather than the three I have. It implies other info may not be precise, either. Anyone with an FSM for 1996 have a table of voltages through a temperature range? My head hurts. Feels like I am dealing with things that violate accepted laws of physics- which implies I may be wigging out in a rather interesting fashion....
|
|
|
Replying to: shaggyman1 (Aug 18, 2009 8:14 am) In view of this; it sounds to me like you are interpreting the voltage drop ACROSS the sensor as the amount of signal the computer sees. But this is just the opposite of what is happening. The higher the resistance of the sensor is at any time; the more the voltage will be dropped across the sensor. And the more voltage that is dropped across the sensor; the lower the voltage will be that reaches the computer. Think of it this way: If there is a 5.0 volt supply to a sensor circuit, and the sensor has enough resistance to drop 3.5 volts across it; the computer will only see the remaining 1.5 volts. IT WILL NOT SEE THE 3.5 VOLTS YOU ARE MEASURING ACROSS THE SENSOR. Similarly, the lower the voltage drop there is across a sensor; the higher the signal voltage will be at the computer. We know this much; sensor resistance decreases with heat; which will produce less voltage drop across the sensor. So if you are measuring lower voltage at the sensor as the engine warms up; you must be measuring the voltage ACROSS the sensor; rather than the voltage in series with the sensor. And that is the source of your headache. Furthermore, I believe the engine temperature sensing circuit is connected in SERIES through the CTS and IAT; rather than in parallel. I also believe the engine load sensing circuit is also connected in series through the TPS and MAP. And this would further confound your attempts to measure the circuit dynamics. But I see one thing that is not right in what you measured: Disconnecting the IAT should NOT have resulted in no change. If it did; this indicates either a short across the IAT sensor, or else the computer was not responding to the change in IAT signal, because it was in "limp in" mode as the result of a trouble code having been set. Does your "check engine" light come on when you first turn the key on, and does it then go out after the motor is started? If it doesn't come on at all; or if it stays on all the time; then the computer is in limp in mode, and will not respond normally to sensor inputs. In this case, you'll need to clear the trouble codes with a code scanner (assuming your 1996 model has an OBD II engine control system) Trouble codes can only be cleared by disconnecting the battery if you have the earlier OBD 1 system. And you can't tune the motor when it has a trouble code set in the computer. I know scanners are expensive; but there is no easy way around this. Actually, some people say that turning the ignition key on and off about 15 times in succession will clear OBD II trouble codes.
|
|
|
|
|
Replying to: zaken1 (Aug 18, 2009 10:36 am) Fine. At last some logic in the chaos. In looking at the wiring diagram i see the O2, MAP, IAT, TPS are all in fact connected to the CTS. So how is one to correctly troubleshoot these components? AAAARRRGGGHH. Removing the IAT had no effect because (as you pointed out) it had set a fail code when I disconnected the CTS, and was in limp mode. (BTW) I've found that an easy way to clear the codes is to first disconnect the negative battery cable, then ground the positive cable to the chassis for a few seconds, thus draining the various capacitors- including those pesky ones in the airbag system. Beats a trip to O'Reilly to borrow their OBDII reader. (Personally, I think they should be built in with readouts available in real time) Back at it tomorrow, with a new TPS which seems to have a much better and smoother resistance curve than the last two. Got my fingers crossed. Again. Thanks Zaken! I thought I was witnessing a miracle gone horribly wrong.... |
|
|
Replying to: shaggyman1 (Aug 18, 2009 2:01 pm) Now, back to today's problem; if I were working on a vehicle that acted the way you describe, I would begin with the simple and direct approach: The symptoms you described sound just like what happens whan a TPS is adjusted too lean (set too far counterclockwise). So, before doing anything else, I would loosen the two mounting screws; and turn the damn thing clockwise until there was a noticeable improvement in the running. Forget the adustment specs; those specs are only valid when the engine is all new and stock. As soon as the compression changes, or any of 1,000 other factors changes; the TPS setting goes off. I have had to reset mine all over the range, after changing other things on the motor. And it ran great in all sorts of different settings, when they were appropriate. It would only be if resetting the TPS as far as possible did not correct the problem, that I would start thinking about defective sensors or other exotic issues. If you set the TPS too rich; the consequence will be that the fuel cut does not come in during decelleration. And that is fairly obvious, as you slow to a stop in gear. Normally, the fuel should come back on as you get down below about 10 mph; and the car will feel different when that happens. But the other consequence of too rich a TPS setting is that the fuel mileage drops severely; and the exhaust pipe becomes wet and sooty. One owner wrote in here some time ago; complaining that his exhaust pipe always looked rich. So I finally helped him to overcome his fears about disturbing the TPS setting; and explained that it has to be done by the "seat of the pants." A few weeks later, he wrote in to thank me for going against what everybody else had said, and reported that it took a series of tries and road tests before he finally got it right; but his car now is peppier than ever, the pipe is brown and clean, and the fuel economy is great.
|
|
|
|
|
Replying to: zaken1 (Aug 18, 2009 6:05 pm) I connected a known good CTS and immersed it in boiling water, thinking to imitate a warm engine and started it. Died as soon as I opened the throttle. So the coolant temerature sensor or it's circuit is not the culprit. Okay, what else is temperature sensitive? What about the Early Fuel Evap Heater (EFE) in the plate under the Throttle Body? Fuel system component, temperature related, has a 30A fuse and a relay..... It's on the car, in the wiring digram, and utterly without mention in Chilton or Haynes. AllData says it preheats the Fuel/Air and reduces the time in open loop, until engine reaches operating temperature when it is deenergized by the ECM. Resistance should be 0.5 to 3.0 Ohms. (seems low for a heater) Checks at 2.6 Ohms across the leads, and open from either lead to battery ground. Connect unit and measure voltage: Voltage coming in while running is 13.4V with a drop to 1.2V on the ground side. (12V drop) Tht's a heckuva drop for 3 Ohms. Sounds reasonable that it might be the culprit.
|
|
|
Replying to: shaggyman1 (Aug 20, 2009 6:46 am) I hope you can see from this that the voltage drop across a 3 ohm load has NOTHING to do with the amount of resistance of that load. It is solely determined by how much ADDITIONAL resistance is in series in that circuit. If the 3 ohm heater is the only resistance in that circuit (which is what I would expect in a heating circuit) then the entire 12 volt supply (minus what is dropped across the plugs and wiring harness) will be dropped across that 3 ohm resistance. Applying Ohm's Law (Current = Voltage divided by Resistance); 12 volts / 3 ohms = 4 amps. 4 amps current at 12 volts is 48 watts of power (Power = Voltage X Current). 48 watts of power is about what a single 12 volt headlight draws. That is not at all unreasonable for a heater that has to bring a steady stream of fuel and air up to a temperature high enough to vaporize the fuel. It probably sounds high to you because most other current draws in automotive electronics are much smaller than that. But I expect the rear window defroster grid draws at least that much. And the starter motor draws 20 to 30 times that much!!! So your test of the EFE heater tells me that it is working right. I appreciate your perception that this appears to be a temperature sensitive problem; but I assure you that the root cause will turn out not to be someting directly related to temperature, and will instead turn out to be related to excess leanness, or sudden changes in air/fuel ratio when the throttle is opened. I'm glad you tested the TPS and eliminated that as a potential cause. I can accept that; and this information helps narrow down the focus. What it now sounds like to me is that the EGR valve is opening too soon, or too suddenly, when the throttle is opened. This would happen if one or more of the EGR vacuum control devices were bypassed or missing. (there should be an exhaust back pressure tradsducer and an electrical vacuum switching solenoid in the vacuum lines to the EGR valve). It would also happen if the ignition timing had been adjusted without first disabling the electronic advance by shorting the check connector wires. |
|
|
Replying to: shaggyman1 (Aug 20, 2009 6:46 am) You can eliminate the EGR valve as a factor by simply disconnecting and plugging its vacuum hose; and seeing if that changes the stalling behavior. However; the EGR valve may also be sticking partly open; and may thus never close fully. And that would continue whether the vacuum hose was connected or plugged. It should be possible to reach underneath the valve actuating diaphragm, and manually raise and release it to see whether the spring returns it all the way down. If you manually raise the valve while the motor is cold and idling; it should make it stall. And if none of these things work, it is probably time to reset the dreaded idle air bypass. |
|
You are here:
Forums
Hatchbacks Chevrolet Metro/Geo Metro Chevrolet/Geo Metro
New? Join Now!
Forum Tools
Search Forums
Browse by Vehicle
1997 Geo Metro
2001 Chevrolet Metro
Browse by Board
Browse by Topic
Today's Chats
