99 Altima hesitation: I think I'm onto something!

It does have such a switch, but, it's part of the same unit as the TPS, and according to the service manual, it's not used for engine control. It may be only used for the transmission in automatic models (mine is manual). FWIW I did adjust it according to the manual, but had no effect on the problem. Also, when the car is cold, the throttle is kept open by that fast idle cam, beyond the point where the closed throttle switch is closed anyway -- and this is when the abruptness is at its worst. The procedure for adjusting the switch requires the engine to be fully warmed up first, which I'm sure is so that the fast idle cam won't be holding the throttle open when you're trying to adjust it properly.

I've wondered about both of those things, but the throttle closed switch seems to be a non-issue.. and I did a quick test of the clutch switch by pressing it with the cruise control on, and it did cancel the cruise, so as long as there is only the one switch on the clutch, that would seem to be fine.

I kind of figured as much, which is why I got into testing the TPS in depth. It seems like that kind of a problem to me.

Save for the PCV, all done.

Sadly no, I just read about that technique yesterday. But, I did try pressurizing the intake manifold manually and there was no leakage of any serious amount... a very slow one around the EGR valve I think, but that may have been due to the wrong "polarity" of pressure in the system.

Absolutely, I certainly appreciate any and all ideas you or anyone else has.

Thanks again,

OK, I found your older thread and was wondering, did you replace the coolant temp sesnor? It's worth a shot. Cheap part, easy replacement. Usually a bad temp sensor has the symptom of bad gas mileage and it may cause a not so stable low rpm operation as well, as you're experiencing.

Also, if it's not that, then perhaps you have a bad fuel injector. Just a mildly educated guess.

CD

Yeah, I have wondered about that sensor for a long time, but haven't replaced it yet. I priced it at $30 one weekend and they would have had to order it in, so I decided against it for the time being. I have tested mine though with my meter and it seems accurate warm and cold, so... ?

Could be I suppose. The idle seems slightly rough but it's not like it's running on 3 cyls. I'm not sure testing injectors is still at the level I want to get into in my driveway with an hour to spare on any given day :)

Another thing I've noticed lately is the engine doesn't seem to speed up when I turn the wheel left or right at idle, and it will often rev way down to 500 rpm or below when I have eg the blower on full with the headlights and rear defrost on. I don't know if this is possibly related in any way to the hesitation / overrevving, but I have been meaning to mention it. It's almost as if the ECU in general isn't getting the proper indication of engine speed, or, the fact that it's at idle...

That sounds like bad electrical connections. When you turn the wheel, the power steering puts more of a load on the motor, the rpms drop. Because the rpms dropped, and they are so low to begin with being just above idle, the ALT output then drops, dangerously low, so low that bad grounds and other electrical gremlins make themsleves more obvious. Because the ALT's output is so low, all the readings to the ECU get off base, the O2 sensor is slightly off, the temp sensor etc, you get the idea. So your car runs funny.

In the other scenario, when you maximize the load on the ALT by putting th fan on full, defroster, headlights, etc, you are working your ALT to the max. Any corroded connections will have a great effect on its ability to do its job delivering ampage to the electrical system.

Solution? Thoroughly check and clean all major electrical connections such as the battery, all the alternator connections, the ground connections to the engine block etc. And I mean dis-connect the connections, clean them, then put them back.

If after cleaning them you still experience these problems, then maybe your ALT needs a rebuild, replace brushes etc.

Hope this helps

CD

or perhaps the power steering pressure switch is bad and not telling the computer to speed up the engine when the power steering is being applied???

The computer should pick the idle back up when it gets a signal from the power steering pressure switch telling it that the engine is being loaded. Could be a bad switch or an idle controller problem.

I can agree that the engine performance problems may be due to some electrical problems, but your reasoning is way off. The computer system operates all it's sensors at 5volts reference, so the battery would have to be too discharged to even fire the injectors before the sensor voltages would be affected. I do see what you are getting at though. When voltage is low resistance has a greater affect, and the problems may well be related to bad connections or grounds.

It would be a lot easier to just check the voltage drop across the ground circuits rather than try to find and clean them all.

I think the alternator output is not suspect in the hesitation and erratic idle complaint originally posted.

When all the obvious and simple things check out ok, then it is time for the big guns. JM probably needs to have a shop with all the necessary equipment trouble shoot this one. All the obvious stuff hasn't turned up any problems. Of course, if he has the time and patience then he can keep pecking at it until he might stumble across the culprit. Good luck JM.

How is it way off? The sensors expect 5 volts, so somewhere in the system the 12 volts from the battery is being reduced to 5 volts. But if the battery is not pushing out 12 volts, like say 11 or 10, then the other end of the voltage divider is not pushing out 5 volts but instead

OK, check and clean all ground connections. That's simpler.

Think about it The ALT's output rating, say like 80 amps, is usually determined when the car's rpm is at something like 3000 rpms. When the engine's rpms go low, the alternator still must produce a minumam amperage, say 15 amps at 1000 rpms. But a sick alternator will go below that and may contribute to a cars erratic behaviour. I'm not sayin that the problem is the alternator, just a possibility.

It's way off because you assume the power supply for the 5 volts is unregulated. You look at it as if you were the first person to think of the negative consequences of not having a stable reference voltage. The fact is, if your alternator dies the car will continue to run normally off the battery until the battery is almost completely dead. And when the battery does get really low its the high voltage ignition system that is the first to fail.

-jim

A 5v regulator costs next to nothing so there's no doubt one would be used. Any voltage between about 7v and 16v from the alternator/battery would give out the same 5v.

Thanks, and thanks also to KM who explained it to me as well in an email.

CD

If I might jump in with a suggestion here. Measuring resistance can be a bit tricky due to the ability of moisture, oil, and even dust to carry a reading across your high impedance volt meter test points. These ghost shorts may be able to carry the low current from your DVOM, but would not be able to carry enough current to affect you TPS signal adversely. It would be better to measure the actual voltage signal at the PCM harness. If you back probe Pin 23 (TPS signal) at the PCM harness connector with the connector plugged into the PCM and the ignition switch on, engine off, you should read 150 mv to 850 mv at closed throttle. As you depress the accelerator, the voltage should climb steadily to a wide open throttle reading of 3.5 v to 4.7 v. Having an oscilloscope would be nice because you could watch the voltage trace for any glitches, but if you are very observant and steady you may be able to catch any glitch by seeing the voltage fail to climb steadily on your DVOM. If your meter has selectable voltage ranges, use the lowest ranger possible to view 5 volts. If it has an analog display be sure to watch it carefully.

If you do not get the expected voltage ranges then be sure to verify that you have a good 5 volt reference voltage at the TPS and then perform a voltage drop test on the TPS negative wire to ground. Any voltage reading across the ground return indicates unwanted resistance that needs to be corrected.

Just testing resistance readings is not always a very conclusive diagnosis for TPS circutry.

Lets see if I am getting this right.. With both the ECM and the TPS disconnected from the harness. You are getting a reading of 1200 ohms between the TPS signal wire and the Referance voltage wire? You should get an infinite reading during that test, unless (and I would have to check a wiring schematic) there is some other circuit in parrallel with the TPS on your vehicle. Keep in mind that the Reference voltage wire is connected parrell to many other sensors. Only the Signal wire (to pin 23) is a direct wire from the TPS center pin back to the ECM. Do you have a good wiring diagram? Look it over carefully and be sure you are testing the correct wires.

Well, I can see where that might be a problem, but don't be fooled by some continuity across the circuit due to possible contamination. That sort of short will not usually carry enough current to skew the TPS signal measurably, but it will show up on an ohm meter. If there is indeed a hard short across the two wires the reading would usually be closer to 0 Ohms. There again, be sure you are testing the correct wires. Double check. Still, checking the signal voltage is a much more reliable test with less chance of error.

My post on testing wasn't aimed at you, I just thought it might be some helpful information to other readers. I hope you didn't take it as a slight on you capabilities. The truth is, you seem to have a pretty go grasp on the situation. Anything I can do to help is offered gacefully.

There in lies the problem. The reference voltage (sensor power supply 5 Volts) wire leaves the ECM and branches out to all the sensors that need reference voltage. The signal ground also leaves the ECM and branches out to all the sensors that have a signal ground. You are reading the parrellel resistance of all the other sensors in the system if you check from reference to signal ground. You would have to disconnect every sensor to show an open circuit between those two wires. You need to be checking from reference (ECM pin 49, Red wire) to TPS signal (ECM pin 23 , Yellow wire) to find the specified 4,000 ohms at WOT. It will all make sense to you if you could look at the whole wiring diagram and get an understanding of the circuitry.

Kevin

BTW, Engine performance electronics is my favorite type of work.

Maybe this will help.

Here is a link to a good explaination of a TPS It is Generic in nature but does apply to your type of TPS.