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98 Nissan Pathfinder RPM problem

I have an auto transmission 98 nissan pathfinder that I bought brand new way back (in 1998 of course) that seems to have problems shifting into higher gears. When I’m going on a level hwy, I’ll get going to 70mph and see the engine rev to 4000 rpm’s. It’ll stay there for a while (minutes to 10’s of minutes). Then the engine will shift to a higher gear and the rpm goes to 2500. Then it shifts back so rpm is 4000. This happens back and forth unpredictably. When it’s reving at 4000 rpms I tried letting my foot off the gas and the rpm’s drop to less than 1000 like there’s no load the transmission. Then I press the gas pedal again and the rpm jumps back to 4000 rpm’s. I’ve had the transmission checked a few times and found nothing. I’ve changed the transmission fluid, had a major tune up, changed the timing belt, but it still happens. People keep telling me it must be happening on a slight grade but, no, it’s on a flat road. There is no ‘problem light’ that appears either (like a check engine light). I’ve driven many other cars and they don’t do this. I’m at my wits end trying to figure this out. You guys are my last resort.

Who, exactly, has checked your transmission? General mechanics are normally not a good bet for transmission issues. “Auto care” chain operations are even worse. “Quick lube” places usually don’t even have mechanics in them.

Regardless of who checked it out did anyone actually take it for a drive and reproduce the issue?

Find your best local, independent transmission shop. Tell them that someone needs to take it out to the highway and reproduce the problem. Preferably this would be done while connected to a scantool.

Thanks for the quick reply. Yes it was a general mechanic. Even the dealer. They all did take it out for a drive at hwy speeds and they claim they didn’t see it. Yet, everytime (and I mean EVERYTIME) I drive it, I see it. It doesn’t have to be hwy speeds but it is more pronounced at hwy speeds. Would AAMCO be a good place to take it. Or do they count as an “Auto care” chain?

Well, AAMCO shops are franchises. So each one can vary. But, in general they are not known to be a good bet. Many would say it stands for “All Automatics Must Come Out” - or in other words, whatever you bring them you will be told that you need a new transmission. Then, they give what are often sub-standard services.

I would just ask around among people you know to see whether you can get any experiences others have had with local shops. And when you get someone to take it out for a drive, perhaps you should drive while they ride.

This isn’t something that happens only when the vehicle is cold is it?

It doesn’t seem to be a cold issue. It happens all times of year and all times of day. Even on a 150 mile trip, in the summer or winter, it’ll do it all the way, kicking up, kicking down, pretty much on a random pattern. When a general mechanic checked it, he started the engine, stepped on the brake, put it in drive, stepped on the gas and watched the tach. From that he said the transmission is fine. Is there anything else I can check?

what about the transmission coolant temp sensor? I read it in the following post, the last entry:

Though it doesn’t seem to be a temperature/cold issue. What about this link:

I asked about the car being cold (not the air temps) because many auto transmissions won’t shift into higher gears until everything warms up.

It is likely that you are having a transmission control issue - rather than an issue with the transmission itself. I don’t know this transmission (and only have general knowledge about them to begin with), but everything they do is computer controlled. The computer needs info from several different places to know what to tell the transmission to do.

For example, you could easily have a problem with your throttle position sensor. As you know, if you punch the throttle the transmission should downshift and the rpms will shoot up as a result. Well, if the computer only thinks that you have floored the throttle (e.g. from just a voltage irregularity), it will downshift the transmission.

Anyway, the problem is that there’s little point in guessing about it and any of the internet info you come up with will just give you all sorts of different possibilities. An actual transmission tech needs to put it on a scanner and drive it. A good transmission tech will know exactly what yo check and will have the equipment with which to check it. All you or I can do is make wild guesses.

You seem resistant to the cold engine theory, but the fact that the problem occurs on flat roads more often than on an upgrade suggests otherwise. I would guess that it happens also on downgrades. If so, the problem is not unpredictable. It occurs when the engine cools enough that the transmission kicks down a gear in order to heat up the engine.

If you have a temperature gauge, look at that while driving. It should be at or a little higher than the halfway mark after fifteen minutes of drving the SUV in any weather with the heater off or on. Anything less is too cool. If you see that the temperature gauge rarely hits the halfway mark, a too cool engine is your problem. Your next step would be to change the thermostat and fluid, making sure of the correct fluid and mixture with water.
A few of the top 20 guys on this site taught me this a year ago, and I was doubtful but they were right. I replaced my thermostat and fluid and it fixed the problem. It cost me all of fifty dollars. Now I’m back to getting 32 mpg and not killing my engine. Like your vehicle, the problem occurred most often on flat highways and going downhill. It happened in all weather conditions as well, though much less often in summer. But I have an old Toyota, not a Nissan.

You have a good point. Thank you so much. At least now I have a target instead of, as you said, making wild guesses. I’ll start looking for a good transmission technician. Would the dealer be able to diagnose this problem?

Dealers. like AAMCO shops, can be very hit & miss. They should be able to figure it out, yes. But quite often they are more interested in selling you stuff. They are also no more or less competent than a regular shop. Actually, in the case of transmission questions you’re probably better off at an independent shop where you’ll find more expertise & experience.

If it was the throttle position sensor, would that activate a “check engine” light?

Its perfectly possible that it would not.