The 2002 Nissan Xterra (4WD), automatic transmission, I drive for work is starting to have trouble traveling up hills. Just the other day I was traveling on a freeway up a fairly steep grade for ~6 miles. It wouldn’t down shift and the pedal was to the floor while I watched the speed slowly drop. I started noticing earlier this year that it will disengage the cruise control on this same steep grade, though not every time. This used to be my personal vehicle and I have been driving it since 2002, so I know how it drives. A mechanic test drove it today and chalked it up to age (our admin assistant took it to the mechanic, so I didn’t get to talk to him). Its got something like 170k miles. That is a lot of miles and it is 10 years old, but I am really wary of any diagnosis based solely on age.
Talk to the mechanic. If it’s not downshifting, then either it doesn’t know the pedal is to the floor (throttle position sensor), or the transmission is not doing what it’s supposed to do.
There is no such thing as a problem on a car that is age related that *cannot be fixed - the question is how much it will cost to fix it. Cars are not like people - if a person is 90, their body is going to crap out on them and there’s not much to be done about it. If a car is 80, then whatever craps out can be replaced with a fresh part, and the car can drive on.
Can you describe it in a little more detail? Is the problem simply that it refuses to downshift to pass or climb a hill, but the motor has normal power? Can you pull the shifter to downshift a gear and then climb the hill normally? If the engine lacks power AND it won’t downshift, that takes us in a sligtly different direction than if the engine has normal power BUT it won’t downshift.
A compression test, a real one with a pressure gauge. Replace the air & fuel filter. Have the exhaust system checked for excessive back-pressure (plugged converter)…
As G37 noted, a plugged catalytic converter is one of the likely possibilities. Before just replacing the cat, a quick test using an inexpensive vacuum gauge would help identify if your exhaust flow is restricted.
I agree that this sounds like a case of a plugged-up catalytic converter, or possibly a weak fuel pump, or an accumulation of the outcomes of lax maintenance, or low compression, or…
One of the possible keys to tracking down the source(s) of the problem is the Check Engine Light. With the symptoms that were described, I would be VERY surprised if the CEL was not lit up.
Is the CEL lit up? And, if so, why did the OP not mention it?
Thanks all for your input. The engine sounds fine but just doesn’t down shift. If I lock out the over-drive before I start climbing the hill then I can usually get to the top just fine, though I still find the pedal hitting the floor. If I lock out the over-drive half-way up the hill after I have already lost some speed, it will stop slowing but I can’t get back up to freeway speeds until I crest the hill. The next shift down is third and I haven’t tried that gear. Another thing that happened recently…I was in cruise control on a relatively flat stretch of road. I hit a small hill and the engine down-shifted appropriately to keep up the speed setting. But when the road leveled out I noticed the engine didn’t shift back into over-drive. When I released the cruise control it immediately shifted into over-drive. I seem to recall having to occasionally back off the throttle to encourage a transmission shift in the past, though not very often, and usually only when traveling at high speeds I think (this has been very infrequent).