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RPM'S Running High

My car has 181,000 miles on it and has never had a major problem. However, the last couple weeks I have noticed that my RPM’s run at about 3500 when cruising down the interstate about 70-75 mph. That is much higher than usual. I have also noticed that the air conditioner is not blowing air very well and it wont change from feet to defrost or any other setting for that matter. I can deal with the air problem but the high running RPM’s I cannot. Could these two problems be related? If so what can be causing it?

They are probably not related. Sounds like it isn’t shifting into 4th gear. Is your check engine light on? When you are driving at this speed, try shifting from “D” to “3” and see if anything changes. If not you’ll have to visit a transmission shop to have it diagnosed.

The direction of the vent air flow is controlled by a little door in the ductwork. In some cars this door has an electric motor, and in other cars it is operated by engine vacuum. The blend door, which controls the mix of hot air from the heater core, and cold air from the vent or A/C, is also controlled by vacuum. In some cars, the transmission senses engine vacuum, and you have insufficient vacuum, the transmission will think you are under a heavy load and may keep you in a lower gear. Also, if the car thinks you are under heavy load, it may shut off the A/C compressor.

So my 2? diagosis is, you have a vacuum leak.

What year is your Grand Prix and is your Check Engine Light on?

It’s a 1999 and yes the check engine light is on.

Thank you for the help. I will try that and then write back.

For starters, many auto parts stores have a code reader that they will plug into your car for free and read off the codes. If you post the exact codes here, we may be able to help more. (They will be in the format P0001, P0101, etc)

The codes that came up were P0507 and P0171

Well, given those codes I’d say StrongDreams… was onto something.

P0171 is about running too lean (too much air in the mix, though it could also be too little fuel)

P0507 is just about the idle being higher than expected - a very common cause? Vacuum leak. Is it idling too high?

You still haven’t given the year so no one could say for sure but the vacuum problem messing with the transmission is a possibility.

Put that with inoperable HVAC controls and I’d say that there is probably a vacuum line going from the engine through the firewall to the HVAC that is either disconnected or broken. You can just look all along the firewall very carefully to see if you notice anything amiss. But if you’re not used to looking at all of that stuff under there you might need an experienced eye.

Hi everyone I am having the same issue as the comment posted above. My car is a 2010 Toyota Corolla and I have a 140,00 miles on her. Driving down the interstate I’m sitting at almost 4000 RPMs and driving around town I sit right at or hair below 3000. My car just started doing this about a month ago and I took it to my local AutoZone and they told me there was no codes reading. I also took it to a local mechanic and they’re telling me that it could just be the age but since there’s no issue presenting they can’t help me… Which makes no sense because they can see it. If anyone can give me any advice that would be absolutely amazing. I am praying that it is not my transmission, I’ve taken really good care of my car and it’s almost paid off. I also don’t want to take it to an offhand mechanic with an issue as they could replace everything but what is actually wrong.

P.s I’m also having an issue with my AC. We have replaced the resistor as well as the fan blower and I still have no AC unless I knock the hell out of it from underneath my glove compartment.

I “fixed” my mom’s Mazda for the same problem many years ago. The overdrive button wasn’t pushed. My sister had borrowed it, left the button unpushed, and my mom had to have me come over to find out what was wrong.

Have you checked to be sure your overdrive button is pushed (if you have one)?

The AC problem will have to go to a shop. The system will need to be tested by a certified tech. After 17 years, its very possible that the refrigerant has left the system. If the problem is airflow, start by checking your cabin air filter.

Check your cabin air filter. It will strain the A/C and ventilation if clogged. Its hiding behind your glove box

Im not worried about the AC so much as I am about the idling but the cabin filter was changed last month with the other one.

I dont have one…

I don’t have one sorry…

My understanding is that your problem is the engine rpm is too high when driving at 40+ mph. If so, that’s not a high idle rpm problem. That’s more likely a transmission problem of some kind. Checking the transmission fluid level is where I’d start.