RPM drops while driving

[Snoop dogg]

Hey guys i'm a newbie so I have a stock 98 grand Prix gtp with 115,000 miles on it and I've notice every now and then that my rpm drops from 2000 rpm to 1500 or 1000 rpm while driving anywhere from 30 mph to 70 mph and when I take my foot off the gas pedal and put it back on it goes back to normal for a little bit so I change my spark plugs and wires and now it does it all the time now and I have no check engine light on. anyone know what it may be

[Explicit_Spade]

Vac leak or dirty maf.

[Scottydoggs]

on the highway @ 65 mph, whats your rmp's? should be about 1800, if its higher and then drops lower it could be the fourth gear hub is dead but still kickes in avery now and then.

[Snoop dogg]

at 65 mph on the highway my rpm are @ about 1800 or 2000 but still drops after a while it use to stay around those numbers before. but it also drops at low speed to like driving at 35 or 40 mph I've been reading on the inter net that a bad coil park can caused this to happen to is this true. And one other thing at idle it shakes a little bit not hard but lightly like its misfiring

[Scottydoggs]

dont think coils would do that, it would shudder and feel weak if the spark was being cut off. and that would be while getting into the gas only for the most part.

[Snoop dogg]

also when on the highway when the rpm drops to 1500 if i give it gas the boost kick in getting back up to 2000 rpm but it moves very slow. i have to take my foot off the gas and put it back on before it back to normal for a little bit and when i take my foot off the gas the rpm jumps up a bit before it goes down.

[Scottydoggs]

the last part sounds like its setting into over drive.

starting to sound like a tranny issue, like the torque converter maybe or its solenoid.

[02NavyBlue]

This whole time it just sounds like you're describing normal torque converter operation. Given it starts at 30 mph, which is the lowest possible speed it will lock, and that the RPM's drop to as low as, well, 1,350 really. It will go to 1k if you let off. Also you say "when I take my foot off the gas pedal and put it back on it goes back to normal for a little bit" The torque converter unlocks when you let off, and when you get back on the gas it waits about 1.5 seconds before re-locking. Still normal. "at 65 mph on the highway my rpm are @ about 1800 or 2000" - That's normal operation. "but it also drops at low speed to like driving at 35 or 40 mph" - Also normal, mine goes real low like 1,350 and you can hear the engine and know it's at pretty low rpms.

[Snoop dogg]

the last part sounds like its setting into over drive.

starting to sound like a tranny issue, like the torque converter maybe or its solenoid.

the last time I took it to the shop for a check engine light it throw a code for the solenoid but when I return and they scanned it again they said it was not there any more

[Snoop dogg]

ill try to put up a video of it

[02NavyBlue]

Which solenoid? Do you know it, or the code? There are issues that can happen and cause the torque converter to stay locked and when it shouldn't, but you've really been describing normal operation. Does it happen all the time, when hot/cold, or randomly? Maybe try and get a video of it.

[Snoop dogg]

This whole time it just sounds like you're describing normal torque converter operation. Given it starts at 30 mph, which is the lowest possible speed it will lock, and that the RPM's drop to as low as, well, 1,350 really. It will go to 1k if you let off. Also you say "when I take my foot off the gas pedal and put it back on it goes back to normal for a little bit" The torque converter unlocks when you let off, and when you get back on the gas it waits about 1.5 seconds before re-locking. Still normal. "at 65 mph on the highway my rpm are @ about 1800 or 2000" - That's normal operation. "but it also drops at low speed to like driving at 35 or 40 mph" - Also normal, mine goes real low like 1,350 and you can hear the engine and know it's at pretty low rpms.

but it never did it before anything else that can cause this to happen?

[Snoop dogg]

Which solenoid? Do you know it, or the code? There are issues that can happen and cause the torque converter to stay locked and when it shouldn't, but you've really been describing normal operation. Does it happen all the time, when hot/cold, or randomly? Maybe try and get a video of it.

ill put the code up when I get home I still have the paper

[Snoop dogg]

yes more now that I change my spark plugs and wires and if I am setting still I feel the car slightly shake like a misfire

[Snoop dogg]

the code it throw the last time my check engine light was on was P1871 solenoid circuit

[FordMan77]

What kind of spark plugs did you install?

[02NavyBlue]

P1871 - Transmission Mechanical Transfer Case 4x4 Switch Circuit Short To Battery

Sure thats right?

[Snoop dogg]

yeah that is what it said on the diagnostics paper i had

[Snoop dogg]

[02NavyBlue]

Ok our cars are FWD since the late eighties, and have never been 4x4 so I'm not sure where that came from.

Edit: just saw the video, watching it now