RPMs dropping at 50-70 MPH - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 10-02-2003, 06:00 PM   #1
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Default RPMs dropping at 50-70 MPH

Have a 97 SSE. 73K miles. Meticulously maintained. Easily driven all it'* life.

Symptoms: Doesn't always happen, but when it does, car is warmed up. While driving at speeds usually above 50 MPH, the RPMs will fluctuate. Say the car is sitting on 3000 RPMs, will all of a sudden drop to 2600 for about a second or so, maybe 2, then will go back to 3000. This will continue at different speeds, but the RPM drop is almost always 400.

Don't think the transmission is slipping.

I bought the car from the original owner, who, when asked about this, said that he brought it to the dealer who cleaned his fuel injectors. That was about 18 months ago.

Second question: What should my temp guage be reading? At normal operation, holds a constant mark just to the right of the 200 degree mark.

Help! and thanks.
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Old 10-02-2003, 07:01 PM   #2
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"Symptoms: Doesn't always happen, but when it does, car is warmed up. While driving at speeds usually above 50 MPH, the RPMs will fluctuate. Say the car is sitting on 3000 RPMs, will all of a sudden drop to 2600 for about a second or so, maybe 2, then will go back to 3000. This will continue at different speeds, but the RPM drop is almost always 400. "

This is obviously not in overdrive or the RPM would be about 1800 with the Torque converter clutch engaged. When the torque converter clutch (TCC) engages, it makes a direct coupling to the transmission from the engine (mechanical) as opposed to a fluid coupling (Torique Coverter). This coupling eliminates the slippage that occurs with the fluid coupling. The rpm drop is about 400 rpm. Right on the money. To me, I would say that you are, at that speed and gear right around the point that the TCC is engaging. If you are just meeting and breaking the permissives of the TCC it may engage and then disengage. I would suggest shifting into Over Drive and seeing if it does the same thing. Really at those speeds you should be in overdrive anyway, no need to be running your engine at 3000 rpm. To test if your TCC has been engaged, while on the highway at about 50-60 mph cruising (not passing or hills) tap on the brake. The engine RPMs should suddenly drop to about 1000 (TCC has disengaged the engine and transmission). After a second or two the engine rpms will then go back up to about 1800 (TCC re-engaging)

Hope this shed some light on it...

Jason McKay
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Old 10-02-2003, 09:04 PM   #3
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i agree about the TCC locking up.

also sounds like you have a cooling problem, ive never seen my dads go past 200 mark and mine only goes to the 200 (rarely) mark for a few minutes at a time. I would suspect a thermostat, maybe a stuck one or one not opening up all the way. change it even if it will end up being for general principals. Your fans should turn on at about that mark as well, are they?
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Old 10-02-2003, 09:37 PM   #4
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First thing I'd look at is the TPS. If it has a dead spot, it won't let the TCC lock up. It'll also hunt for the right throttle setting. Will your cruise control stay engaged while this is happening?

As far as the cooling is concerned, install a 180 thermostat and an aftermarket trans cooler. That'* the best heat insurance you can buy, especially with the failure-prone plastic Series II upper intake.
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Old 10-03-2003, 11:31 AM   #5
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You guys are great. Thanks for the responses. My example of the RPM drop was purely hypothetical and for illustration puposes only. Let me say this, the only time I've noticed the car acting this way is when it is in overdrive. So maybe my example of the RPM reduction from 3000-2600 kind of muddied the waters for you. I say again, have only noticed it when the car is driven in OD. Hope that helps clarify the problem. Nonethelss, think you've pinpointed it to the transmission. So all that said, if it is, in fact, a problem w/the TCC engaging, is that an easy thing to fix or a difficult thing?
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Old 10-03-2003, 12:35 PM   #6
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Quote:
Originally Posted by 74747474
You guys are great. Thanks for the responses. My example of the RPM drop was purely hypothetical and for illustration puposes only. Let me say this, the only time I've noticed the car acting this way is when it is in overdrive. So maybe my example of the RPM reduction from 3000-2600 kind of muddied the waters for you. I say again, have only noticed it when the car is driven in OD. Hope that helps clarify the problem. Nonethelss, think you've pinpointed it to the transmission. So all that said, if it is, in fact, a problem w/the TCC engaging, is that an easy thing to fix or a difficult thing?
That is normal then, although going in and out concerns me. Of course, double-check your tranny fluid level in park with the engine idling. Then do what Will suggested and eliminate or confirm that possibility.
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Old 10-03-2003, 06:41 PM   #7
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Okay, now that I know that it is only in OD that it is happening, I am actually experiencing a similar problem. I have a post in already but haven't figured it out yet. Mine will just drop out of OD into 4th and the TCC will not lock back up. It does not go back into OD or TCC lockup. It is a pain driving at 3000 rpm with no TCC. Car burns enough gas already. The TPS (Throttle Position Sensor), which I have been getting codes in definitely affects the TCC. The TPS basically will tell the TCC to disengage during heavy throttle (passing) to allow the engine rpms to get into the higher horsepower ranges. If, like mentioned, there is a bad spot on the TPS or intermittent faults, this will allow the TCC to disengage and re-engage. Check for any codes.
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Old 10-03-2003, 07:27 PM   #8
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Quote:
Originally Posted by A Bonnie Canuck
Okay, now that I know that it is only in OD that it is happening, I am actually experiencing a similar problem. I have a post in already but haven't figured it out yet. Mine will just drop out of OD into 4th and the TCC will not lock back up. It does not go back into OD or TCC lockup. It is a pain driving at 3000 rpm with no TCC. Car burns enough gas already. The TPS (Throttle Position Sensor), which I have been getting codes in definitely affects the TCC. The TPS basically will tell the TCC to disengage during heavy throttle (passing) to allow the engine rpms to get into the higher horsepower ranges. If, like mentioned, there is a bad spot on the TPS or intermittent faults, this will allow the TCC to disengage and re-engage. Check for any codes.
OD is 4th. If you notice the RPM change here, the TCC is operating. So, it'* dropping into 3rd?? 3rd at 55-60 would yield 3000 rpm. Based on that, what you say makes sense. Never mind me. Damn Jack playin' with me...
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Old 10-03-2003, 07:56 PM   #9
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maybe the igniton control module is bad when ever troubleshooting always keep in mind that in order for an engine 2 run you need fuel,igniton,air thats the basics
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Old 10-06-2003, 11:17 AM   #10
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It doesn't shift from OD to 3rd. The only change is in the RPMs. Didn't drive car this weekend, so wasn't able to check like will asked. But probably going to trade it off this week on a Pontiac Vibe. Wow, great car!!!
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