RPM jumps on deceleration

saturn
sl
sensors

#1

While cruising at any given speed, I will come to a stop sign or red light or a situation that causes me to take my foot off the gas. The idle will stay at whatever the cruising idle was for about 2 seconds, then increase a couple hundred RPMs, then come down to the normal RPM. The car actually accelerates a tiny bit with my foot off the gas. Even with my foot on the brake when coming to a stop sign, the idle goes up a tiny bit with my foot on the brake.



List of parts I’ve thrown at this 2002 SL1 with 86k miles for 1 year now:



plugs and wires twice over 2 years

fuel filter

cleaned carbon off egr and throttle body

pcv valve, helped out a lot, but still trouble

thermostat

idle air control valve

throttle position sensor



What is telling the car to get gas when I take my foot off the accelerator? Thanks for any suggestions.


#2

Automatic transmission?


#3

Does this surge seem to happen when the RPMs are falling in the 1500-2000 range?

If so I think it’s the action of the the overrun fuel-cutoff system. This system saves gas and cuts emissions by cutting off the fuel when coasting with the throttle closed. When the RPMs drop back towards idle speed the fuel cuts back on.

Has it always done this? It might be normal operation.


#4

Based on the description I’m wondering if it isn’t simply the torque converter clutch kicking off as it should. I.e. it sounds perfectly normal to me.

When you are at cruising speed in an automatic, the TCC will kick in & lock the engine & transmission together so they spin & the same speed. The converter then no longer multiplies torque. When you hit the brake, hit the accelerator reasonably hard, or go off the accelerator reasonably suddenly it unlocks the converter from the engine. This produces an rpm variation of a few hundred. On many cars it is easy to see it kick on - a small drop in rpm w/out a gear change, and then see it kick off - a small rpm increase.


#5

Auto


#6

Yes, the surge happens in that range. It has not always done this. It began to happen after the air intake manifold gasket was replaced, then got worse over a couple years. A mechanic suggested replacing the pcv valve with one from the dealer. It helped a lot, but gas mileage is the most affected. I used to consistently pull 34-38 mpg. Now, 29 around the city and 33 highway. I always check mileage after I change a part to see if it did anything.


#7

But, I don’t believe a car should accelerate at all with a foot off the gas pedal. This car does and I’m noticing the extra “jump” now over these past couple months. The jump is very slight. It feels as if a shot of gas is going into the engine seconds after the foot comes off the gas. I have a mpg problem and this problem. I wonder if they could be related?


#8

ALSO!! The car used to have trouble when I would shift into Park. For example, stop in the driveway, shift into Park and the idle would go from ~700 to 1500 RPM. The engine would just race. I have hit the gas pedal and sometimes the idle would come back down. Sometimes not. Shutting the car off stops the racing. It hasn’t done this much since the pcv valve was replaced the second time this past spring.


#9

I’m sorry if you don’t believe that a car “should” do this, but this:
"…a situation that causes me to take my foot off the gas. The idle will stay at whatever the cruising idle was for about 2 seconds, then increase a couple hundred RPMs, then come down to the normal RPM"

describes what happens when the TCC kicks off. It is not abnormal, though the 2 seconds initially might be a little long.

However, I’m also not saying that this is all that is going on.


#10

I can drive down residential streets stopping periodically for stop signs. I take the foot off the gas and coast for a brief moment of time. When it is time to press the brake, the car can actually accelerate enough where I can feel it in the chair. The RPM never increased on this car until about 10k miles ago.

You’re saying this could be from a faulty TCC? And what might explain the jump in RPM when going from Drive to Park?


#11

No - I wouldn’t say that based on that description. And the point was that it would be from a normal TCC - not a faulty one.

Is it always associated with pressing the brake? Is it also similarly the case that the shift to park is with the brake on?


#12

It is not always associated with the brake. It just takes a couple seconds after the foot comes off the gas. At slow speeds, less than 25 mph, I can actually feel the car accelerate, which is a weird feeling when the car speeds up without the gas being pressed down.

The increase in RPM used to be really bad, the jump would be very noticeable. I took it to a mechanic, they put their finger over the pcv valve, and told me to go buy one from the dealer. That mostly remedied the problem, but my gas mileage has not returned to the near 35 mpg I got almost 2 years ago.


#13

have you checked the temp sensor? Orignal sensors have a plastic bulb sites in the coolant cracks thus allowing coolant to short the contacts. replacement units have metal bulb.

Easy to replace, no need to drain coolant, just go fast, part is inexspensive.

Had the same problem, replacing sensor cured.


#14

Yes, I’ve replaced the engine coolant temp sensor. The 2002 model didn’t use the plastic bulb design. I pulled out a brass head and replaced it with one from my auto parts store. No change happened.


#15

Re-check the intake manifold for leaks, or any other vacuum leak.


#16

Ok, I can check the intake with carb cleaner again, but where else can I spray to look for vacuum leaks?


#17

Hi guys im having a simlar issue mini cooper 2006 auto transmission

the car used to bog out when i put my foot on the accellerator to the point it feltlike i was in fifth gear so i replace the trans filter and fluid etc. now the car works through every gear in the slap stick/semi auto mode but and moves like stink but when i take my foot off the gar the rpms shoot up to aound 4000rpm and slowly drop back down. now in full auto matic it cant make its mind up which gear it wants to be in doing the same thing between gears but manually the clutches dont slip its just like when i come off the gas it puts itself in neutral and over revs then when i hit the gas it chooses a gear and books like a new car any suggestions its driving me nuts


#18

Great , revive a 9 year old thread about a different vehicle . Doesn’t use sentence structure or punctuation and expects someone to wade through it.


#22

@Nick_James … suggest to focus on the throttle position sensor or a transmission fluid issue.