RPM surge when slowing down

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jatwow20

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I have a 2005 ls sport, and my car surges forward when coming to a stop... usually around 10 mph. I have a blue driver odb2 tool, and looked at the live data. Below is the data, you can see the rpm jump without the throttle opening up. The car does not have a iacv and the fuel trim is usually negative

Rpm/mph/throttle position

809 17 13.3
789 17 13.3
744 14 13.3
705 13 13.7
665 11 13.7
681 10 13.7
710 8 13.7
868 6 13.7
817 5 13.5
672 3 13.3
644 2 12.9
598 1 12.9
599 0 12.9
600 0 12.9
601 0 12.9
602 0 12.9
603 0 12.9
603 0 13.1



Any ideas?

Thanks

Screenshot_20210331-165509.jpg


Screenshot_20210331-165509.jpg
 
Normal. It's the transmission down shifting.
 
Not entirely sure about that. The car actually gains "power" and I need to actually press a bit harder on the brakes to account for the extra force. I would expect somewhat of an engine braking responses, and a decrease of "power" not the opposite.
 
Both of mine did the same thing under the right circumstances. It corresponded to a transmission shift. (I had both from new to 225K + miles.)
My current car does slow down noticeably when it shifts while coasting below 20 MPH. It's taking me some time to get used to that.
 
If your trans isn't shifting 100 percent correctly like mine you'll get a little kick on the downshift to 1st when stopping during the warm up phase.
 
Hey guys thanks for the feedback so far. Still don't think it's the transmission. My first few post were from the mobile and I was a bit lazy. Let me try explaining again.

Problem: When slowing down I noticed the car would surge forward. This means the cars wheels would actually be putting down more power during that surge (rotating forward with force, not just coasting). This means I have to press on the brakes harder just for a second, until the rpm dropped back down (~1 second before I return to normal braking pressure). I noticed I would hear the engine rev during that surge of power, thus I started to look at the logs I posted.

What I checked: I originally assumed the throttle would open up as part of the IACV system (not an valve, just electronically controlled throttle to control the air flow), but the logs did not catch the throttle opening up, but did notice the rpm jump like I heard and saw on the dash.

Why it's not the transmission: I am familiar with the transmission downshifting and having a normal engine braking force. In that scenario the rpm can jump, but the car would actually lose speed, my issues is the opposite (car wants to gain speed).

Today I was able to test the transmission to make sure it was not late or hard shifts. I was manually shifting as my speed decreased (lets ignore the engine stress/rev matching concerns), and was able to put it into first gear (coasted for a bit and watched the rpm drop steady, so I know it was actually engaged in first gear) and braked normally, but the surge still occurred. This means the transmission was not causing the rpm jump.



Is it possible there is a vacuum leak, and o2 sensors detect a lean condition and throw more fuel at it which causes a bit more power and rpm jump? I also notice the fuel trim is usually negative. Any thoughts?
 
How many miles on your LS? Has the transmission ever been serviced, (to the best of your knowledge). Have the drive bands in the transmission ever been adjusted, (again... to the best of your knowledge).
 
To the OP: Is your car a V8? I have a V6 which does that all of the time, but it is a manual, so I can override the extra torque when stopping, but cruising around, it tries to speed up in low speed driving, like 30-45 mph. Once the car stops, the revs retreat to idle speed. If you have a V8, then disregard.
 
It's a 2005 ls sport v8 with <80k miles. I haven't serviced the transmission since I've owned the car (going to be 4 years in a few months), and I believe maintenance was minimal with the previous owner... As in there weren't any issues which needed to be fixed (not that is was neglected).

I don't know much about the delayed torque converter issue, but since I was able to hold it in first gear and the rpms dropped (steadily with me applying brakes lightly). Wouldn't the torque converter be in the same rpm range? Would that issue occur in that scenario?

It really feels like someone pressed the gas ~50% for half a second and then let off. The rpms jump and power linearly follows, meaning sometimes the rmp revs are higher, so the power is more (to the wheels), other times it's very little and not noticable. It's not like a sudden jerk forward.
 
milehighmikey said:
To the OP: Is your car a V8? I have a V6 which does that all of the time, but it is a manual, so I can override the extra torque when stopping, but cruising around, it tries to speed up in low speed driving, like 30-45 mph. Once the car stops, the revs retreat to idle speed. If you have a V8, then disregard.
Click to expand...

I have the same issue with my 5 speed. I have learned to live with it. Putting in the clutch will reduce the RPMs to normal for me. Or as you have written, coming to a stop will drop the RPM's. I would love to find the cause of this.
 
I have cleaned the MAF, and hit the throttle body with a bit of cleaner to see if that would help... can't say it did anything.

I will try to clean the EGR valve this weekend. I'm thinking it might be stuck and messing around with the air/fuel causing the sputter when decelerating (about to come to a stop/idle RPM). If that doesn't work I'm thinking of checking the TPS, which is what might be causing the issues with milehighmikey, and craigh? It happens to you guys while driving regularly?
 
With our V6s, it's the computer doing the high idle BS. As soon as the car fully stops, the rpms drop from 2k to idle in a second. Good thing I don't use a loud exhaust -- that would sound like ass. Sort of like a fart can equipped 4 banger with an automatic transmission, but different.
 
milehighmikey said:
With our V6s, it's the computer doing the high idle BS. As soon as the car fully stops, the rpms drop from 2k to idle in a second. Good thing I don't use a loud exhaust -- that would sound like ass. Sort of like a fart can equipped 4 banger with an automatic transmission, but different.
Click to expand...

I agree with you on the cause, something in the computer. I have cleaned the MAF, changed the TPS, cleaned the intake, checked for air leaks and also had the ECM reflashed. Nothing has made a difference on this. I am really puzzled as to the cause, but it must be one of the inputs to the ECM.

Mike, have you also had the sudden stutter in light snow or dust? This happens to me a couple times a year and it appears to be an issue only with the 5 speeds.
 
craigh said:
I agree with you on the cause, something in the computer. I have cleaned the MAF, changed the TPS, cleaned the intake, checked for air leaks and also had the ECM reflashed. Nothing has made a difference on this. I am really puzzled as to the cause, but it must be one of the inputs to the ECM.

Mike, have you also had the sudden stutter in light snow or dust? This happens to me a couple times a year and it appears to be an issue only with the 5 speeds.
Click to expand...

No Craig, I have not.
 
jatwow20 said:
It's a 2005 ls sport v8 with <80k miles. I haven't serviced the transmission since I've owned the car (going to be 4 years in a few months), and I believe maintenance was minimal with the previous owner...
Click to expand...

With the age and milage... I'm going to assume that your LS has seen mostly short trip... in town driving. That would mean that the shifter has seen a lot of use over the years.

If that is the case... I would suggest a shifter cable adjustment... and replacing the Transmission Range Sensor.

Most people call this the neutral safety switch, or reverse light switch... but it does much more since it allows the PCM and transmission to communicate with each other.

An out of adjustment shifter cable and/or worn transmission range sensor could possibly be causing your symptoms.

If I am wrong... you are only out about $40, and an hour of your time... if you do it yourself.

Jack up the car... and solidly place it on 4 jack stands !!!

The cable adjustment and sensor replacement should be done in neutral. Once in neutral... disconnect the shifter cable from the shifter arm on the transmission.

Then replace the range sensor. Take note that there are specific "line up" marks on the sensor, and that the new sensor is lined up from the factory... already in the neutral position. You could also use a paint marker and put your own "line up" marks on it... if you want to.

Once the new sensor is installed, (still in neutral)... observe the location of the end of the shifter cable... and see if it lines up exactly with the ball on the shifter arm.

If not... trace/follow the shifter cable back towards the shifter. You will see a little bracket with a white part that has "saw teeth" on it. This is where you adjust the shiffter cable. There is a little lock type piece holding both halves of the adjuster together. Removing the lock will allow you to slide the cable back and forth for adjustment.

Adjust the cable... so that the end of the cable lines up exactly with the ball on the shifter arm, and snap the cable on the ball.

Then lock the teeth at the adjustment spot... back together. Make sure you connect the electrical harness to the range sensor before lowering the car.

Then take it for a test drive and see how it functions.
 
milehighmikey said:
No Craig, I have not.
Click to expand...

You will know it when it happens. More than a stutter and more like a thud. I have had it happen when driving highway speeds with light snow, but also heard that it happens in dusty areas. don;t even know where to start to diagnose it and it has not happened to me in about a year or so. This is my second 5 speed and it happened with my first one also after the mileage got over about 140k.
 
jatwow20 said:
I don't know much about the delayed torque converter issue, but since I was able to hold it in first gear and the rpms dropped (steadily with me applying brakes lightly). Wouldn't the torque converter be in the same rpm range? Would that issue occur in that scenario?
Click to expand...

Well since you are most likely still in 5th gear (automatic transmission) when coasting to a stop... if the TCC is still engaged, (more or less in direct drive), the momentum of the car will power through the drivetrain until the TCC disengages.

Then when the TCC disengages... the engine RPM will will come up some... because you are no longer in direct drive, (no torque converter lockup).
 
My LS, (with the 5R55S transmission), has never done this. However... under the right conditions... my Grand Marquis, (with the 4R70W transmission), would.
 
jatwow20 said:
It's a 2005 ls sport v8 with <80k miles. I haven't serviced the transmission since I've owned the car (going to be 4 years in a few months), and I believe maintenance was minimal with the previous owner... As in there weren't any issues which needed to be fixed (not that is was neglected).....
Click to expand...

My 2005 LS Ultimate had TSB 09-12-12 performed on the trans at 43,600 miles (2011), but, this had to do more with hard shifting issues or loss of gear. I tried to upload the .pdf, but it's too large. You can Google it.
 
I've struggled with harsh shifts since I got my LS. I've changed the fluid, had the ecm serviced, replaced the MAFF. Each thing has improved the shifting for a short time then back to harsh again. The latest thing I've done was address a squealing belt. I noticed that after I sprayed the belt with silicone spray to get it to quiet down that the shifting was better. It didn't last though. I changed the idler (it felt fine but heavy) and belt and still had the squeal. The tensioner pulley seemed fine and the tension did too so I didn't change that. Still problems with squealing and shifting. After I started getting a burning smell I changed the tensioner and the belt again. It is now running great and shifting normally. I am still waiting for it to start acting up again since it seems too good to be true but it is amazing how good this car can run and feel when it is running right.
 
jerryg2112 said:
Still problems with squealing and shifting. After I started getting a burning smell I changed the tensioner and the belt again. It is now running great and shifting normally. I am still waiting for it to start acting up again since it seems too good to be true but it is amazing how good this car can run and feel when it is running right.
Click to expand...

If the tensioner pulley was tying/locking up... that means there was more load on the engine.

More load on the engine means more load on the ignition coils.

More load on the ignition coils means more misfires.

More misfires mean more EMI/RFI feedback to the PCM.

More EMI/RFI feedback to the PCM means more transmission shifting issues.
 
04_Sport_LS said:
If the tensioner pulley was tying/locking up... that means there was more load on the engine.

More load on the engine means more load on the ignition coils.

More load on the ignition coils means more misfires.

More misfires mean more EMI/RFI feedback to the PCM.

More EMI/RFI feedback to the PCM means more transmission shifting issues.
Click to expand...
That's what I was thinking only not in that detail. It is running better but still I have a bad smell like something is burning. I changed all of the idlers and the tensioner and the other components all spin freely and easily. It is a mystery to me. I think I may change the fluid in the power steering pump. When I first got the car it had steering whine and I added the friction modifier. Otherwise I am just going to live with it until something gives.
 
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