RPMs bouncing/Engine Pulsing

[Cable810]

This is all happening while driving. The RPMs will started to bounc 100rpms at a time. So like 1500RPM to 1600rpm then back to 1500rpm. From what googleing I have done other people have had this problem too just not on the trailblazers. I did search Trailvoy and nothing helped. What it seems to be from my google fu is a Tranny, and possiably Intake manifold leak or something. It happens mostly around 50-60mph. I'll get some video later today.

 

[The_Roadie]

I'd guess tranny clutch lockup. Do you have access to a scan tool you can drive around with to check for slippage and TCC lockup?

 

[Cable810]

The shop does. He has let me used it to diagnose the misfire. I'll have to try and see if I can Monday... But I do have the Torque Free

 

[The_Roadie]

.. But I do have the Torque Free

Please consider supporting the Dev by throwing him $5 for the full version. Not just you, but everybody.

 

[Cable810]

Just bught it!! I'll try and mess around with it. Deffinitly will get a video up tonight. Thanks Roadie!

 

[Cable810]

You can kinda hear the engine pulsing in the background.

2012 11 17 15 28 17 424 - YouTube

2012 11 17 15 30 36 521 - YouTube

 

[Voymom]

You can kinda hear the engine pulsing in the background.

2012 11 17 15 28 17 424 - YouTube

2012 11 17 15 30 36 521 - YouTube

Do you have the heater or A/C on? I know my needles moves and my engine sounds like it is acting funny when the heater/ac is on.

 

[Cable810]

Nothing is on. I never use the A/C cause it ain't workin and the Heat I just use to get the cab warm then I turn it back off.
Even with the it off I still feel heat blowing down by my feet... So I'm thinking it isn't turning off completely or is this Normal?

 

[Voymom]

Nothing is on. I never use the A/C cause it ain't workin and the Heat I just use to get the cab warm then I turn it back off.
Even with the it off I still feel heat blowing down by my feet... So I'm thinking it isn't turning off completely or is this Normal?

I have heard of a failing/failed ignition switch causing the a/c and heater to not turn off completely. The ignition switch can cause a world of odd happenings apparently.

 

[freddyboy61]

What you feel, if you're not in RECIRC mode, is outside air going thru the system. This is annoying in the winter before the coolant heats up, you get nothing but a cold breeze if you don't remember to switch over to RECIRC.

 

[Voymom]

What you feel, if you're not in RECIRC mode, is outside air going thru the system. This is annoying in the winter before the coolant heats up, you get nothing but a cold breeze if you don't remember to switch over to RECIRC.

I don't have that issue.....I'm the same way, I never use A/C because I rather have the windows down, and I only have the heat on until my heated seats kick in. Once I turn it off I don't have any air blowing on my feet. Not saying this isn't the issue though...but I sure don't have that problem with mine.

 

[Cable810]

The TCC is not sliping what so ever. I explained what is happening the shop seeing if they would have an idea. The guy said bring it by tomorrow around 1 so I did. We drove about 15 miles Rpms were bouncing and engine was pulsing. When we turned around it didn't do anything. THe mechanic was stumped he had it hooked up to his scanner which is a Snap-on he said it cost him $3,500 Dunno what model.

 

[The_Roadie]

... THe mechanic was stumped ...

You did the right thing, and he would know better than anybody on an internet forum. Hard to believe he had NO useful data in memory to look for misfires, inadvertent knock sensor activation, spark timing changes due to whatever reason, fuel trim changes, injector pulse width erratic timing.

You SURE he looked deeply into the memory data during the time of the pulsing? Very very odd.

Maybe he just looked for codes, and didn't look deeply? Or understand how to properly use his $3500 investment?

 

[meerschm]

are you sure the torque converter is not dropping out? this is very similar to how my TB acted a week or so ago, when it was just a bit too too low on fluid.

only happened when locked up, low 50s speed, and on just a bit of a hill (needing some torque to keep the speed constant.) when the TCC ran short of fluid, the rpms increased, which pumped more fluid, which let the clutch lock back up.

no CEL, no codes. mine moved a bit more, but very similar.

unless the speedometer is moving the same fraction the rpm is, something is slipping, or coming out of lock.

 

[Cable810]

Pretty sure he went through all the things on his Tool. He checked for codes and there were none. I asked about the plugs and coilpacks and he said no misfire. I saw that he check the knock sensor. I wasn't able to STARE at the tool cause I was driving. He said that(I'm sure y'all don't care) if it is the TCC the RPMs would be bouncing all the time no matter what.

are you sure the torque converter is not dropping out? this is very similar to how my TB acted a week or so ago, when it was just a bit too too low on fluid.

only happened when locked up, low 50s speed, and on just a bit of a hill (needing some torque to keep the speed constant.) when the TCC ran short of fluid, the rpms increased, which pumped more fluid, which let the clutch lock back up.

no CEL, no codes. mine moved a bit more, but very similar.

unless the speedometer is moving the same fraction the rpm is, something is slipping, or coming out of lock. .

Usually when it does it Mid to low 50mph slight hill, some times on a straightaway too. No CEL and no codes. I know that the Tranny needs a fluid change checked it and it was brown so it probally needs a change. Don't know if that would have any bearing on it at all.

 

[The_Roadie]

I know that the Tranny needs a fluid change checked it and it was brown so it probally needs a change. Don't know if that would have any bearing on it at all.

I can't see how it could FAIL to be relevant. Why not do the known maintenance first, before spending money on a diagnostic that might not be needed after a fluid and filter change?

 

[Wishbone]

Wow! What a strange coincidence. I started experiencing the exact same symptoms Saturday.

It was extremely foggy out so I was driving 50-65 mph on the freeway. I increased to 70 mph and it stopped. I checked my fluid, it's a bit brown, I have 162000 miles, and I don't think the fluid has ever been changed. At least not since I bought it at 57K.

It appears to not be happening without the fog now, so I'm thinking it may be electronic related. These transmissions are all electronically controlled, right?

 

[jrSS]

Yes they are electronically controlled. That's why u can change ur shifts w/ a tune.

 

[Wishbone]

It just feels like it's slipping in and out of overdrive. I'll pay extra attention to it that tonight as I'm driving around...

I have Torque, what metrics should I be monitoring?

 

[meerschm]

if it is real bad you could see a DTC P1870 code and a check engine light.

I had this and got some good advice, http://gmtnation.com/f23/3-4-transmission-wander-5512/ turns out a quart of fluid fixed mine up, at least for the time being

if it is not too bad, or due to low fluid, you might not see the code at all.

since you know it needs fluid and filter changed, you can do that as soon as you can. (probably want to do some other maintenance you put off)

if it is still acting up, you can (cheapest to most expensive)

1) don't drive between 50 and 60
2) drop the pan again and change some valves
3) change out the valve body
4) rebuild the transmission
5) install a used transmission
6) install a rebuilt transmission

Good luck.

 

[jrSS]

Usually people change out the fluid when the tranny is slipping by then usually its too late. DO NOT do a tranny flush w/ that many miles. I wouldn't.

 

[Cable810]

Guess I'll get the fluid changed. I'll report back if the problem is still there after the fluid change!

 

[Wishbone]

I haven't had any more symptoms. I'm interested to hear how things go Caleb. I also want to change my fluid and filter, but it's scary. Good luck...

 

[CaptainXL]

I had my tranny fluid exchanged at a gm dealer. They ran 14 quarts of Dexron 6 through it. The truck shifts much better than with the old dirty Dexron 5? That was in there. I recommend this service. Much better than just pulling pan and replacing 5 quarts.

 

[Cable810]

I had my tranny fluid exchanged at a gm dealer. They ran 14 quarts of Dexron 6 through it. The truck shifts much better than with the old dirty Dexron 5? That was in there. I recommend this service. Much better than just pulling pan and replacing 5 quarts.

How many miles? I'm sure I'm close to 180K-200K the OD is wrong.

 

[Wooluf1952]

Just an FYI: GM went from Dexron III to Dexron VI. No 4 or 5 AFAIK.

 

[Cable810]

Got the fluid in the pan changed. A good amount of dirt was in the pan, and it was a bit low. I'll find out if the issue us still occuring this Friday. Does our TCC have a drain plug?

 

[CaptainXL]

Just an FYI: GM went from Dexron III to Dexron VI. No 4 or 5 AFAIK.

Your correct. I was just being lazy calling it 5.

Got the fluid in the pan changed. A good amount of dirt was in the pan, and it was a bit low. I'll find out if the issue us still occuring this Friday. Does our TCC have a drain plug?

I have 120k on my truck and the fluid was pretty brown. Not dark but definitely not pink.

You mixed 5 new quarts of Dexron 6 with the supposed old Dexron 3 which to me is a waste of money and does not make sense. I would really suggest you get an entire fluid exchange given the mileage you have. Dexron 6 is expensive and at $120 for the exchange I got it makes good sense.

Some people consider the fluid exhange that dealers perform as a "flush" which the term gets a bad rap. Truth of the matter that is not correct. All it is basically is a complete empty of the old Dexron and then refill with Dexron 6. When they pump it through the new fluid chases the old fluid out the cooler lines. Eventually the new fluid emerges out the cooler line and at that point you know all the old fluid is out.

 

[Cable810]

Your correct. I was just being lazy calling it 5.



I have 120k on my truck and the fluid was pretty brown. Not dark but definitely not pink.

You mixed 5 new quarts of Dexron 6 with the supposed old Dexron 3 which to me is a waste of money and does not make sense. I would really suggest you get an entire fluid exchange given the mileage you have. Dexron 6 is expensive and at $120 for the exchange I got it makes good sense.

Some people consider the fluid exhange that dealers perform as a "flush" which the term gets a bad rap. Truth of the matter that is not correct. All it is basically is a complete empty of the old Dexron and then refill with Dexron 6. When they pump it through the new fluid chases the old fluid out the cooler lines. Eventually the new fluid emerges out the cooler line and at that point you know all the old fluid is out.

My biggest thing is would it be ok to do it on a tranny thats probally got close to 180-200k?

 

[CaptainXL]

My biggest thing is would it be ok to do it on a tranny thats probally got close to 180-200k?

It's perfectly ok. You want to keep all that old fluid in the tranny? It's just a fluid exchange.

Keeping the unchanged fluid in the tranny will hurt more than putting in new fluid. Would you change only a quart of engine oil at a time?

Let me tell ya that it's the best thing I have done to my truck. Shifting is like brand new. No harsh shifts like before.

The fluid exchange is actually highly recommended if fluid changes have not been done on a regular basis. You need some type of baseline for fluid cleanliness. Normally changing the fluid every 30,000 from the time the vehicle is new suffices. But your fluid is extremely old and leaving it in there is not adviseable.

In my case I just had my fluid exchanged a few months ago and have driven over 2000 miles with no issues or even minor shifting hickups. It's great. Best $120 I have ever spent. The filter is just a mesh screen that lasts a long time. I will need to change it soon. So I will drop the pan next year in the summer and do that along with cleaning off the magnet and inspecting for metal shavings. But come next year when i do drop the pan the difference will be that all the fluid was already changed out and I don't have to worry about exchanging the fluid then and I didn't waste 5 quarts of Dexron 6 by mixing it with old fluid.

From there on out I just drop the pan every 30K and put new fluid in to keep the tranny happy. Of course there is still going to be some fluid left in the tranny that just keeps getting older and older. That's where this fluid exchange comes into play.

 

[Cable810]

Thanks for clearing that up Makes total sense now!!

 

[Cable810]

Well more diggin and askin I've done turned up that it might be the MAP sensor. I talked with a guy who runs a shop about the issue seeing if he had any idea. He called me the other day and said he had a truck come in with the exact same symptoms and he said he replaced the MAP sensor and it went away. I'll confirm in the spring when the TB will be out of storage.