RPM surging

[DenaliAK]

Ok, new question. Whenever I'm cruising on the highway, right around 50-55, and I lightly accelerate (like matching the speed of the car in front of me), I get this RPM repeating surge from about 1500-2000 RPM. Voom-voom-voom-voom...it's really annoying. It's like it is trying to decide whether to downshift or not and just can't make up it's mind.

Anyone else experience this or know anything about it? I basically can't use CC because of it, and I'm sure it messes with my MPG.

 

[DenaliAK]

Hmmm...this must not be a common thing.

 

[COTahoeLS]

Tuned or stock PCM?

-E

 

[DenaliAK]

Stock.

 

[JKmotorsports]

Unfortunately, it could be a number of things. What year is your truck? Does it have cable throttle body or electronic? It could be any of the sensors for the ETC system (TPS, APP sensor, TAC module, etc). Could be lack of communication between the ETC components. Possibly tc not locking/disengaging properly. Can you get a scanner to see of there are any pending DTCs? Sometimes a DTC will show up in the pcm w/o illuminating the CEL until the pcm sees the problem happen a certain number of times.

 

[DenaliAK]

It's a 2003 Denali, and that's about all I know. I don't know anything about the engine or computer or what half the abbreviations you used mean...lol.

 

[Rivieraracing]

Have you checked your tranny fluid level and/or changed the fluid recently? Might be low?

 

[DenaliAK]

Negative. I've only owned it for a couple of weeks. I'll check it today.

 

[Rivieraracing]

Get it plenty warmed up and check it while it is running at idle, also pay attention to the color and the smell of it and report back!

 

[DenaliAK]

Alright, so I checked the tranny fluid. I ran through the manual's instructions on how to do it...hot check, run through the gears first, leave engine running....and my dipstick was almost dry. So, I'm guessing I need more fluid badly. What little fluid showed up looked a lot like clean oil, kind of amber, maybe a little darker. No particular smell, other then hot engine.

So now my biggest problem is figuring out how to add fluid. The manual is really unhelpful...it tells you this whole procedure on how to check it, then tells you to add some if it's low, but doesn't say where to add it. Awesome. I didn't see any obvious reservoirs for it, so where the heck to I add transmission fluid?

 

[Modded]

I'm not sure about your year truck but maybe you could pour it from the dipstick holder.

 

[Rivieraracing]

Yeah pour the tranny fluid in the dipstick hole, get yourself a funnel or if it is high enough just pour it in out of the bottle, just do little bits at a time and check the level often, you don't wanna put too much in there! When you think you have enough in there, drive it around a little and check again!!

Low fluid will cause the problems you stated, once the fluid is filled up check for leaks!

 

[DenaliAK]

Thanks for the help, Riviera...much appreciated.

I'll run down to the store and get some transmission fluid and fill her up. Will see if that corrects the surging problem. I hope it does, but then like you mentioned, I have to figure out why it's losing fluid.

 

[DenaliAK]

The problem seems to have been the tranny fluid level. Good call, Riviera and others. Thanks for the help, and for the simple fix. I'll keep an eye on it to make sure I'm not losing fluid badly somewhere.

 

[Rivieraracing]

Glad I could help, that has been the cause of a few tranny problems I've encountered in my short life!! How low was it?

A buddy of mine bought a car for $150 from a lady because she said the tranny was out on it, the first thing we both thought of was "low tranny fluid", we towed it out of the place, went up the street and put in 2 1/2 quarts in it (it was very very low and that was all we brought with us), put it in drive and DROVE it all the way to his house!! 2 days later he posted it up for sale and sold it for $3750!!!

It's easy to forget but always check your tranny fluid when the tranny is acting up!!

 

[DenaliAK]

Glad I could help, that has been the cause of a few tranny problems I've encountered in my short life!! How low was it?

A buddy of mine bought a car for $150 from a lady because she said the tranny was out on it, the first thing we both thought of was "low tranny fluid", we towed it out of the place, went up the street and put in 2 1/2 quarts in it (it was very very low and that was all we brought with us), put it in drive and DROVE it all the way to his house!! 2 days later he posted it up for sale and sold it for $3750!!!

It's easy to forget but always check your tranny fluid when the tranny is acting up!!

lol

That's quite the profit margin.

I'm not sure yet how low it was/is. I haven't driven a huge amount, and it says to check it hot, so I've checked twice. To avoid over-filling, I've been pretty conservative. So far I've added twice for a total of about 3/4 quart. I'll probably check it again when my wife comes home with the truck. I'll keep adding until the dipstick says not to. I've got a 600+mile road-trip to do next week, so I want all the fluids topped off.

 

[Nxtnlne]

lol

That's quite the profit margin.

I'm not sure yet how low it was/is. I haven't driven a huge amount, and it says to check it hot, so I've checked twice. To avoid over-filling, I've been pretty conservative. So far I've added twice for a total of about 3/4 quart. I'll probably check it again when my wife comes home with the truck. I'll keep adding until the dipstick says not to. I've got a 600+mile road-trip to do next week, so I want all the fluids topped off.

What your were experiencing is called RPM flare..My VortecMax was doing this also. Long Story short I was having trouble with the dealer always saying "Nothing is wrong with your truck." It was still under warranty and I knew something was wrong. I requested they gave me a TAP cell print out on what the line pressure was doing relevant to throttle position. Trans Adaptive Pressure. If you are light on the throttle shifting form 1st-2nd the trans line pressure should be low as well. Well mine was not I had way too much line pressure at 1/8 throttle causing that surge. It was happening through all gears intermittently.
After I requested that they called me back confirming there WAS a mechanical issue. Hah it only took 4 dealer visits? They pulled it apart and found debris in the valve body and burnt clutches...Tranny was rebuilt at 27,000 miles.
One more thought, if you get your trans to the appropriate level and find that it keeps going low then you might have an internal leak.

 

[DenaliAK]

I've now added a full quart and it's still showing low, although the RPM flare or whatever you want to call it has gone away. How much tranny fluid do these things hold? I'm at work so I can't consult my manual, but I have no idea at all how much I might need to buy to top it off.

 

[Rivieraracing]

I think it is around 5 -6 qts, more towards 6 I believe! I think with these trucks if it isn't measuring on the stick it is more than a qt low. Just put some in, drive it around for a couple of minutes, pull over letting it idle for a minute, check it again while it is idling (I always do 2 checks at a time just to make sure wiping it off everytime), and add more. When you think you are at the right level, drive it around a little longer, give it some gas and let the motor breath a little and then check it all out again!!

 

[DenaliAK]

Going on a pretty major road trip tomorrow, so I'll probably buy a couple more quarts just to be safe.

Any suggestions on oil? I'm full now, but it's about due for an oil change and I'd like suggestions on what to use. I was thinking Mobil 1 synthetic, but it's got over 100K miles so maybe a high-mileage oil?