Engine rpm increases @ highway speeds

[js216]

I have a 1999 Bravada, 4.3 liter.

I have had all the common problems with this vehicle during my ownership and I have fixed everything. Reaction sun shell failure, remote oil line leaks, intake manifold/dexcool leaks, waterpump, etc. It really is a love/hate relationship and this vehicle has always been maintained meticulously. Now I have a new one: At highway speeds my rpms suddenly increase from about 2250/2500 to 3000/3250. Feels like the transmission slipping or shift into another gear. I just had the transmission totally redone a year ago when the sunshell exploded, so I had it re-checked under warranty and it is not a transmission problem. The shop stated these idle speed issue are "common" with these type of SUV's , but offered no real solution. They stated it was a "fuel control/idle speed sensor" isue. Anyone else have this happen. It normally happens only between 65 and 75 mph (normal highway speeds). I tap the accelerator and it feels as though it shifts into overdrive. It does not happen at any other speeds (35-65 or above 80 mph). Any thoughts? I really hate going to the dealer for a diagnostic. My backside typically begins to ache when I think about it.

[MT-2500]

Did they put it on a engine/transmission capable scanner?
If so that will pinpoint the problem.
There statement on fuel control idle speed sensor is strange.
Get it to a good repair shop and have them check it out for sure?
Got any check engine lights?
Is the engine running good?
Has anybody checked the proper fluid level when hot?

[muff34]

hmm interesting ...........lol . My `00 silvy with the 4.3 has similar rpm issues at highway speed . running down the road it will start to moan up and down, about 150 rpm difference ,no shifting/slipping
I call the old girl Moana now because of it....lol. Not trying to jack your thread ,just thought I`d throw this out there as it is also a 4.3

[js216]

To answer MT2500's questions:

Yes the transmission was checked. The engine was not fully looked at, but they did look for any codes with a scan tool. They did pinpoint another "common" problem (listed below) - I just explain there diagonosis incorrectly. They are a transmission shop.

No it was not the fuel control/idle speed sensor as I originally stated. They stated it was more than likely the crankshaft positioning sensor.

No this shop is not my GM dealer. My transmission shop actually knows how to fix transmissions and apply a valid warranty. The are top notch and the problem is not the transmission, fluid, or any other oddity with their repair. That was my original thought and I had them check everything. My dealer wanted to replace my transmission with a Serta unit and that was not an acceptable offer. My dealer is very competent, but when it came to this repair they follow the corporate guideline to replace vs. repair. On another note the transmission failure was a "latent manufacturing defect" from GM that actually can affect any 4L60E transmission. The covered the repair costs after som legal wrangling on my part (say GM secret warranty).

No CEL

Sometimes I forget that most people believe, in general, that most individuals who post problems are complete idiots. I for one am not. I have an FAA certified airframe and powerplant license as a maintenance technician (aircraft) and I am very competent around an automobile. I normally perform all of my maintenance or can trouble-shoot most issues before I even take my vehicle to a dealer. I do agree in original parts and I agree the dealer know more about my GM product than I do, so they perform service on my car from time to time. My shop/garage has most of the equipment that Dealers have anyway. This issue is something that I could not diagnose.

However I did find a possible solution. Various TSB's describe this issue as "Chuggle" which can be caused by a transmission issue (which was already checked), or the positioning sensor, or an extremely filthy throttle body (already cleaned). Ruling out any "bad gas" everything but the CP sensor has been checked. The dealer does want to check a few other things (fuel pressure (pump) and they still would like to retest the torque converter clutch solenoid, but I should get an answer this week on the likely culprit.

[MT-2500]

Good Luck and let us know how it goes.
Yes there is several different engine running problems that can make a transmissiuon act up.
Also sometime there is computer reflashes to cure a few problems.
For more details and all tsb's and factory manual repair info and all know problems the All Data online one year sub is great on things like that.
Well worth the 24.95$ us plastic for a years sub.
One repair can save you a lot more than that.
http://www.alldata.com/products/diy/index.html