View Full Version : Darkman got one for you!! throttle body is oscillating

11-17-11, 10:23 PM
Got a 2004 CTS-V that I bought off a friend of mine. It had 127K and a miss in #7. Turns out my friend had run a lifter thru #7. Long story short I bought a new GM short block to get it back going and the throttle body is oscillating when you turn the key on or anytime through out the range when accelerator pedal is held in one position for any length of time. So as long as you are moving the pedal its fine but if you stop at any point no matter at 15% or 60% the TB starts oscillating. We have been working on this for a month. We have checked all the wiring, changed the TB, TPS, MAP, APP, TAC Module, some of the plug and it is kick our butts, We have went through any diagnostics step that the service manual told us to do and I can't figure it out. When the engine is cold you get several codes which seem to be sporadic from day to day and most of the time it puts you in REP mode. Once the car warms up you can clear the codes and you will not get anymore codes as long as the engine is warm. But the throttle body start isolating and you can see the voltage and percentage change on the tech2. Any thoughts.

11-18-11, 07:28 AM
The symptoms sound more like a vacuum leak than anything else. What are the code numbers that it throws when cold?

11-18-11, 11:43 PM
It changes from day to day on the cold start but here are some we have gotten

11-20-11, 10:56 AM
I have reviewed (and attached) the information related to those error codes. The common denominator among those codes is the throttle position circuit which the PCM uses for throttle actuation. If have not done so already you might check all the engine-harness related grounds since voltage references, like those used in the throttle position circuit, are interdependent and recheck all the connectors in the throttle position circuit. I could not tell how you are running tests, but running the troubleshooting steps in the attachments would best be done by someone with a GM Tech II scanner. If it were mine, I would have to take it to a dealer for that.

12-10-11, 11:58 PM
Ok Darkman me and my tuner are at wits end. We have got all the codes to clear or we do not get any codes period but the throttle body osculates. All the data on the TECH II is within range and agree as they are suppose to. The car will crank and run but once you start giving it pedal or trying to hold the pedal at a constant RPM the TB starts moving all over the place and when you drive it and give it pedal at above 40% it feel likes it is cutting out but its the TB osculating you can see it on the TECH II. We checked for a vacuum like you suggested and did not find one, also the TB will do it with out the engine running so that should rule out a vacuum problem, When you turn the key on the TB does it click and it fine, but if you depress the pedal with the engine not running and the key on the TB starts moving all over the place, it sound like you are typing Morris code. The volts on the TB motor both 1 & 2 move from 12.60 to 12.40 when you depress the pedal 100%. We have changed the TAC module, APP sensor twice, TP Sensor tried 4, Throttle Body times 3, New TAC module plug, New APP plug, new wiring between the TAC and APP, New Throttle Body plug,. My turner has a Vette and we can verify that the parts that inter change work on his car and still we get the same osculation on mine. We completely remove all the tape from the harness and check all the wiring, every thing ohms ok. Check the ground from the TAC, even cut the pink wire for power from the ignition to the TAC and wired it straight to battery terminal to check and see if we very losing voltage somehow, nothing we have done will make it stop. We have move the wires on all the connectors while watching the DMM, nothing and change some of the plugs. Bob can take control of the circuit with the TECH II and increase the RPMS and it runs smooth with no osculating. That why we felt like it was in the APP sensors even though the all read ok on the TECH II so I bought a 2nd one but we still got the same thing. Bob thinks that is has got to be the TAC Module and based on everything I think he may be right but we have already bought a new and its on the car the other one I have already sent back for the credit. I guess its possible that we got a bad one. Bob had over 400 tunes under his belt and this one is kicking our tails. He cant tune it if the TB is acting a fool.

Before the common code was P0220 and P2125. By the way The GM Service manual has the drawing wrong for the APP plug. It took some work but we have it wired like it should be as all the sensors are reading correctly on the TECH II.
I have a drawing that I can email you but I can't get it to attach here.

You got any thoughts on what we may have missed

12-11-11, 10:06 AM
I doubt that I am going to of much help since you have tried essentially everything I think of to do. However, in reading through your process I have some questions.

You indicate that you have replaced the throttle position sensor, if I am reading correctly, 4 times. The Service Manual P0220 trouble shooting chart indicates that throttle position sensor is not serviceable and should only be replaced with the throttle body assembly.

You write indicates that you are interchanging parts and working with a Tech II scanner. However, the pdfs on the various error codes provide a number of tests involving voltage references and in somecases tests with components disconnected or jumper wires – these tests are designed to isolate certain parts. I cannot tell from your post whether your components are passing or failing any of these tests.

For example: The APP is actually 3 individualaccelerator pedal position sensors within 1 housing. Three separate signal, lowreference, and 5-volt reference circuits are used in order to connectthe APP and the TAC module. The APP sensor 1 voltage should increase at the same time that the accelerator pedal is depressed, from below 1 volt at 0percent pedal travel to above 2 volts at 100 percent pedaltravel. APP sensor 2 voltage should decrease from above 4 volts at 0 pedal travel to below 2.9 volts at 100 percent pedal travel. APP sensor 3 voltage should decrease from above 3.8 volts at 0 pedal travel to below 3.1 volts at 100 percent pedal travel. Does your APP pass this test?
Make sure you throttle pedal assembly is working freely and not binding in any way.

Finally, although it is not mentioned in the literature – you may be dealing with a bad PCM, which works with the TAC to operate the throttle openings.

I have attached P2125 for others reading this thread.

12-11-11, 02:48 PM

The TPS is held on the Throttle body with 2 screws and a changeable part. I bought a new one before I took it to the tuner and my tuner had two spares and he also had one on his vette, which we know works we tried all 4.

We have a set of GM 2004 CTS-V service manuals and we have gone step by step checking everything they list. All components are within the ranges listed in the service manual.

I had changed the "APP sensors" actually the complete pedal assembly before I took it to the tuner the one in the car was bad. So based on the codes and the service manual we changed the TAC. Once we changed the TAC and because of him being able to take control of the APP circuit and it run smooth with the TECH II he felt that maybe the APP the I bought was bad so I got another one, we can interchange both APP units with the same problem oscillating. The APP looks good on the TECH II everything is in range volts wise and percent wise and sensor 1 and 2 agree and sensor 2 and 3 agree. We have tired it under the hood holding the pedal in our hand with the same result.

I had asked him about the PCM and he thought we should get some indication that it was bad from codes, I think I will try that next. We are at the point where we get no codes therefore we don't know what to try now.

I assume the the voltage on the TP Motor 1 and 2 should be 12 volts. There is nothing in the manual that tells anything about how they should work or what the voltage should be. I would have thought that the voltage would change based on what it was being told to open or close but it only drops about .2 volts from not depressed to fully depressed. His vette did the same thing.
The voltage on the TP sensor 1 and TP sensor 2 is what is moving around but we have know way of knowing if it is told too move are not it is constant when he takes the circuit over with the TECH II. Do you know a test on haw we can tell.

i will try to find a PCM tomorrow

thanks again.

12-11-11, 02:57 PM
The PCM comment is just a guess based on it being the only component involved in throttle opening/closing process that you have not changed out. I do not diagree that it is not particuarly likely, but then again you seem to have eliminated all the likely issues. Good luck.

12-14-11, 08:13 PM
We tried the ECM today. Still the same TB doing Morris code.

Today we tried several combination on the wires on the APP got codes like crazy, the way we got it wired is the only way it will not throw codes and it looks perfect on the techII. I talked to the guy from AUTOECM which is where I got the TAC from and he is sending me another one to replace that one, I went thru the whole story of what we had tried and he was really nice and he sent me one out today at no charge, I just have to send the other one back. Will try that again when the new TAC gets here if that don't work not sure what to do next.

The only connector we have not changed is the one to the TB Motor but we took it apart and cleaned it and the pins look good. I could not find anything in the service manual about how the TB Motor circuit is suppose to work. I assume that it should be 12 volts to both 1 & 2 with key in run. But I would have thought that when it is told to move that one side or the other the volts would increase and the other would decease but that not what we are seeing, they both are almost the same just .02 volt different. Can you shed some light on that circuit or tell me where to look in the service manual it only about 9000 pages.

Thanks so much for your help.

12-14-11, 08:47 PM
I have not found any specific reference in the Service Manual. Attached is the wiring diagram, which you probably already have. I also a generic discussion of drive-by-wire setups that suggest that the throttle body servo motor "sees" 12 volts.

12-15-11, 12:09 AM
Thanks and yes its in the manual but other may need it. As noted before the drawing and pin outs for the APP plug are incorrect in my manual as was apparent by the missing pin in C or H. What is interesting in the drawing is that the numbering on the 5 volt Reference is out in sequence with the rest of the numbering.

BROWN 1271 Low Reference Signal 1
PURPLE 1272 Low Reference Signal 2
GRAY 1273 Low Reference Signal 3

TAN 1276 5-Volt Reference Signal 1
YELLOW/BLACK 1274 5-Volt Reference Signal 2
LIGHT BLUE 1275 5-Volt Reference Signal 3

DARK BLUE 1161 APP Sensor 1 Signal
LIGHT BLUE 1162 APP Sensor 2 Signal
DARK GREEN 1163 APP Sensor 3 Signal

It also noted the APP Sensor 1 shares a 5 Volt reference with TP Sensor 1
and APP Sensor 2 shares a 5 Volt reference with TP Sensor 2

That is why we tried swapping those wires today to see if the problem went away but if you swap them you start throwing codes.

Today on the TAC pins with the connector unpluged both TP 1 & TP 2 showed 5.37 volt as well as the pin on the other side of the TAC that goes to the pedal with the connector unpluged APP1 and APP both showed 5.37 volts with a DDM but when you plug things up and read volts you get 4.98 with the meter and the TECH II showed 5.00 volts.

What would be of great help is if someone had a schematic of the pedal itself, then you would know that you had things wired to the right pin.

12-21-11, 07:51 PM
It was the TAC Module, I guess even the mighty GM puts out a bad part now and again. It just took me several weeks to buy in that they did, when you have change a component you assume that it is good and move on to something else this time it bit me.

12-21-11, 09:48 PM
Well congratulations on getting it fixed and ending the mystery.:thumbsup: