Engine Starting & Unstable Engine RPM Issue
As I see comments in this string that cleaning the throttle body has appeared to help, I'm going to suggest the following issue as something that could be the cause.
Incidentally, note that it may not be the throttle body cleaning that is affecting things, but the fact that the Throttle Position Sensor (TPS) is worked through its full range several times during the process. Read on and good luck!!!
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5/2/2014
Problem:
GM 3.4L V6 (2007 Pontiac Torrent) Unstable engine RPM, causing multiple error codes. May also be misidentified as engine “missing”.
Cause:
The throttle position sensor (TPS) design is subject to issues that will prevent it from relaying the actual position of the throttle (butterfly valve). The nature of the issues is such that the problem may be intermittent. A new throttle body assembly may not experience these issues until it has been used for multiple years, but others --or a remanufactured throttle body assembly-- may have this same issue soon after it has been put into service.
Background:
When the issue is occurring, engine RPM is unstable/erratic, possibly more so at particular points in the throttle range. This may happen regardless of conditions or operation. When the issue is present, the tachometer will show the RPM wavering approximately ± <500 RPM, for example. Note that the engine will not appear to be missing/misfiring, but simply unstable in speed only. Some fault codes may result regarding traction control. Also, some other fault codes may be set regarding a throttle that is not responding properly to the electronic pedal demand.
In some cases, when only keying ON but not starting the engine, the throttle can be heard to make erratic noises as it struggles to attain its proper location. This can occur at 0% throttle and also at other points in the throttle range. Further, this can be monitored on a diagnostic tool, which shows the throttle position wavering (although the engine isn’t even running) and the pedal position to be unchanged. As the pedal is worked from 0-100% and back several times, the erratic noises from the throttle body may decrease or cease entirely in some cases and the throttle then operates normally and silently as is expected. To confirm, the diagnostic tool may then show the throttle position to respond immediately and without wavering.
The best way to confirm the issue may be to wait until a resulting service light appears and then --using a diagnostic tool-- turn and the ignition on but do not start the engine. Confirm that the throttle position is changing while the pedal is unapplied or being held stationary at some other location. If no diagnostic tool is available, intermittent “buzzing” of the throttle body may be heard if the problem exists.
Diagnosis:
The fact that the above issue appears to be affected by 0-100% use of the throttle suggests that the TPS sensor was the intermittent cause.
A teardown and analysis of the TPS sensor assembly (which is integrated into the throttle body cover) revealed that its “wiper” pivots on a guide bore on one side and against a wave spring washer on the other side –where it also mates with clocking “flats” on the throttle shaft. It was noted on the subject throttle body that no parts of this assembly were lubricated and the wave spring washer had ground into the facing metal plate. The TPS wiper connection to the throttle shaft is spring-loaded to account for mis-alignment, and allows for tolerance and assembly. Note that this spring loaded connection works well –provided that it is not overcome by force, in which case the throttle shaft can turn independently of the TPS wiper to a significant extent.
As indicated by the ground metal surface and complete absence of lubrication, the drag on the TPS wiper may become sufficient enough to overcome its spring loaded connection to the throttle shaft. This will prevent the wiper from moving with small, but significant movements of the throttle causing unstable engine RPM, etc.
Repair:
Unless an alternative throttle body assembly is available, the TPS sensor wiper must be accessed in order to lubricate the wave spring washer and cover bore area that receives the other side of the wiper. A silicone based grease may be used. Note that the throttle body cover may be removed (held by 6 reusable clips) and serviced without having to remove the throttle body assembly from the vehicle.
Remove any burrs or roughness from the metal plate where it contacts the wave spring washer.
Clean the carbon tracks with electrical contact cleaner.
A custom method of reattaching the metal plate covering the TPS must be devised. There are multiple ways to do this. The most convenient method may depend on what tools and/or materials are avialable.