2002 Chevy Blazer automatic trans/RWD. Intermittent, momentary, single jerking at 50+mph. Some days it doesn't do it at all other days this may happen every 5 to 10 miles. Runs smoothly, no loss of power, seems to shift ok, just this intermittent jerk. This happened very infrequently prior to recent repairs, and in February of this year 2011 had a complete tune up, plugs, cap, wires, the works, and new fuel pump. The jerking disappeared after these repairs. Recently there was a loud buzzing noise which turned out to be coming from a broken distributor(not replaced with Feb repairs). Had the distributor replaced and that very day the jerking returned. HELP!! What could this be?

 

When replacing the distributor, the timing must be set properly. Once the distributor is installed and timed correctly, a scan tool must be used to monitor the CMP Retard, setting it to zero by moving the distributor. After that is done, a CASE Relearn (crankshaft variation learn procedure) should be performed; again requires a scan tool. I am betting that these steps were not performed and there is a misfire caused by inaccurate timing.

 

Thank you...Back to the dealership with your reply in hand........

 

Went back to dealership mechanic, they checked everything out, including timing and state that everything is ok. The jerking is still there. It happened 4 times during my 25 mile commute this morning. Any ideas what else it could be?

 

My Blazer has also done the same thing over the course of my ownership.
Here is a breakdown of events:
Year 2004 - Driving at about 45-55 MPH, either while accelerating, slowing down, or just cruising at speed, the truck would feel like I dropped it into drive gear 2 for about half a second. It even sounds like it, because in a fit of anger I dropped it into D3 while at speed and it had the same "created" effect.
There are various levels of intensity as well. For instance I could be driving and you can just barely feel it. Other times it could be a violent jerk that feels like a downshift.
The real odd part of this is during this event, the temperature gauge will drop to far left (low temp) and then return back to what the temp really is. It has done this at all temps.
Back in 04, I took it to AAMCO. Big mistake. They said my clutch rings were burnt up and I had pieces in my torque converter. Whatever they did, the problem went away, but it returned a year later.
At this time, I had a major problem with a shift solenoid, and had that fixed at the dealership. All problems went away and I thought things would be good to go.

Now back to present time, the truck is doing it again. I do a lot more interstate driving these days, and now it's doing it above 60mph.
Again, varying degrees of intensity; sometimes it will be light, other times you can hear a clunk and really feel the jerking of the vehicle like you dropped it into D2 while at speed, but only for half a second. The temp gauge also "shuts off" half a second before the jerk. When its a small jerk, the temp gauge only falls a little bit. If it drops all the way to the left, it is a hard jerk.

I talked to another mechanic about it and he said it sounds like overdrive is shifting at the wrong times. I'm not familiar with these trannys so I really don't know what's going on. I have replaced many parts over the years, including the following: TPS, IAC valve, coolant temp sensor on the driver side header, thermostat, shift solenoid B, as well as the typical distributor, cap, rotor, wires, plugs, etc.

I'm at a loss. The tranny is the only thing that intimidates me on the truck, as I've changed a lot of things on this vehicle. (View my build thread in sig). I've talked to mechanics, friends, etc. No one can put 2 and 2 together to tell me what's wrong. I've got a great scan tool (Torque on the droid and a scan tool that hooks up to the laptop). I'm going to record my sensor data from now on and hope I can submit it to someone for analysis. I'm ready to throw down the cash to replace the transmission. I am very frustrated with this.

 

How's the battery connection at the battery as well as connection at the alternator? My cousin's transmission was doing crazy stuff and it turned out to be a bad electrical connection. The only reason I bring it up is that the temp gauge moves. The only thing in common are electricity. VSS determines what gear you are in. If the PCM looses track of how fast the vehicle is going it can drop to whatever gear it thinks it needs based on speed and engine operation. ECM recieves a 5V signal from the VSS. A poor connection somewhere resulting in the loss of 1V will make a big difference to the VSS but might not affect other things as much.

Just grasping at straws for you but maybe something to look at in your free time. Unless you know nothing of electricity it could be a costly assumption if taken to a shop or dealer.

 

I haven't actually put a meter on things, but all other electrical components work fine. There is no perceived loss of power anywhere else on the vehicle. Lights both external and internal remain on, radio remains, engine RPM gauge remains the same. (the jerk does not effect RPM's)
I have performed a battery/alternator test and they pass just fine. I DID have a problem with the starter connections at one time where no power to the vehicle would be applied when the ignition was turned. I've since fixed that and there is no problems with the starter now.

I would also like to add that there is a new whine when accelerating. Listening under the hood it sounds like something with poor lubrication. Best way I can describe it is if you've got a generator running in an enclosure; with the difference being the door to the enclosure open and closed. In this case it sounds like a door is opened. If you can understand. This truck has been nothing but a pain in my rear. I've put a lot of work into this thing and she still won't run correctly. About to go test drive a Wrangler. =P

New update: the extra whine seemed to have worked itself out. I think it had something to do with my ventilation system blower motor. Though back to the original problem, yesterday the truck hesitated on me in the neighborhood while going under 20 mph. At this point the temp gauge did not move but the RPM's dropped and bounced the needed for a moment. Same feeling inside the vehicle though. I've been driving with my phone hooked up to the OBD2, (Torque App) and my temp reading coming off the obd2 port fluxes from 177-190 pretty frequently. Is there an allowed amount of flux on the temp? When the truck hesitates the reading goes down to 150F. I've read that 200 degrees is the must reach spot, but my blazer has never been there unless in heavy Las Vegas traffic. My wife's 2004 runs at about 210 all the time. I've changed my thermostat, radiator, water pump, heater core, and hoses over the course of my ownership.

 

Post deleted. Accidentally posted a duplicate.