I have had this 1997 blazer for about 10 months. Within the last month and half it has started to jerk/shudder when the motor is under stress (going up hills and sometimes accelerating). If i let off the gas or press the pedal so it downshifts it will stop. It feels like rumble strips sometimes and sometimes it will only do one or two jerks.

The truck has always idled oddly and was sometimes hard to start. When this jerking problem arose I took action. I serviced the Transmission fluid, that did nothing, then i took it to a reputable transmission shop and there estimate said that it was not a transmission problem but a motor problem.
I replaced the egr before the jerking started to try and fix the idle, it idled better for two days and went back to crap. Two days ago I upgraded to MFI Fuel Injectors and put a new Cat on, since the other one had started rattling. I thought that the Fuel injector would fix the idle, starting, and jerking problem, but it didn't. Now the truck Idles and starts pretty darn good, but the jerking is still there!!1

So, today I put a new dist cap and rotor in it and drove it down the road......and it pulled the old rumble strip move again!!!!

I am at a loss and am not sure where to go next. I will try and put high test in it, thinking it might be detonating. I also have some new AC spark plugs to put in this weekend, but they probably won't do anything.

I know that there are similar posts to this, but they all seemed to have cap and rotor as their solution. Any help would be greatly appreciated!!!!

 

What condition was the old cap and rotor in? Also, have you tried misting water over the engine as it idled, in the dark, looking for spark leak out of the wire jackets? You could also test the ICM and coil. Test, rather than just putting parts on that may or may not do anything.

 

I have never heard of misting water in the dark, I could give it a try, though. The tabs in the cap were wore a little bit and partly black. There was no corrosion, though. I just got a tester, so I will have to test the coil and ICM. Thanks for the response.

BTW I put premium fuel in it and didn't change anything.

 

Premium fuel would let you know if it was a combustion problem in the cylinder that was not properly being accounted for by the PCM. Sounds like you have a spark problem.

I would take the ICM and coil to your closest autoparts store that has the proper tester and have them test each repeatedly to get them up to operating temperature. Some units can test fine at room temperature, but once heated up and stressed, they break down.

I was taught the misting water trick a VERY long time ago. The dark hit just makes it easier to see the spark jump from the wires.

 

Today driving home from work the truck barely did the jerking, due to premium fuel?? It is a pretty hilly road and usually i have to drive so as to keep it from shuddering. Today it barely did it and I was trying to make it do it. (I have noticed that it will do the shudder when it is about to down shift, no matter what the speed)

Thanks alot for the help, and I will continue to keep you informed.

 

Was it humid/raining when it was at its worst and dryer when it was working better?

 

It doesn't seem to matter what the conditions are like.

 

Never had a Blazer do this but have had 2 Buicks and 1 Pontiac with your symptoms, especially the shudder just before down shift with the cruise control. The first one cost me dearly to have the dealer find one bad plug wire. Could have bought many sets of wires for what the diagnostic cost. Since then it's been the look at plug wires in the dark trick to find them and replace the wires myself.
The misting is new to me but well worth doing.

 

I checked the wires real quick tonight, but didn't see anything unusual. I will install the AC spark plugs I have, and then inspect the wires more fully.
I took the truck to get diagnosed a couple weeks ago (Soon after jerking started), and the mechanic said that the rough idle was from the Fuel Injector and that the jerking was probably just a bad plug. So, I put the new fuel injector system in and it does idle much better now. I have noticed that sometimes it will stumble, at idle, and as far as the jerking being from spark plugs.....I will find out when I inspect/change them.

If not Wires or Plugs, does this seem like it could be Torque Converter Solenoid or TPS, or another sensor??

 

I got the ICM tested yesterday, and it passed three times in a row.
Today I cleaned the MAF, not much difference, still doing the jerk.
Then I put new AC Spark plugs in, gapped to .060 and it seems to be back to normal, SMOOTH Operator. Idles smoothly and no jerking, I haven't driven it too much but hopefully it is gone for good.

Thanks for your help.