Well, here goes. I have a 97 Blazer 4.3 LT, has 226K on the motor, trans replaced 20K ago. Engine has been good till about a year ago, miss started being noticed only on the hwy as you crest hills and such when the load on the moter gets real light, originally thought trans issues, but that I belive has been ruled out. Now having gotten cold out, things have escalated. The miss is only above 1800 rpms, typically from 2000 to 2500, but it can be higher, most prominent just as the trans shifts, fine after the shift till back in the rpm range again. When its "missing" if you step on the gas, regardless of speed or gear, it'll take off strong and smooth untill you start to ease off, then the "miss" returns. it's very reproducable, humidity doesn't seem to effect it, temp does to a point, very cold(Wisconsin -5) 2 weeks ago, hardly missed at all, until after it warmed up for a while, now in the 20's-30's it's getting bad again. Now here's the rest of the possibly associated issue, at about the same time the miss started getting bad, it's seems it takes forever to get the torque conv. to lock up on the hwy, it will eventually, but it takes some time, maybe 10 miles sometimes. Don't know if it's a syptom of whats going on with the engine, or not.


I've replaced the cap, rotor, wires, and plugs last year, delco cap n rotor, store brand wires and bosch platinum plugs. I've also replaced the MAP sensor(used but from a good motor), and checked several times for vacuum leaks. I'm in the process of running injector cleaner throught the system, and the fuel pressures are good. Tomorrow I'm going to pull the cap and check for dist gear play, carbon traces, and all the rest of the ugly stuff we see with these Vortecs, also going to pull plug wires and a plug or 2 to verify all is good there. May pull the EGR and check that also, I have a new gasket already here, only a few mins work.


This thing is driving me crazy, how can an engine miss this badly and not throw any codes, even checked them anyway to make sure. Well, maybe someone can get me pointed in a much narrower direction, work is slow so money is tight no room for hit n miss part replacement.


Thanks in advance any help is very much appreciated.....


WhitetailHunter

 

Need to start with the basics on a problem like this. Any one or more of several things can cause a misfire like you're describing, and NOT set any codes or turn the check engine light on. In no particular order, (other than the way they popped into my head) here are a few possibilities:

Crankshaft position sensor relearn
Check/adjust camshaft retard
AC Delco plugs 41-993 with a gap of .060"
Check fuel pressure and leakdown: https://blazerforum.com/forum/2nd-ge...eakdown-88305/ and post all 4 readings.

Make sure the distributor gear is not worn.
Cylinder leakage test to make sure the valves are sealing.
Make sure the ignition coil is not leaking under load. Spray a mist of water over it at night and watch for sparks while applying a load to the engine.
Make sure there is a dab of silicone dielectric grease on each terminal, inside and out, of the distributor cap.

There is a specific reason for each one of these possibilities. If you have a question, just ask

 

Only question I have is about the coil under load, it only happens during light load, when I'm hard on the throttle or getting on the hwy it's smooth and strong till it gets ready to shift, if I'm still accelerating hard it'll shift good and keep going, with only the slightest hint of any miss as it shifts, not untill I ease off the throttle and settle into a steady drive or under light/mod acceleration does it miss, and then only really in the 2000 rpm area, at 35 mph it's at about 1400 rpm and smooth, if I accelerate relativly easy, keep it under 2000, it shifts smooth and no miss. I will try to get the testing done, weather looks good for a few days.


Thanks for the quick reply....
WhitetailHunter

 

Coils can do some pretty weird stuff under varying loads, might be tough to spot, and it might not be the problem, but still a possibility. Wouldn't take much to swap another coil. Whatever the problem is, it will be something minor, but not so minor to diagnose

Another thought... There is a GM TSB for binding valves. The guides need to be reamed, just a hair, to prevent the valve stem from binding under certain engine loads.