I have a 98 Blazer with 154,000 mi. It has developed a slight miss at 60 to 70 mph, but seems to be a steady miss at about 65 mph. Recently it has started to miss at around 30 but goes away once it reaches operating temp. Yesterday the SES light came on at low speed and it was running rough, once it warmed up the SES went off. I have had it to a local GM mech. and he cleaned the injectors but that hasn't worked, he also said the fuel press.regulator was low but still within factory spec.So far this is what I have done within the last 3 months, new fuel pump (old one was bad and the car stopped running) new fuel filter, new gas cap, wires, capand plugs have less than 5000 miles, replaced the dist. due to a cracked housing. Hwy fuel mileage has dropped from 24 to 18 / 19 mpg. I am the 2nd owner the 1st was 85 years old so I doubt the vehicle was beat, I have owned for the last 3 years. Any help is greatly appreciated.
AMXAL

 

Can you get the code read and let us know?
Does it have the miss at 30 if you run it in "D3" instead of "D"?

 

my motor blew at that exact mileage, watch your gages.

 

The highway figures you have are about the average for most, 24 is great mileage on the highway for your truck. Could be tranny issue. Maybe have differential fluids and/or tranny fluids checked. Mine has a miss at that speed too, but my bearings are also being bathed by coolant that isn't supposed to be there. My engine is also around that 154k mileage range.

 

Thanks for the info, I'll try the idea of running in"D" and if that does anything, I stopped at the local Auto Zone to have them read the code but they say they can't because it (SES) is currently not on, is that true??

 

I don't actually know what Autozone can do as we don't have Autozone in Canada.
I know that the dealer can check for information stored in history, such as codes.

 

The problem with Autozone is their crappy generic scantools that they use can't see stored codes, only active (atleast that's what they say). If they used the higher feature one that they sell, you could see stored codes as well as freeze frame data.

 

I was able to get the codes read today, but 1st todays "excitement" here in the Chicago area it has been 50 degrees and very damp, it's raining now, when I started the it was hard starting and when I put it in gear to drive it missed badly until about 30mph then it seemed to straighten out, then at 55 mph it started to miss again up to 65 mph, I did not go faster. It ran this way the 18 miles I travel to work, when I left work this afternoon it did the same thing as in the morning except this time the SES came on, at this point I was able to get the codes read, the scanner showed a 300 and 440, The tech said the 440 idicated a bad or loose fuel cap, I checked that and it was tight( I purchased it 2 weeks ago) he also said the 300 showed multiple cyl. miss fires. I guess I'm lost, I tried driving in "D" and that made no difference, I have also cheched all the fluids, (trans and coolant) and both are correct and clean. Am I to the point of taking her "out back" and shootin her??

 

Running worse in the rain?? Hhhmmm, try opening the hood, starting the engine, and taking a look, when it is DARK out, and see if you see a light show, if necessary, spray the engine LIGHLY with water, like from a spray bottle used for glass cleaner, and see if you get a light show.

What kind of plugs did you put in? Did you reset the timing? Tried resetting the computer? Does it do the same thing on a cold motor? or, only when up to operating temp?

 

how u reset timing?