This is my first post here, and I’m not too mechanically-savvy with vehicles, so please bear with me.
I have a 1998 Chevy Blazer (2WD) with 115k miles. When I turn the key and start the car, the RPMs rev up really high for about 4 seconds and then drop down real low until the car dies. All in all this is about an 8-second process, and it sounds like I’m flooring the accelerator when, in fact, I’m not even touching it. The Blazer just started doing this, but previously for about the last few months about once a month, when I would start the car I would get a loud backfire sound which sounded like it came from under the hood (driver’s side), and this was followed by the smell of exhaust coming into the cab through the AC vents.
I have no idea how to go about diagnosing this since I’m unable to drive it to get the codes read, so any ideas would be greatly appreciated. Thanks in advance for the help. Cheers!
Are you saying the Check Engine light is on?
Tester
Check under the hood for any hoses that may have been dislodged by the backfiring.
The backfiring part is what’s bothering me. And why was that happening?
If you say you heard the backfire from under the hood, it means it came up thru the intake system. And on the intake system is a component called the Idle Air Control valve. This controls the idle speed under all conditions.
I’m thinking each time the backfires occurred with the throttle plate closed, the pressure was sent thru the idle air bleed port on throttle body into the IAC valve, damaging it. And now the engine won’t idle right.
If this is what has happened, you have to find out what’s causing the backfire. Otherwise, if you install a new IAC valve, that too will be damaged from backfire.
Tester
Thanks for chiming in 
Yes, the Check Engine light is on. And whether or not the sound that I hear is indeed a “backfire,” it sure does sound like that: it’s a loud “pow” sound, followed by exhaust fumes that I have actually been able to visibly see (in addition to smelling) coming through the AC vents that are located above the radio. I have owned this car for a little over 2 years now, and it has done this probably about 12 to 15 times total. But this last time it did it is when it rendered the Blazer undrivable due to the RPMs revving up and then the car dying afterwards. I appreciate you guys’ help!
Replace the IAC. You need to get the code read at Autozone or Advance And report back with the code reading. The code will be in a P000 format.
After looking for a big vacuum leak I would take a look at the throttle body.
Make sure the butterfly valve is intact and shut. It could be dirty and sticking.
Thanks everybody for the info! I’m going to replace the IAC valve when I get home today and also clean up the throttle and go from there. Cheers!
Here’s an update:
After I removed the air cleaner to get to the IAC, I noticed this little guy just hanging out while not being hooked up to the spot where it’s supposed to go. I don’t know exactly what this part is, but I’m fairly sure it’s causing some of the problems! Anyways, I went ahead and replaced the IAC and put this renegade valve back on, but I need to replace it because one of the two hard plastic clips is broken, therefore not securing it in place. I’m trying to figure out what it is called so I can get a replacement, so if anyone happens to know the name of this part I’d greatly appreciate the help! Thanks guys/gals!
That could very well be the big vacuum leak I mentioned in my last post.
How about a photo of where that connection is supposed to fit?
Well I think I’ve figured it out. This is the PCV valve cover, and I’m going to pick it up tomorrow morning at 8am. I’m feeling good about it. Thanks again for everyone’s help!