So I have been dealing with a problem on my 2001 ZR2 Blazer for awhile, Its got 193k miles on it. I'd be driving it down the road for 5 - 10 minutes after letting it warm up and out of nowhere the tachometer will start going all over up to 6k rpms. This problem has persisted for awhile it will make a knocking noise in the back of the car near the exhaust and it will backfire bad. When the problem is happening the car won't be able to speed up. It gets especially bad when hitting bumps in the road the tachometer will freak out. This is at speeds of 55 mph usually but can happen at lower speeds. After awhile the problem would stop and the car will drive perfectly fine. I dealt with this problem for a month or 2 which is probably a problem. After awhile of this problem I was out and about with a girlfriend when I noticed the temp gauge going all the way up to 260 degrees and the check gauges light came on. When we were driving home the car stalled on me twice. I eventually got back to her house and didn't touch the car for the rest of the night. The next day when going to leave her house the car started then stalled out. I got out and opened the hood to find the heater hoses were spraying antifreeze all over the engine bay. The old owners of the car had modified the hoses and put a y clamp on the hoses for no odd reason which was pvc. This had eventually cracked causing the antifreeze to spray. Luckily my girlfriends dad owns a towing and he had a y clamp to fix it. After fixing that I drove back home 45 minutes it would stall when stopped but start right back up. I made it back home and didn't drive the rest of the day. The next day I started the truck up for church and it ran for a minute then stalled out. After this it wouldn't crank over at all unless you let it sit for awhile then it would start and stall again. I called my dad up because he knows a lot more about cars than me. We checked for all the norms it has spark to atleast the distributor cap (though the plugs have spark when the car isn't on but when the key is in idk if its supposed to.) The car has fuel pressure but we don't know the exact pressure. We were dumbfouded for days and the car didn't run for a week. We looked through this forum to find a ckp could cause the problem so we installed a new one, thought we don't have a scanner to relearn it. This didn't fix the problem then we looked towards the iac which I had purchased one before and it was laying around so we installed a new one. This made the car idle better but it would still stall out after a bit. When we were changing that out we had the throttle body off of course which we noticed the bottom of it was covered in carbon. We ended up cleaning the throttle body off discovering holes in it for air. This made the car run smoother but still stall. We then took the battery out of the car to clean the terminals as antifreeze got all over it and the surrounding area. This aloud the car to start and run for longer but eventually still shut off. In this process we noticed the lower radiator hose also had leak which the old owner had put a t clamp on, again no clue why. Me and my dad decided to replace this hose with the factory one so it no long would have the clamp. This hose i'm guessing had been leaking for awhile. This required us to flush the radiator then we replaced it and filled the radiator back up. After this the car started and ran the best it has. We let it idle for 30 minutes no stalling, we also cleaned all the antifreeze that had sprayed everywhere. We tested the car in the driveway it still had hesitation when putting it into gear and the rpms would dip like it was gonna stall but it would stay running. We then did a road test to find out the tachometer problem is still there. The next day I decided to drive it up to the gym and when letting it warm up it ended up stalling out but started right back up. I drove it up to the gas station before the gym it drove fine until right before turning into the gas station it hesitated and died on me and it kept losing power and dying on me until I finally got it running but the tachometer was going crazy again it almost stalled in an intersection so I floored it to keep going and made it to the gym. After the gym I decided to put it in neutral to rev it and see at what rpm it does the tachometer problem and usually when it does the problem it wants to stall out. It turns out at 3k rpms it does the problem and misfires. Me and my dad are lost on what could be causing it. We've seen the distributor gear could be the problem and are willing to look at it if that's a possibility. When driving home from the gym I drove really slow and kept it under 3k rpms and it seemed fine and made it home other than it just feels like it has a loss of power. If anyone would know why its doing this it would be an extreme help, as we're both lost. It does have a ses light on and I do have a scanner and if you would like to know what its all reading I can let you know what its throwing up. Thank you!

 

Thats what weak spark looks like. The ignition system loses power as rpms increase as the coil has less time to charge during dwell.

The first cause is loss of voltage to the ignition module caused by a bad ignition switch or wiring connectors.

The second is a bad ignition module, third, a bad coil. Generally the ignition modules misfire at low speeds then just stop working.

Overheating is caused by weak spark leaving unburned fuel that causes cylinder overheating. The fuel washes oil from the cylinder walls and friction causes heat.

First verify more than 14 volts on the charging system. Then measure the DC voltage at the power to the distributor, it must be at least 14 volts. If not, there are bas electrical connections that can be bypassed by a 30 amp relay triggered by the ignition switch like is done with dim headlights.