2004, Chevy Blazer, 4.3L with 4x4, around 106k miles

Now I'm having some issues again with the oil system.

When I sit at idle, the tach bounces around the 400 to 700 mark and shudders the whole vehicle. The oil gauge bounces around either side of the 40 mark pretty badly as well. When I get up to speed, it stops, but when I come to a stop light, it does it again. It used to be only from time to time, nows its every time I start her up.

I checked the oil, and its prob a quart or so (just guesstimating) over the "max" line. And today I discovered the oil cap was missing. I replaced that ASAP.

The check engine light is on, but its been on for months. The mechanics with the OBDII reader say the code is just a bad gas cap issue.

I haven't had the code checked since this issue, I plan to do that tomorrow.

The vehicle temp is normal.

What do you guys think?

Here is a video from two nights ago (around the time the shuddering issue started, but as you can see, its not doing it here) at a stop light with normal acceleration: Click on it, it will take you to my photobucket where you can view the video

 

1st thought is a ground issue, possibly under the fuse box under the hood or wiring at the PCM. When the tach jumps does it actually change revs or is it just the tach having an epileptic moment? Just the tach I'd still say a ground issue. If it does change RPM's then I'd look at the CKP - Crankshaft Position Sensor under the the front of the crank, 2 wires in a plug & a MM socket to pull it out. Look for witness mark/abrasions on it.

This is a good write up about a bunch of issues that were corrected by 1 ground at the rear, yes at the rear & it affected so many things you wouldn't think possible.
https://blazerforum.com/forum/2nd-ge...problem-42968/

Reading it again I'd check that CKP.
Search for that in here & you can get more info... & they aren't expensive but some need shims. I know mine got hit by the crank when it was on the dyno & it messed up a whole whack of things.

 

Went to the shop, had them read the codes.
Not great, but not too bad.

MAF Sensor
Random Misfire
Fuel Vapor Valve Sensor

 

I'd clean MAF before changing it & check the plug & wiring also. The tune up may help the misfire when the cap & rotor get replaced. As for the fuel vapor valve sensor - not a clue lol. Search it out in here.
If you had the actual codes, like the MAF code should be P0101 then you could look them up & see what causes the PCM to throw the codes.

 

If your tach starts jumping erratically, pull the crank sensor and look at it.


The plastic timing cover on these is warping. Some are having the crank ring strike the sensor, causing the tach to jump around.

GM has a spacer kit to shim out the crank sensor.

 

Probably not the issue, but check out https://blazerforum.com/forum/2nd-ge...problem-42968/
Lots of good info there & kinda freaky too, all those grounds at one point & at the rear of the vehicle.
Yes if the tack jumps around that's odd & the CKP might be the culprit. Adding a spacer or 2 could take care of that. $10 or less for 2.
But for the sensor to be impacted by the lands on the crank it reads from... that's not good. Hopefully it's not the CKP getting hit, that = internal engine issues IE: main bearings.