OK - so you are saying that originally the misfire wasn't felt but now it is felt at idle, correct? If you accelerate, the felt misfire goes away?

I am wondering if your observations might be due to a common problem on these Blazers where the CMP Retard value is off. Timing is set directly by the crank sensor and the PCM, but where the rotor points inside the cap is also important. If it's not pointing at the right place when the spark comes through, a misfire can occur. Since timing changes dramatically during acceleration vs idle, the rotor would be in different positions when the spark comes through and there is potential for gong "over the edge" with the spark jumping to the wrong place and getting a misfire. Understand what I mean? In any case the CMP retard is a measure of where the rotor is pointing in relationship to the crank sensor and should be 0 +/- 2 degrees. Since distributor rotation is locked in these vehicles (unless someone has already modified hte distributor hold down to allow rotation), excessive values are typically caused by wear (usually the distributor gear). CMP retard is super easy to check, but you need to have a capable scanner.

For this reason and to allow you to do live displays of misfire counters as well as many other things, I'm going to recommend an inexpensive scanner setup that will do a lot of the Powertrain things on these Blazers. Note that if you have a 1996-1997 - then Car Gauge Pro will not work for you. Note that neither will do a crank sensor relearn. That requires much more expensive software that has bidirectional communication with the PCM.

For Android Phone: BAFX Bluetooth OBD2 adapter as sold on Amazon ($23).
App choices from Google Play of either Car Gauge Pro ($8.95) or Dash Command ($10 + $10 per vehicle for extended GM PIDs).

For iPhone: Take your pick of Wifi OBD2 Adapters I have no recommendation but cheapest is usually not best. Dash Command ($10 + $10 per vehicle).

Get one of these and we can proceed. Otherwise you're just shooting in the dark.

Good luck and best wishes for a sucessful repair.

 

okay thanks, that OBD scanner sounds like a good in-depth reader....I'll order that

Can a hole in the front and the back of a muffler cause a misfire?

 

A misfire is usually caused by one of 3 issues. An issue of fuel, air, or spark. Depending on the engine you have, it could be a plugged/dirty injector. For air, it could be a bad sensor, or vacuum leak, which are quite common on s-series trucks. And as for spark, it could be the cap, rotor, coil pack, plug or plug wire. A hole in the exhaust (unless by an o2 sensor) should not cause a misfire, as it is just an exit for exhaust gases.

 

Okay so i originally posted here about a cyl 1 misfire i still have.

what I've done so far:
Replaced spark plugs, wires, cap and rotor (its was a year since the last tune up so that money wasnt wasted

replaced the fuel pressure regulator and plenum gaskets while i was in there (oil had a rich fuel smell so i thought it was the regulator)

ive tested the compression, fuel pressure and did a fuel leak down test which stayed consistent in pressure

looking at the LTFT B1 from live data, its reading 13.3% and i know its supposed to be at or below 5

The pcv valve rattles a whole lot, so when i removed it with the car on and covered it with a finger LTFT B1 % went up by a lot....

IN CONCLUSION, that leads me to believe theres some sort of vac leak but on a blazer, there arent many vac lines

ideas?

 

A misfire for whatever reason leaves unconsumed oxygen in the exhaust. This can be interpreted by the O2 sensor/PCM as a lean condition and hence can result in your fuel trim reading where fuel is being added to the factory calibration to compensate. Could be a vacuum leak - or still could be pretty much anything else that causes a misfire. PCV valve rattles because of the misfire - it isn't the cause.

If you really believe it's a vacuum leak, then get out the carb cleaner and start spraying.

Did you get the scan tool? If so which one? Ready to start actual diagnosis?

 

You need to answer this question (immediately above).

Also know that your testing in post#3 in this thread makes zero sense and is incomplete.

 

apologies,

the cylinder 1 misfire i had went away, i never got around to buying that OBD scanner, but in the end my final diagnosis came down to a dead injector or a carbed up valve

to see which it could have been i used some seafoam i had laying around in the vac line, and the misfire went away, Smooth idle now

I would try to reply to this post but it wasnt letting me, and a few time the only way i saw your comment was through my email,

again, sorry about all the thread

in other news, after I used seafoam through a vac line, my original cyl 1 misfire died down. Now idle is almost smoth as it used to be, i used a little less than 1/3 of a can i had left

after the p0301 code went off, a p0300 code came on, but not fully. Its still just a pending code in the computer

I just think thats because the miss is way less than before but i didnt use enough seafoam to finish the job

 

Glad to hear the Seafoam fixed your problem. If it's just a pending code and the seafoam helped, keep driving it.

 

Okay got it, thanks for your point of view

 

What I'm saying is if the Seafoam helped it will probably get even better as you drive it more.