98 blazer, has 191,500 miles, when i got it, the owner said it needed an O2 sensor because it was running rich. It idled rough but was fine while driving. So I got a whole tune up for it and 2 sensors. After completing all that work, it starts better but still misses at idle. I'll double check that the wires are seated on the plugs, but after that, I'm kind of at a loss. Any ideas?

 

Get some carb and injector cleaner and see if it makes a difference, could be the spider injector sticking... hard to tell without pressure testing or anything else... small things like air filter and fuel filter are easily forgotten and tend to reduce flow...

 

Misfires can be a challenging journey, there are many potential causes. Some approaches:

Fuel -

You can check fuel pressure per the sticky at the top of this forum
You can run a cleaner through the system like Seafoam to see if it helps as Don suggested
Some swap/upgrade the "fuel injectors" just because but I'm not a fan of parts swapping without a smoking gun
The definitive tests for bad injectors is looking at scope traces (pintle humps, etc) and/or doing an injector balance test

Ignition -

Lots of guys replace the ignition tune up parts if they are due anyhow but it sounds like you did that. Did you use AC Delco parts?
You can monitor misfires with live scanner data to see if its a specific cylinder or random
Again the definitive test here are scope traces (voltage levels, ringing, etc)
A water spray bottle at night can help identify leakage
A drop cylinder test can find a bad cylinder

Compression -

Testing all cylinders for compression can locate the problem cylinder if there is one

Vacuum/air leaks -

Checking vacuum hoses and the air intake connections can clear up problems.
A water spray bottle can help here too

Fuel trims -

Another scanner data lead. Many sources of misfires can be indicated with this test - vacuum leaks, MAF sensors, O2 semsors, air intake leaks, fuel pressure, bad injectors, etc. The data can narrow down the search.

CMP retard -

Another scanner lead that can be way out of spec and cause problems

O2 sensor live data -

Helps determine if the sensors are getting sluggish or have failed

Scan for any other codes -

Can help pinpoint problem areas

Other possibilities -

Dirty EGR
Dirty throttle body
Fuel filter
Exhaust restriction
Bad coil or ICM
Bad ECT sensor
Bad MAF sensor
Bad MAP sensor
Worn distributor

That's probably enough for one post. Let us know if you want to pursue any of these.

George

 

Thanks for the response. I'm thinking it's vacuum lines (mine are HORRIBLE), clogged cat, or a dirty MAF. If it still is missing after that, then I'll move onto the pressure tests, etc. It runs ok while driving but at idle is when you notice. I got vacuum lines today, hopefully I'll be able to change them before work tomorrow, but it looks like I'll be shoveling. I'll keep ya updated. Happy Turkey Day!

 

I know EXACLTY... how you feel. 96 Blazer LS... 295,276 miles. On July 5th of this year I was going to work and I got a check gages light. My temp gage was pegged... I had two more miles to go before I could get off at the next exit where I knew there was a service station I could pull into... and shut the engine down. Lots of steam was coming out of the bypass hose where it split. This was the third time this engine had over heated in the last twelve years. First time I was coming home from work, Interstate, blew the Bypass Hose. Second time was going to Disney, Electric Cooling Fan failed.... (loose connection, my fault). Third and last time... Bypass Hose again. This hose seems to be the weak link in the cooling system, and should be replaced every five to six years according to the timeline and mileage when they have failed. I had already changed the Bypass Hose before it overheated the first time, My point, after the first time, it did not have a smooth idle anymore. I attempted to fix the problem... to no avail. I replaced everything you can think of... with ACDelco Parts only... including the Harmonic Balancer... ACDelco/Amazon... the outer ring on the balancer slipped about an inch and the vibration was unbearable. This solved the vibration problem... not the non-smooth idle problem... not to be confused with a rough idle or a very rough idle problem... and only noticeable while in gear.... vehicle not moving. Off the line...driving on the Interstate... no issues... no loss of power... very strong. After this last overheating... I had a very rough idle. I checked compression... number one had low compression... 45 lbs was my highest reading... taking it six times. 3/170, 5/190, 2/180, 4/180, 6/150... Next step was to bring engine to TDC and run compressed air into the number one cylinder. I hit the cylinder with twenty pounds of air... and it came out the throttle body as fast as I was putting it in... Confirmation that the number one intake valve was leaking badly. As it turned out when I pulled the heads off... number one intake valve was WARPED... but looked to be in good condition when doing just a visual. A day at the machine shop provided me with two like new heads. The heads did no crack from the overheating. Just this one valve had to be replaced. I just fired this engine up November 24th... I took my time putting it back together... photographing everything I did on a daily basis... over 400 photos. Before I allowed the engine to start, I cranked the engine until I got oil pressure... (40 lbs)... coil wire off, coil unplugged, ICM unplugged, Injector body unplugged. First crank took four seconds to get max oil pressure, the second crank took only two seconds. I allowed the fuel pump to run... and purge the lines so I new the fuel would be up to the injector body when I attempted to start the engine. I plugged everything in... made sure the video camera was still going... got back into the vehicle... cranked the engine... started right up. I let it idle for an hour... the first time. Two hours the following week... after I changed out the belt tensioner pulley... (factory original)... bearing almost completely dry... ready to hand grenade. The idle sessions went very smooth... very little vibration observing the engine, hand on snorkel. I took it out on a run today... and noticed that when I came to a stop... I had almost the same non-smooth idle as I had after the first overheating event. This is all I have at this time. I am going to check fuel pressure next... but I changed the fuel pump in May of 2011 at 225308 miles. I did not use an ACDelco pump assembly, I used Bosch. Then I will check what codes have set on my twenty mile trip today. TJ in New Tampa

 

I did some research and came to the conclusion that it has to be the fuel pressure regulator. I found several videos on YouTube that describe the very same problems I'm having and how it causes it. So I ordered a new regulator and parked it for the time being. One reason is because a ton of gas has gotten into the oil and I don't want to lock the engine up, the other reason is because I used a 1/2 tank of gas for 120 miles. So after I change it out I'll let you know if that was the fix. I'm hoping it doesn't come down to ripping the heads off. 😂

 

Replacing the pressure regulator was a success, but now my cruise doesn't work. I replaced the vacuum lines too but have to recheck everything. Purrs like a kitten now though, I'm very happy it's drivable again.