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General ChatChat about all things Blazer (and related vehicles). Off-topic stuff should be in the lounge, and all mechanical problems should be posted in the proper forum.
Hey George, I think I have all this down correctly.
STFT B1 Counts 147-150. 12-20% fluctuating
STFT. B2 Counts 142-150. 11-17% fluctuating
LTFT. B1 Counts 138. 8% steady
LTFT B2 Counts 138. 8% steady
No change between open/cloped loop that I noticed.
Cylinders 4 and 5 only ones showing misfires. Counts consistently went up as engine was running. All other cylinders showing 0 misfires.
Not really sure if I read thr"CMP" retard or not but cam retard was showing 0
Spark advance 16-23 deg, fluctuating.
Not sure if I understand how to read the O2 sensors with the computer but did get some info.
B1 S1 0.295. It was titled low on the machine
B1 S2 0.595 It was titled high on the machine
B1 S1 Counts 122-840
B2 S1 Counts 205-885
Counts fluctuated with B1&B2 rich/lean flip flopping.
I do have a compression tester and can do one tomorrow [ at least on cyl 4&5] if you think its necessary.
Plugs, wires, cap, rotor were all replaced after this issue started. All wires are ran correctly as well as the cap and rotor. I used Autolite single platinum AP605's. If I missed anything let me know. Ive never used a scanner of this caliber before but have over a decade on the parts counter but have never really had to go this deep before. I TRULY APPRECIATE you helping me get this figured out.
Just for kicks, I'm going to bunch the SBC into two categories as either a 5.7~type or a 5.3~type, knowing they both represent a lot of various cubic inches and designs --- OK?
This bothers me:
#4 & #5 show high miscounts and they are next to each other in a 5.3 firing order --- in a 5.7, not so.
A 5.7 has a different firing order/compression pattern --- neither of which (eg; 5.3~typical v 5.7~typical) did I see posted yet - but I'll play along.
5.7
1 8 4 3 / 6 5 7 2
5.3
1 8 7 2 / 6 5 4 3
The OPEN ---> CLOSED LOOP transition should be obvious and then the B1/B2 Oxygen sensors should instantly become chatty --- I didn't hear anything about that.
It HAS to start crosscounts instantly when it goes into Closed Loop --- that's the big holdup for it waiting to get out of OPEN Loop. .
If I wuz you ---> I'd hold off on the compression test yet --- just pull the ignition fuse and crank the engine --- using your ears to listen to the cranking rhythm' let it crank for a good 20 seconds so you get used to the rhythm and can actually hear something is wrong ... or not.
Pray for: "not".
If it "lumps over" or you hear a pop-noise in the intake every other revolution--- THEN we go for a compression test.
I don't remember if you said the CEL was flashing or not --- but with that miss-count, it HAS to.
It isn't a common-wall had gasket failure because those cylinders are on different banks
There "could be a busted rocker or jumped-off pushrod that can affect those two cylinders in a single-plane manifold --
--- but then again, I'm not sure which engine you have (you didn't say, that I noticed anyway).
Sorry for all the color and test-editing --- I had a lot of time to post this and I hope it helps.
Were your STFT percentages changing that much at idle? They do not normally drift around that much. Were all 4 values (%) positive? If so then the computer is adding fuel to correct for a significant lean condition. Have you looked for vacuum leaks yet? If those values get closer to zero at 2500 rpm, a vacuum leak would be suggested.
The 2 front O2 sensors switch between around 0.15v and 0.85v around once per second at idle. When you graph them they look like this:
If you dont have graphing ability then you can watch the values switching in live data at idle. The rear sensor should flatline between 0.5-0.8v. If it also switches there is a problem. The fact that there is no change when switching to closed loop also suggests however that its not the O2 sensors.
You should do the water misting experiment and check for spark at those two cylinders
You can hold off on the compression test until after we get everything else answered. That said, when you changed the plugs, how did they look? Any of them different?
What's the mileage on this engine?
Is there any history with the spider?
Given two specific cylinders, one on each bank we ultimately need to find out if its spark, fuel delivery or a mechanical problem. As I said earlier there are tests for the spider as well.
Also, over the years we have found that for ignition parts and fuel pumps we have the best luck with AC Delco and Delphi.
George
Last edited by GeorgeLG; Oct 28, 2025 at 04:50 AM.
I was able to graph the o2 sensors. [Pics below]. I took pics of B1S1, B2S2 and the downstream sensor as well. I would rather someone look at the graph and decipher whats shown. I have a vacuum gauge, however no connectors that are suitable to go from the brake booster hose to the gauge. I fashionedan uncut nozzle from a tube of rtv [dont think it worked], it was either sucking by or i have minimal vacuum, and a large leak i havent identified yet.I wont be able to look at it again until Saturday. I'm also going to borrow a smoke machine to help locate any leaks that might be present.
B1S3 is your downstream sensor. If the engine was warmed up and in closed loop then this sensor shows a problem. It should be flatlined not moving around. Lets ignore this for now because this in and of itself will not make the truck run bad.
B2S1 is the passenger side precat sensor and its not switching right. The question is always whats actually wrong, the sensor itself, the wiring, the engine computer or the fuel delivery. Since that the one with the heater code we should test the heater circuit and then replace that sensor.
B1S1 looks better but not perfect
What did those plugs look like?
Fuel trims at 2500 will help you on vacuum. Also, there are other vacumm tests like spraying fluids around usual suspects.
Old plugs were worn, gap was worn to .060 and of normal color. I did spray around with the ether and had no change in engine speed. I'm hoping that the smoke machine will show something this weekend, pop the plenum, slap a new gasket on and laugh about such a simple issue causing so much trouble. For everything to be fine when parked then all this showing up is really got me baffled. Is there anything I can do spider wise without a whole bunch of disassembly while im at a stand still with the vacuum tests? I have to drive it in and out of the garage when messing with it. Old blazer has over 300k on it, id like to keep it running as its my daily driver 76 mile round trip to work everyday. Im unsure if the spider has ever been replaced, i did remove the throttle body and replace the gasket hoping it would fix it just because it was cheap. I did see the lines on the spider and they are black in color, I've seen some replacements with white lines but unsure if that means anything or not.
At 300K I am leaning back towards the compression test. I am surprised that the plugs on #4 and #5 did not look different.
Yes the spider and the control circuits can be tested with the spider in place. Probably the most useful test is a pulse balance test. You hook up a fuel pressure gauge and pulse each injector in turn, recharging the line between each and see if the pressure drop is uniform. The pulsing can be done with a pulse box or some scanners. You can also test each control pulse with a noid light. Lastly you can measure the coil resistance of each injector coil. This is one of those areas where the replacement needs to be of high quality. AC Delco is not made anymore but there is one higher quality after market unit that has been getting recommended by I have not seen any follow up comments.
If this was my truck, I would do these in the following order:
Fuel pressure leak down test (complete)
Compression test
Injector balance test
Water misting in the dark
Replace at least the B2S1 O2 sensor after testing the heater circuit
Vacuum leaks. Check for unmtered air right after the MAF (quick)
Today was a productive day as far as getting info.
VACUUM TEST:
16lbs at idle. Open throttle a little dropped to 15lbs then level back out at 16. Needle very steady No jumping or quivering. SMOKE TEST: I bprrpwed a smoke machine and found and no leaks found.
COMPRESSION TEST:
compression may be a little low but not terribly sure.
Those compression readings are pretty low. Please do all six cylinders, engine, warm, all plugs out, throttle open, fuel pump relay pulled, same number of compression pumps on each attempt for each cylinder if any of the answers are more than 10% below the high cylinder, poor teaspoon of oil in the spark plug hole and repeat the test.
if all the values wind up hovering around 120 psi then you got a well worn engine at 300,000 miles but if none of them are 10% different from the high value, then it’s not a cylinder problem. That said with that kind of engine wear things will never run completely smooth. Let’s wait and see what kind of total test results you get, but if nothing is catastrophically indicated then we move onto the spider.
Sounds like a plan. I'll get the other cylinders this evening most likey. I had to return the scanner but if need be shouldn't have a problem borrowing it again. Of course I cracked a plug when I took cyl. 4 out. Believe it or not I maybe use a little over a quart between oil changes just using gtx hm lol. Luckily most of my trips to work are highway miles and I set the cruise at 60 just ride. Gotta be gentle on the ol gal 😆
