Good Old Random Cylinder Misfire Mystery

[Nicolai8775]

The trims are a bit weird in that usually with a vacuum leak the trims would be high positive at idle, and go lower (closer to zero) at higher rpms. I think you said the MAF was cleaned but do you know it's working properly? Might want to swap it out for a known good one.

Also, perhaps a standard vacuum test would be helpful, maybe just to rule it out as the cause. Any exhaust leaks?

Replaced with a nearly new MAF and both long fuel trims are steady at -1.58 but everything is misfiring now lol

 

[Nicolai8775]

Replaced with a nearly new MAF and both long fuel trims are steady at -1.58 but everything is misfiring now lol

Ok since it's warming up a bit bank 1 is at +1.58 and bank 2 is at +7.6

 

[iamdub]

Any idea of the brand of crank and cam sensors?

 

[Fless]

Is it still misfiring after it warms up?

Have you watched or graphed the upstream O2 sensor activity? Should be between 0.1v and 0.9v and active.

 

[Nicolai8775]

Ok since it's warming up a bit bank 1 is at +1.58 and bank 2 is at +7.6

Test drove it. ltfts dropped considerably. Never going higher than +11.5 and for the most part stayed pretty even between both banks.

Graph showing a slight miss on 5 still though nowhere near as frequent

 

[Nicolai8775]

Is it still misfiring after it warms up?

Have you watched or graphed the upstream O2 sensor activity? Should be between 0.1v and 0.9v and active.

I will do this on the drive home!

 

[rockola1971]

Ive seen this before...On one of my 2003 Tahoe 5.3L's. Turned out the spark plug wires were trashed. I would inspect your wires. Pull the boots up and way from the spark plug and Ign Coil connectors(terminals) on the wires. See if any have a broken connection to the center conductor of the spark plug wires. Some numbskull could have pulled a wire or 2 or 3.... and broke the terminals off the wires or just broke them away and didnt even notice it since the boots at the spark plugs tend to hide this unless the terminal flat out just stays on the spark plug. Swap spark plug wires and see if the misfires move.

 

[Nicolai8775]

Ive seen this before...On one of my 2003 Tahoe 5.3L's. Turned out the spark plug wires were trashed. I would inspect your wires. Pull the boots up and way from the spark plug and Ign Coil connectors(terminals) on the wires. See if any have a broken connection to the center conductor of the spark plug wires. Some numbskull could have pulled a wire or 2 or 3.... and broke the terminals off the wires or just broke them away and didnt even notice it since the boots at the spark plugs tend to hide this unless the terminal flat out just stays on the spark plug. Swap spark plug wires and see if the misfires move.

I bought new wires just now cause when I was trying to gently pull the boot off the terminal stayed behind. Pulled one off my other truck just to get her going for the drive

Also bought an ECT for shitts and giggles cause that's about the only sensor that hasn't been replaced by previous owner besides knock sensors, TPS, and IAC lol

 

[Nicolai8775]

Upstream O2 sensors ran between .1 and .9 but the voltage jumped all over the place. Is this normal?

 

[Nicolai8775]

Is it still misfiring after it warms up?

Have you watched or graphed the upstream O2 sensor activity? Should be between 0.1v and 0.9v and active.

Ok I figured it out that the sensors are supposed to be active, as in they are supposed to jump around.

At idle Bank one is currently sitting at .6volts, hasn't moved and I've been idling for about 10 minutes.

Bank 2 is bouncing around as it should between .1-.9 though it did show it hitting 0volts occasionally.

Edit: bank1 starting bouncing around while cruising.

LTFTs are about +9%- 12%.

Misfires occasionally on multiple cylinders, but it's nowhere near as bad as it was.

Getting balls for gas mileage though.

I didn't like the plugs or wires, there was a random AC Delco in cylinder 4 that definitely wasn't gapped at .040. replaced all that.

Noticed that that canister on top of the intake was extra wobbly and realized it was missing the little rubber piece to keep it tight. Fixed that.

Things seem much better, still having a hard start long crank issue occasionally.

 

[Nicolai8775]

Any idea of the brand of crank and cam sensors?

I don't know unfortunately, I was having trouble ordering the AC Delco through O'Reilly's so I'm assuming it's the generic auto parts store brand

 

[Nicolai8775]

 

[Nicolai8775]

LTFT is currently +8.59 in bank one and +14.84 in bank 2, at idle. Does this indicate a vacuum leak? I think Fless mentioned this earlier. The trims don't drop when I accelerate, they increase slightly

 

[Nicolai8775]

Also wanted to add. The fuel tank depressurization when I open the gas cap issue. Everytime I fill up on gas and it nears full, you can hear the gurgling fuel coming up the filler neck as if it's close to overflowing. That's not normal

 

[Fless]

LTFT is currently +8.59 in bank one and +14.84 in bank 2, at idle. Does this indicate a vacuum leak? I think Fless mentioned this earlier. The trims don't drop when I accelerate, they increase slightly

Great that you're getting some improvement!

Couple of thoughts... that Bank 1 S1 O2 sensor seems to be a bit lazy waking up, so as long as the heater circuit is working you might want to consider replacing it and the other upstream sensor. Best done in pairs, but not totally necessary. A sleeping or dead sensor will stay around 0.450v; weak or lazy sensors will switch slowly or not make full range under various rpms.

The somewhat high fuel trims seem to indicate a vacuum leak or unmetered air getting in past the MAF. Could be a leak in the intake tube (is everything tight and sealed; no open ports or cracks in the tube?). Less common, could also be a dead or weak fuel injector causing the cylinder to run lean; that would pump air into the exhaust and the PCM is going to add fuel to compensate. I'd expect that only on the bank that has the issue, though.

 

[Nicolai8775]

Great that you're getting some improvement!

Couple of thoughts... that Bank 1 S1 O2 sensor seems to be a bit lazy waking up, so as long as the heater circuit is working you might want to consider replacing it and the other upstream sensor. Best done in pairs, but not totally necessary. A sleeping or dead sensor will stay around 0.450v; weak or lazy sensors will switch slowly or not make full range under various rpms.

The somewhat high fuel trims seem to indicate a vacuum leak or unmetered air getting in past the MAF. Could be a leak in the intake tube (is everything tight and sealed; no open ports or cracks in the tube?). Less common, could also be a dead or weak fuel injector causing the cylinder to run lean; that would pump air into the exhaust and the PCM is going to add fuel to compensate. I'd expect that only on the bank that has the issue, though.

It's got the stupid K&N intake. I want to get a volant, heard those are good.
The tube looked worn where it hits the fan shroud, I will make sure it's not worn through. The rubber is in good condition on the intake tube. The clamps are all right.

I still really don't want to believe that I did the intake gaskets wrong lol. I don't think I did cause of the water test and even propane around the manifold. Could it be the pcv valve/hose or perhaps the EGR tube?

I'm not so sure about injectors now cause torque is showing very minimal misfires if any the last couple trips. When cylinder 5 was throwing a misfire code I listened to a few of the injectors with a stethoscope and compared to #5 and heard the same consistent clicking sound.

 

[Nicolai8775]

What's more likely to cause a long crank? A vacuum leak, or bad injectors?

If it were injectors, why isn't it misfiring? I don't see any seepage around the injectors indicating bad o-rings. I will check more thoroughly when I get home. Or is it internal failure, like a clog. Cause my fuel system did leak down a little after like 45 minutes or so

 

[Nicolai8775]

O2 sensors changed, no difference

 

[Nicolai8775]

I once again tested intake for leaks by unplugging iac and spraying the crap out of the intake with carb cleaner and any other vacuum source I could find. No change in ltft in torque. I have 6 injectors left from my 2001 Tahoe if I need to replace. How do I diagnose?

 

[Fless]

One thing you can do is an injector balance test (injector removal not required), which measures the difference between the initial fuel rail pressure and the pressure change when each injector is fired. Ultimately an injector flow test might be needed to measure the volume of each injector's contribution, but that requires removal of the fuel rail.