Hey all,
I have an 02 that started having a rich condition driving home from work the other day. Started misfiring with extreme loss of power. Codes are rich on both banks and P0300 random misfire. I'm still learning so feel free to poke holes in my troubleshooting. I'm here to learn.

Long Term Fuel Trims on both banks go to -10 on both banks at idle and go more negative as you give it throttle.
I checked the cap and rotor for damage or arching stains, but looks clean. They were just replaced with Delco brand about a year and a half ago.
The injectors were replaced March 2021 (about a year ago) with the AC Delco upgraded version. I did a leak down test and held 60psi for 10 minutes with no leak down.
Plugs and wires were replaced last year as well with Delco plugs and NGK wires.
O2 sensors are reporting normally, switching between high and low voltage.
I've swapped in a known good ignition coil with same results. I have not swapped in a known good ignition module yet, but this is my next step after it cools down this evening. I'm pulling these known good parts off of my other Blazer that is running perfectly.
CMP offset is ~4.
MAF and air temp sensors are reporting correctly. Same readings as the known good blazer.

Edit: Swapped in a known good control module and same results.

Seems to do it after it warms up for a few minutes. Trims will be OK, then starts shifting negative the longer it stays running, about 10 minutes and it's undrivable.

More Edit: I introduced a vacuum leak by taking the valve cover hose off the air intake and fuel trims evened out so I do believe it's dumping fuel and this isn't an ignition problem.
I speculate the FPR may have a small pin hole leak that only shows under vacuum.

Even more Edit: I don't suspect the FPR anymore. The MAF is reporting fine, but when it's plugged in my trims go way negative, but if I unplug it my trims go way lean. Gonna swap MAFs from my good blazer and see what changes.

 

Looks like it was indeed the MAF. After swapping the MAF from my good blazer in it's running like a bat out of hell. It looks like there maybe some fuzz stuck in the other MAF. Going to brake kleen it and blow it out with some compressed air. If that doesn't work I'll go ahead and replace. I still find it odd it was reporting fine in Torque. Maybe Torque can't update fast enough to see glitchy readings. Oh well. Thanks for letting me talk this one out and maybe if someone comes across this from Google it will help them out too.

EDIT: see reply below. It was not the MAF.

 

Good work and glad your fixed up. When looking at fuel trims you always need both LTFT and STFT on each bank. The actual fuel delivery is the sum of the two numbers.


George

 

It wasn't the MAF.

I've gone through about everything on this engine now. Everything seems to be reporting normally. O2 sensors are switching like they are supposed to.
I went ahead and threw injectors at it. New plugs, wires, cap and rotor. Unplugged the evap purge valve, still running rich. Runs more rich with throttle. Strong smell of gas out of the exhaust.

It seems to only start doing it when it goes into closed loop. I've attached an image of some live data I took. The engine was in closed loop when I took this data.



There's not many things that could cause a rich condition like this and I've gone through just about all of them. Fuel pressure is good 56psi, and drops ~6psi when running. I can't find any information on what a normal drop would be when running, so I can't tell if the fuel return line is clogged from this data.

The only work I did on my blazer prior to this issue was the lower intake gaskets. I noticed a large decrease in fuel economy after doing this. The old set were leaking coolant, and I was running lean. After the lower intake gaskets were replaced, I was no longer burning coolant and my lean condition went away. I didn't have drivability problems directly after doing this and fuel trims were almost exactly 0 +/- 2%. The rich condition started about 2 months after. And the only other work was replacing a leaf spring on the driver's side.

 

Couple of questions:

Are those 4 graphs time aligned. IOW, you go from -3 ish total fuel trims at idle to -27ish at 1500 rpm?

What does the other bank look like?

How completely have you evaluated all relevant sensors that are involved in fuel delivery calculations?

What the history on the injector spider? Have you tested the injector function since the new spider?

Does it run better in open loop even if the truck warms up?

George

 

The graphs are time aligned. Other bank looks about the same, couldn't fit it in Sheets. I've looked at the sensors in Torque and compared them to my other blazer and the readings are about the same, +/- 1% on readings between the two.

Injectors are brand new. Old set was AC Delco only about a year old, new set is Standard Motor because AC Delco doesn't seem to make them anymore. They hold the same pressure the Delco set held for 10 minutes. Is there another way I can test injector functions? The spider isn't like individual injectors like on the LS platform so I'm unsure how to do any sort of test lamp test.

 

There are a few ways to evaluate injector function:

Do they leak at rest after the fuel rail is charged by the fuel pump. This is the fuel pressure test in the sticky

Does the PCM give the proper low side control pulses to each injector in sequence to open them for the fuel delivery. This is a noid light test This would result in a lean condition though if faulty

A pulse box and pressure gauge test where you pulse each injector after charging the rail and compare the pressure drops and by implication, the fuel delivery quantity. Are they uniform? This can also be done by some scanners.

You can measure the resistance of each injector coil to see if they are uniform.

A high end shop can watch the ignition waveform to analyze much of this including the pintle hump showing the injector firing but most diy'ers don't have a scope and high V probes.

A rich condition may also be the lack of ignition as RPMS go up

It can also be mechanical

it can also be excess fuel rail pressure from a faulty regulator.

It can be a sensor such as the ECT or the O2 sensors

Have you looked at the plugs, are they uniformly sooty? What about a compression test?

Let me know if you want to pursue any of these.


George

 

I'm unable to test the injectors without a high end scanner right? Do you know of a scanner that can pulse each injector individually?

I've pulled two spark plugs. #1 and #3 as they were two of the cylinders that misfire. #3 misfires the most.


These cylinders always seem to be misfiring. #3 is the worst.

#1 sparkplug.

#3 sparkplug.

Borescope of cylinder #1

Borescope of cylinder #3

Sorry for the picture spam. I plan on doing a compression test tomorrow. I'm very worried how wet cylinder #3 is. I replaced the #1 and #3 sparkplugs and wires without change.

As far as leak down test, fuel pressure holds 56psi for 10 minutes without any loss.

 

I have used this pulse box with a fuel pressure gauge:



You charge the fuel rail with key on then pulse the injector and measure the pressure drop. repeat for the other five injectors.

As far as scanner high end functions, Les Myer knows the most about those, you can PM him for input.

George

 

Happen to have the pin-out for the spider?


Looks like it doesn't work for the spider?