MAF sensor and O2 sensor bad?

[jlglr4]

I believe sensor 1 in the GAP tool is the pre-cat/lamda sensor (one in the same) in the manifold. I think the other is the post-cat sensor (or the mid-cat sensor - I just learned we actually have three o2 sensors on each side, though we seem to only have two live data values on the tool). As I understand it, the pre-cat sensor controls STFT, and the LTFT is actually calculated from stored rolling averages of some sort based on the STFT.

It probably makes sense that you’re not seeing the idle problem until it heats up and goes into closed loop. Before it heats up, in open loop, its using set fuel mixture values. When the fuel mixture is turned over to the sensors, it starts running into a problem.

But I don’t see why it’s taking so long to reach temp. I know you’ve replaced the T-stat not long ago, and it’s not electrically controlled as I recall. And you replaced the rear coolant temp sensor, which I believe is the one used for the temp gauge in the dash. Unless that front temp sensor is somehow messing up the calculation from the rear temp sensor - manual just says they check against each other, but doesn’t elaborate. Either way, if the engine temps are not accurate, that could be causing your idle problem. Could be that front temp sensor, unless you’ve already changed that as well.

 

[Michael Gain]

I believe sensor 1 in the GAP tool is the pre-cat/lamda sensor (one in the same) in the manifold. I think the other is the post-cat sensor (or the mid-cat sensor - I just learned we actually have three o2 sensors on each side, though we seem to only have two live data values on the tool). As I understand it, the pre-cat sensor controls STFT, and the LTFT is actually calculated from stored rolling averages of some sort based on the STFT.

It probably makes sense that you’re not seeing the idle problem until it heats up and goes into closed loop. Before it heats up, in open loop, its using set fuel mixture values. When the fuel mixture is turned over to the sensors, it starts running into a problem.

But I don’t see why it’s taking so long to reach temp. I know you’ve replaced the T-stat not long ago, and it’s not electrically controlled as I recall. And you replaced the rear coolant temp sensor, which I believe is the one used for the temp gauge in the dash. Unless that front temp sensor is somehow messing up the calculation from the rear temp sensor - manual just says they check against each other, but doesn’t elaborate. Either way, if the engine temps are not accurate, that could be causing your idle problem. Could be that front temp sensor, unless you’ve already changed that as well.

That's right. There are three per side. Two before the cat and one after. So much fun!

Since I love throwing parts at the truck, I just ordered the other air temperature sensor and the front coolant sensor. Coupled with the other temp sensor, that should rule out all of the temperature sensors.

I will trouble shoot the o2 sensors tomorrow. I also plan to finally re-smoke the vacuum lines, clean the bank 1 MAF sensor connector, and swap in the new gas cap (why not?).

I also received my prospeed ladder. So, at least I get to install something functional this weekend; vice throwing parts at this idling problem (stupid grin emoji)

 

[jlglr4]

So, I have some info for you. I ran all these values, and mine looks the same as yours. Almost exactly. Those misfire fields I believe are some kind of counters for keep track of the cycle - not actually counting misfires (my engine was running very smooth while these values just keep counting up). And the catalyst value judgments at 8500 I think must be some kind of limit to set a flag (or is meaningless or wrong).

My secondary o2 sensors even looked the same on my first pass - out of balance. Then I realized that the gap tool was simply not updating the values properly - and then completely froze. After I got it going again, I went to those values first, and the two o2 voltages were the same (and fluctuating some, not as much as the first o2 sensors). So, you might want to read those again.

Throttle position - same story. Both look like yours, both move when I hit the gas (until the tool freezes up).

Bottom line, it doesn’t look like any of these values are causing your idle problem.

 

[Michael Gain]

That is great feedback! Thank you for this. You've been very helpful, across the forum! I'll let you know how smoke test and post-maf-connector-cleaning goes.

 

[Michael Gain]

In between installing the ladder, I sprayed down the maf connector with electrical connector cleaner. After about the 10th time, it was not any better. Now that the dirt is off, it does look corroded.

I spent 30 minutes trying to get the internal portion of the clip to release so I could remove the keeper and pins, but that was less than fruitful. I'll probably end up ordering a new pigtail and pre-terminated leads, but we'll see what happens once it's back together.

I also installed the new gas cap. Seemed to seal much better.

Finally, I completed the smoke test of the vacuum lines. No leaks in the lines from pushing smoke. I'll let the smoke dissipate tonight and reassemble in the morning. I'll then use smoke over the running engine to see if it gets drawn in anywhere.

I will also check the fuel trims again at that point.

 

[Michael Gain]

Ok. So here is the analysis, but no definitive cause: as the engine heats up, it goes lean. And, I mean super lean. I replicated the idle bog by revving the engine and then letting the throttle slam shut. When this happened, the STFTs went very negative and only slightly rebounded--causing the idle bog.

I ruled out a massive air leak via smoke test. So, if it is fuel related, that leaves the HPFPs (as everything else was replaced). Or, could that be a symptom of something with the fuel injector replacement? Maybe I did not clean the port well enough and the injector seals are not dealing completely; this, leaking fuel? Even though I see no evidence of this around the injectors.

If not fuel related, maybe it is timing? The GAP tool will not show me any timing variables. It just displays "N/A". I had a compression test performed during the misfire debacle and all cylinders came back solid. Leakdown test confirmed that all valves were dealing properly as well.

I hope that the new coolant and air temperature sensors fix it. As I am at a loss. Here is the actual data. All performed while in neutral:

Idle, cold engine.

~1500 rpm, cold engine

~2500 rpm, cold engine:

Added STFTs, idle, cold engine:

~1500 rpms, cold engine:

More follows.

 

[Michael Gain]

Post 2:

~2500 rpm, cold engine

Idle, warm engine

~1500 rpm, warm engine

~2500 rpm, warm engine

Part throttle, rpms steady

 

[Michael Gain]

Final post for today:

Accelerate back to 2500 rpms:

Idle bog

Also, here is a video of my noisy engine. Granted, the ignition and injector covers were missing when I bought it, but it is still noisier than I think it should be:

https://youtube.com/shorts/WKaNnElOsDk

 

[jlglr4]

Definitely running lean when you’re having the idle issue. The other numbers aren’t terrible (a little lean, but almost in a normal range of less than 10%). Then there is that one outlier where it goes rich - not sure what that was about, but the STFT does bounce around some when moving the throttle.

I’m still hoping it’s that second coolant temp sensor or the air temp sensor. At the warmest readings (still only about 189F), the second coolant temp sensor is only at 68F. That strikes me as a big difference between the cold side and hot side of the coolant system. Maybe it just didn’t have enough time to warm up.

You mentioned before that it was taking 15 minutes to warm up (I assume that was according to the dash gauge). Wondering how long it took for the hot side to hit 87c/189F, and where the temp gauge on the dash was at that point? Comparing the ECT1 temp to the dash might tell you if the engine is actually warming up faster, but the dash temp gauge is not showing it. If ECT 1 shows hot, but the dash is still showing cool, that might be more evidence of a bad ECT 2. If ECT1 shows the engine is really taking 15 minutes to hit those temps, I think the only possibility is a bad t-stat or maybe the fan running too early (fan clutch or bad relay or something). My dash temp gauge hits the middle temp position in about 5 mins, and I’ve seen others on here say the same thing.

 

[gsxr]

My dash temp gauge hits the middle temp position in about 5 mins, and I’ve seen others on here say the same thing.

Same here. My dash gauge also hits the middle (actually around 70°C, IIRC) within ±5 mins driving at 40-50mph, even in cool ambients.