Short Term Fuel Trim Graph Analysis - What is it?

[craiganator]

Any ideas what is causing the short-term fuel trim to be so erratic shown in the graph? (lower graph line in red)

99 Ford Explorer
230K miles
symptoms: surging and skipping. Threw a P0171 and P0301 code before resetting. Have changed the intake manifold gaskets per the common problem threads, yet problem still exists.

Does this graph point to anything obvious for the experts?

 

[96eb96]

What are your LTFTS? Are they climbing after your reset your computer?
If you are sure there are no other vac leaks - rubber hoses checked, etc. then that could implicate a bad 02 Sensor. An O2 sensor that periodically flatlines(unless there is a reason like a vac leak) is usually due for replacement.

 

[2000StreetRod]

Pre-cat O2 sensors cycling

Which engine? If the SOHC V6, did you replace the intake manifold upper and lower gaskets?

I'm no expert but your pre-cat O2 sensors are cycling between about 0.1 volts (Lambda=1.02) and 0.8 volts (Lambda=0.98) which is a typical range for an aged sensor when the PCM is in closed loop. A new sensor would probably reach 0.9 volts for Lambda=0.98 (A/F=14.4). I doubt replacing the pre-cat O2 sensors will correct the issue.

P0171 System too Lean (Bank 1)
P0301 Cylinder 1 Misfire Detected
Is the graph for the time after the codes were cleared?
Clearing the codes may also reset the fuel trims. I know disconnecting the battery to clear the codes also resets the fuel trims to 1.00 or using Auterra Dyno-Scan 0%. If the trims were just reset then the graph shows the PCM adjusting them based on the received O2 sensor outputs. I agree with 96eb96 that it would be helpful to show the LTFTs. The PCM may be increasing them to allow reduction of the STFTs. According to the 1999 MY OBD System Operation Summay DTC P0171 is set when the LTFT exceeds 25% and the STFT exceeds 5%. I agree that the Bank 1 STFT varies significantly more than the Bank 2 STFT.

Is the intake system stock?

 

[2000StreetRod]

smoke test

I just found and read your other related thread: HELP: Not your normal Head Gasket issue, 99 EXP w/ 230K miles and learned that you have the SOHC V6 engine.

I watched the video and read that smoke is escaping between the head and the block on the driver side (Bank 2) of the engine. This is inconsistent with the lean code and misfire on Bank 1 (passenger side). It would be unusual for the head gasket to fail in an area away from the combustion chamber. Also, good compression and no loss of coolant does not support a blown head gasket. It seems more plausible to me that the leak is in the front timing cover or between the front timing cover and the block. The photo below by CDW6212R shows where the timing cover mates to the block and the shape of the gasket between them.

I would not pull the head until I was certain of the source of the smoke leak. Head gasket replacement requires new head bolts, gaskets, and retiming the camshafts which takes a special tool set.

 

[craiganator]

Responses to input

Answers for 2000StreetRod

Thanks for the response and input. See my answers below...

Is the graph for the time after the codes were cleared? Yes. I had cleared the codes before graphing.


It is the SOHC V6 and I have changed the upper and lower intake manifold gaskets.

I'll need to run it again to get the LTFTs for more useful data. Will do next.

The intake system is stock.

I agree to a certain degree that the smoke escaping on Bank 2 seems inconsistent with the lean code and misfire on Bank 1. Here was the rationale until we concluded that it was not the issue. I was thinking that ambient air was entering an opening in the head oil gallery on Bank 2, traveling through the crankcase, then coming out the valve cover hose on Bank 1 and enter the intake hose between the MAF and Tbody. It was a stretch but confirmed it was not the issue by running the engine with this valve cover hose blocked, thus no additional air entering this way. It still had the skip and looked like the graph. The smoke test showed only the area between the head and block to leak smoke. The intake did not smoke or any other area of the vacuum lines/system. There is a slim possibility we mistook the head/block area and it could be the timing cover area. Even so, the same blockage of the valve cover hose would have eliminated this extra air coming into the air stream between the MAF and TB. Does that make sense?

I guess what I am saying is either the timing cover mating surface or the cylinder head mating surface only could pull air into the crakcase and then out the Bank 1 valve cover.

It is still interesting that only Bank 1 STFT is erratic.

Do you think there is any merit in the MAF causing only Bank 1 to have an issue?

 

[craiganator]

for 96eb96: I will need to re-run the graphing software pulling the LTFT numbers also. Will post afterwards...

thanks for the input.

 

[craiganator]

Ok. Drove the exploder for about 3.5 miles around the block while collecting data including the Long Term Fuel Trim values for both bank 1 and bank 2. Bank 1 is where the issue lies. And before resetting the codes, they were p0171 and p0301 which indicates an issue in cylinder #1 right?


Does this mean it is constantly trying to add fuel to cylinder # 1 but there is too much air in cylinder 1 to get the mix ratio at 14.7:1? and does that possibly indicate a clogged fuel injector on cylinder 1?

 

[craiganator]

more data...

 

[2000StreetRod]

lean at low flow

It looks to me that your engine is running lean at low airflow rates. As the airflow increases the long term fuel decreases from rich (+) toward neutral. When the airflow decreases the LTFT increases toward rich. Both banks do it but Bank 1 is significantly worse than Bank 2.

At low airflow a vacuum leak is more significant (percentage of total flow) than at high airflow. Did you inspect the green O ring that seals the EGR pipe where it enters the intake manifold? If not careful, the O ring can be damaged when removing/installing the upper intake manifold.

 

[craiganator]

Problem solved.

"This is inconsistent with the lean code and misfire on Bank 1 (passenger side)." It was your idea here, 2000StreetRod, that got me pointed in the direction of cylinder number 1.

It did throw the codes P0171 and P0301 before I reset them. I used a mechanics stethascope to listen to the injectors and all in Bank 1 clicked and didn't check them electrically as I thought they were functioning. To be sure, I Ohm'd out the injectors and they all read 15.3 Ohms except, you guessed it, cylinder #1. No resistance. So, I spent $73 on an injector and replaced it. Below is the graph of the drive around the block afterwards. Both Bank 1 and 2 STFTs were duplicates and the vehicle ran good.

Thank you for the input and help pointing me in the right direction.

As a side note: the smoke test still showed there is a leak between the head and the block on Bank 2, but it has no impact on excess air flow into the system nor loss of compression or coolant leaks. I believe this is a spot where the exploder has an oil leak and it is just pure luck the head gasket is not compromised in the combustion chamber or water jacket areas.

 

[2000StreetRod]

fuel injector cleaner

I'm glad you solved the problem. Those long term fuel trims look much better and as you posted, the short term fuel trims seem to track. If you have more than 50,000 miles on your fuel filter, I suggest you replace it. Also, if you haven't added a can of fuel injector cleaner to your tank in the last year I suggest trying a can of Techron.