wsrgklt

After a suggestion from @lnengineering on another thread, I decided to check the fuel trims on my '99 C2.

I already had a wifi OBD adapter, so I picked up the OBD Fusion app to gather some data (BTW I don't have a lot of experience with scan tools, but this seems like a ton of functionality for $10). I took a 30 minute drive at various speeds, and then took the data series starting from after the engine had come up to full operating temperature. As you can see from the charts below, Short Term Fuel Trims on both banks are running in mostly +/- 3% but Long Term Fuel Trim is mostly at 8% or more.


I think this means my engine is running lean (that's why it's adding more fuel). I've read that you should keep the fuel trims within +/- 5%, even though the CEL won't trigger until 10%. I want to address this before I put too many miles on my rebuilt engine. My questions for more knowledgeable folks:
1. Am I gathering and reading this data correctly?
2. Assuming there is an issue to be addressed here, what are the next steps for trouble shooting? I gather the primary suspects are the MAF, O2 sensors, and fuel injectors. How do I test these systems with a scan tool?

Charles Navarro

This is a new engine, right? I talk to so many people and read so many threads/emails, that I lose track. :-)

Although I am no authority when it comes to fuel trims and vehicle diagnostics, with the fuel trims adding fuel, it thinks it's lean, when in fact, it's probably running rich and has a vacuum leak or bad injector.

IMHO, first step would be to smoke test the engine to look for vacuum leaks. Usual suspects are oil fill tube and AOS vent hose if they are original.

wsrgklt

@lnengineering As always, your insight is very much appreciated.

This is a new short block with refinished heads and a new AOS. The oil tube is original, but I'm not sure if the AOS vent hose was replaced. Of course, I will address this with my mechanic who did the engine rebuild, and I'm sure he'll test for vacuum leaks. Since this isn't an emergency and we're under shelter-at-home here in Chicago, I'm using this opportunity to learn about my car and see if there are any things I can do to diagnose from home.

Does anyone have suggestions on how to tell if the injectors are bad? The fact that fuel trims are off for both banks would lead me to believe it's an issue with multiple injectors, or something upstream of the injectors, right?

Charles Navarro

If it was me, I would replace all the injectors. Don't risk it. We mandate it for all rebuilds.

I for one wouldn't install a fresh engine without a new MAF or set of primary o2 sensors either. They all are critical to the tune and performance of the engine.

jumpy chunky

If you want to track fuel trims a Durametric is what I would use. The posted graph is tough to decipher, at least for me.
​​​​Your RKAT value or adaptation at idle should be 0. This value is taken at idle, engine at operating temp. RKAT values are most sensitive to intake leaks and will be positive if adding and negative if leaning fuel input. If numbers are below 0, you will need to check MAF numbers. At idle MAF should read around 17 KG/HR.
FRA is adaptation under load and should be 1 or nearly so. Your RKAT value can deviate by about 4 and a half percent before setting a fault. Pretty sure when fuel mixture is balanced O2 sensor will read around 4.5 volts.
This info may be of little or no use without a Durametric.
. edit:. Decimal point misplaced. Should read .45 volts equals balanced mixture....oops.

wsrgklt

@lnengineering I considered replacing the injectors during the rebuild, but last summer the injectors for the 3.4 were $240 each. Luckily, it looks like they've come down from that astronomical price to a merely exorbitant $95 each. MAF looks pretty inexpensive and easy to DIY. I'll probably go ahead and do that next weekend. Which would you consider to be the primary O2 sensors - the ones pre or post catalytic converter?

@jumpy chunky Thanks for the detailed info. My charts aren't pretty because I dumped the data into Excel and didn't spend much time on formatting. I should have left RPM out and taken shorter time window. In any case, I'm not looking to spend Durametric money on a scanner, but I wonder if I can back into the RKAT values from other readings? I can definitely get the MAF and O2 sensor numbers from my current setup and will compare those against the values you suggested.

wsrgklt

Looking at the data logged from that drive, my MAF numbers (warmed up, at idle) are around .8 lb/min or 22 kg/hr. Is that a significant amount of deviation? The O2 sensor values for sensor 1 are reading .75v and sensor 2 around .12v and .16v. I wonder if I'm not reading those correctly.

jumpy chunky

Your MAF value is higher than it should be for a
3.4. Shouldn't deviate more than 1 or 2 KG.
You may want to measure again or clean or replace MAF. Your O2 sensor value shows a lean condition.
I bought new injectors last Summer for $76.00 each, from Sonnen Porsche. A sound investment for the health of your 996. I think following Charles' ecommendation would be wise, injectors, O2 sensors and clean MAF or replace. Used Durametrics can be found for $200.00 on Market place, every now and then.

808Bill

Considering the cost of a rebuild vs the savings on new injectors is small money, Protect your investment!

yelcab

I had some experience with fuel trim issues and they are very hard to troubleshoot without some sophisticated tools, such as gas analyzers. As you want to learn about your engine, I suggest a slower methodical approach is better than a "change everything at once" approach. The slower approach will let you learn about the working of the individual components.

I did not read your other thread so I cannot tell why you want to do this. Is there a running issue? Is there a CEL code? Is there a hot glowing Cat? What leads to this point?

The computer uses the front O2 sensor signal to control the fuel mixture. You should start there by observing the live reads of the O2 sensor signals. It should swing in the range of 0 to 1000 mV during the course of normal operation. 1000mV is a lean condition (a lot of Oxygen) and 100mV is a rich condition (not much Oxygen). Although, if you have an air leak in the exhaust near the O2 sensor, the O2 sensor will read more Oxygen and crank up the fuel which results in a long term +% fuel trim issue.

Start there before you change O2 sensors, injectors, etc

Charles Navarro

Primary are pre-cat.

Scott at Team Harco

Check Pelican for Bosch injectors:
https://www.pelicanparts.com/catalog..._pg1.htm#item1

I got the last Bosch MAF from Amazon at about $120. There are others there going for about $200.

Hope you get it sorted.

Charles Navarro

Just make sure you buy any of these components from a trusted source. There are counterfeits on the market...

jllphan

The injectors you linked are designed for the 3.6, OP has the 3.4. While it is possible to use the 3.6 injectors, some modification is necessary, just a heads up.

Scott at Team Harco

Yes. I've been careful to try and trace the sources.

I'll need to check this. I recall reading about this in the past. More research is needed. Thanks for the heads-up.