Hello,
I have a 96 Z28 that i have been fixing up. The car has been running great until the other day i got codes for bank 1 and bank 2 rich. At idle, the i had around -15 on both sides and under throttle, i was getting close to -30 under light throttle going down the road. If i unplugged the MAF and forced it into speed density mode, fuel trims were plus or minus 1.6 on both sides at idle and at high RPM. After replacing the MAF, i have the same result. The first MAF was a Duralast from Autozone and the second was a BlueStreak from Oreilleys. Has anyone had trouble with new MAF's on these engines? Is this a situation like the optisparks where you have to buy a certain brand to get things to work right? Car runs great, but the fuel trims need to be addressed...
Repairs made so far-
-New Precision fuel pump
-Petris Opti
-New Bosch O2 sensors -all 4
-Spark plugs- ACDelco
-Water Pump
-Flow matched injectors from Fuel Injector Connection
-Remanufactured 4L60E transmission

 

Are the numbers you posted the percentage fuel trims? Are they the long term or short term fuel trims? Generally the LTFT’s are limited to -15%.

Did you get codes P0172 and P0175?

I've never used anything but AC Delco O2 sensors, but based on reading and helping with posts over the 24 years on this site, many people complain about less than good results with any other brand O2 sensors.

Don't recall any issues with brand of MAF sensor.

Have you checked the rubber intake elbow for cracks, and the bottom edge not being fully over the throttle body on the bottom? Possibly a leak in the silencer tube attachment point? All of those leaks bypass the MAF sensor, but would not affect the speed-density calculations.

Have you checked the fuel pressure?

 

Test your fuel pressure regulator.

Possible the diaphragm failed and it is allowing fuel into the intake via the vacuum line, causing it to be rich.

 

Yep, there is a bit of fuel in the vac connection for the regulator. KOEO- fuel pressure is holding at 45psi. I will replace the regulator and report back. It was wet but not squirting out or anything. Intake elbow is in good shape. I would love to get an ACDelco, but i was informed by my dealer that they can't get them anymore. I did see that there is a ACDelco Reman for sale on rockauto.

 

You lost me. You “would love to get an AC Delco”….. what? O2 sensor, MAF, intake elbow?

The original part numbers for the GM MAF sensor were superseded by 19207203, according to Shoebox's parts list. AC Delco part #213-4657.

1993-1997 F-Body Part Number Collection

If you meant MAF:

https://www.carparts.com/mass-air-fl...elco/AC2134657

I would think if the FPR diaphragm was leaking, the pressure would not hold KOEO.

 

Ok, i replaced the FPR as suggested- you guys were right about the pressure leaking down- i guess i just didn't watch long enough. Fuel trims are now Bank 1 & 2 short term -1.6 to 0 Long term 1&2 is 0.8. so thats good. Now my next problem is that the system still isn't holding pressure KOEO (just falls to 0). Im guessing that my new fuel pump has a defective check valve. *Sigh* Good thing i put in the access door B)

 

What brand fuel pressure regulator did you get?

 

I got a Standard motor products FPR. Fuel pump is a Precision. I tend to get my parts from Oreilley's because they cover my labor on bad parts with a special warranty.

 

Could also be a leaking line, including the flex hose in the tank. While the injectors are new, maybe one is leaking.

 

-New Precision fuel pump
-Petris Opti
-New Bosch O2 sensors -all 4
-Spark plugs- ACDelco
-Water Pump
-Flow matched injectors from Fuel Injector Connection
-Remanufactured 4L60E transmission

Give us the specs and part numbers of these injectors. If you have a link to them, please post it up.

 

Here is a link to the injectors-
Fuel Injectors

 

Edit for the below post....I totally missed seeing that you said the trims were better after the FPR work.....so I'll leave my post as is below just as food for thought.

I wonder about the injectors and the O2s. Like Injuneer mentioned, I've seen multiple complaints about Bosch O2 sensors on our machines over the years. Based on this, I've only used AC Delco O2's on mine.

I know they claim stock replacement on the injectors, but until I see some characterization data on them.....they could be causing some over fueling issues. Being rebuilt makes me raise an eyebrow too....I would not put rebuilt injectors on my machine....but that's just a me thing. Seeing as you said it runs better in SD mode, that may be evidence that the injectors are fine. How long did you run it in SD mode? Was it long enough for it to warm up and go into closed loop?

Be sure to verify what Injuneer mentioned above, then I'd get AC Delco sensors for the pre cat O2 positions left and right banks. The post cat positions can wait if you need to save money as they are just watching for cat effectiveness. The pre cat ones are the ones controlling the fueling in closed loop.

 

The SD trick is something that i have been using as a troubleshooting measure on GM vehicles for sometime now- except it has been on 99+ engines. Because GM uses both the MAP and MAF sensors to calculate the VE tables, you can unplug either and the ECM will automatically set a fault and use the other sensor for fuel trims. So, if you suspect a bad MAF, you can simply pull up your fuel trims on a scan tool and unplug the MAF while the engine is running. If you see a significant change, then you have found your problem. THe newer rigs will continue in fuel correction. This 96 ECM seems to be different though and threw me off- it seems that unplugging the MAF on this system sends it into open loop and is not really a very good troubleshooting measure for fuel trims, although, if you are having a major drivability (rough running engine) and unplug the MAF and then have a good running engine- then i suppose that is the only place it could be useful here. This also goes for most every newer import that i have delt with.
The car sat for about 6 years with a bad trans and the old gas pretty much destoryed everything in the fuel system. I wanted to get a flow matched set from here in the states in hopes of decent quality control. THe way i look at it is; somebody ran these on a test bench whereas the new ones from China rely on you, the end customer, to find the bad ones. I have been burnt twice with new fuel injectors. So far, I have used this company on three different rigs and everything has worked out great. It is getting more and more difficult to get good parts for anything. My parts guy at the local Chevrolet dealership informed me that AcDelco is now just rebranding Chinese junk and i was able to confirm this when i got a Chinese thermostat for my 97 Grand Prix 3.8 in an AcDelco box from the dealership (the engine will bounce back and forth between 215 and 175- very lazy cheap thermostat). The parts they sell there for all of the new vehicles just say GM and there is no reference to AcDelco. While i love OE parts, i have become very skeptical on AcDelco parts as of late. Maybe some new old stock stuff would be the exception.
As far as the Bosch O2's go, i have been using them for 8 years and have only had one problem and that was on a 2018 Silverado 1500. I guess that could be something to consider, but the car runs strong and idles nicely so im reluctant to condemn them. I replaced all 4 sensors earlier as the car still had the originals in it when i got it- had to break them off and put my Milwaukee 2767 on what was left to get them to come out of the pipes. THen had to run a tap through the bosses to repair the damaged threads.
When i have time, I am planning to disconnect the return line at the fuel rail, place a hose on it and run it to a bottle. THen ill hit the key a few times and watch my fuel pressure gauge and the line running into the bottle to see if the pressure is bleeding through the regulator. If that checks out, I will pull the supply line off the rail and dead-head it into my pressure gauge to see if it holds pressure that way- If not, i guess Oreilley's will be paying me to replace a defective fuel pump/sending unit.

 

FIC seems to be a very reliable source for injectors. He states the Bosch III 24 #/HR injectors will work with stock 24 #/HR programming. I guess the offsets must be similar. As I recall he also has a video on his site showing the spray patterns of the stock Multec and the Bosch III injectors. He feels the Bosch injectors should actually improve performance.

In OBD-2 the PCM uses the speed-density method to calculate the mass air flow. It compares the MAF sensor indicated flow to the speed-density calculated mass air flow. If the two numbers do not match (within an unstated %) it sets P0101 for MAF sensor performance (rationality). The speed-density method relies on RPM, cylinder displacement, and the VE table to calculate volume air flow, the converts volume flow to mass flow using air pressure (MAP) and air temperature (IAT) and the perfect gas law. The unknown (at least to me, not in the factory service manual) is the % difference in the two methods that triggers the difference. I've seen numbers like 15% and 25%, but have not verified them.

Point is if the LTFT's are maxed out at 25% and the PCM is using the STFT’s to add even more fuel due to the MAF error, it would seem logical that the P0101 code would set.

As far as the Bosch O2 sensors, I have no firsthand experience. But over the 25+ years I've been on sites like this, I've seen a number of threads where someone indicated they had problems with the Bosch sensors on an LT1 engine. I've used Bosch O2 sensors on other vehicles, but never had to replace them on my 94 because it's been running open loop on an aftermarket engine management system for 22 years.

 

It's easy to test for a leaking injector -

Take out the 4 screws that hold the rails to the manifold, lift the rails with injectors out and place paper towel under the injectors.

Cycle the key on and off several times to run the fuel pump.

Look and see if you have any wet paper towel.