1996 chevy c1500 5.7l lean trim readings under a load

Morning All:
I have a 1996 chevy c1500, 5.7l engine. The engine is a new factory GM engine it has about 600 miles on it right now. I am getting High Positive LTFT numbers under a load ( normal highway Driving). Fuel trims at Idle, and when I raise RPM up seem to be ok to Me.

Here are my readings:
Cal LD 25.1
ECT 194
MAP 15 IN HG
RPM 2052
SPEED 66
SPK ADV 38
IAT 79 DEGREES
MAF 5.28 LB/MIN
TPS 10.2

STFT BNK 1 - 0.8 TO 0.8
LTFT BNK 1 13.3
STFT BNK 2 - 0.8 TO 1.6
LTFT BNK 2 14. 8

O2 SENSORS ALL SEEM TO BE SWITCHING AS THEY SHOULD, i DO HAVE IN MY OPINION A HIGH IDLE WHEN COLD STARTING FOR A FEW SECONDS AROUND 2000 RPM THE DROPS TO ABOUT 1000 UNTIL ENGINE WARMS A LITTLE THEN DOWN TO ABOUT 680 TO 700 RPM. I PULLED AND CLEANED THE MAF SENSOR YESTERDAY AND AT IDLE FUEL TRIMS LOOKED PRETTY NORMAL AS WELL AS AT 1500 RPM TO 2500 RPMS WHILE IN PARK. UNDER A LOAD AT HIGHWAY SPEEDS WHILE THEY WERE LOWER THE LTFT WERE STILL HIGH RUNNING ABOUT 9.4 TO 10 T0 11.
ANY THOUGHTS WOULD BE APPRECIATED, i AM STILL LEARNING ABOUT FUEL TRIMS, WHAT CONFUSES ME RIGHT NOW IS HOW THEY LOOK OK AT IDLES RPM"S BUT HIGH POSITIVES UNDER A LOAD. LEAN IS SUGGESTING UN METERED AIR, BUT NOT SURE WHY IT IS NOT SHOWING UP AT IDLE SPEEDS. HOPE i GAVE ENOUGH INFO.

I'm not very familiar with those motors but a lean condition can be caused by either more air or less fuel. I would check the fuel pressure, make sure the injectors are not clogged. On the air side you can check the EGR operation to make sure it's not sticking but at those higher RPMs it shouldn't really matter.

I INSTALLED A NEW AC DELCO MPFI SPIDER INJECTION ASSEMBLY WHEN I INSTALLED THE NEW ENGINE. I ALSO REPLACED THE FUEL PUMP ABOUT 2 MONTHS AGO. FUEL PRESSURE IS AT 62 PSI. I DON'T THINK ITS A FUEL PROBLEM FUEL PRESSURE WILL DROP 4 TO 5 PSI AND HOLD FOR 10 MIN OR BETTER. EGR IS THE OLD ONE SO ITS A POSSIBILITY. THANKS FOR THE REPLY

Question, should I or shouldn't I be seeing a bigger swing in the STFT readings attempting to correct the lean condition ? I realize the LTFT is responsible for keeping the STFT at or near 0% , I was just wondering if I should see a bit more swing in the STFT.

That was my thought too when I first saw your post i.e. the STFT is not reflecting what LTFT reading is. Could they be from before? Have you reset the counters?

Other thoughts 1) both banks are showing lean so the issue is likely upstream i.e. the MAF/MAP or an air leak. 2) you mentioned when you first fire it up the RPMs are unusually high, if so, again could point to an air leak that seals up as the engine warms up but under load it's sucking in air.

Yea I pulled and cleaned the MAF and reinstalled I also sealed the air cleaner indicator it was cracked at top ( it's the little plunger type). It did help but still high LTFT and stft basically the same. I'm thinking something other then a simple vacuum leak. I'm actually wondering if my brake booster might be the issue. I dont have a smoker but I will try to DIY one this weekend and smoke the engine or just try water or carb cleaner.

The idle issue is at start up and doesn't last but a few seconds before dropping down to a normal idle range. I agree with the air leak suspect or the IAC it's a aftermarket. I been told by more then one that these vortex engines perform better with OEM parts. I am going to work on swapping back to OEM parts.

Do you have a cam retard offset pid ?

Yes, cam retard is set to 0. I haven't had much time to work on it lately but, its going to be a MAF problem or a fuel delivery issue most likely. I am going to smoke the engine this weekend just to check for leaks, as well as check fuel pump and MAF, saw a video Paul did on cleaning a MAF with a small brush going to try that. I will post my result.

Hi guys, let me know if i'm wrong but this engine is not a speed density one (not map comanding fuel injectors).

That mean, when you have a vaccum leak, rpm goes down.

if this is a MAF engine and your trims look good at idle, I would move away from vacuum leaks. A vacuum leak would have high trims at idle that get better with RPM. You have the opposite, indicating a lack of fuel or restriction in air.

Can you check fuel pressure under load? It's possible to not have enough volume, but have good pressure, if that makes sense. How about an injector balance test?

What was the reason for! he engine replacement?

Nissan Technician

Yes it is MAF engine, and I agree we are looking at either a fuel delivery issue or a under reporting MAF sensor. I did clean MAF with just MAF cleaner trims got better but still high, however, they are not high enough to set any codes, it's more of a poor fuel mileage issue at this point. Truck has almost 326000 miles on it body and all are in good condition, I had some issues with original engine leaking intake and some other things, was going to have it rebuilt. After some research found it more cost effective to just purchase a new engine. Turns out the motor that was in it was a reman from autozone. I will dive into the fuel delivery diagnostics this weekend, to see if it is a pump issue or injector issue. I dont have a scanner that is Bi- directional so I cant do a injector balance test, myself. Unless someone can tell me a way to do a test on a MPFI system with out a scanner.

Guafa, This engine is a MAF system.

Tutti 57, Just a thought in regards to the fuel injector balance test, is it possible to disconnect the harness to the injector assembly, and use a jumper wire and fuel pressure gauge to safely conduct a balance test ? I know you would only be able to pulse each injector once during a cycle.

Hey Everyone, just wanted to add an update on my lean condition. After startup this morning I had a P0102 code low voltage circuit. I hook scanner up and monitored the trims while heading into work. What I noticed was the LTFT were correcting the STFT numbers alot better. Bnk1 Lt 6.8 t0 7.0 St - 2.3 to - 3.1 bnk 2 , LT 7.0 to 8.0 ST - 2.3 to - 3.1. Under load. That said I will pull MAF and clean it good with a small brush and MAF cleaner, see how that does, then go from there. I will go ahead and check for possible fuel delv issue to be sure but I think this may be a MAF issue.

Would you mind to report Maf readings after clean it?

I have the idea this engine should report 4.8 to 5 lb/min at idle and something about 9 lb/min at 2000 rpm

Thanks