2003 4.3 Vortec, Low throttle, high MAP

[AK49BWL]

This engine is going to drive me up a wall in a minute. I just had the engine rebuilt about 500 miles ago, replaced a ton of parts, but nothing seems to want to fix a problem this 2003 Chevy Blazer (2 door, 2wd, LU3 4.3 V6, 4L60E Auto, 3.42 rear) has had since I bought it with 152k on the odometer.

With very little throttle, my MAP kPa is flying up. At idle, 0%, it's at between 38-41 kPa. With as little as 15% throttle, MAP jumps to 60 kPa. At 35%, MAP is 90 kPa. This issue has remained CONSTANT through replacing the catalytic converter, o2 sensors, every single gasket in and on the engine, the injectors, EVAP purge solenoid, PCV valve and its pipes (including the one that goes to the intake hat), MAP sensor (twice), IAC, then the entire throttle body, and all of the vacuum pipes and their connecting parts at the intake manifold. NONE of this has changed the TPS/MAP curve. I know it's not right because my mom's 2005 Blazer has the exact same engine and PCM configuration, and doesn't do this. I couldn't even get DFCO to activate AT ALL until I heavily dropped the throttle follower and cracker tables in the tune, because the MAP would NEVER drop below 28 kPa under ANY circumstance before I did that. If I shut down the IAC while cruising at 2000 RPM or so, let off the throttle, MAP will drop to 15-ish.

With the MAF on (essentially no tuning done yet), my LTFTs+STFTs are massively enriching, +12 to +20 across the entire table. MAF off, high spark copied to low, trims are leaning out, -4 to -12, mostly between 35-65 kPa... Higher than that and the VE seems to be almost set perfect from the factory. It does seem to respond to changes I've made in the VE table, but I swear with each progressive change my MPG gets worse and worse, and its ability to pull a hill without downshifting does the same. My next up part swap is going to be the plastic upper intake manifold, because I have a spare here on hand... But I have a very strong feeling that ALSO will not change a single thing. So what am I looking at... Cracked lower (aluminum) intake manifold? Bigger internal issue?

Now, it is worth noting that the engine runs smooth as glass above 750 RPM, very little knock detected (by the sensor at least), and it does have very good acceleration, but I feel like it could do so much better considering I've taken the clutch fan off and opened up the intake filter box for better airflow. It just feels like the power isn't there where it should be. My 1999 4 door 4x4 Blazer got better mileage on the highway than this thing does, and I never even got to tune that one. Any ideas? Should I just throw this thing against a wall and V8 swap? lol.

[Fast4.7]

This engine is going to drive me up a wall in a minute. I just had the engine rebuilt about 500 miles ago, replaced a ton of parts, but nothing seems to want to fix a problem this 2003 Chevy Blazer (2 door, 2wd, LU3 4.3 V6, 4L60E Auto, 3.42 rear) has had since I bought it with 152k on the odometer.

With very little throttle, my MAP kPa is flying up. At idle, 0%, it's at between 38-41 kPa. With as little as 15% throttle, MAP jumps to 60 kPa. At 35%, MAP is 90 kPa. This issue has remained CONSTANT through replacing the catalytic converter, o2 sensors, every single gasket in and on the engine, the injectors, EVAP purge solenoid, PCV valve and its pipes (including the one that goes to the intake hat), MAP sensor (twice), IAC, then the entire throttle body, and all of the vacuum pipes and their connecting parts at the intake manifold. NONE of this has changed the TPS/MAP curve. I know it's not right because my mom's 2005 Blazer has the exact same engine and PCM configuration, and doesn't do this. I couldn't even get DFCO to activate AT ALL until I heavily dropped the throttle follower and cracker tables in the tune, because the MAP would NEVER drop below 28 kPa under ANY circumstance before I did that. If I shut down the IAC while cruising at 2000 RPM or so, let off the throttle, MAP will drop to 15-ish.

With the MAF on (essentially no tuning done yet), my LTFTs+STFTs are massively enriching, +12 to +20 across the entire table. MAF off, high spark copied to low, trims are leaning out, -4 to -12, mostly between 35-65 kPa... Higher than that and the VE seems to be almost set perfect from the factory. It does seem to respond to changes I've made in the VE table, but I swear with each progressive change my MPG gets worse and worse, and its ability to pull a hill without downshifting does the same. My next up part swap is going to be the plastic upper intake manifold, because I have a spare here on hand... But I have a very strong feeling that ALSO will not change a single thing. So what am I looking at... Cracked lower (aluminum) intake manifold? Bigger internal issue?

Now, it is worth noting that the engine runs smooth as glass above 750 RPM, very little knock detected (by the sensor at least), and it does have very good acceleration, but I feel like it could do so much better considering I've taken the clutch fan off and opened up the intake filter box for better airflow. It just feels like the power isn't there where it should be. My 1999 4 door 4x4 Blazer got better mileage on the highway than this thing does, and I never even got to tune that one. Any ideas? Should I just throw this thing against a wall and V8 swap? lol.

Seems about normal to me. In the 1,500-2,500 rpm range you should be to near atmospheric pressure by 40% throttle. Even the modified 5.7 in my Express van that pulls a good bit more airflow for a given rpm than your V6 is very near atmospheric pressure at 40% throttle. With 3.73 gears cruising 2,250 rpm @ 70 mph on flat land I ran about 18-22% throttle and in the 55-65 kpa range. Idle KPA sounds about right. It almost always took a high speed deceleration in a gear lower than OD to activate decel fuel cut. Those values stayed fairly consistant with 3 different cams as well. Factory Vortec cam, a 395' Marine cam, and a factory LT4 Vette cam. Even Lunati's version of the LT4 Hotcam only really changed the idle KPA up about 8-10 KPA. Cruise KPA stayed very similar.

Also is there a chance they used something along the line of the 1995 L35 CPI cam in that engine? The cam in that engine had a fair bit more duration and lift than the stock later model Vortec cams.

I will also say that if your engine has the MFI spider, make certain your injector flow rate is set for 22.1 lb/hr rather than the 23.1 of the older spider. My stock spider ran at 66 psi where the replacement ran at 62 psi. Finally if you are running gasoline that contains up to 10% Ethanol, you should be running with the Stoichiometric AFR set to 14.1:1 instead of 14.7. Stock 2002 Express van tune on my 1997 Express van with the MFI spider and E10 was adding 10-15% fuel in places. Once I discovered the spider difference and corrected the Stoichiometric air/fuel ratio the tune fell in to place much more easily.

Curious as to what you call poor fuel mileage? I took a 2,200 mile road trip running at 14.1:1 at highway cruise with 42* of timing on flat land and averaged 17-18 mpg nearly every tank running between 75 and 85 mph most of the way. Marine cammed 5.7, 4L85E, 3.73 and 30.5" tall tires at the time.

Unless things are far too rich you should see increased torque and less downshifting running slightly richer than you do lean.

Seems like its time to invest in something like an AEM Wideband and see where your fueling actually is.