Need help diagnosing after rebuild.

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joesenior79

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I have a 97 5.7L L31 I just finished completely rebuilding, (with a Black Bear Tune).

The issue I'm having is its a hard start and stalls anytime I put it into gear and I have hardly any power under load, struggles to make it up any decent size hill. It's great first thing in the morning but gets worse after warming up.

I have an Autel MX808 and it's not throwing any codes. However, there's a couple cylinder misfires at idle but goes away after giving it a little gas.

The only things that I've noticed abnormal are: the IAC counts are high, around 136 at idle and gets even worse while under load, and my barometric pressure is also reading high at 12.33 at idle. I'm not sure what my TPS volts at idle should be but it's reading .55v.

All these sensors are brand new and AcDelco. Am I missing or overlooking something here or maybe some faulty sensors?

I've looked for a vacuum leak with smoke and nothing.

Any advice or thoughts would be much appreciated.
 

SAATR

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What unit is you barometric pressure, PSIA? Unless you're at 4000ft or above, that seems low. I'm near sea level and am at approx 30inHg, 14ish PSIA as of 5 minutes ago. If you are at elevation, then that may be correct.

What is your idle speed vs. desired idle speed when your IAC counts are high?

Here's some sample data from a good running 98 L31 engine, at operating temperature, with AC on, 87F ambient, close to sea level:

Idle - 630rpm
Desired idle - 625rpm
Coolant temp - 192F
Intake temp - 130F
IAC position - 64
TPS voltage - .52V
Baro - 30inHg
MAP - 10inHg
Spark Advance - 23 degrees
MAF - 7.5 to 7.8 g/s
Calculated load - 3 percent

Hopefully that gives some basis for comparison.

I'm interested to know what your fuel pressure is, and whether or not you're in closed loop during the loss of power.
 

Schurkey

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I have a 97 5.7L L31 I just finished completely rebuilding, (with a Black Bear Tune).
Reinstall stock tune, see what happens.

my barometric pressure is also reading high at 12.33 at idle.
I'm old. I'm used to thinking in terms of manifold vacuum instead of manifold pressure.

IF (big IF) your weather shows 30 inches of mercury, and you have 12.33 inches of mercury pressure in the manifold, 30 - 12.33 = 17.67 inches of vacuum. Not horrible, weather/altitude dependent. Could be better. Is your scan tool reporting pressure in inches of mercury?

You can get the actual barometric pressure (weather) by checking the MAP sensor when the ignition is on, engine not running.



What are the cranking compression pressures for each cylinder?
What is your fuel pressure?
What are your short- and long-term fuel trims?
What is the coolant temperature?
Are your O2 sensors appropriately active?
Are the outlets of the catalytic converters hotter than the inlets? Converters not plugged?
 

joesenior79

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Sorry for the screenshots, I'm on break at work. I do have P0300 (multiple random misfires) and runs extremely rich at idle but goes away after while driving or of i unplug the MAF. But then I have no power. My fuel trims are usually -3 but I was messing with them this morning and reset them.
 

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joesenior79

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Reinstall stock tune, see what happens.


I'm old. I'm used to thinking in terms of manifold vacuum instead of manifold pressure.

IF (big IF) your weather shows 30 inches of mercury, and you have 12.33 inches of mercury pressure in the manifold, 30 - 12.33 = 17.67 inches of vacuum. Not horrible, weather/altitude dependent. Could be better. Is your scan tool reporting pressure in inches of mercury?

You can get the actual barometric pressure (weather) by checking the MAP sensor when the ignition is on, engine not running.



What are the cranking compression pressures for each cylinder?
What is your fuel pressure?
What are your short- and long-term fuel trims?
What is the coolant temperature?
Are your O2 sensors appropriately active?
Are the outlets of the catalytic converters hotter than the inlets? Converters not plugged?
I can get a more recent compression numbers and fuel pressure after work today. I had check them both before Ave they were in spec.
 

joesenior79

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What unit is you barometric pressure, PSIA? Unless you're at 4000ft or above, that seems low. I'm near sea level and am at approx 30inHg, 14ish PSIA as of 5 minutes ago. If you are at elevation, then that may be correct.

What is your idle speed vs. desired idle speed when your IAC counts are high?

Here's some sample data from a good running 98 L31 engine, at operating temperature, with AC on, 87F ambient, close to sea level:

Idle - 630rpm
Desired idle - 625rpm
Coolant temp - 192F
Intake temp - 130F
IAC position - 64
TPS voltage - .52V
Baro - 30inHg
MAP - 10inHg
Spark Advance - 23 degrees
MAF - 7.5 to 7.8 g/s
Calculated load - 3 percent

Hopefully that gives some basis for comparison.

I'm interested to know what your fuel pressure is, and whether or not you're in closed loop during the loss of power.
I'm Utah which is about 5,000 feet.
 

SAATR

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Your MAP is high for idle.

If you subtract barometric pressure from MAP you'll get manifold gauge pressure.

My engine:

5psia - 15psia = -10psig manifold pressure

Your engine:

7psia - 12.5psia = -5.5psig manifold pressure

Either your suspicions about a vacuum leak are correct and you haven't found the source, your vacuum problems are being caused by the tune or engine mechanicals, or your MAP sensor is faulty.

I would plug the brake booster line, the PCV vacuum hose, and the crankcase fresh air supply on the passenger valve cover and run the engine. You should get positive pressure out of the PCV port in the valve cover. If you get a vacuum, you have an internal vacuum leak.

Failing that, I would verify the MAP readings with a known good gauge and see if you don't have a faulty sensor.

Compression test, as suggested by Schurkey, should verify good valve and ring seal.

I'm assuming that the data you are showing is a cold start and run by the low 14.1 A/F ratio and high desired idle. Hot running numbers would be more helpful. If I'm wrong and that IS hot idle, I'd like to know what the target A/F ratio is.
 

joesenior79

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You're right, .cold start. Didn't have much time at break to let her warm up. But the numbers are even worse after warming up. Looks like I'll be looking for a vacuum leak again this weekend. I'm hoping to hell it's not an internal leak, tearing into this motor for the third time this year does NOT sound like fun.

Thanks again.
 

joesenior79

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So I'm curious, did my local machine shop over port my heads? I asked them to gasket match my heads and now I'm wondering if that might've been too much and could be causing an internal vacuum leak? If so, is there an easier and less expensive remedy for this other than purchasing new heads? For example, maybe add some metal back to the ports via welds? I'm assuming not, probably too many issues that could cause if not done correctly. I will need to verify if this is the issue tho.
 

stutaeng

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Those fuel trims look out of whack. I usually see these in percentage point. Less than 5% +/- is ideal. 10-15 is getting out of range, but some ECUs seem to still compensate up to 25%.

Over that, causes driveability issues.
 
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