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Gm 5.3 low manifold vacuum

by voltageviking on Nov 10 2017, 2:27 AM

Hello fellow techs and DIYers.

I am a technician and recently I was running a diagnostic on a 2004 Chevy Avalanche 5.3L 4WD automatic. Stored code p0456, p0300, and a B1S2 HOS2 signal code (don't remember the numerical code). During my diagnostic, I found Volumetric Efficiency at 52%, with fuel trims increasing with higher RPMs indicating a MAF sensor failure. Diagnosed the B1S2 HOS2 as faulty due to no response to artificially created rich and lean conditions.

During the diagnostic, I found MAP at 12-14 inHg pressure (16-17 inHg vacuum) at idle. Baro reading was accurate. MAP sensor was checked and verified as accurate via a vacuum gauge. I removed the exhaust post-manifold on both banks to verify no clogged exhaust. Fuel trims at idle (with a new O2 sensor) were near 0% indicating no vacuum leaks. Idle speed was normal. I spent at least two to three hours trying to chase down why the manifold vacuum was so low. The customer opted not to replace the MAF sensor, so I don't know if that could have caused vacuum to be out of wack. I checked the induction hoses and air filter and no problems were found. Idle speed was able to be controlled via a scan tool. Cylinder contribution testing yielded no issues. Relative cranking compression test using an amp clamp on the battery cable showed no abnormalities. No noticeable acceleration issues other than slight misfire. I don't think this engine has VVT.

My question is if anyone else has come across this problem of high manifold pressure/low manifold vacuum, or if this is the normal reading for this engine. I have been scratching my head on this one.

Any advice or pointers would be much appreciated!!

-Nate from Baltimore

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7 Replies

By spark derk Sep 21 2018, 3:38 PM

These from the factory had a set of plastic intake gaskets with orange inlays. They will typically show a lean condition, eventually leading to a misfire (p0300). I would spray the intake gasket area with carb clean/propane and watch your fuel trims. If bad, replace the intake gaskets.

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By 1986chevyman Aug 14 2018, 5:29 PM

I would take a close look at the MAF with a low VE reading. Sounds like it could be dirty.

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By voltageviking Nov 30 2017, 6:24 AM

as a side note to BPlusTech (Joe from NY): in terms of Volumetric Efficiency readings, I tend to investigate or at least raise suspicion over any test that shows less than 65% efficiency. There are a number of good calculators out there. I personally use the one on OTC's website. It has come in handy quite a bit. Hope this helps you in your future quests on the diagnostic trail!

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By voltageviking Nov 30 2017, 6:20 AM

Karrpilot:

thanks for the insight! I did remove the MAP sensor and use a mechanical gauge just to make sure it wasn't a faulty sensor. The sensor readings were accurate, at about 16.5-17 inHg at idle. The vehicle has since been picked up, but is a repeat customer of mine and if this developes into an ongoing case, I will update here with my findings. Good to know about the rocker arms checks!

Joe from NY:

Thanks for the feedback as well. I did not scope the MAF for the hertz readings or signal integrity. I do have a lab scope that I am proficient with, but didn't use it in this case. I was going off of the bad VE readings (with verifying no exhaust or intake restrictions) and escalating fuel trims to call the MAF skewed. The valvetrain was my last option to consider about the low Manifold vacuum. Unfortunately I ran out of time with the vehicle and didn't get to pull valve covers. The customer just wanted the O2 sensor replaced. The customer is a repeat customer of mine, somif the truck returns and I get the opportunity to dig deeper I will keep you posted. Thanks for the tips!

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By bbq kid 99 Nov 29 2017, 3:54 PM

When did all these codes happen?? Look at the freeze frame records and see if these codes happen when the engine was cold. You might have felt the misfire when the engine was cold but once it warms up the misfire will go away, if this is the case it might be the intake gaskets leaking vacuum when the engine is cold and it will seal once it warms up. Fel-Pro has an updated intake gasket set to fix this problem MS98016T.

If you look at the voltage for the MAP sensor it should read real close 5.3 Volts

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By karrpilot Nov 25 2017, 2:27 PM

Have you checked the vacuum with an actual old school mechanical guage? If so, and the readings are low, take the valve covers off, disable the ignition system, and have some one crank the engine while you view the rocker arm movement. Those engines are also known to take out lifters and camshaft lobes. A rocker arm that barely moves is the culprit.

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By bplustech Nov 23 2017, 12:22 AM

Hi Nate, the volumetric numbers are way too low to the best of my knowledge. Can you scope the MAF? Also, Those engines are known to have valve train problems. I have seen them run smooth at idle, have 100psi on a compression test and have a off idle missfire that could be causing your P0300. It's usually broken valve springs but could be more serious. Sometimes even pulling a valve cover won't allow you to see the broken spring or the problem with the valves. Good luck and I would love to hear the final outcome. Take care.

Joe from N.Y.

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