4.8L, 5.3L, 6.0L GM Truck rough idle? Trouble Codes PO300, PO171, PO174 and PO332?

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These two conditions seem to be very common, and requires similar repair procedures (removal of the intake manifold). Gm has recognized these problems and has issued a parts revision and TSB (Technical Service Bulletin) to repair these symptoms.

First we will talk about a rough idle. Most vehicle owners complain about a rough idle or a misfire when the engine is cold, and as the engine warms up the problem goes away??? But, leaving your check engine light on, with codes PO300, PO171 and PO174. PO300 is a misfire code that is usually fixed by new ignition parts like spark plugs and wires. But, when accompanied by PO171 and PO174, this indicates a Fuel Trim problem. The engine may be running to lean, due to a intake vacuum leak. When your vehicles engine is cold, the PCM (power control module) is running in ‘closed loop’ a predetermined run program. As the engine reaches normal operating temperature the PCM switches to ‘open loop’ using the engine control sensors to read the data from the engine to refine the running condition. One of these sensors is a oxygen sensor, this monitors the air/fuel ratios in the exhaust to calculate the amount of fuel to deliver to the engine. With a vacuum leak the the computer controls can not calculate for the extra air in the engine, making a lean condition. The oxygen sensor sees the lean condition and the PCM increases the amount of fuel to the engine to compensate. This is called fuel trim, as the fuel trim enrichment reaches 25% or more adjustment the PCM sets a code for the problem.

GM has recognized two part problems with sealing of the plastic intake manifold, the intake gaskets and bolts. The original red/orange gasket has be revised to a teal green colored gasket were the material has been updated. The bolts are sleeved with a rubber crush washer and require a special torque sequence to insure a proper seal. The intake bolts should NEVER be reused!

GM has also issued a TSB (02-06-04-023A) for code PO332 (knock sensor), which may be heard as a mild to severe engine ping usually sounding worse on acceleration. This condition is a result of corrosion on the rear bank knock sensor due to water intrusion into the sensor cavity. This condition is more frequent on vehicles that the owner washes the engine compartment.

To correct the Knock sensor problem requires replacement of the rear knock sensor (their is also a front knock sensor,  we also recommend replacing) and building a dam (wall) around the sensor using RTV to divert water away from the sensor.

APDTY’s kit comes with Intake Gaskets, Foam Pads, Intake Bolts, RTV and Knock Sensor(s) and Optional  Wiring Harness

Tighten the intake manifold bolts in sequence, tighten on first pass to 44Lb in. On the final pass torque to 89Lb in.


ASE Certified Technician

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25 Responses

  1. James Trueblood says:

    I have heard of cleaning the intake inside. How do you do this. Can this cause an air restriction which causes low idle and stalling?

  2. Jason says:

    Have you tried taking off the throttle body from the inake manifold and removing the idle control valve from the throttle body and cleaning both with some throttle body cleaner and a tooth brush? This may take care of your low idle and stalling….

  3. julio says:

    I change the intake manifold gasket and checked all the lines i disconnected and my check engine light is till on and its also burning up more fuel now…

  4. tk says:

    codes on my truck are PO 171/174/ 300/ ive changed all 4 O2 sensors, fuel pump ,filter plugs an wires/ truck runs fine, but after clearing codes enging light came back on. ive sprayed a lot of either around intake tryin to find a vacuum leak with no luck/ wat to do?

  5. Mark says:

    To all that are still having the Bank 1 & 2 Lean codes after changing the intake manifold gasket. I suggest buying new intake bolts and new rubber washers/spacers that go under the bolts. I talked to a tech at GM and he says if you replace the IMG and do the correct torque sequence and are still throwing the codes, it is more than likely the bolts and rubber spacers under the bolts. The rubber spacers actually apply force down equally to each part of the intake manifold. I just had this happen to me on my 07 Gen III 5.3L and had to go to the dealer to order all 10 bolts and Rubber spacers. It now has no codes. Supposedly the rubber has to be a certain durometer in order to apply the correct force down to make the IMG leak proof.

  6. Justosaga says:

    Why you said to change intake bolts?

    • Joel says:

      The intake bolts are crush type bolts and they cannot or should not be reused. Depending on the repair you are making, GM has revised the gaskets to seal better, but this might not work unless new bolts are installed.

  7. Mike says:

    I replaced the entire intake manifold with one that has been totally re-engineered. This would be your best fix. My 2500hd showed significant increase in power and a bit of better fuel economy. The manifold is totally different then the oem

  8. jaw978431 says:

    I just changed my intake manifold gaskets, and have checked every inch to make sure there is no more leak.I changed both knock sensors as well. My truck (2000 silverado 2500 6.0L Vortec, is now showing multiple codes. P0102(mass air flow circuit low), P0122(Throttle/pedal Position sensor circuit low), P0327 (Knock sensor 1 circuit low input bank 1), P0332 (Knock sensor 2 circuit low input bank 1), I have both knock sensor codes thrown twice in a row?? Also an Airbag code, and also P0122 again?(throttle position sensor circuit interm.) All of these things had to be unplugged while doing my gasket change. They ironically are all in the same bundle of wires that runs on a bracket attached to the top of the intake manifold. I have a scan tool that allows me to clear these codes. I also can read live data. For instance, i am throwing the mass air flow alarm, however, my mass air flow readings are normal. Should I be able to just clear the codes and everything be ok? Will the codes come back if there is a furthur issue?? After the repairs i did, I fired up the truck and it started up perfectly and sounds and idles perfectly. It is not overheating or anything. Can someone please give me some advice? I would really appreciate it. This forum is what helped me diagnose and fix my truck. THANKS SO MUCH.

  9. Alexis says:

    Could this problem also cause a ” reduced engine power?” I have been dealing with identical problems with what you described with the PCM. I have codes P0180 P0300 and P1071. Along with those issues I also have a problem where the vehicle will suddenly have a “reduced engine power” message. The car will not respond to the gas pedal and after pulling over and resting for a moment, then restarting it will start and run fine. I have replaced the spark plugs, cleaned MAF sensor, replaced head gasket ( had a mis-threaded spark plug) and am still running rich and rough… any other ideas??

    • Alexis says:

      Oops meant to say cylinder head and gasket. BTW I have a chevy Tahoe 5.3L flex fuel with 140,xxx miles.

    • Jeremiah says:

      I have a 2004 Chevy Tahoe 5.3L flex fuel with right around the same mileage. I was having the exact same problem and getting the “Reduced Engine Power” message. I changed the spark plugs and all that with no luck. Turned out to be a bad fuel injector. The best way I have found to test for a bad injector is with a “Noid Light Kit”. Most auto part stores will let you borrow theirs or rent it for a small charge. It is very easy to use and will tell you real quick if that’s your problem. Good luck!

  10. john brown says:

    Hello…I have a 2003 tahoe, 5.3l. I have a strong raw fuel smell, codes p0200, and p0300. Is this going to be PCM problem? Bad injectors? It had the smell a few days ago along with white smoke. Before I got home it went away. I’ve checked wires involing the fuel injectors, and PCM. Went ahead and changed fuses just to make sure. Help!!

  11. rob says:

    Hi I’ve got same problem Alexis have you solved

  12. ROBM says:

    My understanding is, the vehicle goes into closed loop shortly after starting and that is the mode where the sensors are monitored and the PCM adjust fuel trim to acheive 14.7 to 1 air/fuel mixture. Not open loop as it says in the article. In open loop the car is running a basic program until some of the readings are stable enough to go into closed loop.

  13. Brandon smith says:

    Another issue with earlier year 4.8L,5.3L, and 6.0L is the knock sensor harness they like become open causing a the same knock sensor codes so when replacing your knock sensors don’t forget to check the wiring. Also when removing any intake manifold the gaskets or O-rings should always be replaced. Keep in mind if you had time to do it twice you had time to do it right the first time :)

  14. Gino says:

    Hello, I have a 2008 Chevy Silverado 6.0L vortecmax with 119,000 miles, I am having this shaking problem with my engine. I bought an OBD tester and it showed code p0306 (cylinder 6 misfire detected). So what I did was replace the spark plugs, but that didnt fix it. So I went and replaced the wires, but still nothing and having the same code and shaking/knocking. I then replaced the ignition coil to see if that could be the problem. But I’m still having the same problem. Now am not so sure what it might be. Before I take it to a mechanic I want to see what more I can do to try and fix it myself without taking out the engine. Some help would be greatly appreciated.

    • Bill says:

      I have the same problem as Gino, a 6.0L in a Chevy Express with 94k miles. The mechanic did all the plugs and wires etc. and it still throws the misfire. It runs a little rough at idle once it warms up, so I am going to clean the MAS and the Idle body (guessing on the name there).

      The mechanic said he would have to do a pressure test but he indicated it is throwing due to low pressure in that cylinder. Mine fires the code almost every time I drive on the first acceleration from idle.

  15. Josh says:


    Since youve almost eliminated the ignition system I would check injector pulse from ecm. You can do this with a test lamp. Note that on this engine the ecm pulses ground to the injectors so keep that in mind when testing.

  16. Don says:

    Does the knock sensor wiring just run out the back end of the intake manifold? Just replaced my intake manifold gaskets (Felpro). I have a whistling sound coming from the back end of the manifold. Re=checked the torque and used the proper sequence and that is ok. Just wondering if that wire is causing a leak.

    • Tyler says:

      It would only cause a leak if you didnt lay it down the middle of the penulum when you put the intake on, or if it got pushed over into the gasket. The center penulum is open from the back going to the knock sensors, so if that harness was routed straight back, its fine. More than likely a piece of debris on gasket during the reinstall. How careful were you at getting everything spotless and dry around the valve intake ports and the intake itself?

  17. Ryan Bowling says:

    Ecm, just went through all yalls problems and spent to much money before going to the dealership

  18. trudy day says:

    i have a 2000 gmc 6 liter engine it sat for over a year i had fuses stolen out of it i replaced them i replaced a
    battery changed a cranking sensor ,fuel pump works but it wont stay running it starts every click of the key but keeps shutting down can you help me find the problem ty

  1. July 11, 2013

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