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Michael Moore
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code 171 and 174 on a chevy avalanche 2003 with a 5.3 engine

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code 171 and 174 on a chevy avalanche 2003 with a 5.3 engine does that also mean that a mass air flow sensor is bad as well check engine light stays on and will idle rough when cold in mornin
This code is for a lean engine, possible vacuum leak. How many miles on the engine?
Customer: replied 7 years ago.
100 thousand i had the intake manifold checked and it is good readinbg so i was told by so many people that it is either a mass air flow sensor or a oxygen sensor it idles rough in mornin when it is very cold and smooths out once it warms up but still idles a littl rough and check engine light use to come on and stay on till weather outside warmed up and then it would go out but now it stays on all time

The mass air flow sensor can cause this issue, if you have a K and N style air filter the oil can get on the sensor ans cause a false reading. You can clean the sensor to make sure there is nothing on the sensor such as dirt or oil. Remove the sensor from the air cleaner, spray some carb or throttle cleaner in to the sensor. You should be able to see small wires inside, clean the wires with the spray. It does not take much to clean the sensors.

The O2 sensors are less likely to cause this issue with out setting a O2 sensor code, and both front o2 sensors would have to have the same problem since you have lean codes on both banks of the engine.

The intake gasket, How was it tested? You may have heard that its been a problem. GM has redesigned it and has a bulletin for your symptoms. Cold rough idle, p0171 and p0174. Lets explore it a little. If you have never had them replaced before, then they are bad. The gaskets go flat and can no longer seal. The new gaskets do not have this problem. The gaskets normally leak at the bottom of the intake and is very hard to test. If you have some time, remove the intake, its not hard and the gaskets are reusable. remove the intake and look at the gasket. If they are orange, they are the original, if they are green, they are the new style. Look to see if the rubber part is flat in any area, if it is, then they are not sealing.

Customer: replied 7 years ago.
it was tested by a guy i know that had a gauge that yu hook up to the vaccum line coming off the intake and it read good but i took it to a dealer for a diagnostic and they said it was a mass air plow as well as intake manifold and they also said i might have a catalytic converter bad as well so i think they are trying to get alot of money from me now another mechanic told me he had some people with same problem and it was water getting in the fuel on the 5.3 engines and said i need to clean the injectors as well as the fuel line im am very confused on this and i just dont want to spend money on something that is not the problem so do yu think it is the intake manifold gasket that is the problem i had the mass air flow taken apart by a mechanic about 3 weeks ago and he cleaned it for me with some cleaner spray that was used ofr mass air flow sensors only

Lets stop talking to other techs unless they are GM techs. The GM techs are going to know this truck very well. i work on about 10 of these trucks week on average. However that does not mean the other ideas are good or bad, but lets look at what normally goes bad on these trucks.

Water in the fuel will not cause the lean codes p0171, p0174. It will cause the engine to mis-fire and the check engine light to flash, code p0300 to set.

A catalytic converter can cause some major issues, normally you will get a lack of power and or a p0420, p0430 to set.

One cause is also possible, you used E-85 fuel in a non flex fuse truck. This should have cleared up at your next fuel up with normal gas.


Now lets get down to the most likely cause. The intake gaskets were not checked properly. You can not test for a vacuum leak with a vacuum gauge. You need a scan tool that reads engine data such as the fuel trims. Wile looking at the fuel trims, you need to spray carb cleaner to the under side of the intake wile the truck is runing and see if the fuel trims change. This is the only test to check the gaskets with out a visual inspection.

Here is some information on this issue that GM has put out.



#PIP3232B: Cold Engine Idle Misfire Or DTCs P0171 P0174 - keywords 4.8 5.3 6.0 air cold crack hesitate idle intermittent internal intake leak lean misfire miss P0171 P0174 P0300 power rough surge vacuum - (Feb 7, 2007)




99 through 05 GEN III truck engines may experience any of the following:

  1. A cold rough idle that diminishes as the engine warms up.
  2. Possible DTCS P0171, P0174, or P0300.
  3. Random misfires at idle.



The intake gasket seals may be leaking around the intake ports, causing a lean condition. The intake gasket seals may leak around the bottom side of the port, making it a challenge to detect the vacuum leak. The technician may also notice lower than normal MAF readings if the intake seals are leaking.

  1. If leaking intake gasket seals are found, replace the intake gaskets. During the repair be sure to use the current GMSPO part number for replacement intake gaskets. The current GMSPO part number for intake gaskets will have a teal green intake gasket seal. The old version of the intake gaskets were orange in color. Use only the teal green colored gaskets for a repair.
  2. The intake plenum should also be checked for excessive warpage that may cause an uneven clamping pressure of the seal. If the plenum is found to be warped it will need to be replaced. Service Information states, "An intake manifold with warpage in excess of 3 mm (0.118 in) over a 200 mm (7.87 in) area should be replaced". Intake warpage is measured at the gasket sealing surfaces, and should be measured within a 200 mm (7.87 in) area. This measurement is across only two of the intake runner port openings. Measurements taken across the entire distance of all four intake runner ports will lead to unnecessary manifold replacements.



Customer: replied 7 years ago.
ok ty for the info i guess i need to take it to somebody that has a good diagnostic computer to check it out and also when it does idle rough when it is very cold outside it sounds like a rod or lifter knocking bad for about 2 min then it smooths out does that tell yu anything further as far as gasoline goes i have been using reg unleaded do i need to use a tank of premium in it at least once a month

The noise is most likely a lifter. This is common on this engine, should not affect engine operation unless the noise does not go away. You may try adding some Reslone oil treatment at you next oil change, this will clean out the lifters and may help. At this time, I would not want to replace the lifters, the cost is around $2400.

Using premium gas will not help any, if you are using a good brand of fuel, you should not have any problems. Stay way from the Mom and Pop fuel stations.

Customer: replied 7 years ago.
ok i have been going to thes stations all over nashville called thorntons they are all over so i assume they have good gas is ther a recall on that intake manifold gasket from GM beings how they came out with a bulletin on it in 2007 that they will pay to have it replaced? Also i put some lucas oil additive in the oil a quart about 2 weeks ago so i noticed before that when it is warm outside and i dont have that rough idle that the knocking sound is not there so i think air is getting in it some how . So do yu feeel that the intake manifold gasket is deffinately the issue?

Fuel station is good, there is no recall, just information. Oil additive is good.

I would replace the intake gaskets if they have never been replaced before.

Customer: replied 7 years ago.
ok ty do yu have any photos of how to replace the intake i can do it myself or not

I no not have any photos, however it is easy to replace the intake gaskets. Remove the engine cover, unplug the injectors. Remove the fuel lines at the drivers side rear of the engine(you will need a release tool, not expensive at your local auto store), 2 electrical connectors at the rear of the intake, 12 small bolts and the intake can come out.

Should take about a hour (takes me about 10 mins).

Customer: replied 7 years ago.
ty so much for yur answer i will get the intake replaced i know a guy that said he will do it for 125 is that to much
Thats a good price, Dealers and most shops will charge around $200.
Customer: replied 7 years ago.
1 more question do i need to add fuel injector cleaner as well every now and then as well and what are the symptoms if a catalytic converter is goin bad i have 2 on this vehicle

Its a good idea to add the cleaner about every 3-4 fill ups.

If the catalytic converter was going bad, it can cause a lack of power, worse when hot. It can also cause one bank of the engine (left or right side cylinders) to have a mis-fire. Both cats do not go out at the same time so it may only affect the one side, this is why you may have mis-fires on only one bank of cylinders.

The dealership may have recommended a catalytic converter because some times if the engine has run lean for a long time, it can burn out the cat, this will give you a check engine light and codes P0420 and/or P0430 which means the cats are no longer working.

Customer: replied 7 years ago.
ok ty so much yu have ben very helpful so i will get the intake gasket replaced on it what is that 1 thing yu were talkin about on it that may be warped and how much is it to be replaced yu called it a plenum im not sure what that is how much is it to replace
This is extremely rare and would not even check it for warpage. This is just a guideline if you find the gaskets are normal. I would ask the tech to see the gaskets or tell you what color the rubber is to make sure you have the old style gaskets and not the new ones. Old style are orange, new ones are green.
Customer: replied 7 years ago.
ty so much i really appreciate it i hope it takes care of issue if it dont then i guess it might be a O2 sensor right or mass air flow
More likely a mass air flow then a O2.
Customer: replied 7 years ago.
ok ty so muchbut i think and hope the intake will resolve the problem
Let me know what happens.
Michael Moore and 3 other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
i will ty so much