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BOBBYSS
BOBBYSS, Master Technician
Category: Chevy
Satisfied Customers: 14118
Experience:  ASE MASTER CERTIFIED, AA DEGREE AUTO/DIESEL TECH
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Chevrolet Tahoe: Ive got a 2006 Tahoe with a 5.3l. There are

Resolved Question:

I've got a 2006 Tahoe with a 5.3l. There are 2 issues that may or may not be related. When I put the engine under hard acceleration, the check engine light comes on. When I read the codes, it indicates a rich condition on both banks 1 and 2. The first time I read the codes, it indicated a faulty reading from the MAF sensor. I have replaced this and I no longer get the MAF code, but I still get the rich condition codes (0172 and 0175).

The second issue is that when I start the truck, the engine struggles to run smooth and get to a good idle condition. This lasts for about 2-3 seconds and then the engine idles fine, and seems to run fine. On some occasions, particularly when the engine is cold, I have to crank it for quite a while and then open the throttle a bit to get it to start. In this case the idle is rough for about 2-3 seconds, and then the engine runs fine.

Any ideas?
Submitted: 1 year ago.
Category: Chevy
Expert:  BOBBYSS replied 1 year ago.

BOBBYSS : Hello, and thank you for letting us help you with your automotive question. I would start by checking the fuel pressure regulator. It is very common for this to leak on this engine and cause a rich condition. The regulator is located on the fuel rail by the injectors. It is round and has a vacuum line going to it. It is on the top drivers side of the engine. With the engine running pull the vacuum line from the regulator. If there is any fuel present then this is the problem. Please let me k is what you find and we can continue from there.
Customer:

From what I understand, the fuel pressure regulator for the 2006 is located in the tank as part of the pump assembly. There doesn't appear to be one on or near the fuel rails.

BOBBYSS :

Can you tell me if the vehicle is a flex fuel vehicle? Does it have a yellow gas cap?

Customer:

No. It is not a Flex Fuel vehicle.

BOBBYSS :

Okay, then you are correct that the regulator is on the fuel pump assembly. You need to start by checking the fuel pressure. You will need a fuel pressure gauge installed on the fuel rail to check this. If the pressure is above 62 psi then there is a problem with the fuel pump assembly. If the pressure is fine then suspect a bad coolant temperature sensor. If it is reading a colder than normal temperature then the computer is going to add too much fuel. You will need a computer scanner to read what the computer is seeing for sure. Do you plan on fixing this yourself and do you have these tools?

Customer:

The fuel pressure is around 55 psi. Unfortunately I don't have a scanner that will let me monitor the coolant temp in real time. I plan on fixing this myself so I need to buy or borrow one.

Customer:

Also, do you think that a bum coolant temp sensor would cause the hard starting problem?

BOBBYSS :

Yes. If the sensor is not reading correctly then the computer will load up the engine with fuel and cause a hard start and these codes.

Customer:

The part itself is pretty cheap. I'll replace it and get back to you.

BOBBYSS :

Okay. Let me know. I typically do not recommend just changing parts to see what happens though. I guess that is cheaper right now then purchasing a scanner.

Customer:

OK, so I went ahead and bought a decent scanner that lets me look at live data. The coolant temperature is at 63 degF which is reasonable (the garage is around 68 degF) and the Intake Air Temp is 66 degF. Seems that the sensor is OK.

Customer:

What next?

Customer:

I forgot to mention that when I looked at the freeze frame data, the LTFT for both banks 1 and 2 was around -19%. The STFT values were -6.8% for Bank 1 and -3.4% or Bank 2. It also showed some values for Banks 3 and 4 but I have no idea what those mean.

Customer:

The throttle position sensor reads 20% when the engine is off and at idle. Is that normal?

BOBBYSS :

Can you smell fuel from the exhaust or does it smell fairly normal?

Customer:

When I tried to start the the Tahoe, it gave me a hard time. It behaved like it was flooded. Had to open the throttle a bunch to get it to start. Once it did there was a fair amount of white-ish smoke. Definitely smells like it is running rich, but I didn't smell raw gas.

BOBBYSS :

When you checked the fuel pressure did you happen to check and see if the pressure held when the vehicle was turned off?

Customer:

I just rechecked the fuel pressure. It actually went up to 60 psi while it was running and dropped to 56 psi when I shut it off, and stayed there. It also stays at 56 when the ignition is on but the engine is not running.

BOBBYSS :

That is fine fuel pressure. Does the STFT move constantly? Does the voltage from the O2 sensors fluctuate when the engine is warm?

Customer:

I drove the truck this morning and got it warmed up. Starts rough when cold, but is fine once it warms up. The STFT moves constantly for both banks. The O2 sensors (pre-catalytic cvtr) fluctuate from .9V to around .785 volts while idling. The LTFT started out around -6% and was at -19.5 by the time I got home.

BOBBYSS : And both sides, bank one and two, basically read the same way, correct?
Customer:

Both sides read the same.

BOBBYSS :

Okay. The next thing you need to check is the circuit. I would start by checking the ground at 103. Make sure that the connection is clean and tight with no wire fraying. I would then check the other harness around the transmission.

BOBBYSS :

Full Size Image

Customer:

The connection is good and there is no fraying.

BOBBYSS :

Sorry for all of the questions, but I do not want to jump to conclusions just yet. Can you tell me what the TPS and the MAF read at idle?

Customer:

No problem at all. This is a tough problem and I appreciate you help. At idle, the MAF is reading 1.1 lb/min, the TPS is at 14.1%, and the MAP sensor is reading 10.9".

Customer:

In case it matters, I am in Albuquerque which is at 5000 ft altitude.

Customer:

One other thing I tried was to disconnect the hose to the PCV valve to create a lean condition. When I did this the LTFT for both banks immediately started going to the positive side. Once reconnected, they returned to their large negative values.

BOBBYSS : Do you have a vacuum gauge to see what your engine vacuum is reading?
Customer:

The vacuum gauge is reading just shy of 12" so I guess the MAP sensor is OK.

BOBBYSS :

That sounds a little low. I an sort of suspecting two things here. If the vacuum is this low then you may have a timing chain that is worn out. It is quite possible that the chain is worn just enough to cause low compression at idle until it warms up. If the compression is fine then I am suspecting a problem with the PCM.

Customer:

The truck only has 60,000 miles on it. I'd be amazed if the timing chain was worn out. I am starting to worry that it might be the PCM, which sucks! Do you know what the Throttle position sensor should be reading at idle?

Customer:

I just looked at the voltages out of the TPS, ignition on but engine not running.

Customer:

TP1: 1.23 volts throttle closed, 3.5 volts wide open, 1.38 volts when throttle released, and returns to 1.23 volts after about 6 seconds

Customer:

TP2: 1.09 volts throttle closed, 3.5 volts wide open, 1.22 volts when throttle released, and returns to 1.09 volts after about 6 seconds

Customer:

Seems that when the throttle is first opened, the throttle plate is slightly bound as it "pops" open at first. This does not occur after the throttle is released and then immediately opened again.

BOBBYSS :

This is a little off for these sensors, but it is not too bad. Look inside of the throttle body bore to see if there is a ridge where the throttle plate rests on the bore. If there is a ridge in the throttle bore then replace the throttle body. If it seems fine I would check the evap canister. It is located under the vehicle next to the gas tank. I would remove it and see if it is heavy. If it is very heavy then it is saturated and causing it. If it is fine then I am at the PCM for the problem. If you test the items I mentioned and they are fine let me know and I will opt out and allow another expert to try and continue with you.

Customer:

I'll check the evap canister tomorrow. I'll let you know what I find. I'll keep hoping it isn't the PCM.

Customer:

Didn't find a ridge in the throttle body.

BOBBYSS :

Okay. Let me know about the canister and we can continue from that point. This is definitely a tricky problem.

Customer:

I've been too swamped at work to get a chance to check the canister. If I simply disconnect the line from the canister to the valve and cap it off, will it accomplish the same thing?

Customer:

Should I consider resetting the PCM? Does it store info that could be reset? If so, how do I do that? Disconnect both battery cables?

BOBBYSS, Master Technician
Category: Chevy
Satisfied Customers: 14118
Experience: ASE MASTER CERTIFIED, AA DEGREE AUTO/DIESEL TECH
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