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Discussion Starter · #1 ·
I bought it used a couple of months ago.

Initially it was throwing P0101. I replaced the MAF sensor and it went away.

Then it switched to P0171 & P0174. I replaced the two upstream O2 sensors which helped a little.

I also added some fuel-injector cleaner to the gas tank, and soon after [coincidence/dirty fuel at fillup/cleaner loosed up dirt in tank or lines] got P0300 and P0307 engine misfire codes. I removed the fuel injectors, soaked them in Seafoam, cleaned out the fuel rails, reinstalled them and filled the fuel rails with Seaform after reinstalling everything. This fixed the misfire problem.

I was still getting P0171 & P0174, so I fixed the exhaust system by repairing the end bolts of the exhaust manifold which had all broken. And replaced the exhaust manifold gaskets while doing so.

This helped a bunch, but the engine still doesn't have much power, and now it comes up with P0101, P0172 and P0175 codes.

And by not having much power, I mean that for longer uphill grades, the truck doesn't have the power to maintain highway speed [with only about 1000lb load], and pressing the gas pedal will make it worse [truck will have even less power] and if I keep pressing it I'll hear a popping sound from the motor. If I release the gas pedal and just keep the motor at around 2000 rpm, it seems to have the most power and will generally maintain that speed [between 80-90 kmh].

Things I've checked:
-fuel system pressure with key on/engine off, goes to 58 ft-lb, then gradually goes down to about 40 in 3 minutes
-air filter and plugs are new. old plugs were all clean, but I think the seller had just recently replaced them, but there was no sign of oil on any of them
-hasn't used a noticeable amount of oil after about 2000km
-sprayed around the top of the engine with it running, testing for vacuum leaks, didn't find one

What should I try to check next?

Things I'm considering doing:
-replacing input manifold gasket, which I understand has a history of leaking on the underside of the manifold, so it is hard to test for
-replacing fuel injectors, as they might be leaking, resulting in that fuel pressure drop. I would also install an '03 fuel filter in the fuel line, just to make sure they stayed clean, as the existing filter that is part of the fuel pump may not be totally up to the job anymore.
-maybe take out the crank position sensor, as some people have had problems due to rust forming on it [or rather, on the disc it senses]

I can also post some live data readings if it would help.
 

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Really sounds like cat problem. Could have be caused by the missfire if it did it for a while. But not all that uncommon. We stocked a couple versions back in the day. I was the guy that had to cut the cats off the exhausts to send back to GM so I had a pretty good feel for what we were installing.
 

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check the MAP manifold pressure sensor for "sticking" during part vacuum conditions
check the TPS throttle position sensor to make sure it is linear in operation
I have seen BOTH failures cause issues like you describe
 

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Discussion Starter · #4 ·
Really sounds like cat problem. Could have be caused by the missfire if it did it for a while. But not all that uncommon. We stocked a couple versions back in the day. I was the guy that had to cut the cats off the exhausts to send back to GM so I had a pretty good feel for what we were installing.
It only was doing the misfiring for less than a week, and the live data for the downstream O2 sensors still look fine [so it looks like the CATs are still working]. But I also have read that some people have had plugged CATs. Would there be some way to test or check that they need to be replaced?
 

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Discussion Starter · #5 · (Edited)
check the MAP manifold pressure sensor for "sticking" during part vacuum conditions
check the TPS throttle position sensor to make sure it is linear in operation
I have seen BOTH failures cause issues like you describe
I'll log just the MAP sensor with my reader, and see how the data changes, which should show how responsive the sensor is [just logging the single sensor should save the data more frequently than saving all the sensors].

I have used carb cleaner on both sides of the throttle plate and where it makes contact, then wiped it down with a rag to clean off anything stuck to them. And the truck has a drive-by-wire setup, so would GM still have a TPS as well [and if so, wouldn't they also throw a code if the throttle plate didn't respond to how open it was supposed to be]?

Edit: Actually used CRC Throttle Body Cleaner on the throttle plate, so the intake manifold should not be dissolved...
 

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a drive-by-wire throttle system does have a TPS BUT the 6.0L came both ways in that GEN (I can NOT remember the change over) being electronic I would expect a throttle position error code OR a TPS faulty/in range mis read code but could still be of interest to monitor the TPS and the commanded throttle position
and for the MAP yes watching the MAP and commanded throttle as you drive "through" the rough parts of the throttle and see is throttle goes up and vacuum goes down in unison
and for collapsed/plugged CATS usually you will see LOW VACUUM on a MAP at high engine speeds/loads (near ZERO Vacuum at <70% throttle where as a good CAT will still "pull vacuum" to "test" down shift at hi way speeds into a lower gear and "HOLD" the revs with the throttle and check vacuum pulled on a Gauge OR data logger and repeat the same conditions with a LOWER engine speed and the vacuum "recovers" the cats may be plugged but if you get good vacuum in both conditions CATS should be OK)
a short read on vacuum gauges
http://bbkworks.blogspot.ca/2012/05/vacuum-gaugereading-and-interpretation.html
 

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davester,

Your fuel pressure seems out of spec. This may or may not be the main problem but should be diagnosed.

You should be seeing around 55-62 psi, key on, engine off.

Key off static press. should be around 55-60 psi.

The acceptable limit of psi loss in 10 mins. is 5 psi. Your bleeding off 18 psi in 3 mins.!

Pull the vacuum hose off the FPR and check for the presence of fuel, it should be dry.

Also not sure about your 2005 but I know 1999 models have a check ball in the fuel pump that can fail and cause press. loss as well.

To diagnose the check ball, relieve the fuel press. @ the fuel rail to 10 psi. Anything over 2 psi loss in 10 mins. is out of spec.
 

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Discussion Starter · #8 ·
davester,

Your fuel pressure seems out of spec. This may or may not be the main problem but should be diagnosed.

You should be seeing around 55-62 psi, key on, engine off.

Key off static press. should be around 55-60 psi.

The acceptable limit of psi loss in 10 mins. is 5 psi. Your bleeding off 18 psi in 3 mins.!

Pull the vacuum hose off the FPR and check for the presence of fuel, it should be dry.

Also not sure about your 2005 but I know 1999 models have a check ball in the fuel pump that can fail and cause press. loss as well.

To diagnose the check ball, relieve the fuel press. @ the fuel rail to 10 psi. Anything over 2 psi loss in 10 mins. is out of spec.
This truck has the returnless setup, where GM put everything into a single hard to access location, namely, the fuel tank [fuel filter, fuel pump, pressure regulator, all in one unit, in the fuel tank]. I was thinking that maybe just pulling the injectors from the intake manifold, but still hooked up to the fuel system, turn on the key, and see if any of them are leaking.
 

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This truck has the returnless setup, where GM put everything into a single hard to access location, namely, the fuel tank [fuel filter, fuel pump, pressure regulator, all in one unit, in the fuel tank]. I was thinking that maybe just pulling the injectors from the intake manifold, but still hooked up to the fuel system, turn on the key, and see if any of them are leaking.
Right.

I didn't realize they had switched to the returnless system by '05.

I was going by memory from when I had to trouble shoot my '03. Turned out to be a bad FPR.

Seems you are on track with the injectors since DTC P0172, P0175 indicate a "rich" issue.
 

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Discussion Starter · #10 ·
Well, on Friday I replaced the intake manifold gaskets and the knock sensors and harness on the truck. The installed knock sensors didn't bad at all [a little rust/scaling on the sensors and the plastic looked yellowed (assuming it was originally white like the new sensors] and I hadn't received any ODB code for the knock sensors.

The valley cover was just dirty, with none of the rust or scale I've seen online on other trucks. I guess the previous owners weren't big on washing the motor every week or two.

I was mildly annoyed that I looked at the Haynes manual, which showed a water line with 4 connections on top of the knock connector plate [or valley cover, underneath the intake manifold], and I wanted to also remove the cover to check how the inside looked, so I spent an evening and a morning tracking down the gaskets for that pipe [we have TERRIBLE parts distributors here in Canada vs the US], only to take off the intake manifold and find that rear two ports were covered with plugs and the front two were connected with a pipe, but I didn't have to remove it to take off the valley cover. Blah.

I replaced the knock sensors because:
a) the truck is 10 years old, and likely didn't get the best maintenance, more like, just enough to keep it going
b) it's not cheap to take off the intake manifold to replace them [the distributors make up for poor selection by having high prices for the parts they do stock]
c) I could get the sensors/harness online from the US for less than 2/3 the price here
d) I had no idea how good/bad they would look, and I needed to get the truck back into service right away

So I figured I might as well get cheaper replacements up front instead of opening the truck up, then possibly having to buy more expensive ones after opening it up.

Overall, it took me about 8 hours to remove the intake manifold, knock sensors and valley cover, clean them [well, what I could reach with my fingers in the intake ports and manifold] and reassemble them all. I took it apart with the fuel injectors installed on the manifold, but then removed them for installation (the fuel vapor solenoid on the intake manifold really didn't want to come off the line without me feeling like something was going to break, and the line routes underneath the fuel rail and it wasn't easy getting it out, and I didn't want to damage the injectors or manifold gaskets while I worked it back under the fuel rail]. And I would suggest splurging the couple of bucks on getting the plastic dohickey for disconnecting the quick-release fuel line connector, vs trying to work it free with one or more screwdrivers jammed into it.

The only thing I'm not sure if I should do is to replace all the intake manifold bolts. They are three-piece jobs, the bolt, a guide shaft that positions it correctly in the manifold, and a rubber washer. The rubber washers seemed reasonable [didn't look damaged or worn, not that I've seen a new one] so I cleaned and reinstalled the ones I had. But I also haven't been able to find a source for replacement units other than the dealer [who actually manages to make the distributors here look cheap].

Anyway, after doing this, I've run the truck about 80 km, both on the highway and in the city, and I no longer get a mass air flow range error or that the fuel trim is rich in both banks. Not sure if my mileage has gotten better, but the engine also is running better, when I step on it, the engine doesn't really bog down or 'pop', and it seems to have reasonable power now.

I'll still check the fuel injectors if they are leaking and make some time to check how the engine vacuum changes.

But that will have to wait until after I get done doing snow.
 
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