No start???

[C3pilot]

So this has me stumped.

2004 Silverado 3/4 ton 6.0L automatic 4x4 with 222k miles. Got this truck about 6 months ago for my son’s first truck. It ran and drove but needed some refreshing. Engine work so far.
-heads rebuilt by machine shop, we installed w/new gaskets and bolts.
-new knock sensors
-new intake gaskets
-cleaned the injectors
-new plugs and wires
-oil and filter
-exhaust manifolds machined flat w/new gm gaskets
Engine went back together, compression seemed good and consistent across all cylinders. Truck starts up fine and we’ve driven it several hundred miles. But, it’s still not running right. Seems like there’s a miss especially after it heats up, say after 5-10 miles driving.

I have no check engine lights on. It feels like it’s under powered and running rich, 11.9 mpg. I’ve pulled plug wires one at a time to see if I can find a dead cylinder with no luck. I’ve removed the exhaust heat shield and temped each cylinder exhaust manifold to see if I can find a cool running cylinder, no luck. I’ve checked for vacuum leaks, no luck. I suspect cats might be clogged but haven’t been able to get a good look at them yet.

So here’s what has me REALLY stumped. In trying to figure out what might be causing the rough running condition I pull/clean/reinstall the grounds on the frame and block. Grounds I pulled: driver side frame horn (2 wires), batt to engine block (1wire), under driver door (3 wires 2 bolts). I didn’t pull the wire behind the pass cylinder head initially. I try to start the truck and it WONT start!
Engine turns over but no start at all.

I’ve checked for spark, it’s there. I’ve checked the fuel pump, it’s good. It shows approximately 53psi on the prime and slightly higher during cranking. Battery has been removed and FULLY charged. I’ve tried starting in Park and neutral, no luck.

I’m looking for some suggestions. Anything that seems obvious that I’ve done/missed that I can check. I’ll try just about anything. I’m thinking at looking at the cam shaft sensor next.

Thx!
C3pilot

[C3pilot]

Doh…..missed the PCM ground that connects at the battery negative to block connection. Fired right up, still not running great yet still no check engine lights.

[hatzie]

11.9mpg is not out of line on the LQ4. Mine is averaging 9-11 around town and 12-14 on the highway. Typically 10 City 12 hwy.

It may be time to log some fueling and possibly misfire and knock sensor data to see if you can narrow it down.

[C3pilot]

Originally Posted by hatzie

11.9mpg is not out of line on the LQ4. Mine is averaging 9-11 around town and 12-14 on the highway. Typically 10 City 12 hwy.

It may be time to log some fueling and possibly misfire and knock sensor data to see if you can narrow it down.

Man that seems low on the MPG! But we’ll keep an eye on it. About to focus on getting the exhaust sorted and go from there. Thx for the numbers!

[kev2809]

curious if yours is lifted any or does it have oversized wheels and tires?

my avalanches MPG dropped drastically after a body lift and 20s with 33s.

[C3pilot]

Originally Posted by kev2809

curious if yours is lifted any or does it have oversized wheels and tires?

my avalanches MPG dropped drastically after a body lift and 20s with 33s.

No lift and stock tires on mine. How about before your lift and tires? I’m not expecting 20+ mpg, but I am expecting way more than 11-12… Again mine isn’t running right and should hopefully improve when/if I can get it dialed in.

[kev2809]

well tbh I never clocked my MPG. but I noticed so much more fuel burned after the wheels and lift.

I also have a 2002 sierra with a 4.8. that one gets waaay better gas mileage but it's completely stock.

[hatzie]

The engine size and vehicle weight drastically affects the fuel mileage. Not to mention the availability of 6 speed automatics on the 1/2 ton trucks and SUVs at the tail end of the T800 production.
The 4.8 & 5.3 in a 1/2 ton truck will get significantly better fuel mileage than an LQ4 6.0L in a 9600lb GVW 3/4 ton truck. Add in the extra gears and optimized PCM tables for the 6Lxx transmission and you again bump up the mileage noticeably.

If you lifted and installed larger tires the speed and odometer are affected quite a lot. Both read lower than actual speed and miles traveled. 3.45 gears with 245/75R16 tires (29.47") changed to 33" tires drops your original 3.45 final ratio down to something roughly similar to 3.08 gears. If I did that with the NV4500 in my GMC the RPMS would drop from 1,103 to go 5mph in 1st gear to 985.

You need to change the final drive ratio correction factor in the PCM and adjust the tire size in the ABS computer to compensate for the larger tires. Otherwise the PCM will be using the wrong portions of the fueling and transmission torque reduction tables. Not to mention getting tickets because the speedometer is reading several MPH too low.

[biketopia]

my 07 classic 2500 hd 6.0 2wd gets 14-16 unladen and about 11-12 when towing with mild weight. If you have a temp gun shoot the cats when it's up to temp, there should be higher temp after the cat if they are functioning. Also you can watch the O2 data to see if it's swinging. The 02's might just be flat wore out at this point too. Did you do a cam or tune or anything when you pulled the heads, go back with the same 317 heads or change em up? Pull the valve covers and double check torque on the rockers to make sure one didn't loosen up, and feel each push rod to make sure you don't have loose one that maybe is bent.

[C3pilot]

Originally Posted by hatzie

The engine size and vehicle weight drastically affects the fuel mileage. Not to mention the availability of 6 speed automatics on the 1/2 ton trucks and SUVs at the tail end of the T800 production.
The 4.8 & 5.3 in a 1/2 ton truck will get significantly better fuel mileage than an LQ4 6.0L in a 9600lb GVW 3/4 ton truck. Add in the extra gears and optimized PCM tables for the 6Lxx transmission and you again bump up the mileage noticeably.

If you lifted and installed larger tires the speed and odometer are affected quite a lot. Both read lower than actual speed and miles traveled. 3.45 gears with 245/75R16 tires (29.47") changed to 33" tires drops your original 3.45 final ratio down to something roughly similar to 3.08 gears. If I did that with the NV4500 in my GMC the RPMS would drop from 1,103 to go 5mph in 1st gear to 985.

You need to change the final drive ratio correction factor in the PCM and adjust the tire size in the ABS computer to compensate for the larger tires. Otherwise the PCM will be using the wrong portions of the fueling and transmission torque reduction tables. Not to mention getting tickets because the speedometer is reading several MPH too low.

I agree with pretty much all of the above but most of it doesn’t apply to my situation. Stock tires and no lift. However it is a used truck with 222k on the clock so I’m not expecting like new performance or gas mileage. 4L80E in it and stock gears as far as I know. Could a previous owner “tweaked” the tune, sure. Not sure I mentioned it but it’s a non HD 3/4 ton.

[C3pilot]

Originally Posted by biketopia

my 07 classic 2500 hd 6.0 2wd gets 14-16 unladen and about 11-12 when towing with mild weight. If you have a temp gun shoot the cats when it's up to temp, there should be higher temp after the cat if they are functioning. Also you can watch the O2 data to see if it's swinging. The 02's might just be flat wore out at this point too. Did you do a cam or tune or anything when you pulled the heads, go back with the same 317 heads or change em up? Pull the valve covers and double check torque on the rockers to make sure one didn't loosen up, and feel each push rod to make sure you don't have loose one that maybe is bent.

I have checked the cats and I, pretty sure that their shot. I don’t have a scan tool (yet) that will read the real time O2 info. Could a malfunctioning O2 sensor operate without throwing a check engine light. Went with 317s no mods, cam, or tune. Probably not a bad idea to check the rockers and pushrods. I was think 15+ mpg should be doable…right???

[biketopia]

Originally Posted by C3pilot

I have checked the cats and I, pretty sure that their shot. I don’t have a scan tool (yet) that will read the real time O2 info. Could a malfunctioning O2 sensor operate without throwing a check engine light. Went with 317s no mods, cam, or tune. Probably not a bad idea to check the rockers and pushrods. I was think 15+ mpg should be doable…right???

With a non HD, yes 15 should be obtainable. I'll ask my brother what he gets, he has an 01 2500 non HD, his is also 2wd and has 230k on it. Usually they will tell on them selves, but I've had cases, it's typically Fords though, that will have issues and not throw a code. Is your check engine light working? Have you tried scanning for codes anyways? After you check the rockers, I'd take it for a drive and get it up to temp then let it idle while you crawl under and see if you can shoot the entrance and exit of the cats with your temp gun. Remember the rear o2 is only to tell the ECM if the cat is working, the fronts are what control AFR. When you did the heads did they get the broken bolts all of them always have out, and what manifold gaskets did you use? And you're sure you don't have any exhaust leaks, that can introduce false air and cause it to go rich.

[kev2809]

I know this might not be related.....

does it take a few cranks, then it turns on? I had to crank mine 2 or 3 times before it would run. I would get random codes, (02 sensors, misfire, etc.) I did a tune up, changed the 02 sensor, fuel pressure regulator, gas cap. couldn't figure it out. turned out to be the fuel pump. the truck just died on me and wouldn't start this time.

what happened was the filter sock on the pump completely came apart and the trash of it was being sucked through the fuel system. when I pulled the pump it was completely gone and I can see particles of it everywhere in the gas. new pump, a few new filters later and the engine codes went away and truck fires right up.

maybe something to look at?

[C3pilot]

Originally Posted by biketopia

With a non HD, yes 15 should be obtainable. I'll ask my brother what he gets, he has an 01 2500 non HD, his is also 2wd and has 230k on it. Usually they will tell on them selves, but I've had cases, it's typically Fords though, that will have issues and not throw a code. Is your check engine light working? Have you tried scanning for codes anyways? After you check the rockers, I'd take it for a drive and get it up to temp then let it idle while you crawl under and see if you can shoot the entrance and exit of the cats with your temp gun. Remember the rear o2 is only to tell the ECM if the cat is working, the fronts are what control AFR. When you did the heads did they get the broken bolts all of them always have out, and what manifold gaskets did you use? And you're sure you don't have any exhaust leaks, that can introduce false air and cause it to go rich.

I’d really like to hear what your brother is getting for mpg!
Yes check engine light is working, it came on for the bad knock sensors that I replaced. Yes I have scanned for codes but nothing is coming up. I still like the rocker suggestion and will be checking them. I did check the cats and I would say they seem to be bad, lower temp at back of cat. The broken exhaust manifold bolts was what really lead to redoing the heads. Yes….overkill I know to pull the heads for broken exhaust bolts. But….that’s how I roll! Haha. And yes there were exhaust leaks at the manifold prior to doing the heads. I initially used Engine Pro exhaust manifold gaskets and they leaked. Pulled the manifolds off and had them surfaced. Reinstall with same Engine Pro (yes now used for about 2 hours of run time) gaskets and still leaking. Removed the EP gaskets and installed GM original exhaust manifold gaskets. I use new manifold to exhaust pipe ring gasket every time I pull the exhaust apart, this will be my 4th set… the only exhaust leaks I have prior to the cats was at the block to manifold, hence the resurfacing of the manifold and GM gaskets.

I know the most logical thing would be replace the cats and O2 sensors. However, I’d really like to have the smoking gun identified before spending the coin to throw a bunch of parts at and pray.

But plz keep the suggestions coming! We’ll get it nailed down sooner or later.

[C3pilot]

Originally Posted by kev2809

I know this might not be related.....

does it take a few cranks, then it turns on? I had to crank mine 2 or 3 times before it would run. I would get random codes, (02 sensors, misfire, etc.) I did a tune up, changed the 02 sensor, fuel pressure regulator, gas cap. couldn't figure it out. turned out to be the fuel pump. the truck just died on me and wouldn't start this time.

what happened was the filter sock on the pump completely came apart and the trash of it was being sucked through the fuel system. when I pulled the pump it was completely gone and I can see particles of it everywhere in the gas. new pump, a few new filters later and the engine codes went away and truck fires right up.

maybe something to look at?

I too thought that the FP might be bad. I checked the pressure during prime and cranking and it seemed ok. 55ish psi. Haven’t checked while driving. It really fires right up with no hesitation. I’ll keep an eye on it though.

[biketopia]

The fact the cats are cooler at the rear is a pretty definitive test that they aren't doing a damn thing. I would check the rockers, then start looking for cats and quality o2's. Or some long tubes and a y pipe from speed engineering, new front o2's and a tune.... Even a good tune on the stock set up will net some power and MPG gains, especially with free flowing exhaust. But I know you probably don't want to do that, just a suggestion though. Does the 04 have an external fuel filter, if so have you replaced it?

[hatzie]

2004 doesn't have a fuel filter from the factory. GM stopped putting them in before the 2004 models. Not sure about the 2003 models.

If you have the right Mastercool hydraulic forming dies you can cut a short section out of the fuel line and form the GM fuel line ends to fit the stock 2002 GM fuel filter cartridge. That's still on my to-do list.

[biketopia]

Originally Posted by hatzie

2004 doesn't have a fuel filter from the factory. GM stopped putting them in before the 2004 models. Not sure about the 2003 models.

If you have the right Mastercool hydraulic forming dies you can cut a short section out of the fuel line and form the GM fuel line ends to fit the stock 2002 GM fuel filter cartridge. That's still on my to-do list.

I don't know why I never thought to do that... I'll add that do the list for my 07. You can also do it without the mastercool tool. There's kits and repair parts to swap in sections of nylon fuel hose for repair, and there's unions in the kit for connecting steel to nylon and the there's premade nylon lines with the right connection's or with the right additional tools you can make your own custom set up.

[C3pilot]

Originally Posted by biketopia

I don't know why I never thought to do that... I'll add that do the list for my 07. You can also do it without the mastercool tool. There's kits and repair parts to swap in sections of nylon fuel hose for repair, and there's unions in the kit for connecting steel to nylon and the there's premade nylon lines with the right connection's or with the right additional tools you can make your own custom set up.

Great idea! I’ll look into it too.