03 Tahoe 5.3 blinking check engine light misfire

[Motojoe86]

Hello all, I am new to this forum as a member however have been on here a ton to read on issues. I have read several of your forums regarding check engine light and am unfortunately creating a new one with hopes there might be more insight. Here goes:

Vehicle:
2003 Chevy Tahoe 4wd z71 with 5.3l flex fuel engine 160k miles

Symptoms:
I am getting a flashing cel whenever i am on the highway over about 70 mph. If i rev it in neutral it will flash being held over 2400 rpms as well. Also, whenever i have it in tow/haul mode it flashes almost all the time. Lastly, towing in regular mode it will flash about 50mph+. It seems to run good minus a very minor stumble in idle (unnoticeable rpm fluctuation) and terrible gas mileage. 2 side notes- i have an srs light on and my alternator is going out which I am replacing this weekend. I cant imagine these would have any affiliation with cel though would it? It usually doesnt store a code however I have been able to pull random misfire code twice.

Work done to no avail:
-New iridium plugs gapped at .40
-New plug wires
-New coils
-New air filter
-New pcv valve
-Cleaned MAF
-Cleaned throttle body
-Fuel pressure tested within spec
-Seafoam through motor oil
-Crankshaft sensor tested within spec

I am running out of ideas on what to replace and the costs are beginning to add up. Any thoughts or ideas on the culprit would be greatly appreciated. Thanks in advance!!

 

[Chubbs]

what are the exact codes that have come through?

hopefully something other than a generic misfire/spec' cylinder misfire.

have you ruled-out any vac' leaks and faulty intake manifold gasket?

 

[Motojoe86]

Unfortunately I am getting generic multiple misfire. I believe p0300. I havent ruled out intake gasket yet as I am a little scared to dive into that. However if that seems likely culprit I will have to give it a go... As for vacuum leak, I have only done the "spray around with carb cleaner" technique lol. I didnt get any noticeable rpm changes though. Is there any way to test intake gasket without actually tearing it apart? Also, thanks for the help

 

[Chubbs]

I have not yet done the intake gasket R&R on this model, myself. But, from past experiences, there isn't much to it on this truck. I have this other vehicle with 1 of those upper "banana manifold" and coil packs bolted to the back of it up against the firewall, then the lower mani' AND about 36-feet of EVAP plumbing criss-crossing over the top & around said manifold. Not to mention the throttle body and coolant plumbing for that. It took me like 12-hours to remove all of that crap just so that I could unbolt the valve covers & replace the gasket set.

you could get a workshop manual from eBay or whatever, maybe $10 used. I found 1 on Craigslist & had the guy mail it to me. Try a search on the gasket leak and see if that holds any plausibility. you may even find a thread for confirmation. Gaskets are cheap! I like cheap fixes.

 

[Chubbs]

You really need a diagnostics flow chart like the techs use for getting to the source of these problems. The really good techs have experience & wisdom in their arsenal and that's like magic. Other than that you have to be really lucky or have a lot of extra time & money where you eventually stumble on the responsible party and have a lot of new parts as result haha.

Edited: really though, everything you have done so far is like 2% of what I did on my truck just as preventative maintenance when I purchased last April with 125k miles. At the very least fuel filter needs replacing on your MY as part of your diagnosis. As you have time flush and replace fluids on every system except the transmission which is a fluid/filter replace ONLY.

You prob don't depend on your truck for daily commute so little-by-little is fine. I drive mine M-F @ most dangerous hwy in USA so I have to know its not going to break down on the roadside where I get myself killed by some idiot. Old, 4WD trucks have a ton of $hit that needs to be serviced if you want to jump in it anytime, be able to turn the key & get where you want to go.

 

[Musicars]

This may be a long shot but I recently had the same code and it was the result of a bad battery. My battery could not handle the cold temperature starts here in Minnesota last month. It tripped the misfire code and was running terrible. So, don't rule out the electrical system as in battery or alternator. Higher rpm require more juice. Same with towing. Just a thought.

 

[Rocket Man]

A flashing cel usually means you're damaging the catalyst in your converters. You're not supposed to drive at all when it's flashing, it's trying to really get your attention. The only time mine did it, it also went into reduced engine power mode. Mine was a massive intake leak.

 

[iamdub]

The only time mine did it, it also went into reduced engine power mode. Mine was a massive intake leak.

A massive intake leak can be overcome if you have enough boost. Jussayin.

 

[Chubbs]

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.

Just a heads-up if you get around to the gasket. It's all about the details bub. And yes, a flashing CEL means you are doing damage.

 

[Motojoe86]

This may be a long shot but I recently had the same code and it was the result of a bad battery. My battery could not handle the cold temperature starts here in Minnesota last month. It tripped the misfire code and was running terrible. So, don't rule out the electrical system as in battery or alternator. Higher rpm require more juice. Same with towing. Just a thought.

Thanks for the info. Unfortunately I forgot to mention in initial post, I do have a new battery on it. Seeming from posts and narrowing down its either intake or cat/o2 sensors. Really hoping its not intake because Im afraid of that job lol