2006 Chevy Colorado 2.8L - Rough cold starts... Nobody can solve it.

[Wolf_22]

About 2 years ago, I decided to splurge on a new re-manufactured engine from Jasper Engines & Transmissions. I did this because the old engine had all kinds of problems, the timing chain skipping teeth being among the least...


On the day I picked the truck up with the new engine, it had problems. The worst was how "rough" it was whenever you'd cold-start it. What happens is, the moment you turn the key to the ignition, it starts up but after about 3-5 seconds during the initial start up, the tachometer falls by about 1-300 RPMs (almost like it stalls). But it doesn't shut off--instead, it acts like it revs up and down for a few seconds within the 500 - 1000 RPM range, but eventually, after maybe 8-10 seconds of the start up, it evens-out and falls down to the normal 6-800 range at idle. If I give it a little gas during that weird moment of revving up and down, it smooths-out instantly (which is what I normally do once I know it's acting up like that).



The odd thing is that it only does this (usually) once throughout the entire day, especially when it's cold outside.


Here's some things about the new engine that's worth knowing:
- It barely has over 20k miles on it.
- It has new spark plugs, new injectors, new coils...
- The waterpump and thermostat in it is new as it came from Jasper.
- New power-steering pump.
- New radiator.

- New engine mounts.
- Using oil filters that have the backflow prevention to maintain oil pressure.
- No CELs--nothing pending OR saved; the computer thinks everything is perfectly fine.



And yet, despite all of this, the shop that installed the engine has been inside the crank case at least 4 times screwing around with the timing chain components because they've heard noises that led them to believe that the chain had too much slack. The first time was when they discovered a snapped timing chain guide. The second time was to revise a tensioner. The third, a new timing chain assembly (chain, guides, and tensioner). The fourth time was to not only replace the tensioner again, but to also replace a camshaft phaser. The last time it was in the shop, they had the valve cover off and the guy I spoke to said that his mechanic told him that everything still looked brand new. They also said that both oil pressure looked good and fuel pressure was irrelevant since the fuel pump pressurizes the rail upon start up.


I also took her to a Chevy dealership to be looked at by a gold senior tech after hearing what I *thought* was a lifter noise coming from near the top of the engine--nothing super scary, just a noise I was a little concerned about that sounded like a minor tapping sound. He listened to it and said he thought it might be a lifter that was either sticking or possibly going bad, but since I sometimes hear it and sometimes don't, I'm convinced it's just sometime to live with for now, especially since that tech wouldn't dive into the head due to Jasper's warranty processes (the dealership didn't want to mess with the engine because of Jasper, so after he tried to diagnose the problem, he reached a point where the Jasper rep told him that to continue diagnosing it, he'd have to basically do things that the Chevy dealership didn't want to mess with). All of this forced me to go back to the original shop, which is when I had the camshaft phaser installed.


The phaser installation helped for awhile, but after doing another oil change a couple weekends ago, the startup noise is back. I don't know if it's because of my oil changing--I used 5 quarts of Mobil-1 5w30 with one of the better oil filters that have that backflow prevention thing; all by the book.


The one thing I haven't done yet is have the shop do either a compression test or a leakdown and primarily because I've been one of those customers who's just nodded at everything they've done (I've tried to trust their decisions). I also haven't done any valve job where you take the entire head off to be sent to some fab shop to have everything decked and reseated, etc.


Any ideas what might be causing this?


I still have 1 year left of warranty with this new engine... And I also have some inheritance coming my way soon, too, so if I need to, I can force a hand on the shop and say, "Look, I don't care how much it costs. FIX THIS." If push comes to shove, I could buy a new truck today as I also have some cash saved up that I could throw at something new if I want--I just want to avoid that for now. Thing is, I like this stupid truck and if not because of that, I'd like to understand and fix the problem simply out of spite.


Where would you start in doing that if this was your truck? The rough cold-starts aside, when she's running, she runs like a sewing machine.