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Discussion Starter #1 (Edited)
3.7 has a rough idle, not bad, but it is there. Also throws a code 'cyl 2 misfire,' only seems to throw the code on cold start. Idle was really bad before, pulled the coil out and found water in the spark plug hole, cleaned up and swapped coils with cyl 1 and 2. Idles better, but still rough and still throws code (cold start this morning). Not sure if this is related but can also hear the faint rattle of what I assume to be the timing chain when first started, especially on cold mornings. Goes away after a few minutes and cannot hear it through the oil fill plug.

What I have done with no change:
  • swapped coils with cylinder 1 and 2
  • cleaned MAF
  • cleaned throttle body
  • swapped MAF with parts truck
  • Seafoam injector cleaner

Any help would be appreciated before I put more money into this truck (body work, remote start, etc). Thanks

Edit: Truck has 260,000kms but engine has 160,000kms from Hummer H3.

Edit: Ran Seafoam injector cleaner through a few tanks of fuel upon purchase.
 

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Did the misfire (P0302) move with the coil? The rattling is probably the timing chain as the hydraulic tensioner is not yet pumped up with oil pressure.
 

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You need to get a compression gauge on cyl 2. It could be the fuel injector as well (leaking or clogged).
 

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Discussion Starter #5 (Edited)
That is my next step hopefully tonight, just have to find my compression tester.

Thanks for the input, will report back once complete.

That start up timing chain rattle is considered normal?
 

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Discussion Starter #6
Compression checked:
  • #1 - 140psi
  • #2 - 140psi
  • #3 - 135psi
  • #4 - 140psi
  • #5 - 140psi

I am surprised they are all that low, wondering if I should be questioning my compression tester? Although it is a quality Mac Tools one.

Or could this in fact be the timing chain?

Thoughts?
 

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Discussion Starter #7 (Edited)
Will be doing another compression test tonight with a different gauge to confirm before proceeding.

Edit: Confirmed compression numbers with a different gauge, pretty much the same.
 

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I agree with the above response. If your code did not change with the coil swap, I'd next try an injector swap to see if the code changes with the injector. Yes, the "chain slap" noise that you hear goes away after about a minute of running, it is normal. The timing chain tensioner is hydraulic and is "relaxed" as the truck sits and oil pressure diminishes. Once the truck is running, the oil pressure builds up and the timing chain tensioner takes the slack out of the chain. Colder weather that we're having will make it take longer for the noise to go away. I have an '04 3.5 Colorado with 244,000 miles on it and it is still running great. I did a compression test on it the last time I changed plugs just for curiosity sake and the compression was 135-140 on all cylinders. I think your numbers are good. The most important thing to look for when doing a compression check is to look for large variations. If you have a variation of more than 15%, you'll want to dig in and figure out what's going on. Again, I've got 244,000 miles on mine with nothing other than routine maintenance to the motor and transmission.
 

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Discussion Starter #9
Thank you for the responses.

If the consensus that the timing chain rattle is normal then we will leave that be. I have experience with timing chain engines, this just seems abnormally loud for an engine with 160,000kms.

There seems to be a lot of conflicting information regarding compression numbers for these trucks. From what I have researched, average should be above 170-180, with 150 being the absolute minimum and 215 being basically brand new.

How To: Do a Compression Test on a 2.8/2.9/3.5 and 3.7 up to 2008 | Chevrolet Colorado & GMC Canyon Forum (355nation.net)

Do not want to be putting more hours or money into this truck until I am confident that the compression numbers are good.
 

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I'll add putting an oil pressure gauge on the system to see if its low oil pressure causing the chain tensioner to not put adequate force on the timing chain. If its low oil pressure, then there a few other considerations in the mix for how to move forward.
 

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Discussion Starter #11
Thanks @Kenzen, I will add that to my list.

Do you have any comments regarding the compression results?

Edit: can oil pressure be checked with a scan tool? Or is there a port to add a guage?
 

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While doing the injector swap, also consider checking the torque on the intake and exhaust manifolds. Make sure they are present and snug. Inspect the exhaust
manifold for cracks.
 

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Thanks @Kenzen, I will add that to my list.

Do you have any comments regarding the compression results?

Edit: can oil pressure be checked with a scan tool? Or is there a port to add a guage?
The compression is low overall, but consistently low - my fear was #2 was going to be an outlier. I'd get a second gauge and do it again.

Our trucks only have low pressure switches, so you cant read the pressure from the OBD2 port. There is a port on the front of the oil filter mount closed off by a plug - I believe it has an M12-1.5 thread - but please verify that. From that you can install a mechanical pressure gauge. There's also the option of screwing in a gauge where the pressure switch screws in, but that is an oddball thread (M12-2?) and you'd lose the low pressure alert when running the engine - not a big deal if your standing there watching. The factory manual identifies an adapter plate that goes between the filter and the filter mount that provides a gauge port - I have no idea what the availability or cost of that is. I bought a cheap $25 oil pressure gauge kit (Amazon) that has the M12-1.5 adapter for the first option I mentioned, but I haven't used it yet on my Colorado.
 

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Discussion Starter #14
Completed the compression check with a second gauge and readings were pretty much the same.

Found some info regarding adding a gauge for oil pressure and my research shows you need a M16 x 1.5 adapter for the port near the oil filter mount just below the belt tensioner, but this should also be verified.

oil pressure gauge | Chevy Colorado & GMC Canyon (coloradofans.com)

Unfortunately do not want to be putting in more time and money if the engine is low on compression. Does not make much sense to be checking oil pressure, swapping injectors, etc if the engine has no compression and is in need of a rebuild or a head.

Thoughts?
 

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I think your fuel injector will wind up being the source of the P0302 code - maybe throw a bottle of Techron in the gas tank and drive it for a while and see if it cleans up.

In the big picture, if its running well (besides the rough idle), and the compression is low but acceptable, I'd just drive it and enjoy it until it warms up a bit. (I have every motivation not to work on my vehicles in cold weather unless necessary.) Since your working off a used but relatively low mileage (or km'age in your case) engine, my "mild concern" is the engine wasn't driven often or it was primarily used just for short trips, and what it really needs is to be taken on some longer highway trips, and have a few frequent oil changes to clean things out. The compression may come up - it could be some coked piston rings that need to be used to clean up.
 

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Discussion Starter #16 (Edited)
Thanks @Kenzen, ordered a new AC Delco injector.

However, now the P0302 code has changed into a P0300 (random misfire). Have barely driven the truck, just started it up this morning and the new code came up. Deleted it and it came back. Have not changed anything on the truck since the last post.

Think I will still install the new injector in cyl #2, will update my findings.
 

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Did you clear the code while the engine was running? Or did you clear it while it was running, and the code came back? The P0300 code can be the result of a weak battery or starter, where during the starting the crankshaft position sensor isn't producing a strong signal to the PCM since the engine is spinning slowly and the vehicle voltage is low. The weak signal causes the P0300 to happen. If you've been starting/restarting it to test it (along with the cold weather) you may have drained it a bit.
 

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Discussion Starter #18
Cleared it multiple ways (including running and not running), still the P0300 code. Only thing I have not done was clear it just before driving. Driving vs idling might produce a different code.

Confident it is not my battery or starter. Battery is brand new, temps are warming up here in Alberta, Canada, and truck has been in the garage recently.
 

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At this point, I'd check the fuel pressure. This is a simple test since there is a test port to hook the gauge up to. Rent/buy a guage - another $20 on ebay or amazon.
 

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Discussion Starter #20
Went to check fuel pressure with my OTC gauge, but have the wrong adapter. For anyone out there with this gauge the adapter looks close but it is not. Will have to pick another gauge up or adapter.

Decided to take it for a drive to see if I could get the P0302 code back. Under moderate load with out down shifting (ie going up a hill) the truck will shudder and check engine light starts flashing (pretty sure this means a rich mixture and damage to catalytic converter), got it back home and still have the P0300 random misfire code. Also now every time it misfires/stumbles at idle I can hear a very faint knock or rattle. Can actually hear it better from inside the cab. Kind of sounds like its coming from the intake manifold area, but extremely hard to pin point.

Injector comes Thursday and at the same time ill pick up another fuel test gauge. Unfortunately do not seem to be nearing a solution at this time.

Edit: the more I listen to the rattle at start up, I really do not think it is the timing chain. Cannot hear it at all through the oil fill plug. Can hear it more clear around the exhaust manifold, especially from underneath the truck.
 
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