Texas69

Have a P0303 misfire in cylinder 3 of a 2003 Buick with 3.1L V6. This was also used in some 90's vintage Olds.

Have done some basics like replace plugs and wires (wires were due anyways at 170k miles). Also replaced the #3 Coil and fuel injector, and upper intake gasket.

Result is that 5 cylinders work better than in years, but #3 still misses. The idle is rough, the exhaust chugs, and car dies after a little use. I inspected all vacuum lines in the R&R of the above parts and things look fine. No coolant in oil and no obvious oil/coolant leaks in the rear bank head. No smoke out of exhaust. The #3 plug was gunky and looked like had oil residue baked on the tip. I checked the #3 plug again after replacement and had a drip of oil on the tip. There was gas in the #3 cylinder as viewed through the injector hole in the manifold, but I do not know if it dripped from the fuel rail when replacing the injector or if it had been collecting there previously.

Things are looking more ominous now. I thought next steps would be compression test and/ or inspection of rockers. However these are a waste of time if I am really dealing with bad rings or seals.

Not worth an engine replacement or rebuild due to age, but if it can be fixed through the replacement of something relatively easy I might try it.

Any further suggested diagnostic tests or possible causes?

Thanks all.

72cutlassconv

Check for spark. If there's a good spark, then do a compression test.

1970greensupreme

Comp check. I have seen rocker bolts pull out of the heads on these. Any noise?
Oil could also sneak in from intake port from internal vac leak. Rings are not a really big problem on these engines. Head gaskets are but your not using coolant I assume

Texas69

1970..., There is a fairly loud tapping noise that started with the miss.

frankr442

Are you still getting the P0303 code after replacing the coil?
Try replacing the valve stem seals on #3.
While you have that valve cover off you can look for any obvious cause of the tapping noise.

edzolz

I had a 98 year with the 3.1, that burnt through the intake EGR portion and filled the cylinders with water. Had to replace the upper plenum and the lower plenum to repair the problem. I would suggest you replace those if they haven't been done already.

Texas69

Update... have not been able to remove valve cover for inspection yet, but did do a compression test. Got a whopping 0.0 compression on Cyl 3. Both wet and dry.

Thinking of cylinder leakdown test and look under valve covers next to further evaluate.

1970greensupreme

I've seen that happen a bunch of 3.8 with plastic intake but not the 3.1

1970greensupreme

You maybe on to something. Like I said, these intakes are prone to leaks, when people re-install rockers after intake gaskets they bugger up the threads and you end up with a rocker not opening the valves

BillK

Sounds like there is a good chance that the rocker arm bolt pulled out of the head. The early engines used an 8mm bolt, the newer ones 10mm which fixed the problem. I used to go to the dealer and fix them for them. A helicoil will take care of the problem if it is the pulled bolt, just make sure you put it in deep enough to clear the "tang" on the bottom of the rocker pivot assembly. Also, a common mistake is mixing up pushrods. They are slightly different lengths and if you swap them you will bend intake valves. The longer pushrod is exhaust.

Texas69

I got the valve cover off and confirmed a rocker bolt pulled out of the head. Will check the other bolts and get an insert added where needed.

Yet another known issue along with manifold gaskets on these GM 3.1/3.4L motors. Miserable jobs, but possible for the DIYer