Replaced head gaskets now I have a miss on clylinder #3

[mcbassin]

Hey guys I picked up a 2006 GP on Craigslist a few weeks ago. Decent body and interior and would make nice daily driver.

The previous owner, (young man in his 20s) said the water pump went out and he overheated the engine. He thought the head gasket(s) were toast. It was missing and had gas bubbling up through the coolant tank.
I assumed at least the head gaskets and possibly the heads were cracked.
I made a deal to buy it and took it home on my trailer.

I removed the heads and gaskets and took them to my engine guy. He found both head gaskets had bad spot but the head were NOT cracked on the magniflux check. The heads were checked for warpage too. That's as far as I went, I'm low on cash and looking for a super cheap driver.

I ordered new gasket kit and a new water pump and spark plug wires. So I finished this project last night. I have low idle miss on cylinder #3. The engine runs strong when given throttle but at idle it's missing. I drove the car and it runs great but when we get back to idle it's still missing....

Here is what I check so far.
1.) Confirmed I have spark to #3
2.) Confirmed the plug is not fouled (swapped the #5 plug and it still misses on #3)
3.) Confirmed I have fuel coming from the #3 injector

My next check is to see if I have a bent push rod or bad rocker? Not sure what to do after that, I suspect a bent valve or similar. I really didn't check the valves out that closely but I suspect that's where the problem lies.

Suggestions???

[Scottydoggs]

swap the spark plug as well to see if the mis moves. you can also swap the coils around to see if the mis moves.

[mcbassin]

swap the spark plug as well to see if the mis moves. you can also swap the coils around to see if the mis moves.

Thanks for replying Scottydoggs.
I did swap #5 spark plug with #3 the miss stayed on the #3 cylinder. I confirmed we have hot fire on all the coils too.
I'm afraid I'm going to have to dig into the front head again....

[Fivefingerdeathpunch]

Swap the wire and injector. Lower spark plug gap to .050.

Check compression.

[Scottydoggs]

double check the wires are on right too. we got a waste spark system you can cross wires and it will still run.

id still physically move the coil. its a free test.

you can also move the injector as well.

then your off to see if all your rockers are tight. if it was a valve i think your would hear that.

a leak down test would tell you if its the valves or not. or a compression test.

[mcbassin]

Swap the wire and injector. Lower spark plug gap to .050.

Check compression.

I have not checked the gap on the plug but I did an injector test on the #3. After I soaked them in injector cleaner. It is firing a nice atomized stream at 60psi.
I double checked the spark plug wires, I will triple check them and gap the plugs. I did test for fire at the plug and the wire is hot.

double check the wires are on right too. we got a waste spark system you can cross wires and it will still run.

id still physically move the coil. its a free test.

you can also move the injector as well.

then your off to see if all your rockers are tight. if it was a valve i think your would hear that.

a leak down test would tell you if its the valves or not. or a compression test.

All good points, I will triple check everything mentioned. I doubt it's a rocker arm, since there's no tapping noise. I can do a compression test, but I don't have a leak down set up.
Is it possible the block could have a crack? It's highly unlikely since there is no oil or coolant present in the exhaust stream....

It's just so weird that the car seems to have unlimited power when stepping on the throttle... so strange. Usually when I've had a bad valve, it never goes away. It just seems to get worse.
I will try to get some video tonight if I have time.... thanks guys

[Scottydoggs]

got that little ground wire bolted back on at the coil bracket from the icm harness? its a easy one to miss or forget about.

[mcbassin]

got that little ground wire bolted back on at the coil bracket from the icm harness? its a easy one to miss or forget about.

I'm pretty sure I did mount that ground wire. I will double check it tonight too.

[BillBoost37]

The suggestion to swap coils is a good one. Even though it appears to be a great coil, swap it to triple check.

Ask me about the transmission I swapped out because of a bad coil that didn't show up anywhere... or as a miss.

[mcbassin]

The suggestion to swap coils is a good one. Even though it appears to be a great coil, swap it to triple check.

Ask me about the transmission I swapped out because of a bad coil that didn't show up anywhere... or as a miss.

Wow! Can't imagine that.
So, help me out here. I have confirmed I have spark at the #3 plug wire going into the spark plug. It's possible to have intermittent spark voltage or intermittent spark?
I have been a long time user of these little GM engines all the way back to the 2.8 days in the late 80s. I remember the modules going out on them and even the crank position sensors giving me fits.
This one has me a little baffled. For the record I have another 2004 GP I can borrow parts from to test.

[mcbassin]

Update from 2-1. I double checked all the plug wires. Then I changed the coil pack. The symptom remained the same. I did a compression check.
Here are the results from the front head. #1-153 PSI #3-120 PSI #5-153 PSI. Bingo. I have a lower compression situation on cylinder #3.
I pulled the valve cover, nothing really out of line with the push rods or rockers for #3.
My next step, I pulled the intakes and then the head.
Nothing jumped out at me on the bottom side of the valves. I am confused again.... I'm losing 30 psi somewhere... suggestions.
I figured I would pull the valves today and see what I see there.

Is it possible there's a broken ring? Seems like blow bye and smoke would be present in the exhaust? The spark plug had no sign of oil consumption.

[mcbassin]

anyone ever have this problem?

[QUICKSILVER462]

Cylinder pressure probably leaking past a valve seat.

[mcbassin]

Cylinder pressure probably leaking past a valve seat.

I pulled the valves late yesterday afternoon. The exhaust side is a little pitted but not so much it shouldn't seal. The intake side looks a little weird at the seating surface. Almost as though it's warped.
I checked both valve stems and they are really straight.
This morning I did a test lap on these valves and seats. It looks as though they will seal.
I'm going to try to take a closer look at the piston rings today with a bright flashlight.

I really can't imagine this thing having a broken ring without some evidence of it. Either a scored cylinder wall or oil in the exhaust.

[mcbassin]

Just to update this thread, I purchased new valves for cylinder #3. Lapped them all and installed new seals. Last night I put the engine back together. It took a bit to get the lifters all pumped back up, but now it runs smooth and has no miss.
It purrs like a kitten.....thanks for all the advice on here guys.
I don't know if the overheat distorted the intake valve or what. Visually it was good but it leaked enough to cause the low idle miss.

[Scottydoggs]

theres a good chance that this car had sat for a long while before you got it. were the valves rusted and pitted up? either way good job fixing it up.

[BillBoost37]

Odd...but nice job finding and fixing it.

[mcbassin]

theres a good chance that this car had sat for a long while before you got it. were the valves rusted and pitted up? either way good job fixing it up.

Thanks, I'm pretty glad to have saved this one from the wrecking yard. I love these old cars. I put 230K miles on my 2004. My old grand am I had 190K miles before I gave it to a family member. before that I had a 1988 Buick Regal (2.8Liter) I put 324K miles on that puppy. All original drive-train too.

The exhaust valves were a little pitted, the car sat from mid summer last until I purchased it about a month ago. I'm pretty certain the intake valve was the culprit but either way we are rocking and rolling in the new daily driver for about $1100 I'll grab a picture when I remember.

Odd...but nice job finding and fixing it.

Thanks. It's rewarding to fix them, frustrating when I knew better than to half ass the job...
In hindsight, I should have spent a few extra hours checking the valves and lapping them.
I haven't done one of these in about 10 years so I got lazy. Way easier than the 2007 5.3 swap we did in my garage last weekend.....now that was fun.

[Fivefingerdeathpunch]

Eh, 5.3's are just bigger but they are just as simple as a 3800 at least.

Having owned 4 of them LS motors, they are my favorite things to work on.

[Turbocharged400sbc]

ive seen this a few times with lifters that are pumped up/full of oil.....

you tighten the rockers too fast and the lifter doesn't compress so you end up shoving the valve into the piston on the holes where its at tdc.


you keep it from happening by rotating over the engine till the far left and right rockers are on base circle (valve closed, then slowly lightly snug them down letting the lifter bleed down when you start to compress the spring. once the two end rockers are snug (they are what align the rocker pedestal plate, start with them first) then you can tackle the others on closed valves. rotate crank till the rest are on the base circle, repeat.

yes we are not an interference engine, and yes you do not NEED to turn the engine over to hit base circle for tightening rockers....but you must snug them up slowly so the lifters bleed down to keep from mashing a valve.....

before installing the head you can turn the engine 30* past tdc and 4 slugs will be over an inch down in the hole while the other two are at BDC. prevents any chance of contact.

and also, tightening rockers that are on the nose of the cam lobe is a wonderfull way to make sure they are out of the correct position/alignment once you snug em down....that's why I start at the end ones (the one with the tight dia hole and the other end with the slotted hole) and work through em with em turned to base circle.... youll also help tip alignment if you hold the rocker slightly counterclockwise while at the snugging point to counter the twist imparted by tq'n the fastener.

**** like this is stuff that should be in the writeups