Leakdown testing GM 3400 - Update - Running

[Blue85]

This 3.4 Liter has had a poor idle for more than a year. In the last couple of months, it has started throwing a code for misfires on cylinder #4. Looking on the net, it seems that these engines love to blow the lower intake manifold gasket and also the head gasket. Since there is no water in oil or vice versa and no coolant disappearing, I assumed that #4 was just being diluted with air from a leak in the manifold gasket. So I replaced the gasket, but didn't find any problems with the old one or loose bolts. Before finishing up the re-assembly, I rigged up a leakdown tester.

1, 3, 5 and 6 all hold pressure fine, but 2 and 4 both seem to leak into the intake. #4 seems intermittent in this behaviour. I can't catch any air leaking from the cooling system or the oil. Doesn't this suggest that the intake valves aren't closing properly? I don't notice any transfer of air between those two cylinders.

What else should I try? I'm want to really understand this before I pull the head. I thought maybe the intake leak is really just the gasket blown between those two cylinders. One can be on intake while the other is on compression, but not the other way around. Firing order is 1-2-3-4-5-6, so #4 fires 240 degrees after #2

 

[oatsk5]

i dont have any helpful info bt is taht the dual quad that is found in the z34

 

[Thunder]

Beings the 2 cylinders in question are right next to each other and are failing the leak down test. I would suspect a bad head gasket. It could be leaking between the cylinders but the Oil and water passages are still intact and sealed.

 

[Blue85]

Yes, this makes the most sense. I was drawing this out on paper and concluded that the #4 intake valve is open while #2 is at TDC (firing), so a leak between the two explains the path to the intake. However, when #4 is firing, #2 should only be open on the exhaust valve. I will repeat this testing to confirm everything because I remember getting mixed results for #4

 

[Blue85]

i dont have any helpful info bt is taht the dual quad that is found in the z34

Single Cam - 6 valves per head, 185hp, very boring. Comes in Grand Am, Impala, Minivans, etc.

 

[Blue85]

I repeated the leakdown testing tonight and couldn't get #2 or #4 to seal up - always heavy leaking into the intake. The rest of them all seem normal - just a little leaking into the crankcase. Knowing that pulling the head would cost me a couple hundred in gaskets alone, I was hesitant, but ended up yanking it off.

My thought is to bring the head to a shop so they can check it out. Nothing looks too bad, but I don't know if the features I see in the casting are normal.

Here's the pics:

 

[Blue85]

The more I think about this, the more I want to just get a remanufactured head and bolt it all back together. I don't know what a local shop will charge to clean and inspect the head, but with remans around $200, that's about a cheap as all the gaskets required if I have to pull it apart again.

How do I know if the reman has been milled? Any advice on this either way?

 

[Blue85]

Ordered gaskets and a remanufactured head today. Everything is supposed to be here by Friday.

 

[Blue85]

OK, if I can't find new head bolts - how bad is it to re-use the old ones? Do I add torque to the spec since the threads will be smoother?

 

[Thunder]

I could be wrong on this but. I believe the 3.4 uses torque to yield head bolts.
If they are TTY bolts. You can't reuse them. You must buy a new set. Torque them to spec. They should be easy to find. cost around 20.00 for a set.
There is a procedure to torquing them also.
Tighten to torque spec then tighten either 1/4 or 1/2 turn. Or Whatever the manual says

If I am wrong. And the 3.4 uses regular head bolts it is OK to reuse them. Torque them to specs.

 

[Blue85]

Well I did find a set of bolts and I put the engine back together. It runs, but there is a vacuum leak - the aluminum plenum has cracked. I used my in-lb wrench, so I dunno. Maybe it has had a crack for a long time. I know that a head gasket was done before on this vehicle. I also found a few minor things that had been put together wrong (broken fuel injector clip, mis-assembled connectors), so who knows what else has been done to it. I'm thinking about sealing off the throttle and putting a few psi of air on the intake to look for leaks without the engine running. Now if only I could find something to make some smoke to inject...

 

[Blue85]

For some reason I always end up dragging a job out with cleaning and paintng. This ride has been on the road for about a week and it is running like it always did. This means two things:

1) the repair was successful
2) still no torque

In the back of my mind I want to swap in a 3800 if any internal engine work is needed again.