98 1500 5.7 vacumn check - internal leak

[jmrich56]

I have a 98 1500 w/5.7 - need to resolve a PO305 -cylinder 5 misfire.
Having checked out the typical suspects - spark, spark plug, plug wire,
distributor cap / rotor, fuel pressure, fuel pressure bleed down, & compression,
2 mechanics suggested an internal vacuum leak. Didn't think to ask how to
perform a check for that. Came home and googled - found a test that makes
sense but would like to confirm. I pulled the air intake hose out of the right bank valve cover and pulled the pcv valve out of left bank valve cover. Started engine and put my thumbs over both valve cover inlets. There was noticeable
vacuum at both valve cover inlets.
Can someone confirm that there should not be vacuum at the valve cover inlets with the pcv hose removed from the valve cover?

[Mr_Rich]

Wouldn't that kind of hint that you have a leaking intake manifold gasket? I would think without the PCV hooked up you might have positive crankcase pressure than negative which the PCV system would take care of.

[jmrich56]

Yes, gasket most likely suspect. A local mechanic instructed me to alter the
check by sealing the valve cover inlets with duct tape along with tape on inlet of the PCV valve. Then remove oil dip stick - start engine - check for vacuum at dip stick. I confirmed vacuum at dip stick. I believe my 5.7 is a 1st. gen. vortec and GM only produced them for a few yrs. Curious if cracked intake manifolds were a common problem.

[crsgmc]

I have a 96, and yes the intake gaskets are known to leak. Had one that was leaking out of the back of the engine and the oil would run down to the pan and on to the floor. I replaced the gasket and it still leaked. I did not see any cracks in the intake manifold, but the gasket surface was pitted quite a bit. I gave up and purchased a new intake manifold and it sealed up perfect. Replacing the gasket is not that hard but the heater hose fittings usually fall apart when you try to remove it. I have not heard of any cracked intake manifolds but that does not mean much. It would be best to get the felpro steel backed gasket, they are supposed to last longer. I used this gasket so time will tell.

[speedygonzales]

You mentioned you checked quite a few items but you never mentioned the injectors. I would feel much more inclined to think it's an injector problem before a gasket problem (see the third paragraph) which you did not mention.

You said compression was tested but didn't mention a leak down test. However a leaking intake gasket will not be found with a leakdown tester.

Typically if the intake gasket is leaking, the engine will consume either oil or coolant. As a result, the plug will usually show signs of it.

Not sure if you knew about the new(er) style vortec intake gaskets which are metal sandwiched with rubber. Far better then the plastic POS the factory used.

https://www.summitracing.com/parts/f...make/chevrolet

I've only seen them in a kit never by themselves.


Here is a bit of reference material

https://www.onallcylinders.com/2016/...auge-readings/


So now let's get back to the "vacuum" issue you mentioned.
There are 2 distinct areas where vacuum occurs. One is in the intake manifold and the other is in the crankcase. They are independent of each other. Positive pressure from the crankcase is vented into the intake. The negative pressure (vacuum) is not. That is the function of the PCV valve. It is a check valve that only allows the positive pressure to escape the crankcase. Thus it creates a small amount of vacuum in the crankcase.

So if the intake gasket were leaking causing a loss of charge to the cylinder, there would be passage from the intake to the crankcase. This is the assumption your post was making. That the intake gasket is leaking causing a lean missfire.

However the likely hood is that the intake leak would draw oil vapor into the cylinder. That would be noticeable on the spark plug.

The intake leak would not pressurize the crankcase since the intake valve is closed when the piston goes upward.

The other type of leak on the intake gasket could be from the water jacket to the intake or from the intake to the outside.

Those would show up as either coolant going into the cylinder which would show on the spark plug as well as a decreasing amount of coolan or. It would show up when spraying a lubricant around the intake gasket. The misfire would come and go when spraying the intake gasket. If the leak were external.

[jmrich56]

With the long holiday break I found the time to pull the intake. Gasket was the problem. About 5 yrs ago I pulled the heads due to a burnt exhaust valve and
just didn't get the install quite right. Speedy, I did find a set of Fel-Pro at AutoZone which made for a much easier installation. The sheet metal skeleton eliminated the risk of gasket sagging between the bolts.
I did not mention in previous posts that I had found some info from other Chevy truck websites regarding the "cylinder misfire" codes. If a cylinder has more than 5% less compression than the average of the other cylinders, the OBD will throw the code. I will try to post a pic of the gasket - believe the leak was at the bottom which prevented me from locating it while performing test w/running engine.
Bottom line - no light at this time and rough idle is gone. Should be able to get the smog check this weekend.
Thanks for the suggestions and input.

[jmrich56]

https://drive.google.com/open?id=1te...RhxLgyUu527k13