Ok here’s my situation...
I have an 87 IROC 5.7 L98
I just recently drove the car about 200 miles back home from college and I noticed that I lost about a quart of oil in the process, so I began checking things over and then I noticed that there was a strong vacuum coming from the dipstick, which I figured was odd. The same thing was happing at the oil cap. My car would actually try to stall when I removed the oil cap while it was running. So I disconnected the both PCV hoses and the vacuum was still there. So that says one of two possibilities either I had a bad intake gasket or a crack in the manifold. Right?. When I removed the Plenum I noticed where the quart of oil went to. The plenum was full of oil which was odd because it wasn’t smoking to bad. I checked the PCV hoses to see if it came from there and they didn't really have that much oil in them. So I continued on disassembling the intake. When I removed the bottom manifold I didn't see any visible problems with the gasket or any visible cracks in the manifold. So I am stumped. Any ideas where the oil and the strong vacuum came from??
Thanks for any help!
Matt

 

Is it vacuum or is it positive pressure(blowing out)? If it's positive pressure then I think you're getting blowby.

 

1987 manifolds in particular to cold climates had this problem with vacuum but not oil loss.. it included a crack in the manifold right near the 56 divide internal of the intake.. it was caused by a Mal threaded Slug which goes in from the underside on the intake which was designed for an internal Engine re-breather. instead of having the external design on the valve covers..

I am more inclined to say your having a leak on the headnear the intake or at the intake gasket. You need an area where medium flow oil passages would be present... just a little at a time and under good vacuum conditions..So start by locating what is the failure by registering the idle Vacuum fluctuations for consistancy and check which plugs are fouled up

 

Try checking the Heads for Loose Inside Bolts.. Your head gasket might be screwed and you might have a small fracture in the head.. Check the Plugs they will tell you and do a compression leakdown test with PSI attached to the sparkplug and check for air loss at the valve covers..

 

Here’s an update...
I have bead blasted and inspected the intake and I still have not found any cracks in the intake. Hrock I checked around the studs that you talked about and no problems there. I tried placing pressure in the cylinder while the intake valve was opened and placed my hand over the intake port, but once again... i couldn't find any leaks... So I reassembled the tpi hoping it might have been something stuck in the gasket that I missed. But once I got it together I started the car and the vacuum was still present. With a cheap vacuum meter i was getting a reading about 10in at idle. After about two minutes of the car running I turned it off and disassembled the TPI again. Once again the plenum had a lot of oil in it. Any more ideas on were I should look for a leak or possiblly any other type of test that I can try that might help me figure this problem out?
Thanks
Matt

 

sorry.. double posted

 

Mine was the same problem.

I posted the topic "Vacuum at oil dipstick tube. What it is?". You can check the replies here.

I disassembled intake and heads. Cause was a bad machined block surface with a small "mountain" in it. Then the head was missaligned. For this, the "intake port matching" between head and intake manifold was bad (the "sandwich" with the intake gasket doesn't was complete). Note that heater changes sizes in parts, bolts, gaskets, etc. and you can't see this condition into teh engine.


You needs check the compression too. You could have bad rings.


Regards,

Denis V.

 

I have found the problem. My #5 piston looks melted and there is a small hole in it on the top. Looks like i am doing a motor swap. Which leaves me with a couple more questions. I have an 84 L69 motor, and i would like to use my TPI on it. I have a feeling that i might run into problems with the intake matching the heads on the L69. Am I right?.. Also I put a cam in the L69 and i am not sure if it will work with a TPI. It is has an Adv. Duration of 270º/276º and a Gross Lift of .440''/.454'' what do yo all think? Also i attached a picture of the piston. What might of cause the problem. Possibly the injector not fireing? The head it fine. Thanks Again Matt

 

ya burned a piston. the car exhibited no engine miss prior to the discovery of vacuum at the stick? it must have been running lousy with the looks of things on top. irregardless, the most sensible deduction you could make, over the internet, that is, is that the intake leak that was the root cause of your initial symptom-vacuum at the d-stick, was in the intake tract for that burned hole. extra air makes you lean, lean gets things hot, heat makes things melt. pressure check that head before you consider it as suitable for a rebuild i think would be cheap insurance. flow ck. that injector. how many miles are showing on the clock? piston tops say valve stem seals are pretty much not there.