Is this possible?
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Cruising
Joined: Feb 2006
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Is this possible?
I had a Vette shop replace the intake manifold gasket on my '90 L98. It began leaking oil again immediately and I took it back to shop, they redid the seal free of charge but told me it would likely blow out again due to excessive ring blowby and excessive crankcase pressure forcing the oil to push out the intake manifold seal. They noted that the pcv valve once again has oil residue in it despite having been replaced with the 1st gasket job. I had the shop perform a leakdown test--all valves were well within norm, and also a compression test--1 cyl 200 psi and the rest 205 psi at 101K miles. I'm skeptical--excellent compression, no visible exhaust smoke, not terribly high mileage with regular Mobil 1 oil changes, car runs fine--can there still be excessive blowby, or are they trying to cover for a screwup?
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Cruising
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Actually had the grommet replaced as well. But I'm still suspicious about the shop's claim that's there's enough ring blowby to cause the intake gasket to blow out on a car with great compression that does not smoke.
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Cruising
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No, only have had the pcv valve and grommet replaced. The shop thinks the problem is the pcv system has too much pressure to handle due to ring blowby. Is it even possible for a car with like-new compression to have ring blowby and resulting elevated crankcase pressure? That's what I'm having a hard time buying.
Race Director
Joined: Sep 2000
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From: The Top of Utah
It certainly doesn't add up. I don't know that oil on/in the PCV valve is all that abnormal. How did your determine that the valves were holding pressure from a leak down test and get no indication on the rings??? What kind of leak down test was that? I only know of one kind and it gives the total cylinder leakage, then YOU (or the technician) have to find the leaks. What was the range of the percentage of leakages? Those compression numbers raise questions, too. They're too high. I don't know what's going on here, by it doesn't all add up.
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Instructor
Joined: Oct 2005
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From: Woodmere New York
I had a similar problem with my 90'...actually was trying to port and polish the tpi system...had a mechanic working on it that was completely ignorant and I had no idea..told me that he had experience with chevy 350's....anyway, after he put everything back together, I had an issue with the intake manifold seals..I started to get a significant oil leak and after bringing the car to my regular mechanics, I learned that the job was botched and the intake had to be taken off and re-aligned...I know that you are having a different issue...However, it seems that the intakes on our C4's are very temperamental....
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Cruising
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It certainly doesn't add up. I don't know that oil on/in the PCV valve is all that abnormal. How did your determine that the valves were holding pressure from a leak down test and get no indication on the rings??? What kind of leak down test was that? I only know of one kind and it gives the total cylinder leakage, then YOU (or the technician) have to find the leaks. What was the range of the percentage of leakages? Those compression numbers raise questions, too. They're too high. I don't know what's going on here, by it doesn't all add up.
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The mechanic pulled the valve covers for the leak down test and had an instrument to measure combustion bypass around the valves when the pistons moved. He just said the leakdown was well within normal range. The compression was recorded after 3-4 rotations to get a full reading (the 1st stroke yielded 130-135psi across the board), the 200-205 psi readings as I understand yields 9.5:1 stock compression and is close to new-engine compression. I didn't press the guy too hard on these test results as it was HE who claimed the engine was likely worn out and therefore had the ring blowby creating the pressure that broke the intake manifold gasket seal he repaired. His own test showed otherwise and weakened his worn engine assumption. The car does not blow out oil from the exhaust, just broke 100K miles, and runs fine, so I found this "worn engine" reasoning hard to believe in the 1st place.
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Thread Starter
Cruising
Joined: Feb 2006
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I had a similar problem with my 90'...actually was trying to port and polish the tpi system...had a mechanic working on it that was completely ignorant and I had no idea..told me that he had experience with chevy 350's....anyway, after he put everything back together, I had an issue with the intake manifold seals..I started to get a significant oil leak and after bringing the car to my regular mechanics, I learned that the job was botched and the intake had to be taken off and re-aligned...I know that you are having a different issue...However, it seems that the intakes on our C4's are very temperamental....
I should've just let the b&^ch leak. A little over a year ago I had another shop do the same intake manifold gasket seal job as this shop did just 3 weeks ago (and re-did again just last week). It was botched the 1st 2 times by 2 different shops, I can't see oil leaking from the intake yet on this latest 3rd try, but the car did have 3 small oil drops under it over the weekend. To top it off the 2nd shop tells me the intake manifold gasket will likely blow out again because the engine is shot and likely needs an overhaul.
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From: The Top of Utah
Originally Posted by corvetteguy90210
The mechanic pulled the valve covers for the leak down test and had an instrument to measure combustion bypass around the valves when the pistons moved.
Originally Posted by corvetteguy90210
The compression was recorded after 3-4 rotations to get a full reading (the 1st stroke yielded 130-135psi across the board), the 200-205 psi readings as I understand yields 9.5:1 stock compression and is close to new-engine compression.
You have a real "Catch 22" here. He won't redo his work because he says the problem is caused by a worn out engine. Then his very questionable tests say the engine isn't warn out. So why isn't he tearing into it?
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Thread Starter
Cruising
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This guy seems to be coming even more questionable. I would like to know what kind of an instrument can measure combustion bypass around the valves, when there is no combustion taking place. Does this "magic" device "see" through metal to detect leakage between the valve and the valve seat, or only past the valve stems and seals, into the valve cover area? Also, the pistons don't move during a leak down test.
More BS from this guy. There is no "correct" compression pressure for any given compression ratio. The cam timing has more to do with cranking compression numbers than the compression ratio. A brand new, L98, I don't care what year, with what factory compression ratio, will not have 200+ psi cranking compression.
You have a real "Catch 22" here. He won't redo his work because he says the problem is caused by a worn out engine. Then his very questionable tests say the engine isn't warn out. So why isn't he tearing into it?
RACE ON!!!
More BS from this guy. There is no "correct" compression pressure for any given compression ratio. The cam timing has more to do with cranking compression numbers than the compression ratio. A brand new, L98, I don't care what year, with what factory compression ratio, will not have 200+ psi cranking compression.
You have a real "Catch 22" here. He won't redo his work because he says the problem is caused by a worn out engine. Then his very questionable tests say the engine isn't warn out. So why isn't he tearing into it?
RACE ON!!!
His story didn't make sense to me neither--he's telling me my engine is worn even after his own tests show the opposite. The leak down test was his idea at first then he backed away from doing it, then I insisted he do it as well as a compression check since I found his worn engine blame hard to buy. Actually since his bill for this work is on a credit card bill that I haven't paid yet and isn't due for another 10 days, I got some ground to stand on if it starts leaking within a week like the last time. I'm not interested in having him try to get right a 3rd time, would rather just dispute the bill with the cc company. Thanks for your input.
Le Mans Master
Joined: Oct 2003
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From: seattle WA
and I question his readings of 200-205, those are some pretty damn high numbers, now the 130-135 I could believe! And if in fact those were the actual numbers I'd say they're starting to get a little low. And at least in my experience it's quite common to see a little oil residue in a PVC, as a matter of fact, I can't remember ever seeing one without, accept when they're new outa the box. I just have to question this guys abilities.
Le Mans Master
Joined: Oct 2004
Posts: 5,841
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From: CLT, North Carolina
Sounds like this shop does not know what they are doing and they are covering up by trying to con you on top of the failed attempt at intake gasket replacement. I bought a 96 project car a while back that had supposedly had the intake gaskets replaced but the job was so sloppy it probably leaked worse after they finished. I have it fixed now.
Race Director
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From: The Top of Utah
Originally Posted by corvetteguy90210
Actually since his bill for this work is on a credit card bill that I haven't paid yet and isn't due for another 10 days, I got some ground to stand on if it starts leaking within a week like the last time.
Good for you. During that 10 days, check with your CC co. I think even if you pay the bill, you have something like 60 days to file a dispute. It might even pay to put the CC co, on notice that you suspect the possibility of problems with this transaction. I hope like heck you aren't sitting still for being charged for the leak down and compression tests. He conducted those tests to to back up, and prove his diagnosis, that the worn out (NOT!) engine caused the gaskets to leak again.If this guy were really a pro, and he even suspected the engine was unhealthy, he should have SOLD you a quickie compression test, before hand, to verify the the intake gasket operation had a good chance of holding. And if not, to SELL you an overhaul. After the fact, in order to back up his excuse making, the tests, regardless of the outcomes, should be on him.
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Team Owner
Joined: May 2002
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
More BS from this guy. There is no "correct" compression pressure for any given compression ratio. The cam timing has more to do with cranking compression numbers than the compression ratio. A brand new, L98, I don't care what year, with what factory compression ratio, will not have 200+ psi cranking compression.
It sure as hell can. Depends on the temperature of the engine, strength of the starter & battery, etc. I pulled 190-200 on my stock engine doing it hot. PSI readings dropped quickly as it cooled off.
I know what you're thinking, 14.7 x CR = Compression reading (psia).
If this was true, all our motors would read 120 psi and even that assumed perfect VE at cranking speed (not gonna happen).
The numbers are closer to 200 because the air heats up as it's compressed quickly. A hot motor will allow it the air to heat up more, thus showing a higher reading. The compression readings should be done as quickly as possible on a hot motor for this reason. Also, as you get to the last cylinders, the battery charge will be weaker and the DC starter motor won't turn the engine over as quickly.
For best results, do the test, then do it again in reverse order, and keep the battery on a charger as much as possible.
Cold compression checks don't make much sense to me.
Team Owner
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
BTW, it's normal to get oil residue in the PCV system, and in the throttle body from the vent side of the PCV. The vent side is there for when the PCV system can't handle it, and will dump the crank vapors in front of the throttle body.
This happens mostly at WOT, especially lower rpms. The PCV doesn't get much vacuum here so it can't pull much.
I get quite a few oil leaks from what I think is high crankcase pressure also, yet my compression checks are good and pump up in 4 cranks. I have yet to do a leakdown check.
This happens mostly at WOT, especially lower rpms. The PCV doesn't get much vacuum here so it can't pull much.
I get quite a few oil leaks from what I think is high crankcase pressure also, yet my compression checks are good and pump up in 4 cranks. I have yet to do a leakdown check.
Le Mans Master
Joined: May 2004
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From: Hellinois
Finalist 2020 C4 of the Year - Modified
As everybody has already told you, your "mechanic" doesn't amount to a hill of beans. Just to show you how wrong he is I just had an experience that will shed some light on your dilemma. Recently I had to do a second overhaul of my engine, also due to an idiot who claimed to be a machinist. After he did the machine work on my engine it had blowby from worn out bores so bad that it would puke oil out of the dipstick tube. It was so bad that it even blew a wad of paper out of the dipstick tube when I revved the engine up. Even with all this pressure I never had a bit of leakage from the intake seals. I know for a fact that your engine does not have this much blowby just from your description so cut your losses and run, let the cc company fight him.
Team Owner
Joined: May 2002
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Who is the CC company going to believe? A customer or a mechanic? You better get a qualified person to look at it if you don't trust him. This is your burden, not his.
A CC dispute is not an automatic refund.
A CC dispute is not an automatic refund.
Race Director
Joined: Sep 2000
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From: The Top of Utah
Originally Posted by CentralCoaster
I know what you're thinking, 14.7 x CR = Compression reading (psia). If this was true, all our motors would read 120 psi and even that assumed perfect VE at cranking speed (not gonna happen).
Originally Posted by CFI-EFI
There is no "correct" compression pressure for any given compression ratio. The cam timing has more to do with cranking compression numbers than the compression ratio.
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