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Even Compression but Irregular Leakdown - Common or no?

25 July 2017
I'm considering an NSX that has even compression across all cylinders (175-180 lbs), but irregular leakdown. 6-9% on all cylinders but 46% on cylinder 1.

It also has a weird history where the dealer has been replacing the valve cover gaskets after every service. Guy who owns it is pretty clueless about cars, so I could see this just being them taking advantage... but they also have never done other services like a timing belt or valve adjustment. 50k miles on the car.

Does anyone know the odds of this being a simple valve adjustment issue, or is this likely to be a damaged cylinder/wall and grounds for a rebuild? I'm hoping it's just a valvetrain issue, but I don't want to bite off more than I can chew. The shop that did the PPI claims the valve covers are on backwards. Not sure how realistic that assessment is.

I'm not sure what $ amount to ask the seller to discount from the car's price. First thing I'm going to ask him to do is get a valve adjustment done to see what changes.
A leakdown test is detecting more potential issues than a (simple) compression test. 46% indicates a problem. The amount to correct it varies between a few hundred $ and a 5-digit number.
So it's hard to guess the odds.

Weird history, never done TB should ring the alarm bells. I wouldn't have a problem with a neglected car is the price is right but regarding the potential amount of money...I'd stay away but that's just me.
The potential advantages of the leak down test is that it facilitates a more specific diagnosis; but, one that requires user skill and good hearing. With pressure applied to the leaky cylinder, you can listen into the intake and exhaust for hissing indicating the escape of air past valves. On other engines you can listen in the open oil filler hole to try and determine if air is escaping past the rings - that would not be impossible; but, would be an interesting challenge on the NSX. The problem with the leak down test is that unlike a compression test the numbers are not absolute. The percentage values are not comparable between different testing machines and the test results are subject to a lot of user error / interpretation.

The service manual spec for compression test results on the C30 is 142 - 199 psi presumably at sea level. Depending on your altitude your test numbers might be spot on for a 50,000 mile engine (199 psi drops to 190 psi at 2000 ft). Given that your compression test results are balanced and subject to the altitude correction, are somewhere between OK to good, the 46% leak down number on one cylinder has me wondering whether the leak down test is flakey. Did they repeat the test on that leaky cylinder to confirm the result? Depending on the test equipment the results can be very non linear, i.e. a reading of 46% is not 9 times worse than a reading of 9%. Normally you proceed to a leak down test when the compression test indicates a problem and you use the leak down test to narrow down the source of the problem. If they did not provide you with an indication of where the leakage was occurring (did the tech indicate that it was one of the valves that was a problem or is that your speculation) I have my suspicions about the leak down test results.

The business with the 'valve covers' is also suspicious. You would not remove the valve covers for a compression or leak down test. You would remove the coil covers and flipping the coil covers front to back is easy to do and not uncommon. The covers will switch without any problem, the only issue being that the gaskets will not seal leading to potential moisture problems. When I replaced the plugs on my car last year I discovered that during the previous service by Acura the coil covers had been flipped front to back.

Having the valve clearances checked / set won't hurt; but, I would have a wet / dry compression test repeated and then have a repeat of the leak down test to see if there is a change (or if the original test was in error). Right now I am still thinking the leak down test smells like fish!
Good info of [MENTION=26435]Old Guy[/MENTION] here.

I wouldn't question the results of the leak down test. -46% is far off compared to the other cylinders. A leak down test WILL detect more problems than a compression test as it takes much longer to perform while the compression test works with only short cycles where a small problem can be 'catched up'. A leak down test will also allow you to search for the cause like [MENTION=26435]Old Guy[/MENTION] described.

Leaning out quite far out of the window, I'd suspect that the seller knows of the problem, they tried to fix it several times but with no success and now sells it.

Good workshops also use a professional endoscope to have a look in the cylinder to inspect the walls and the valve seats while they are open. Really good ones can turn their head like a worm. :D

The bad news is that if it's only an valve not adjusted correctly there's a risk that it has burnt its seat over time.
Sticky valve?

I pondered the sticky valve thing. However, the leak down test is done with the test cylinder at TDC. If a sticky valve / bent valve is holding the valve open enough to cause the high readings on the leak down test I would expect that sticky valve to manifest itself as a low compression test result on the same cylinder. The OP did not report an anomalous compression test result on the cylinder matching up with the high leak down test result which is why the test results smell fishy to me.
Thanks for the feedback on this. I'll check to see if the leakdown was double checked. I'll check again on this as well, but the tech did not give any meaningful feedback. Sounded like he was guessing without having done any more detailed work. He is the one that suggested that it could just need a valve adjustment. The valve covers were not removed by this shop.

Until last month, the dealership had done everything on the car. Seems like they loved to do valve cover gasket replacements, and the red finish on one of the valve covers is dirty and flaking (rear valve cover, which presumably may be the front valve cover). I'm skeptical of the test, too. Only catch being that they have an experienced NSX tech and I can't imagine he did not redo the test. If it's just a burnt valve, I'd still buy it. I just would really like to avoid the car if it's coming from the block.

My impression is the blocks are pretty stout - and given that the car has never been modified and has been dealer maintained, I'm guessing it could be fine. The problem for me is the leakdown combined with the weird valve cover gasket replacements and reversed valve covers... makes me think that [MENTION=10201]goldNSX[/MENTION] isn't leaning too far out the window. Seems like there have been some "hacks" to try to cover up the problem, but I just don't know these engines well at all so I can't really make a calculated guess.
I would avoid this car unless it's well below market value. A healthy C30 at sea level should put up 230psi in a compression test and <5% leakage across the board. (if you are at very high altitude then 175psi is somewhat acceptable but still not great)

46% leakdown is awful, and since that isn't reflected in the compression test I'm inclined to believe that there is some sort of cylinder wall damage at the top of the cylinder, possibly rust since this car seems to have spent a lot of time sitting. This sort of damage won't usually show up on a compression test. 6-9% on the other cylinders also isn't great.
Sorry for not clarifying. It's a 3.2 - the altitude is not high, just a couple hundred feet. Yeah, sounds pretty rough. It's considerably below market value - maybe $15k below. Not entirely sure since the market is so wild right now. I just don't have time to deal with an engine replacement / rebuilding an engine, so it's not so much about the cost.
one of the great assets of prime is the shared knowledge...please post the VIN of this nsx for future shoppers.
That is very helpful...you would be easily fooled by that FS post....esp the 1 owner..No mention of a sick engine
also no mention of a) mismatched body panel, b) improperly painted rear bumper, c) indifferent and careless owner, d) covered in scratches from essentially being treated like a honda civic (most have now been buffed out), e) damaged and missing interior pieces, f) curbed wheels. there were a number of other things that indicated careless ownership with poor maintenance. the fact that the dealer maintenance was very sketchy as well + potential engine issues....

owner does not care what it sells for but the person selling it for him was quite optimistic... at 65k i think it was worth buying and dropping off with ramon, but at the end of the day if i had to rebuild / replace the engine... even at 80k+ you would still have a pretty rough NSX... but if it were a civic it would be quite a nice one.
with older nsx you will find some examples (usually many owners) that have had very little $ spent on them. To own these properly esp now means $ mixed with effort.... with replacement parts hard to find means more will end up FS that sadly have issues.


looks like it is pending sale for 66k today after discovering all the engine issues. not bad for a 2000 monaco blue - i just can't deal with the headache. i'll jump off a cliff if someone buys it and does a valve adjustment or rebuilds the head and it works great. lol
So did YOU buy it? Not a bad deal and no need to jump off a cliff. :) Give it some time and put it the hands of a good mechanic who will take care of the possible engine problem.
Last but not least: congrats! :)
So did YOU buy it? Not a bad deal and no need to jump off a cliff. :) Give it some time and put it the hands of a good mechanic who will take care of the possible engine problem.
Last but not least: congrats! :)

I didn't. I wanted to post here before jumping into it. Given it looks like more of a pain in the butt than it's worth, I didn't pick it up. Could have been a really good deal, but there were just too many other signs pointing towards something being wrong. If it were just a leakdown I would have bought it, but the valve covers being on backwards, and repeated valve cover gaskets. Seemed like something else was wrong. Between that and the unreported accident, I told him I'd offer 60 but not more than that. Wanted to leave room for a new engine or rebuild.