TurboTommy

Just for ****s and giggles I had a leakdown test performed.
To my surprise (25k miles on new engine and burns no oil), the numbers were terrible and all over the board, except for number 4.
The air was leaking past the rings and coming out of the dipstick and oil filler.
What I don't understand is that shouldn't I have seen some symptoms when I'm running the car? Like I said, I practically burn no oil and seem to have very little blow-by. (I have this homemade little test where I have a piece of foam fitted in the oil filler tube, leave the cap off, and drive the car hard---foam stays in place and leaving it there for awhile renders very little oily residue in said foam)
Anyway, technician saw lots of carbon on top of the pistons and thinks there's carbon build-up on the rings/lands and are sticking in a non-running engine. (When the engine is running combustion pressure forces the rings out to the cylinder wall)

What say you all about this?
Has anybody else experieced this?
We did the test twice; cold engine and warmed up. Numbers were slightly better warmed up but not much.
Tech suggested a carbon cleaner treatment. What are the pros and cons in doing such a cleaning?

Any other explanations?

Black51

What do the plugs look like in the problem cylinders? I assume you have a full standalone engine management system. Has that been properly tuned? Have you set your own timing? My best guess is you're not getting proper combustion. At 25k miles, it seems like the new engine should be broken in by now. I notice you have many "unconventional go-fast mods", which sounds like monkey business to me for a 3.0 build... I'm not a pro though.

JDS968

"Lots of carbon" on the pistons definitely hints at incomplete combustion. How does the throttle body and throttle plate look? How are the combustion chambers? Valve heads? How about the faces and stems, intake and exhaust? And what about the exhaust ports and headers? I'm guessing you'll find the combustion chambers, and valve heads baked with carbon, and the exhaust valves, exhaust ports, and headers full of soot. Throttle body and intake valve stems/faces will probably be relatively clean.

Do you have a wideband lambda sensor, and if so, is it set up to output to a gauge or to a data logging system?

Start out by Seafoaming the car, it will probably belch an unbelievable amount of smoke, and then temporarily fix the bad leakdown numbers. If so, you'll know it was the carbon buildup, and you can go about tracking that down.

fast951

Thomas, what are the leakdown numbers?
You are running a water injection system, it should clean the combustion chamber. How did you determine there is excessive carbon buildup?

Since you have no crankcase pressure problem and not experiencing excessive oil consumption it tells me there is no engine problem.

86 951 Driver

Try to use Seafoam to clean that junk all out once you get the problem figured out. I would assume running rich, but you may also not have enough timing. Could go either way really. Has it been strapped to a dyno for any kind of tuning? Street tune somthing?

User 41221

How experienced is the tech at doing leakdown tests, and how well do you know the shop? Did the tech use a boroscope to confirm his diagnosis of carbon build-up on the piston tops? I am not bashing the guy, but plenty of folks have problems doing a valid leakdown test, even experienced mechanics. Did he do a compression test first?

Like John said, if you are running water injection, your piston tops and combustion chambers should be very clean, as that sort of like running constant carbon cleaning all the time.

If you are getting decent power and not using oil, I wouldn't spend a lot of time worrying about this. The only thing I am curious about is that the air was escaping through the AOS, which would indicate a ring sealing problem. If the tech has a problem doing a leakdown test, you will usually get air out of the intake or exhaust, not the crankcase.

TurboTommy

I do trust that the tech did the leakdown properly. I followed up with some questions to sort of make sure he did it right. We discussed it and he wasn't defensive. He knows I'm **** and I've been working with this shop for almost 10 years.

We didn't do a compression test. Should have done it and still will when I get a chance.

All four plugs look good and identical.

I have a wideband, and the engine does not run rich. In fact I run a little leaner than most here would run. I replaced the O2 sensor for the wideband a few months ago; still same readings.
Running a Vitesse MAF.

No boroscope. Just used one of those high powered narrow beamed lights. Yes, I know, it might not be a proper visual aid, but one could see an obvious carbon issue. What I don't have experience with is how much does it take to be detrimental.
Obviously could not see the condition of the valves. Had the intake manifold off, for an unrelated issue last spring and everything looked clean there.

As for the merits of water injection, the general consensus is that it's supposed to "steam clean your engine". A couple of things with that, however: you have to be close to full boost in order for WI to be activated. I can honestly say that I'm not "on it" often enough and also my "boost fluid" is only a small percentage of actual water.

There is something that could have caused the possible excess carbon:
I've had a problem since day one that it would run excessively rich the instant I would throttle lift (in that time until the injectors close at the throttle idle position). I'm talkin' pig rich, off the charts rich, flames out the exhaust rich. Only recently have I been able to rectify this somewhat (still rich, but not as bad). But, in all this time, I wonder if this could have caused the excess carbon (even if the time spent on throttle lift is a small percentage compared to the time spent at other throttle conditons when the air/fuel is spot on)?

Anyway, are we to assume then, that it's the carbon that's causing the bad leakdown #? Does it make sense that when the engine is running that combustion pressure around the rings could neutralize the stickiness of the carbon and still give a good seal?

Is it even a good idea to do a carbon cleaning of the combustion chamber. Does the procedure even work properly? What happens to the carbon? Dissolves? Breaks loose? (If it does, could it scratch cylinder walls?).What's this belching of smoke I'm hearing about?

Thom

I would be interested to know how you got rid of the excessive rich mixture when lightly lifting off the throttle as I (and other friends who run the same EMS) have had this "issue" since day one as well.
I replaced the headgasket two months ago and noticed some carbon build up on the top of the pistons but compression and leakdown tests done afterwards gave excellent results.

TurboTommy

Well, like I said, when lifting I still have a rich condition; it's just inherent to the MAF system. It's just that in my case it was ridiculus.
I ended up going with a stiffer throttle return spring and increased the play between fully closed throttle (up against the stopper) and the "click" position.
This was in an effort to ensure that the injectors would turn off as quickly as possible, between shifts,etc. This helped somewhat.

Anyway, still wondering if I should go ahead with a carbon cleaning treatment. Are there any detrimental side effects to this procedure?

And in a nutshell, is there even any other causes to have bad leakdown #s in the face of a seemingly good running engine. Is it possible I don't know what a good running engine really is? When running in the driveway, cracking the throttle a little bit produces a noticeable movement of air from the oil filler; but, I checked my other cars and they do the same thing (maybe even more).
Leakdown numbers were............are you ready for this.......... between 25 and 45% !!! Number four was 10%

fast951

Is this correct Leakdown between 25% and 45%? Or do you mean 2.5% and 4.5% ?

If you have between 25% and 45% leakdown, then there are some serious problems! Where is the air coming out from Intake, Exhaust, Crankcase?

10% leakdown on #4 is a bit much. Was it cold or hot?

Tedro951

I'm trying to wrap my head around your problem, and I'm struggling. The car runs well, but has a rich issue at throttle close. Oil consumption is minimal. Life was good until a leakdown test was performed, returning numbers that should probably have air howling out of the oil filler in a manner that would blow your hat off.

Was each piston at TDC when THAT cyl was checked? Not that it matters. I'm betting the car is fine.

As for Seafoam, good luck reading through 1.4 million pages of info and 2000 youtube videos of people smoking up the neighborhood. I've done it. Have no idea if it helped anything, but it was cool. If I had a serious ring/carbon issue, I'd do the procedure GM used on Northstar engines, but I still don't think your car needs it.

User 41221

I'd really be curious what a basic compression test would show. If you really have an issue, a compression test would be the best way I can think of to validate the leakdown results. If you really have loss of 25-40%, I have a hard time believing you wouldn't know it.

Black51

Something seems to be contradicting itself here... Have you thought about doing a re-test, maybe even elsewhere to get a second opinion?

TurboTommy

Well, I would agree that something is strange. I'm going to do a compression test when I get the chance and report back.

Still wondering what a carbon cleaning entails. Are there any negative implications? Is there any chance that loosened up carbon could do any damage.
Anybody?

Tedro951

Have you even looked at the Cadillac Northstar procedure I recommended above?
Pouring stuff in your tank won't work. Pouring stuff in your intake wont work. If your ring lands are truly carboned up, I believe you have 2 options. The procedure mentioned above, or physically take the rings off the pistons.

If I was even going to bother with it, I'd try the GM thing first.