disclaimer: There very well could be some noobness here on my part but im ready for some flaming if so required.

So my motor has terrible blowby on my #4 cylinder. I found a motor on craigslist that claimed around 90k on it (99 motor). I got permission to do a leakdown from the seller. 1&4 were leaking severely out of the exhaust side and 2&3 were leaking severely out of the intake side. all about 80% leak. The motor was on the stand and hadn't been started inabout 6 months. I did a leakdown on my motor cold this morning & got excellent results on everything but my 4th cylinder... which was to be expected. Did I overlook anything?he claimes the motor started 6 months ago when he bought it before it got pulled. with those numbers i'd doubt it would even start. thoughts please...

and yeah I made sure the crank was at stock timing and made sure my cams were in the correct position.

 

You sure the valves were closed on the cylinder you were testing? 80% leak is rediculous.

 

it had been sitting for a while.... do they tend to get stuck open when sitting?

 

Hi
Seller here for input as well.

I promise I'm not trying to screw him over, I was just as surprised as he was over the results.

Yes, it was at TDC. We even took off the timing belt and set both cams for the cyl we were testing to lobes straight up (Valve Cover off)
Looked like a new belt and and the timing seemed off on the intake cam.

 

You're supposed to do a leakdown on a warm motor. This one isn't, so panicking over results should be tempered a bit.

If the engine has sat for a while, I would personally put a bit of oil through the spark plug holes (just a bit) and turn it over a couple of times. Rings don't expect to see totally dry cylinders.

All plugs out at this point, so you're not fighting the motor.

Wait a little bit (have a beer) so any extra oil can leak past the rings, then get the motor at TDC for the cylinder you're testing. Note that there are two TDCs in the 4-stroke cycle, and one will have valve overlap, probably. You want the other TDC.

Depending on whether you have a 2-gauge or single gauge leakdown tester, make sure you know how it works.

Put some air into the motor via the spark plug hole on the cylinder under test.

You should try moving the crank a bit back and forth around TDC in the hope that in doing so the rings will seal against the bore.

Put the plugs back into the cylinders you're not testing. This is because rings use compression to help seal against the bores, and adding air is likely to push the pistons down in the bore on either side of perfect TDC. If that happens, you will get very bad false leakdown numbers.

I wonder if removing the belt may have caused you to miss that, if it happened. The plugs in the other 3 will help keep the engine from rotating. Wouldn't hurt to do some observing of the crank position, maybe mark it at your TDC position to help determine movement.

Do your leakdown test. Observe your numbers, of course, but also very important: listen.

If there is a hissing sound from maybe the oil dipstick hole, it's getting past the rings.

If you open the TB and hear hissing coming from the intake ports, it's getting past the intake valves.

If it's getting past the exhaust valve, you probably can hear it even back at the tailpipe. Maybe the exhaust manifold in this case.

If you have the valve cover off, and there is leaking past one of the valves, use a rubber mallet and give the valve a tap (gently, son; there's a piston right there) to see if you can get the spring to get it seated properly. The air pressure should seat the valves, but things happen in an engine that's been sitting.

How's the valve lash?

If you hear gurgling you may have a coolant leak; can be bad head gasket or a crack in the head or block. Pull the head and investigate. You should be able to see if the gasket has blown away somewhere. A blacklight is handy with coolant questions.

Hissing in an adjacent cylinder might also mean head gasket.

Repeat the above for the other three cylinders. Since you don't need to wait this time, the beer is optional.

Leaking even 30% is probably OK for a used engine. More an issue if you get intake valve leaking than exhaust or rings; for intake 20%+ might be worth investigating. These are numbers for a warm engine, remember.

If it's just one cylinder with issues you may just decide to run the motor. People have fixed a single leaky valve, and then found zero power on the dyno from the fix.

The top of the bore is a high wear area, so don't expect perfection with ring seal at that part of the cylinder. But it is the part of the bore you do want to measure.

And remember, it's all going to leak down; don't expect it to hold pressure for minutes or even a minute. Get your instantaneous numbers and stop worrying.

Hope that helps, maybe some things there you didn't try.

 

^ Probably one of the best, most concise descriptions of how/what/why of a leakdown test. Text should go into one of the DIY stickies.

 

What did you set the regulator to? You only need like 30-40psi for a leakdown test. If you blast 90psi into the cylinder you'll probably get some bleed off.

 

i set the regulator to 100, 80, and 20. All showing the same results. The valves were stuck open. Not sure why. Really I guess the best bet is to put some oil in the cylinders... i'm not sure what else to do. I'm sure the seller isn't trying to rip me off and the engine ran when he bought it but i really don't want to buy a motor without knowing it has good compression and is in decent condition.

 

Did you wiggle the cam around while testing?

My experience with leakdown tests has been that valves can get stuck slightly open, and if you wiggle the cam back and forth a bit that will often seal it. I'm not entirely sure why, but my theory is that a bit of carbon is getting stuck between the valve and the seat and preventing it from closing. Cracking the valve open allows it to get blown out by the pressure. If you think about it, the engine never sees a situation like a leakdown test during real use, in which the cylinder has ambient pressure in it when the valve actually closes, there's always air/exhaust flowing over it.

--Ian

 

Well if the valves are open (off the seat) you're not going to get a good leakdown number.

I guess in that case myself I would want to know why they are open. The problem, if there is one, might not be difficult to address.

 

Not so much wiggle it but like I said we took the timing belt off and rotated the cam a couple times each. Thank god for non interference ha ha. But yeah, same thing. That didn't change anything.