Scruit

My 96 is going for $1k in frame work on weds, so I've been doing a bunch of tests to see if the car is worth saving.

Previously I've checked all the suspension and subframe mounts for rust - all clear (except the one I'm having fixed). I also cut open the oil filter to check for metal debris - nothing.

Today I did a compression test

1 185
2 185
3 190
4 195
5 190
6 185


Then I did a leakdown test:

1 7%
2 7%
3 70%
4 5%
5 5%
6 7%

ARGH! 70% loss, all coming out the tailpipe - sounds like a bad exhaust valve.

After some more I checked the ending was still at TDC. Nope - the leak test air had pushed the piston down and turned engine away from TDC, opening the exhaust valve a tiny bit.. I set it back and locked it in position, then got 5% from #3.

The leaking air went into the crankcase and out the filler cap, as expected.

So far so good - this is great news for the engine as it proves the quality if the seal on the valves and rings is as good as new. Even at 131K the engine is in excellent condition.

At least now I'll be confident that I'm sinking $1k into fixing a car that is worth fixing.




















Uh, I prolly I just jinxored myself...

speed racer

iTrader: (10)

I never heard of a leak down. Can you explain what is needed to do this test and possible a little write up. I did my compression test about 4 months ago and it came back good as well. There was on cylinder which was around 15 but I'm not worried yet about it.

Scruit

Leak Down Test

The leakdown test is another way of measuring the sealing capacity of the cylinder. The basic concpet is to feed shop air into the spark plug hole while the piston is at TDC on the compression stroke, and to see how much pressure it can hold.

If you don't want to spent the $100 on buying a leakdown tester, you can make your own.

http://en.wikipedia.org/wiki/Leak-down_tester

The tester has the following components, in order:



1) Shop air supply, regulated to 100psi
2) Pressure guage to read the pressure that the shop air is supplying
3) Damper valve between the pressure guages to keep the pressures seperate
4) Pressure guage to read the pressure of the cylinder
5) A quick-release to allow the spark plug fitting to be threaded in easily, then connected to the air supply. Without this you will have to turn the whole tester as you screw it in!
6) A threaded fitting to screw into the spark plug hole. This has to be straight, about 9" long for the deep spark plug wells in the maxima.

The parts can all be purchased from hardware store (1/4" NPT brass fittings) for about $20 - except the damper valve. All this is is a tube section inline with the tester between the guages that has a severe restriction. This helps keep the pressures from equalising on the two guages - for the test you want to read the difference in pressures. I made a damper valve by plugging a 1/4" NPT male-to-male fitting with solder then drilling a 1mm hole in it.


Steps to test (allow 1-2 hours):

1) Remove the fuel pump fuse then run the engine till it dies. Remove all coil packs and spark plugs. Open the oil filler, the radiator cap and prop open the throttle.
2) Rotate the first cylinder to TDC on the compression stroke using a wrench to turn the crank pulley clockwise.
3) With the shop air supply regulated to 100psi, and the engine locked in place using the wrench on the crank pulley, apply the shhop air.
4) Read the lower guage, it should read 80-100 psi. (80psi = 80%, therefore 20% loss. 95psi = 5% loss)
5) Listen to where the air is escaping to.
6) Repeat 2-5 on the remaining cylinders.


The loss should be less than 20% - any more than that is bad. More that 40% loss means you are doing something wrong, or your engine is crap. Less than 10% is great. Less than 5% is spectacular. 0% loss on a road engine means you must be doing the test wrong, because no engine can be that good, not even a racing engine.

5-20% loss, with the air escaping through the oil filler, is normal and good.

Any air escaping through the exhaust means you should suspect a poorly sealing exhaust valve. Similarly, air escaping through the throttle body means poorly sealing intake valve. Air coming through the radiator means bad head gasket.


The leakdown test is a good follow-up test to a compression check. It will help you pinpoint problems that exist, but also give you a better idea of the cylinder's condition than a compression check alone. A compression check can give artificially good numbers if you crank too many times, or if you have fluid pooling in the cylinder (leaking oil or fuel). Also you can get artificially high numbers from carbon buildup. If your engine passes a compression check AND a leakdown test then you can rest assured your engine is in good shape.

2da mizzax

iTrader: (4)

http://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm

speed racer

iTrader: (10)

Thanks for the link. That's a very good article. I'm sure Scruit you did it yourself. Damn I'm thinking I should probably look into this b/c I plan on putting the vq35 cams in. Don't want my engine to blow up.

I also have a quick question. Cutting the filter open on all oil changes provides a good indication for metal shavings?

I probably have to go to a shop b/c I don't have an air compressor.

Scruit

yes. That's where the metal shavings would get caught, assuming they didn't get caught on the oil pump pickup screen - and if they did then you are already in huge trouble. Another good way of checking is using a magnetic drain plug.

If you cut open the filter and all you find is soot, then that's a good sign that your engine is not eating itself up.


I had a small compressor when I lived in an apartment. But yeah, the shop is nice.

speed racer

iTrader: (10)

Excellent Write UP Now I need to get all my parts ready.

TurTLe*

iTrader: (11)

nice, i was about to stay retest that #3 because your probaly on TDC another stroke or cylinder. Normally you check which piston is at TDC by insert a long straw in there and watch it move up, untill the piston stops going up, then you have 2 options your either on the intake or compression stroke. So once you put the tested in. if you get throw back, your on the wrong one and try again.