Did a leak down test on my motor, was having problems with a misfire, poor fuel mileage and smoking all the time. (blue smoke). Truck is a 85 f150 with a 89 engine which i made carbed. have a holley 4brl with clifford intake. Did the leakdown on cyl 1 and at tdc on the compression stroke. Applied about 65 70 psi down into the cyl. and immediately air came rushing out the oil cap. I also cked for air coming out the carb and tailpipe. none present just the oil cap and breathers. it was alot of air too, if i would put my hand over it and plug the hole air would try to push my hand off and would eventually come up outta the carb?? dont know why. same results on 3 other cyls. all at tdc on compression stroke. Compression test results are 90psi on cyl 1 with the choke plate open on the carb. 115 with it closed, once again dont know why. I dont suspect a burnt valve or anything, head is reman and has about 250 miles on it. what do you guys think. obviously something with the rings, broken, cracked piston?? let me know, time for a rebuild
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To check the rings do a compression test with a c. gauge. Take off the coil wire, screw in, crank the engine a bit. Ck the compression and compare with book. If low, squirt in a couple of squirts of oil and repeat. If the c. did not change, then oil is not coming past your rings.

 

well when i did the leakdown test and it came outta the valve cover how else would it do that other than getting past the rings??

 

Engine Leakdown Test



Solving cylinder leakage problems


An internal combustion engine makes power by first drawing air and fuel into the combustion chamber. Next is the compression of the mixture and the addition of a spark. The harnessing of the resulting contained explosion is ultimately what powers the car. As an engine gets on in miles, the containment of this power can be lost due to piston ring, valve, or cylinder wall wear. Engine performance will suffer as a result.

Pressure Reading
A common test of an engine's ability to compress the air-fuel mixture is called an engine compression test. A pressure gauge is connected in place of the spark plug. The engine is then cranked to create a pressure reading. A compression test is a good check the ability of the engine to create pressure. The gauge reads the positive pressure created by the cylinder.




Loss Reading
A leakdown test is a compression test in reverse. Instead of measuring the ability of the engine to create pressure, compressed air is introduced into the cylinder through the spark plug hole. One gauge on the tester measures the pressure of the air entering into the cylinder and the other measures the percentage of the air escaping—or leaking from the cylinder. The loss percentage will indicate the condition of the cylinder and overall condition of the engine.

Top Dead Center
Before sending air into the engine, the cylinder being tested must be placed at top dead center. The piston must be at the top of its travel. The intake and exhaust valves must be closed. When the air is compressed into the cylinder, the leakdown tester will measure any loss of air escaping past valves or piston rings. If the cylinder is not at top dead center, air escaping past an open valve will give a false reading.

Reading Results
No engine will have perfect sealing with zero percentage loss. Five to 10 percent loss indicates an engine in great to good running order. An engine between 10 and 20 percent can still run okay, but it’ll be time to keep an eye (or ear) on things. Above 20 percent loss and it may be time for a teardown and rebuild. Thirty percent? Major problems. The percent of leakage should also be consistent across the cylinders. Any great differences indicate a problem in that cylinder.

 

Nicely done kevin, as usual one can always count on you. OBCB

 

dont mean to be a jerk but i know how to do a leakdown and compression test. my question was why is soooo much air coming out of the oil cap?? my piston was at tdc on compression with not 1 but 2 valves closed!! and air came rushing outta the oil cap and breathers. again not trying to be a jerk i appreciate the help guy!

motor smokes alot all the time also. constant blue smoke. and above all i got a loud as s knocking/ clacking noise only at 2500 and above.

 

I would guess cylinder bore, broken rings, flat cam....

I would speak to a pro, or wait until some of the other guys chime in here. One way or another, something is amiss. I think it is time to tear down the engine, but speak to others, do the tests, evaluate. How many miles are on it? Blue smoke is a major indicator.

Good luck.

 

I think you already know the answer to this one. If engine was fine before replacing the head, did you up the compression with smaller chambers? That will do a number on old rings.

 

ya it shot out blue smoke from day 1. when i first got the engine and removed all the efi stuff and the cyl head the head was plugged up, hard sludge crusty. right there i knew it wasnt good but i had no option. i pulled the oil pan off and i did not see any metal shaving in the pan but the pickup screen was completly plugged, that right there sent up a red flag. but again i bought this engine and couldnt give it back so i put in a new pump and screen in along with the head and put it in the truck so my guess is this guy lied right to my face and told me that it ran great and didnt smoke. Lesson to everyone buying a used motor. If you have the option to see it run before hand DO IT.

 

I'm guessing at least rings, perhaps pistons themselves. My dad's old 82 300 DIDN'T SMOKE at all, but had no power. EVERY ring in the engine broken in several pieces and actual ring lands broken too. Had to punch it .030" over to clean up bores. Believe it to have been due too lean condition/spark knock. Only had 130,000 on it. Everything else (crank, cam, etc.) was like new. Richened up carb a bit on re-assembly and still running great years later when he sold it. Just tore down and re-built my 300 (hope to get her running tomorrow). It was the SLUDGIEST engine I have EVER seen. Embarassing that it's mine. LOL! Anyway, sounds like yours was high-miles/abused. Mine was 225K and very much abused. Everything went back standard, although the piston to bore clearance was at the outside of the acceptable range. I hate to tell you but you have 3 options: 1. Drive it and live with it as is for as long as possible. 2. Rebuild immediately and plan on the possible expense of boring/new slugs. 3. Get another used engine and check closely before buying. Best of luck with this one!

 

There's a lot more to rebuilding an engine than just boring and installing new pistons. Have the block magnafluxed to ensure there are no cracks before anything else. If the bore is good--checked by a machinist--you might be able to simply install new pistons and rings, then bearings, oil pump, etc. I would also have the rods checked and reconditioned if need be. I would replace the cam, lifters, t.gears, flywh. resurfaced, and the bottom end balanced if you're going to keep it. New p.rods and rockers would help keep o.pressure up.

 

in regards to having the rods reconditioned. whats all involved in that process and for what reason. the cyls will have to be bored out no doubt about it. And i was looking at those master engine rebuild kits on ebay for about 375. would i have to get that balanced or would they come balanced.

and also when i switched over from a serp belt sys on the 89 to a v belt. i used the v belt dampner, would that throw off any balancing issues

 

Google 'remanufactured rods' and read. A bit of homework is good. Read. Study. Put out an effort.

If you change from s. to v. belts, you have to change to an earlier year w. pump., because the v. will turn the p. the opposite direction.

V belt dampener for v belts, and visa versa.

 

All excellent points and advice! I would like to say that I wasn't suggesting that new slugs are all that is required. Perhaps I should have elaborated a bit more or kept in mind that some of our audience might have little/no engine building experience. For those of us who are more budget-minded than others (meaning poor) it is important to get the most bang-for-the-buck/return-of-investment out of a build. I have built a LOT of engines over the last 25 years for myself, customers, family, etc., both industrial and automotive, 1,2,3,4,6 & 8 cyl., 2 & 4 stroke, diesel, air & water cooled, etc. etc. So far, none have failed and every one has run great. I'm pretty proud of my record there, and I have re-used a LOT of parts. My philosophy is that the most important steps are CAREFUL and close inspection, accurate measurement and careful re-assembly. Much of this is due to having been trained by my ****-retentive, near-genius machinist older brother. One of the best investments anyone can make if they are going to build engines is a decent set of inside and outside micrometers, dial indicator, machinists' straightedge and so on. Then get someone who knows how to use them to teach you. You also have to develop an eye and a feel for all of this. There is also a level of experience that has to develop over time. You also have to "build to purpose", meaning you put into it what is required for the application (plus a little). Also a little logical thinking about what you have to work with.
Forged steel cranks and H-beam rods are a good idea for an 8000 RPM race small block, but a waste of money on a 4000 RPM daily-driver 300 I6. My main point in all of this was just to say that an engine with mild to moderate wear can be gone through and everything put back within factory tolerances pretty economically through knowing what can or can't be re-cycled, but from the sound of the description of this particular engine there is a high probability that it will need an overbore and new pistons in addition to the bare minimum, which might make it cost-prohibitive. I mostly wanted to warn the original poster that he might not really even have a viable core to do an "el-cheapo, shadetree" rebuild on. Only a tear-down will tell the tale.
I'll end this over-long rambling with a disturbing tale. Once, while visiting my bro-in-law down in "Deliverance Country" we went to borrow a c-clamp from his neighbors. These guys were in a dirt-floored barn, gathered around an old Chevy pickup. There was a dirty, greasy orange SBC laying in the dirt in front of the truck. I asked the guy if it was a 350. He said "Nope, 327." I asked, "You guys about to rebuild it?" His answer: "Naw, we just did. We're fixin' to put it back in." I am NOT one of those people! I couldn't help but wonder if they were going to re-install the used spark plugs and pour the dirty oil back in!

 

200000+f150 question for you and whoever else might know the answer. i did the leak down test again tonite and when the piston is at tdc at the verry top of its stroke at 0 degrees both valves closed and i pressurize it, air comes outta the breather fast. BUT when i put the piston say 10 degrees btdc and while holding the crank (if i didnt it would spin the engine since the piston isnt straight up and down) i pressurize it and very little air comes out the breathers. maybe a 5 percent loss vs a 50 percent loss at tdc. Why is that, Cylinder bore at top wider that at bottom?? i rechecked the compression too i found that my cheap 25 buck compression tester from pep boys was leaking badly at the hose end. took it back got a new one and compression is at 150 cyls 1 thru 5 cyl 6 at 120 with the spark plugs removed and throttle wide open. throttle closed there all at 120 and 6 is at 90 but thats just because theres not enough air coming in to compress. so maybe i just had a bad gauge but still why soo much pressure loss through the breathers at tdc on compression stroke vs 10 btdc on compression stroke???