I am not trying to suss out any spcific issues, just looking at the overall health ov the truck in deciding what to wrok on next
I did a compression and leakdown test last weekend. Here are my results
Cyl Compression Leakage
1 145 psi 35%
2 150psi 42%
3 142psi 45%
4 140psi 35%
5 140psi 40%
6 150psi 40%
7 145psi 45%
8 140psi 35%

My manual states between 160-200psi is expected with a variance no greater than 20psi between highest and lowest readings. I have no info on leakdown. I listened for leakage at each cylinder and it was primarily going to the crankcase in each instance. Cylinder 2 sounded like a slight intake leak as well, and I could not hear any leakage from exhaust pipes on any cylinder. I used the HF leakdown tester and it seemed pretty horrible. Issues were
*90psi at compressor but it could only put out 10-15psi
*needle on leakage dial continues to descend slowly for a bit after zeroing the tool
*when plugged to open connector (100% leakage) the dial indicated 70%

The only running issues I have are
Can't get idle below 900rpms
Slight surging on low speed
I have an oil and presumably vacuum leak on the intake manifold that I am hoping is the cause of those issues
Any opinions on my compression and leakdown readings?

 

Doesn't sound too good if you're actually getting 35% leak with only 10-15 psi during leak down testing.

Silly question: Your engine was at operating temp during testing?

I usually do leak down at 100psi. For fun I did it on my 6 cylinder mustang at around 100,000 miles. It had zero leak down...couldn't believe it.

I checked a freshly broken in FE engine a while back and had less than 5% leak down in each cylinder.

Might just do a check for anything messing with idle like vacuum leak, carb tune or timing but you might be heading for a rebuild if you want to drive it a lot.

 

I think those compression numbers are pretty good; the estimates from the manual seem high to me. My high-mileage 352 tests at 130 on all eight and it runs well with adequate power.
The leakdown test results don't jibe with the compression test, seems to me.
Eric

 

For fun you might remove and duct tape off any crankcase vents like the PCV and crankcase breather. Then put a pressure gauge on the dip stick tube, and fire up the engine. (Do this after the engine is warm). Rev the motor to about 2000 rpms and see what pressure you get. I would expect less than 2psi on a healthy motor. Not a real test but can tell you if you're getting serious blow-by.

 

High chance of blowing the rear main seal by doing this. That requires pulling the motor to fix.

 

Actually, on an FE you can replace the rear main seal without pulling the motor. I've done it. If it did blow your rear seal, your motor is definitely ready for a rebuild due to worn rings.

You can also rev it with your hand over an open breather hole and feel if there's a lot of blow by. But if you're getting much blow by, you'll know because it will overwhelm the PCV valve and oil will ooze out your breather cap and all over the engine.

Or get a better leak down tester and repeat the test. I'd be curious what your leak down is when tested at 100psi instead of only 10-15psi.

Or just skip it. First FE I had was a 352. Leakdown on every cylinder was 40-50 %... mostly past the rings. It ran and idled okay but was definitely well worn.

 

Thanks for the responses
The tests were done with the engine stone cold. I have never done a leakdown test before. I did not know they were supposed to be on a hot engine. I can try again warm.
Are listed compression readings intended to be on a warm engine also?
I cannot test higher than 15psi with this tester from Harbor Freight. I have 90 at the tank and it reads 15 at the tester. I am not sure why. I may just get myself into more trouble trying to rely on this cheap leakdown tester.
If you had a leaking main seal where would you see the drip from? I am getting some oil from the weep hole in my clutch housing.

 

You would get dripping out of your clutch housing if your engine rear main seal is shot. The rear main seal is at the back of the block where the crankshaft end shaft is for the clutch

 

Yeah, test compression and leakdown on warm engine. I like for engine to cool before re-installing spark plugs.

Rear main leaking will cause oil to show on clutch inspection cover but so can leaks from other places since it's in the back and down low. Valve cover leaks will collect there, oil filter adapter leaks, oil pan leaks, etc. so it can be tricky to diagnose.


Just a safety note... if you do pressurized a cylinder to 100psi, the engine will try to rotate and will move the truck if it's in gear. I just make sure the truck is blocked...like with a tree so it can't move.