Help deciphering compression test

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Jul 17, 2005 | 08:39 PM

colad

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Joined: Jan 2004

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Help deciphering compression test

Hi all,
My engine is losing oil and get Horrible mileage plus I have no oil pressure at temp even after changinf the oil pump so I was trying to determine how far I have to go to fix this engine.

I did a compression test and in pulling the plugs, one two plugs were BLACK as night (not really oily) and the other six looked good.

compression came back 175-190 psi in all cylinders with a 190 and a 185 reading in the cylinders that had the black plugs..

What am I looking at here?

Thanks!

Jul 17, 2005 | 09:44 PM

MBBFord

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Joined: Mar 2005

Posts: 8,542

Likes: 5

From: Louisiana

I did a compresion test on my old engine in my 77 F-100 and as long as all the psi are reading fairly close, your cylinders are fine. I had one cylinder not firing and another one wasn't even half of what the best one was, about a week later the crank shaft came out the block... needless to say I got a new block.

But buy some new spark plugs they're to cheap to try to clean and re-gap your old black ones. Sounds good to me though.

Just a little thing is your catalitic converter might be clogged, best thing to do is get regid of it.

Jul 18, 2005 | 03:48 AM

Chris_ce

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Joined: Apr 2003

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From: Indiana

Sounds like you are running rich.
For the oil pressure get a good mechanical guage to check it. If it is still not any better main bearings.
What motor and year?

Jul 18, 2005 | 08:57 AM

colad

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1990 351W (I am poretty sure now), 230,000 miles....

I do have a mechanical oil pressure gauge on it...it gets about 30-35 Psi when cold and then goes to 0 when warm at idle...I get about 7 Psi on the highway at 60 mph (auto tranny). This is running 20W-50...

Jul 18, 2005 | 10:11 AM

MBBFord

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Joined: Mar 2005

Posts: 8,542

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From: Louisiana

I run 10w-30 and when my engine is hot after running a whille it drops from about 50psi on start up to 10psi, give or take alittle. While I'm just going down the road with the engine warm it's around 20-25 psi, which many people I've talked to said is normal while warm, but also normal to drop to around 10-15 psi when hot. I can't say why your psi drops to 0, but low is normal, but not that low.

Jul 18, 2005 | 12:59 PM

Kemicalburns

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Joined: Jul 2002

Posts: 14,285

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From: Bend,OR

with that many miles your bottem end is probably worn out. worn crank bearings will cause a low pressure reading. try running strait 30w in there and see if that helps. probably a good idea to look for either a wreckingyard motor or get a reman.

Jul 18, 2005 | 03:44 PM

Bronco Man

Junior User

Joined: Apr 2004

Posts: 69

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From: Fresno Ca., San Joaquin V

Try some of the oil that is made for older engines. Maybe try 20/50 weight. My rig has 80 psi at cold star and 40 psi when warm. Check out the distributor cap and wires for your fouled plugs. Might be a good idea to replace spark plugs with a hotter range.
Good luck.

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Jul 18, 2005 | 04:00 PM

dwoodler

Senior User

Joined: Dec 2004

Posts: 460

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From: Edmonton, Alberta

The plugs are definitely due to spark/fuel issues and not compression. Check your wires out, the rotor and try swapping a couple different plugs into the holes where the black ones came from. You are right it is not oil, its carbon cause the cylinder is not burning properly (IMO). The low oil pressure is as Kemicalburns said, low oil pressure is caused by worn main bearings usually. It could be a clogged pickup, a worn out oil pump, or worn bearings. To fix the low oil pressure problem properly, you are going to have to pull the oil pan and check these things. Unfortunately in the fords with 302's the easiest way to do this is by pulling the engine. If the main journals on the crankshaft are within manufacturers specs, you can just roll a new set of bearings in (if that's the problem), the oil pickup is easy to clean with some varsol, brake clean or gasoline; and the oil pump is just a couple bolts and it pops out (when the pan is out). These are the things I think it could be as you have already ruled out a faulty oil pressure transducer by using a mech gauge. Good luck.

Woodler