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Help on a compression test

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Old 04-19-2012, 07:42 PM
shawn55 shawn55 is offline
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Help on a compression test

Here's my problem. I replaced the plugs on my 89 f-350 5.8l all the plugs look real good, light brown. I had 1 plug that was jet BLACK and full of crud, so I went and rented a compression gauge set and the best I could get was 60PSI and 0PSI on the one with the bad plug. Witch 60psi did not make sense to me so I went back and got a different gauge set and the same thing. So then I went and bought a new one and still the same thing. My dad had in old gauge that you had to hold it in the plug hole and that one read 120psi and 60psi on the bad plug hole. that gauge has the hard air tube on it so I can only do the 2 because its to big to get in to the others.
None of this makes sense to me why did the new gauges read so low. Anybody have an answer it sure would help.
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Old 04-19-2012, 10:01 PM
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What was your procedure?
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Old 04-20-2012, 06:33 AM
shawn55 shawn55 is offline
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took all the plugs out and started at the #1 cyl. Put the battery charger on to get a little faster crank.I put the gauge on and crank it over till the gauge did not move anymore, then read the gauge. Then I pushed the air valve in to release the air and started on the next one. And I did a wet test and nothing.
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Old 04-20-2012, 07:20 AM
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First the "rental" gauge has problems from what you said; Your dad's showing 60psi on the dead hole. Either rely on your dad's or get another gauge and your money back.

To properly take a reading you need to have a fully charged battery, have all plugs removed, hold the throttle plates wide open, and crank the engine over the same amount of compression strokes; I use 5. Repeat with a couple of squirts of oil for the wet test.

Sounds like you have a sticking/burnt valve on the one cyl. You could try putting a new plug in the bad hole and fuel injection cleaner in your tank.

Maybe you had a plug fail and the low reading is from the same crud you see on the plug; but it is doubtful.
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Old 04-20-2012, 10:11 PM
shawn55 shawn55 is offline
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well got another new gauge that makes 4 now and this one works. here they are

Pass.Side - Wet test - Driver Side - Wet test
125- 130 - 125 - 128
115- 125 - 105 - 110
115 - 125 - 58 - 58
125 - coudnt get oil in this one - 122 - 130
how does this look. Anybody have a good answer.And OldStyle yes I put new plugs in and ran it for 3 miles and then took them out to do the compression test and that bad hole the plud started looking brown and the rest look white and new.
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Old 04-21-2012, 11:15 AM
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You have a burnt valve. A wet test is only going to raise compression if your rings are bad. Squirting oil in the hole does nothing for a burnt valve.
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Old 04-21-2012, 11:15 AM
shawn55 shawn55 is offline
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That didnt turn out I will try it again.
Dry test
#1 125
#2 115
#3 115
#4 125
#5 122
#6 58
#7 105
#8 125

Wet test
#1 to hard to get oil in no wet test
#2 125
#3 125
#4 130
#5 130
#6 58
#7 110
#8 128

Maybe that makes more sense
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Old 04-21-2012, 01:55 PM
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Can anybody tell me what they think of those numbers. Good or bad
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Old 04-21-2012, 02:33 PM
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If those numbers are accurate, cylinder #6 has big problems with the valves in the head.
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Old 04-21-2012, 03:59 PM
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Thanks for your input, thats kinda what I was thinking. with that bad valve will that make the truck run a little ruff?
Also I found a motor with 126,000miles for $375 Would that be better to put in vs the valve job cause I heard that a valve job can really rack up a bill?
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Old 04-21-2012, 06:17 PM
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How do you know this great find of yours doesn't have the same problem? Or some other problem?

Myself, I'd ask around for recommendations on good machine shops & machinists, give 'em a call and see what they'd charge (assuming you bring the head(s) in to them). It's probably cheaper than you think.

Yes, that much of a compression difference (~45%) can cause a noticeable operational degradation.
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Old 04-21-2012, 06:30 PM
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All of the values seem on the low side and #6 looks like the valves or head gasket aren't sealing well. IIRC 60 psig is needed to fire the mixture.
My current 95 4.9 is in the 150 psig range with 238k miles. My old 83 302 was around 120 psig on 7 cylinders and 85 psig on #8 and you could feel the low one and its plug would foul quicker than the others. It had 200k miles at the time and the rings on #8 were shot. The wet test did bring it up to 110. The engine actually still ran decent and rust finally retired the truck at 218k miles.
YMMV
rikard
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Old 04-21-2012, 06:39 PM
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You might also pull the valve cover on that side and check the push rods for that cyl. Maybe you bent or spit one. Worth looking.
.
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Old 04-21-2012, 09:20 PM
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Well that other motor I found is a 1992 runs good its still in the truck. I will have to pull the truck in and tear it apart. But what shoud I be getting for compression on this motor. But Before I buy the other motor it will get a compression check done first.Its a 92 5.8l what should i be getting for compression?
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Old 04-23-2012, 07:24 PM
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i had the same problem but i had two cylinders (4and 6) with 60psi and 30 psi went to do a valve job and both heads were warped and the cylinder walls were scarred very badly. so i replaced the motor now all my tests run around 140psi on all cylinders (dry) btw i have a 92 E150 motor if that makes a difference
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