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Old 08-27-2010, 12:14 PM   #1
96meangreengt
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Default compression test results

well i decided to do a compression test because i have been getting a knocking sound coming from the motor when it sits for a long peroid of time and does it for the first 3 mins or so of running. not really a loud knock but you can hear it with the windows down.

Drivers side
175
170
175
175

All good right, this is untill

passenger side
150
70
180
150

Spark plugs look great
no smoke or anything car does have 130,000+miles

If the compression was that bad wouldnt it run like crap and not idle worth a crap

It runs good beside the knock early in the morining and has plenty of power What do you guys think. also I get 25mpg average with every tank and thats with 4.10 gears.

The way I did the test was after I got home from work I let it sit for about 45min, took out all sparkplugs, discconected coil packs, held throttle wide open and let it crank about 8 times each cylinder. I had the air intake of if that matters because its hard to get to that side of plugs with the intake in.
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Quote:
Originally Posted by Bladerunner View Post
With 115K+ miles you should be ready to trade in that mustang and get
a newer mustang with lower mileage.

1996 pacific green gt,
Cobra brake swap/stainless lines
sniper tuned
Mishimoto radiator
frpp 4.10 gears
corsa black mufflers
steeda tri-ax
pi intake swap
msd wires
MM road and track package
ram hdx clutch, ralco flywheel
MM torque arm

Last edited by 96meangreengt; 08-27-2010 at 12:40 PM.
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Old 08-27-2010, 02:48 PM   #2
cliffyk
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I assume you double/triple/quadruple tested that cylinder?

If the compression really is that low, and there's no smoke/steam from the exhaust, then I'd start looking at the valves for that hole.

Connect a vacuum meter and examine the vacuum at idle with the engine hot. It should be 18 to 22inHg and steady--if not here's a document explaining how to interpret the readings.
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Old 08-27-2010, 03:01 PM   #3
96meangreengt
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Quote:
Originally Posted by cliffyk View Post
I assume you double/triple/quadruple tested that cylinder?

If the compression really is that low, and there's no smoke/steam from the exhaust, then I'd start looking at the valves for that hole.

Connect a vacuum meter and examine the vacuum at idle with the engine hot. It should be 18 to 22inHg and steady--if not here's a document explaining how to interpret the readings.
I checked it twice cuz i was WTF? I thought maybe i didnt have the compession tester screwed all the way in but it read the same, well second time it read 75. Went outside and checked it again cold and it reads 90. I will have to get a aftermarket vaccum gauge because the auto parts stores dont rent them out.

Im just baffled because like it said it idles fine and pulls hard and doesnt smoke, all sparkplugs looks exacty the same sorts grayish, does burn a lil oil though probly 1 quart between changes

I would think it would run like crap with that one being that low and not to mention the other 2 being pretty low. 15% difference than the rest
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Quote:
Originally Posted by Bladerunner View Post
With 115K+ miles you should be ready to trade in that mustang and get
a newer mustang with lower mileage.

1996 pacific green gt,
Cobra brake swap/stainless lines
sniper tuned
Mishimoto radiator
frpp 4.10 gears
corsa black mufflers
steeda tri-ax
pi intake swap
msd wires
MM road and track package
ram hdx clutch, ralco flywheel
MM torque arm
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Old 08-27-2010, 03:15 PM   #4
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I have seen stranger things over the years--you might want to run a "wet" compression check, put a couple teaspoons of motor oil in the cylinder then perform the check letting it crank 5 or 6 full turns.

If the compression goes up that's an indicator of worn or broken rings...
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
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Old 08-27-2010, 03:26 PM   #5
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Quote:
Originally Posted by cliffyk View Post
I have seen stranger things over the years--you might want to run a "wet" compression check, put a couple teaspoons of motor oil in the cylinder then perform the check letting it crank 5 or 6 full turns.

If the compression goes up that's an indicator of worn or broken rings...
Ok will do. but if the rings was broke or worn out I should see some smoke right expecialy on hard acceration. I will try to get a vaccum gauge.

I was planning on put a new motor in the winter anyway just wanted to be able to drive it untill then.
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Quote:
Originally Posted by Bladerunner View Post
With 115K+ miles you should be ready to trade in that mustang and get
a newer mustang with lower mileage.

1996 pacific green gt,
Cobra brake swap/stainless lines
sniper tuned
Mishimoto radiator
frpp 4.10 gears
corsa black mufflers
steeda tri-ax
pi intake swap
msd wires
MM road and track package
ram hdx clutch, ralco flywheel
MM torque arm
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Old 08-27-2010, 07:55 PM   #6
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Quote:
Originally Posted by cliffyk View Post
I assume you double/triple/quadruple tested that cylinder?

If the compression really is that low, and there's no smoke/steam from the exhaust, then I'd start looking at the valves for that hole.

Connect a vacuum meter and examine the vacuum at idle with the engine hot. It should be 18 to 22inHg and steady--if not here's a document explaining how to interpret the readings.

Ok went and got a vacuum gauge and at idle it reads a steady 20inHg but when you trottle it real hard and fast it goes past the o into the other side,(i hope thats because its a cheap $25 gauge) then come back up to about 22-23inHg then leverles back down to 20inHg. according to the chart if the needle jumps to 0 when throttled hard it is worn rings. tomorrow im gonna do another compression test and do a wet test to see what i get.

Its just funny if its worn rings surely it would smoke a lil. i have taken it to 5,000rpms several times these past few weeks and it pulls hard and doesnt smoke at all.
__________________
Quote:
Originally Posted by Bladerunner View Post
With 115K+ miles you should be ready to trade in that mustang and get
a newer mustang with lower mileage.

1996 pacific green gt,
Cobra brake swap/stainless lines
sniper tuned
Mishimoto radiator
frpp 4.10 gears
corsa black mufflers
steeda tri-ax
pi intake swap
msd wires
MM road and track package
ram hdx clutch, ralco flywheel
MM torque arm
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Old 08-27-2010, 09:37 PM   #7
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Quote:
Originally Posted by 96meangreengt View Post
Ok went and got a vacuum gauge and at idle it reads a steady 20inHg but when you trottle it real hard and fast it goes past the o into the other side,(i hope thats because its a cheap $25 gauge) then come back up to about 22-23inHg then leverles back down to 20inHg. according to the chart if the needle jumps to 0 when throttled hard it is worn rings. tomorrow im gonna do another compression test and do a wet test to see what i get.

Its just funny if its worn rings surely it would smoke a lil. i have taken it to 5,000rpms several times these past few weeks and it pulls hard and doesnt smoke at all.
vacuum is highest at idle so naturally when you throttle up the vacuum will go down. as long as its where it is supossed to be at idle i won't look into that as the culprit. do a wet test as see what happens
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Old 08-28-2010, 08:32 AM   #8
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Quote:
Originally Posted by massred98gt View Post
vacuum is highest at idle so naturally when you throttle up the vacuum will go down. as long as its where it is supossed to be at idle i won't look into that as the culprit. do a wet test as see what happens
However the intake vacuum should not drop off to 0 during acceleration-this is diagnostic tip #4 in the shop manual page I linked to above.

And at the risk of being overly technical, vacuum is highest under closed throttle deceleration...
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.
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Old 08-28-2010, 08:47 AM   #9
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Quote:
Originally Posted by 96meangreengt View Post
Ok went and got a vacuum gauge and at idle it reads a steady 20inHg but when you trottle it real hard and fast it goes past the o into the other side,(i hope thats because its a cheap $25 gauge) then come back up to about 22-23inHg then leverles back down to 20inHg. according to the chart if the needle jumps to 0 when throttled hard it is worn rings. tomorrow im gonna do another compression test and do a wet test to see what i get.

Its just funny if its worn rings surely it would smoke a lil. i have taken it to 5,000rpms several times these past few weeks and it pulls hard and doesnt smoke at all.
Vacuum gauges are cheap, mostly because they only have 4 moving parts (the Bourdon tube, link, rack and indicator):

Click the image to open in full size.

I have one from JC Whitney that cost just $10 over 35 years ago--it works fine--that one would be inaccurate is unlikely--it should read 0.0 of course when not connected.

The wet test will tell us a lot, but that the vacuum at idle was steady pretty much rules out a valve problem...

How many miles on the engine?
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.
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Old 08-28-2010, 09:28 AM   #10
96meangreengt
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Quote:
Originally Posted by cliffyk View Post
Vacuum gauges are cheap, mostly because they only have 4 moving parts (the Bourdon tube, link, rack and indicator):

Click the image to open in full size.

I have one from JC Whitney that cost just $10 over 35 years ago--it works fine--that one would be inaccurate is unlikely--it should read 0.0 of course when not connected.

The wet test will tell us a lot, but that the vacuum at idle was steady pretty much rules out a valve problem...

How many miles on the engine?
I bought it with 128,000 but the odemeter quit working, the guy i know is a good buddy of mine and thought it had about 135,000 cuz he was in iraq for a year and a half. and he bought it before he left and his dad drove it every once in awhile.

OK just came in from doin the wet test, weird results

drivers side dry Drivers side wet
1. 175 1. 195
2. 185 2. 205
3 185 3. 205
4. 180 4. 195

Passenger side dry passenger side wet
1. 180 1. 180
2. 180 2. 185
3. 170 3. 180
4. 170 4. 175

didnt do anything different than yesterday besides just start it up and let it get warm, got to the O on the water temp gauge and then took everything out, takes like 10mins. I dont see how it could of went up without it being something it did like not having the compression gauge screwed all the way in or something.

On ever cyclinder the first stroke was always above 120psi.
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Quote:
Originally Posted by Bladerunner View Post
With 115K+ miles you should be ready to trade in that mustang and get
a newer mustang with lower mileage.

1996 pacific green gt,
Cobra brake swap/stainless lines
sniper tuned
Mishimoto radiator
frpp 4.10 gears
corsa black mufflers
steeda tri-ax
pi intake swap
msd wires
MM road and track package
ram hdx clutch, ralco flywheel
MM torque arm
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Old 08-28-2010, 09:28 AM
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