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Engine Low compression in all 8 cylinders

  • Thread starter Thread starter zlightlystoopid
  • Start date Start date Feb 25, 2020
Z

zlightlystoopid

Member
Jan 29, 2020
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California
Feb 25, 2020
#1
  • Feb 25, 2020
  • #1
Whats up guys after my gt40 heads, upper and lower intake and tfs1 cam swap the car runs good but idles rough and seems to be misfiring so i checked the compression today and all 8 cylinders were about 60psi even added a bit oil in the cylinders and still around 60psi like wtf, at that psi i wouldnt even think the car would start or run, and it starts and runs pretty good thats what confuses me. Any ideas?
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
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Feb 25, 2020
#2
  • Feb 25, 2020
  • #2
I would check the valve adjustment. You may need different push rods.
just my opinion and not based on any actual mechanical experience.
 

dgollem

20+ Year Stangneter
May 5, 2004
211
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Waldorf MD
Feb 25, 2020
#3
  • Feb 25, 2020
  • #3
With all cylinders close to the same, I would suspect the cam is out of time allowing the intake or most like likely the exhaust valve to stay open too long and bleed off compression through the valve. I don't know the specs of the tfs1 cam but old school hot cams would drop compression because the duration was longer. , The corrective actions was changing to dome pistons or some other compression increasing method, also the longer duration pushed the power band up the rpm range and the engine would be a dog at lower rpms. 60 psi seems low even for a long duration cam that's why I suspect cam timing,Jjst some thoughts, hopefully others will chime in.
 
Reactions: General karthief

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,868
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polk county florida
Feb 25, 2020
#4
  • Feb 25, 2020
  • #4
^^I agree, this is also something to think about^^
 
Z

zlightlystoopid

Member
Jan 29, 2020
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Feb 26, 2020
#5
  • Feb 26, 2020
  • #5
General karthief said:
I would check the valve adjustment. You may need different push rods.
just my opinion and not based on any actual mechanical experience.
Click to expand...
For sure i will take the valve covers off and recheck my adjustments
 
Z

zlightlystoopid

Member
Jan 29, 2020
18
3
13
California
Feb 26, 2020
#6
  • Feb 26, 2020
  • #6
dgollem said:
With all cylinders close to the same, I would suspect the cam is out of time allowing the intake or most like likely the exhaust valve to stay open too long and bleed off compression through the valve. I don't know the specs of the tfs1 cam but old school hot cams would drop compression because the duration was longer. , The corrective actions was changing to dome pistons or some other compression increasing method, also the longer duration pushed the power band up the rpm range and the engine would be a dog at lower rpms. 60 psi seems low even for a long duration cam that's why I suspect cam timing,Jjst some thoughts, hopefully others will chime in.
Click to expand...
Hmm yeah that makes sense, you think it would help if i just advance the cam timing a couple degrees, i have the comp cams timing chain and it has the 3 key-ways on the crank sprocket to advance or retard
 

dgollem

20+ Year Stangneter
May 5, 2004
211
33
49
Waldorf MD
Feb 26, 2020
#7
  • Feb 26, 2020
  • #7
For a point of reference I would do the whole top dead center drill and make sure the cam timing is right. being off a tooth will make it run poorly but should not cost that much compression. A couple of teeth maybe. Definitely at least check the alignment of the cam timing marks. Look up degreeing the camshaft. Also karthief may be correct if your push rods are too long they may not be letting the valves fully close. Did you change lifters to lifters, cams and push rods have to match, they don't always play well if you mix manufactures. Also I would check the valve lash adjustment if the rockers are too tight the valves won't close all the way either. Could also be a combination of the events.

Also double check that your compression gauge is good, it would suck to pull things back apart to find out later you had a mis-calibrated/broken gauge. The rough idle and misfire could be caused by other issues.

I just googled lost compression after cam install, there is a lot of information about making sure cam timing is correct and appenrenty a common mistake of using the wrong marks with multi keyway timing chain gears when lining up timing, also common to set it out by 180 degrees.

Keep us posted on your progress.
 
Last edited: Feb 26, 2020
Reactions: 2Blue2 and General karthief
Z

zlightlystoopid

Member
Jan 29, 2020
18
3
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California
Feb 27, 2020
#8
  • Feb 27, 2020
  • #8
dgollem said:
For a point of reference I would do the whole top dead center drill and make sure the cam timing is right. being off a tooth will make it run poorly but should not cost that much compression. A couple of teeth maybe. Definitely at least check the alignment of the cam timing marks. Look up degreeing the camshaft. Also karthief may be correct if your push rods are too long they may not be letting the valves fully close. Did you change lifters to lifters, cams and push rods have to match, they don't always play well if you mix manufactures. Also I would check the valve lash adjustment if the rockers are too tight the valves won't close all the way either. Could also be a combination of the events.

Also double check that your compression gauge is good, it would suck to pull things back apart to find out later you had a mis-calibrated/broken gauge. The rough idle and misfire could be caused by other issues.

I just googled lost compression after cam install, there is a lot of information about making sure cam timing is correct and appenrenty a common mistake of using the wrong marks with multi keyway timing chain gears when lining up timing, also common to set it out by 180 degrees.

Keep us posted on your progress.
Click to expand...
Oh yeah i will definitely try another gauge, if i get the same numbers then ill pull the timing cover off this weekend and see whats going on. Thanks man ill let you know!
 
Reactions: 90sickfox and General karthief
Z

zlightlystoopid

Member
Jan 29, 2020
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California
Mar 1, 2020
#9
  • Mar 1, 2020
  • #9
Guys! Okay so i tested the cylinders with another gauge and numbers are now around 150 psi so the other gauge was definitely bad, still need to find that terrible misfire tho any thoughts? Ive changed plugs and wires and the distributor and coil are like a year old
 

2Blue2

will be trying this sex one when I can find it
Mod Dude
Mar 5, 2019
4,430
2,893
163
Oahu
Mar 1, 2020
#10
  • Mar 1, 2020
  • #10
dgollem said:
Also double check that your compression gauge is good, i
Click to expand...

Way to go Dgollem! Nice call on the compression gauge being bad
It b hard to diagnose over the inters-web.
 
Reactions: General karthief

dgollem

20+ Year Stangneter
May 5, 2004
211
33
49
Waldorf MD
Mar 2, 2020
#11
  • Mar 2, 2020
  • #11
Glad is was just the gauge. The misfire could be a lot of things based on what you describe since it's not valve timing related check for vacuum leaks next. keep us posted on the progress
 
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