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View Full Version : Continuing Battle, compression and Leak-down


PhatFJ
12-11-2011, 12:20 PM
Hey all, I am continuing the battle on my 1975 FJ-40 :mad: Just checked the compression again with the following results:
#1- 119lbs
#2- 119lbs
#3- 110lbs
#4- 119lbs
#5- 119lbs
#6- 119lbs

So, I did a leak-down on #3 and #2
#3 was 80lbs input and 61lbs output air heard through the crank case (rings).

#2 was 80lbs input and 64lbs output air heard through the crank case (rings).

according to the leak-down tester information, you should be concerned if the difference is over 5lbs between cylinders and I am at 3.
Anyone with more experience doing compression and leak-down tests have any information they can share about these results.
I really do not want to tear into the motor unless I have to, I think it has many many more miles on this motor. It has less than 80k now..
Thanks for the input!!

subzali
12-11-2011, 12:34 PM
Yours has better compression #s than mine has...I've never done a leakdown, but I'm sure yours is better than mine too. I'd run it and don't overthink it :D

PhatFJ
12-11-2011, 02:06 PM
Yours has better compression #s than mine has...I've never done a leakdown, but I'm sure yours is better than mine too. I'd run it and don't overthink it :D

Thanks bro!! as always I really appreciate your input... I cannot seem to get my in/Hg up over 12. I put another gasket on (Remflex), I have pulled ALL vacuum lines and plugged them and still cannot figure out why it is so low.. I will keep trying though.. :thumb:

nattybumppo
12-11-2011, 03:33 PM
Remember that you have to adjust for altitude when you take compression readings. I think 119 here is close to 145 at sea level where the factory spec is designed to be read. Your compression is fine, even that low cylinder isn't too bad.

Air Randy
12-11-2011, 03:35 PM
Are you adjusting your readings for the altitude? 119 adjusted for our altitude is 145 lbs. New from the factory is 150, whats the problem?

You should also stop worrying about the vacuum reading. I've never seen one attain that level at this altitude. If it runs OK call it good.

SteveH
12-12-2011, 06:45 AM
FYI - on a '78 FJ40 with very similar compression readings, I see 15" vacuum (idle) at 7000' feet - I have an in-cab vac gauge. If the weather is really nice (high barometric pressure, sunny day), I can see a little more.

RicardoJM
12-12-2011, 07:58 AM
I have read all the posts indicating that even at altitude you should be able to hit 15. I've never acheived that. In the back of my mind (from time to time) I will get to thinking that there is something amiss with my engine - but when I get in it - the truck starts, runs well and my mind transistions to enjoying the drive:D. Having driven your truck, I can tell you the fit, finish and solidness :bowdown: is very enjoyable and I don't think there is anything wrong with the power plant:thumb:.

I know you started the thread looking to get more insight and I don't have any to add. From my limited experience, nothing short of a full tear down, inspection and rebuild would bring total piece of mind in the context of "is my engine giving me everything that it can". If you want to go down that route or the incremental steps (redo the head, then 6 months/1 year later dig into the bottom) - I can understand the underlying motivation.

I would agree with some of the other posts - the numbers are not bad and far away from terrible - run it:D. Truly run it until the engine gives you a solid reason to open it up. Wost case it throws a rod through the block a long way from home and the ensuing rescue/recovery will be a great story. I believe the engine will give you many more years of solid service; and when it does go belly up in short order it can be replaced/rebuilt and be back on the road for many more years.

Jacket
12-12-2011, 11:23 AM
No insight either, but I have similar numbers and a 10 lb variation on mine. Mine ranged between 110 and 120 across the 6 cylinders, and runs like a top. I concluded that I was "within reasonable spec." :shrug:

rover67
12-12-2011, 01:07 PM
I don't think those numbers would really bother me personally.. Especially if it ran well and didn't burn too much oil. It has good compression, but it is a bit odd that it is leaking down that much. How did you do the leak down test exactly?

How wide did you set the gaps on the rings when you built it? Did you do the leak down test on a cold motor or when it was warmed up? How much did adding a little bit of oil to the cylinder change things? Did you tap on the valves to make sure they were snapped shut nicely?

Again, you could go through all of that stuff if you wanted to, but honestly I think it sounds pretty good with those compression numbers..

PhatFJ
12-13-2011, 04:38 PM
Thanks again guys for the input!! I tore back into this because I am having a terrible time getting it to idle well. All indications seem to be vacuum leak, I have replaced ALL gaskets some even twice, pulled the carb off at least 5 times looking for something. One other thing I found is that I was able to get an idle at 7 to 14* advance (with vac retard line removed) it now will not idle unless I am around 34* advanced, go figure, I am thinking that the dizzy is off by a tooth. I have pulled the dizzy, it looks perfect. I did see in the PO paperwork that they changed the stock cam out for an Isky cam. I have ordered a new STOCK cam and plan on getting this back to stock. I read on Mudd that the Isky cam can cause more problems than good, don't know but based on what I am seeing :(,. Right now I seem to be grasping at straws, I have gone through all of the emission components and they seem to be functioning as designed..

Are you adjusting your readings for the altitude? 119 adjusted for our altitude is 145 lbs. New from the factory is 150, whats the problem?
You should also stop worrying about the vacuum reading. I've never seen one attain that level at this altitude. If it runs OK call it good.
No, the readings are not adjusted for altitude

I have read all the posts indicating that even at altitude you should be able to hit 15. I've never acheived that. In the back of my mind (from time to time) I will get to thinking that there is something amiss with my engine - but when I get in it - the truck starts, runs well and my mind transistions to enjoying the drive:D. Having driven your truck, I can tell you the fit, finish and solidness :bowdown: is very enjoyable and I don't think there is anything wrong with the power plant:thumb:.

I know you started the thread looking to get more insight and I don't have any to add. From my limited experience, nothing short of a full tear down, inspection and rebuild would bring total piece of mind in the context of "is my engine giving me everything that it can". If you want to go down that route or the incremental steps (redo the head, then 6 months/1 year later dig into the bottom) - I can understand the underlying motivation.

I would agree with some of the other posts - the numbers are not bad and far away from terrible - run it:D. Truly run it until the engine gives you a solid reason to open it up. Wost case it throws a rod through the block a long way from home and the ensuing rescue/recovery will be a great story. I believe the engine will give you many more years of solid service; and when it does go belly up in short order it can be replaced/rebuilt and be back on the road for many more years.
Thanks Ricardo! Yes I started this trying to see why I could not get into the secondary circuit with lack of power and it seemed that every time I touch something it gets worse.

I don't think those numbers would really bother me personally.. Especially if it ran well and didn't burn too much oil. It has good compression, but it is a bit odd that it is leaking down that much. How did you do the leak down test exactly?

How wide did you set the gaps on the rings when you built it? Did you do the leak down test on a cold motor or when it was warmed up? How much did adding a little bit of oil to the cylinder change things? Did you tap on the valves to make sure they were snapped shut nicely?

Again, you could go through all of that stuff if you wanted to, but honestly I think it sounds pretty good with those compression numbers..
Marco, the 40 was running fine but was gutless going up a hill. does not burn oil or smoke at all. I did do the leak down hot, I did not add any oil, the motor has not been rebuilt it only has about 78k on it so it is stock gap, I did not tap on the valves but the leakage was into the crank case not the valves.

rover67
12-13-2011, 05:42 PM
Have you checked for vacuum leaks with the carb cleaner or propane trick? I had a he'll of a time with my reflex gasket and finally ditched it...

TIMZTOY
12-13-2011, 05:57 PM
dont know your history with the truck, but why did you check it ? having issues or drivability isues ? and the numbers apear to be ok to me. also did you check your compression with the motor hot ? and did you check your leakdown hot and at TDC compression ?

Air Randy
12-13-2011, 07:05 PM
The Isky cam, especially if it is high lift/long duration will effect your vacuum numbers. But unless it is a full race cam I doubt it would account for your inability to get it to idle.

As far as the no power on uphill sections, have you checked the jetting in the carb? After JimC went through my carb it started, idled and ran perfectly but seemed to be lacking power. We went up like 3 jet sizes on the primary and secondary and it gave a huge increase in power. It still runs fine at altitude and shows no sings of plug fouling so I am convinced it is right.

Uncle Ben
12-13-2011, 07:38 PM
The Isky cam, especially if it is high lift/long duration will effect your vacuum numbers. But unless it is a full race cam I doubt it would account for your inability to get it to idle.

As far as the no power on uphill sections, have you checked the jetting in the carb? After JimC went through my carb it started, idled and ran perfectly but seemed to be lacking power. We went up like 3 jet sizes on the primary and secondary and it gave a huge increase in power. It still runs fine at altitude and shows no sings of plug fouling so I am convinced it is right.

So what is a "full race" cam? :rolleyes::hill:

RicardoJM
12-14-2011, 07:23 AM
Regarding idle, in the cold weather my FJ40 needs to be choked to start and idle smooth and when it is really cold (teens and below) the fully choked idle doesn't get really smooth until it has been running a minute or so. Is there a difference in how it idles once the engine has warmed up? If it idles better and smoother when it is warmed up - the issue may be in the choke system or the high idle settings. Have you confirmed the that the choke cable is moving the choke plates as it should? DAMHIK - but it is possible to pull the choke knob all the way out and not close the choke plate.

PhatFJ
12-14-2011, 07:29 AM
Regarding idle, in the cold weather my FJ40 needs to be choked to start and idle smooth and when it is really cold (teens and below) the fully choked idle doesn't get really smooth until it has been running a minute or so. Is there a difference in how it idles once the engine has warmed up? If it idles better and smoother when it is warmed up - the issue may be in the choke system or the high idle settings. Have you confirmed the that the choke cable is moving the choke plates as it should? DAMHIK - but it is possible to pull the choke knob all the way out and not close the choke plate.
The entire choke circuit, linkage, butterfly and cable seem to be working fine. It is only after it has warmed up and the choke is pushed back in that the idle turns to poo poo.

RicardoJM
12-14-2011, 07:46 AM
The entire choke circuit, linkage, butterfly and cable seem to be working fine. It is only after it has warmed up and the choke is pushed back in that the idle turns to poo poo.

Check:thumb:. With the engine warmed up and the choke knob pushed in - if you increase the idle speed, are you able to get a smooth idle? If you can achieve that, it is likely achieved at the point the carb starts to feed fuel through the primary venturi and the issue is likely in the idle circuit.