Imperial Homepage -> Repair ->Engine -> Performance Checks
Question from Dan:
I have a basic question regarding compression testing. The mechanics I've talked with in my area tell me that a cold engine will always produce a LOWER reading than a warm one. They explain that heat causes the rings to expand and create a tighter compression chamber. This makes sense and my question is; Is this ALWAYS true, or are there exceptions to this rule (assuming testing is done correctly - equal conditions for each cylinder)?
I ask this because my cold readings were 105 and the warm ones were 85. All other plugs were in during the tests, but I was using an old battery the seller included. It goes down fast and I know it was cranking slower during the warm engine tests. The deductive reasoning I'm using is that if a cold engine always reads low, then it's safe to assume the compression of this engine is at least 105.
Can anyone shed any light on this? Is there a flaw in this thinking? Am I overlooking something?
I was told by a mechanic that a compression test on a cold engine will give a false LOW READING. I double checked with another source at my local Napa store and he agreed, saying the rings expand with friction to produce a higher reading. As an armature, I sort of have to trust what I'm being told, but it seems the vehicles I've owned had lower oil pressure when they were hot. Maybe that has nothing to do with compression, but I did as they said and ran it until the temp read midway on the gauge.
Besides burned knuckles, here's my problem: When I checked the same cylinder cold it read 105. Hot it reads about 80. I checked it several times and the highest reading I got was about 84. It seems the longer I crank the engine the higher the readings get. Is this right? I'm using the kind of tester that screws into the plug hole. I'm tightening it by hand because I can't get a wrench down there to tighten it without smelling burning flesh. But, I also tightened it by hand when it was cold, too. Would a hot block and cold tester fitting account for this? Does this make sense to anyone? Am I getting bad advice, doing something wrong, or ??????
Replies:
From Roger:
I would have an answer to your question. We always did the compression check cold. We did a dry compression check- take out plugs and check each cylinder while recording each. Then a wet reading- Shoot 1-2 squirts of motor oil from oil can in cylinder and check each one then record. If you have a 25% variance in the reading between cylinders you have a problem. That is 25% between #1 and #5 on dry test. If it does not come up on the wet test it is a valve. If it does it is rings. You can tell which valve it is by going on Top Dead Center on the weak cylinder (Piston up both valves closed) shoot are in the cylinder with blow gun, if it comes out the carburetor it is an intake. If it blows out the exhaust it is an exhaust valve. If it goes into the crank case bad rings. For a true idea as to what is wrong I would do both tests.
From Jim:
Two things.... First there are TWO compression tests -- (1) DRY and (2) "WET". The dry one is just the regular compression test that you run on a warm engine. THEN, you put about a teaspoon of engine oil in the cylinder to seal the rings. This is the WET test. The WET reading will always be higher than the dry one. Second, it is not the absolute level of compression that matters--it is the difference between the cylinders. If all the cylinders are within about 10-15psi of each other you are good to go.
From Charlie:
After the gauge pulses about 4 times that's your reading , there is no oil pressure on your cylinder walls hot or cold, if your reading get s worse hot you may have a burnt valve in that cylinder, loosen your rocker arms so both valves are shut on the hot engine and put compressed air in the cylinder, air rushing out the carb-intake valve, exhaust pipe- ex valve, bubbling in the radiator-head gasket or crack in head or block, dipstick tube or oil cap hole broken ring or hole in piston (would have bad comp all the time).
From Demetrios:
The reason why the compression reading will be higher for a hot engine are two. First, the piston (that is made of aluminum) expands faster than the iron block (not the piston rings). Most of the blow-by in an engine is in the little gap formed by the end gap of the piston ring and the piston-cylinder clearance. As the piston expands, the gap shrinks a bit, so the blow-by decreases. The second reason why the reading may be higher when warm is that the engine may crank a little faster. The faster the engine cranks, the less time for blow-by.
Now, why is your readings lower when warm? May be your starter is weak and it actually cranks slower when warm. Who knows? However, as suggested by others, the reading compared to readings from other cylinders are important. Because all these factors vary (temperature, starter effectiveness, oil viscosity -> which affects cranking speed) it is impossible to always have the same conditions when you do a compression test. However, when you compare cylinders of the same engine, it is likely that all above conditions are similar, and thus the comparison is valid.
As said by others, oil pressure is irrelevant to compression gage readings.
From Robin:
Usually you are looking for a bad cylinder so you compare all the cylinders and see which ones are the odd ones. Because an engine will usually wear fairly evenly. A good engine will generally have the same readings within 10psi of each other. That is a very general statement. If you have 7 readings of 150 +/- 10 and 1 reading of 100 you have 1 weak cylinder for example. Hot or cold you should be getting around 150 per cylinder on a "good" engine. Again very general statement. More is better. Oil pressure has nothing to do with it. The more you crank it the higher it will climb until it reaches its peak. Just watch the gauge as you crank. When it stops climbing stop cranking. Check all cylinders the same way. You will get best results with the throttle wide open when cranking. Make sure the ignition is disabled. A piston ring actually functions properly when the engine is running as compressed gas from the combustion chamber gets behind the ring in the piston land and forces the ring out against the cylinder wall. That is the design theory anyway and has nothing to do with compression testing.
Question from Paul:
My '72 440 runs strong but has a poor quality idle. It stales sometimes. The car had been sitting (never run) for about 12-15 years. What can I do to smooth out the otherwise great running motor?
Reply from Dick:
Step one is, have a compression test done on the engine. If all 8 are within + or - 10% of each other, the car can be made to idle well. If they are further out of agreement than that, you most likely have a burned exhaust valve, which you can also hear at the tailpipe if the exhaust system is in good shape. A burned exhaust valve will make a chuff-chuff sound every time the affected cylinder fires. Other causes of low compression on a cylinder are of course also possible, these can be sorted out by holding each cylinder in turn at TDC and pumping air into the spark plug hole, while listening at the crankcase vent, carburetor, and exhaust pipe for where the air is leaking out.
If you have even compression, it's time to investigate the Carburetor. If you feel up to it, get a kit and rebuild it yourself (store bought rebuilts are almost always poorly done, in my experience), or failing that, find an old time carburetor shop and take the car there. If you want to try something first on the carburetor, take out the idle mixture screws, first noting how many turns each one was set from all the way seated. Then take the straw from your handy-dandy can of spray carburetor cleaner, and stick it in each screw hole, and give a short squirt in there. Then reinsert the idle screws to the same setting. I hope the setting is the same on both screws, and somewhere around two turns out from seated. If not, try that as an initial setting, then fine tune them watching the vacuum gauge, trying for best idle (highest RPM with smooth idle.)
If the power is normal at higher RPM, we are assured that you have no serious timing error, or valve timing problems, so I am basing the above on that information. If you think the power might be down from normal ( a 440 should go like stink when you step on the loud pedal!), then you might have other problems, and the car should be seen by someone with an engine analyzer.
Question from Jay:
For those of you gear heads that know engines real well, I have some data for you and some questions about my apparent dramatic drop in compression in our '62 413.
I tested the compression of all cylinders of our '62 413 about a week ago. I did both wet and dry condition tests. I was discouraged with what I found. Significant drop in all cylinders since my last test done in Nov '97. (All figures that I will quote are dry/wet.) I can't be 100% conclusive of the changes since in Nov I did a wet test only on #7 which at that time seemed troubled (85/120#)
Here is a table showing November's test compared to July's test:
COMPRESSION
Nov '97 Jul '98
DRY WET DRY WET
#1 152 110 120
#2 143 130 147
#3 151 125 152
#4 136 82 100
#5 147 125 155
#6 142 102 106
#7 85 120 40 50
#8 148 132 136
# DIFFERENCE FROM NOV '97 to JUL '98
PSI DROP (dry) July diff between wet & dry
#1 42 10
#2 13 17
#3 26 27
#4 54 18
#5 22 30
#6 40 4
#7 45 10
#8 16 4
I'm no mechanic but I know the difference in wet vs. dry indicates ring wear, while overall drop indicates a valve problem(s). I think that #7 is the burnt valve. In Nov all except #7 were in spec. Now all but #8 and #2 are out of spec.
When the valve started to make noise, I didn't know that it was the valve. I thought that I might have a sticky lifter, so I added some "valve medic" to the crankcase in hopes of freeing it up. This valve medic was still in the crankcase at the time of the last test. I don't know if the properties of the oil changes with the inclusion of the valve medic enough to show up in a compression test. Could this cause the low numbers, or am I barking up the wrong tree?
Thinking that might be the case, I changed the oil & filter yesterday. (I only use 30wt Penzoil in all our cars). This morning it seemed to have a little more "guts" than it did than yesterday (of course I'm comparing yesterday afternoon's weather (hot) against this morning's (relatively cool). Might that have an effect on performance enough to notice? Is it worth my time to test compression again since I changed the oil?
I gave the engine a tune up and changed the plugs at the same time that I tested compression (July '98). A few of the plugs (two, but I can't remember which ones) showed a slight bit of oil fouling, but not much. Other plugs appeared dry with a sandy or light tan color and slight hard deposits that I could scrape off only with a screwdriver. The carb was rebuilt about 12,000 ago.
For those of you familiar with compressions in the 413, what do my numbers tell you about my engine? By the way, I use a screw-in compression tester. Could I have a flaw in my test method? (I screw the tester in as much as I can, which is more than 10 turns, but it's hard to get enough leverage to make sure that I have it all the way in.)
It seems to me that the drop on compression is dramatic for only 8 months. The car has been running for about 15 months since it was parked/stored for about 4 years by the previous owner.
I started using 8 oz of Marvel Mystery Oil per tankful of gas around May '98. Could that have an effect either positive or negative?
Does anybody have any recommendations? Should I look into getting a valve job? Do I re-ring while I have the heads off, or should I go with an entirely rebuilt engine?
Reply from Dick:
Large variations from one compression test to the next are not unusual, although your variations are a little more than I would expect.
I do not think this means that your engine is about to self destruct. You obviously have one sick cylinder, but as I said before, one poorly seating exhaust valve is not going to make much difference in the way the car drives, until it gets so bad that it turns into a complete "miss". You will not hurt the car by driving it that way.
If you want to fix it because it gets on your nerves, you would be happiest in the long run to do a complete engine overhaul or rebuild. If the car is in this condition, quite likely just doing a valve job is going to immediately cause an increase in wear rate in the bottom end of the engine, and you will soon be doing rings and bearings anyway. Also, most likely, the timing gears/chain are worn badly too. So, you might as well wait until you can spend $2000 or so and get a good rebuild from a professional mechanic, or if you want, we can coach you through doing it yourself, but you will need to invest in some tools, and you are going to need a large, well lit, clean place to work, and a large amount of time to work on it, plus rags, hand cleaner, compressor etc. if you don't already have those. But the point is, it is not Rocket Science - you can learn how to do it by getting a little advice and using your common sense.
Question from Glenn:
While I am driving my '67 Imperial, I notice an extensive amount of blue smoke wafting from under the hood of the car. I am attributing this to "blow-by". What can be done to correct this problem. When I purchased this fine example last October, the previous owner informed me that he did aa compression test on the car and came out with the following results:
Left Bank going from front to back:Cyl. 1 - 115 -145 PSI
Cyl. 2 - 45 - 60 PSI
Cyl. 3 - 120 - 160 PSI
Cyl. 4 - 180 PSI.
Right Bank going from front to back:Cyl 5 - 165 PSI
Cyl 6 - 170 PSI
Cyl 7 - 165 PSI
Cyl 8 - 160 PSIThe FSM states that the Compression pressure with the engine warm, spark plugs removed, and a wide open throttle should be between 130 - 165 PSI with a maximum variation between cylinders of 25 PSI. I have been using Rislone in the motor since I purchased the car in the hopes to increase compression. I have been told by a number of people to use "Marvel Mystery Oil" to a) help to increase the compression. and b) to help clean the valves. I am told that the low compression is being caused by the Valves not closing properly. Any ideas??
Replies:
From Dick:
The stuff in the bottles will help your car only by lightening the load it has to carry (by removing money from your wallet). Your cylinders are numbered 1,3,5,7 front to back on the driver's side, and 2,4,6,8 front to back on the passenger's side. Your # 3 cylinder almost certainly has a burned exhaust valve.
If you listen to the tail pipe at idle, you will hear a chuff-chuff-chuff sound, right? (If this is not true, you have another, probably more expensive problem). When you crank the starter, you hear an unevenness to the cranking RPM before it starts, too, right? The only cure for this is a valve job, sorry. You can do most of the labor yourself, if you have a good set of tools and don't mind getting dirty and sore. You would be best off to pull the heads (I'd do both sides) and take them to a shop. If you do this part of the labor, you'll be out about $500 by the time you are back on the road, maybe less, and you'll have a perfect running car that will amaze you with its power and smoothness . The blue smoke from under the hood is indeed blow-by, and if your PCV system is fully functional (shake the PCV valve to make sure it rattles, and verify the suction hose is hooked up and producing vacuum at the valve cover end), then you have so much blow-by that your engine needs a total rebuild, now we are talking real money here. Again, if you do the R&R and disassembly - reassemble, you can save a lot of money, but you are still going to be out $1200 or so before you are back on the road. You'll also know a lot more about engines than you do now, unless you have done this a few times before.
From Phil:
I used to do a lot of cylinder heads, but have never seen a Mopar head from the factory with a "hardened" guide. I've seen the later models with an induction hardened exhaust seat, but never a guide. It wouldn't have been cost effective to do it that way. Don't know what you're talking about a "typical" guide problem" either with big block Mopars. If they're sucking oil, the valve seals are bad. Any car is like this, all the big block Mopars I've seen with bad guides had way in excess of 100,000 miles. Any 383-400-440 I have ever had that had bad oil consumption either had bad rings or a real bad oil leak. I have a 77Dodge truck with 145,000 miles and a 63 300 with a 115,000 miles and neither one has an oil consumption problem. If you do run them hard, they will use a little more oil, but if you're using a quart of oil every 300 miles something is wrong, that is not typical oil usage for a big block Mopar. I find one quart every 2000 miles is more typical on a used motor and have had several that didn't use any. I have seen several GM motors, especially Chevy's, with their O Ring (rubber band ) seals that eat oil like it's going out of style, with around 60,000 mile on them. I find oil eating to be a more typical GM problem anyway..
From Kerry:
Several things could cause low compression. Valves are certainly one but valves don't generally make it smoke. Valve seals will but valve seals won't drop compression. I hate to suggest it but given the compression loss on #2, I would suspect a broken or stuck ring or two or even possibly a cracked piston. There are some shade tree tricks you can do to test. Pull all the plugs so you can do the compression tests again. Squirt some motor oil into #2 and check the compression. If it is worn rings sometimes the compression will show higher because the oil seals the gap a little. Don't know what it would do to a crack. but since you have 60 PSI, it must not be worn too bad. Does it smoke out the tailpipe? If it only smokes from the breather, that also might point to blow-by caused by a ring or piston problem hate to be the one to say that it doesn't sound good.
Question from Luis (440):
Recently purchased 1968 convertible but have not had the time and weather problems to work on her much since I got her. She starts up great but the white smoke and the fumes are killing us in my house through my closed garage door. I had a 1969 at one time and I don't remember it smoking like that. Could that be a bad sign? I would like to do a compression test on the engine how much would you think it would cost at a service station or can i do it myself. I know some thing about doing a compression test but don't the proper procedure. How does a compression tester cost?
Replies:
From Stu:
I picked up a compression tester at Auto Zone for 25 bucks. White smoke does sound like you have bad rings to me...
From Demetrios:
As an easy test, try disconnecting the brake booster line. If the smoke goes away, you may have a bad booster that sucks brake fluid.
From Bob:
A compression check is not expensive, check with a few of your local mechanics on cost.
From Willliam:
NAPA has a "block check" kit that includes fluid that you draw the vapors from the radiator coolant through to check for combustion gasses in the coolant. I think the one we bought last week was about $50.00 for the kit. A definitive test.
Question from Greg (Flathead):
I completed a differential compression test on my old straight eight this weekend. The test is performed with the cylinder in question at TDC, compressed air is applied through the gauge and the drop in pressure is read by the second gauge. By carefully listening to the escaping air the defective valve or piston ring can be pin pointed. In my case four of the cyls. had no reading, I determined by listening, that the exhaust valves and possibly to a small extent the rings were the culprit....
Replies:
From Mikey:
Assuming you have done the oil down the plug hole trick to see if anything changes? We used to take flathead Fords from Windmachines that had sat all winter and pull the plugs, add wd-40 , etc and crank the daylights out of them. Sometimes a stuck valve or ring would decide to play, other times it wouldn't.
From Kne:
For some reason I've seen more than the usual amount of burned exhaust valves in flatheads, many with big cracks and V-notches in them. Don't know if this is inherent to the design, old metallurgy, or just looking at old high mileage engines that were run hard and maintained poorly. ???
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