70bigblockdodge
Old Man with a Hat
Check the torque on your intake bolts esp. 5&7 . If plug is oily it could be the cause move a clean one to that cyl clean #7 and put it in the other cylinder. These are just easy suggestions not a miracle rebuild.
pulsating 383 engine
- Thread starter jct
- Start date
Redo the compression test on the 70lb cylinder. First time just measure it as you did before, then squirt a bit of engine oil in through the plug hole and take the compression again. If the shot of oil makes the compression increase, then the rings are bad. However, if the reading stays the same, the problem is a valve. A valve problem could be a burnt valve, bad seat, bent push rod, bad lifter. The oil on the spark plug could be a bad valve oil seal.
In any case, none of these issues will result in catastrophic engine failure, unless you decide to quarter mile it. So my advice, drive it.
In any case, none of these issues will result in catastrophic engine failure, unless you decide to quarter mile it. So my advice, drive it.
won't hurt to try the wet compression test but my guess would be rings also. a bad valve would cause a slight bounce in the vacuum gauge and you said yours was steady. the oily cylinder is probably what one or both of the reasons stated above. the compression is probably higher on that cylinder because of the oil. it's like doing a wet compression test.
Ok... I'm up and this has been bothering me. Please don't take offense to this, but... the information hasn't been adding up in quite the logical way I would have hoped for. It is true that nobody has seen it all or knows it all and we all get good and stumped from time to time. But some of this information that we are answering to may be flawed.
First off... the test drive symptoms: while driving in various engine load conditions pay close attention to the feel of the engine...does it seem low on power?(many causes) Is the pulse feeling always there, or at least when somewhat under load(sign of one or more cylinders weak)? Do you feel it while sitting still and does it clear when shifted to neutral or reverse(good chance for motor mounts)? Does hot or cold engine have a great affect(many causes)? Power brake the engine not quite to the point of spinning tires for a few seconds(pronounced miss would likely be ignition)
Second... provided the cause was thought to be engine condition, like compression, vacuum gauge testing has lots of guides all over the internet. But to understand it better, just think of it as a quick way to check on your engine's ability to "breathe" properly. One cylinder not making power should show a wiggle on the gauge WHILE the engine is pulsating... more cylinders more wiggles. There is a rhythm to your engine when running that matches engine speed... the wiggles will match that rhythm and can be used to tell one vs several cylinders as suspected cause... a lower compression engine is a lower vacuum engine, but lots of factors affect this like cam profiles. Rumpty cams, make poor vacuum at idle because they don't "breathe" well at idle...they are optimized for higher RPMs.
Third, if I felt convinced the problem could be compression related, I would use a shop vac to clean any debris from around the spark plug holes and maybe degrease the area on a grimy motor. Then make sure you have a charged battery(critical) and hopefully can run a charger during the test, remove ALL spark plugs, keep them in order and photograph them so the folks here can help diagnose. Use a compression gauge that screws into the spark plug hole, cheap rubber cone ones are great for lawn mowers, and make sure the holes are clean before installing the gauge, or debris could skew results. wide open throttle, ignition coil disabled, crank about 8-10 revolutions judged by ear but should be enough that you cant go wrong. Record all readings, if the reading seem low do a wet test by dropping a teaspoon of oil or less into the spark plug holes and retest...record the readings. To interpret readings they should be within 10% of each other and close to factory specs... or without specs 90 psi ish for low compression and up to 200 psi ish for high compression... lower with rumpty cam (poor breathing at cranking speed)...its more about similar readings than specs. The wet test will usually raise compression by 5-10 psi... if more its a good indicator the rings are not sealing well. Young Mr. Horvaths has recently posted what appears to be an old sitting engine with poor results... his tests look valid and I understand "freshening up" but if it were started and run, it might just straighten itself out (compression wise) by reseating and oiling dry rings.
Fourth, if the compression results indicate a problem on any cylinder, a leakdown test would be called for. The cylinder is charged with air using a specific tool that can indicate how much air is lost. The cylinder would be positioned at TDC compression to do this so valves are closed. there WILL be leakage at the rings, so don't be fooled. Listening at the exhaust and intake can indicate valves leaking, big cracked heads,block etc can bubble through cooling system, but a lack of bubbles doesn't prove its good. If too much air leaks down...30% ish or more and only comes from crankcase (oil cap) rings are further proven. Passing leakdown results tell that its not combustion chamber related and could be cam,pushrod,valvetrain related. This is why compression results alone are usually not used to determine teardown.
If you think the head gasket or block is damaged there are combustion gas leak detection kits available cheap at your part store they are used to detect CO2 gases at the radiator fill point and used a blue fluid that turns yellow in CO2 concentrations... you and your engine produce CO2. CAUTION do not let the fluid get contaminated or it wont work.
This, my friends, is not the most comprehensive diagnostic available, but covers much of the ground in this thread. A professional will NEVER disable an engine until the cause has been pinpointed because if he doesn't find it at teardown, it will like return after repairs. I am not an engine builder, but have built a few and repaired many, you wont see or measure a problem you haven't diagnosed beforehand too often for a engine pro to fall into that trap.
Feel free to make corrections or point out omissions... or hell just argue... but maybe this will help
First off... the test drive symptoms: while driving in various engine load conditions pay close attention to the feel of the engine...does it seem low on power?(many causes) Is the pulse feeling always there, or at least when somewhat under load(sign of one or more cylinders weak)? Do you feel it while sitting still and does it clear when shifted to neutral or reverse(good chance for motor mounts)? Does hot or cold engine have a great affect(many causes)? Power brake the engine not quite to the point of spinning tires for a few seconds(pronounced miss would likely be ignition)
Second... provided the cause was thought to be engine condition, like compression, vacuum gauge testing has lots of guides all over the internet. But to understand it better, just think of it as a quick way to check on your engine's ability to "breathe" properly. One cylinder not making power should show a wiggle on the gauge WHILE the engine is pulsating... more cylinders more wiggles. There is a rhythm to your engine when running that matches engine speed... the wiggles will match that rhythm and can be used to tell one vs several cylinders as suspected cause... a lower compression engine is a lower vacuum engine, but lots of factors affect this like cam profiles. Rumpty cams, make poor vacuum at idle because they don't "breathe" well at idle...they are optimized for higher RPMs.
Third, if I felt convinced the problem could be compression related, I would use a shop vac to clean any debris from around the spark plug holes and maybe degrease the area on a grimy motor. Then make sure you have a charged battery(critical) and hopefully can run a charger during the test, remove ALL spark plugs, keep them in order and photograph them so the folks here can help diagnose. Use a compression gauge that screws into the spark plug hole, cheap rubber cone ones are great for lawn mowers, and make sure the holes are clean before installing the gauge, or debris could skew results. wide open throttle, ignition coil disabled, crank about 8-10 revolutions judged by ear but should be enough that you cant go wrong. Record all readings, if the reading seem low do a wet test by dropping a teaspoon of oil or less into the spark plug holes and retest...record the readings. To interpret readings they should be within 10% of each other and close to factory specs... or without specs 90 psi ish for low compression and up to 200 psi ish for high compression... lower with rumpty cam (poor breathing at cranking speed)...its more about similar readings than specs. The wet test will usually raise compression by 5-10 psi... if more its a good indicator the rings are not sealing well. Young Mr. Horvaths has recently posted what appears to be an old sitting engine with poor results... his tests look valid and I understand "freshening up" but if it were started and run, it might just straighten itself out (compression wise) by reseating and oiling dry rings.
Fourth, if the compression results indicate a problem on any cylinder, a leakdown test would be called for. The cylinder is charged with air using a specific tool that can indicate how much air is lost. The cylinder would be positioned at TDC compression to do this so valves are closed. there WILL be leakage at the rings, so don't be fooled. Listening at the exhaust and intake can indicate valves leaking, big cracked heads,block etc can bubble through cooling system, but a lack of bubbles doesn't prove its good. If too much air leaks down...30% ish or more and only comes from crankcase (oil cap) rings are further proven. Passing leakdown results tell that its not combustion chamber related and could be cam,pushrod,valvetrain related. This is why compression results alone are usually not used to determine teardown.
If you think the head gasket or block is damaged there are combustion gas leak detection kits available cheap at your part store they are used to detect CO2 gases at the radiator fill point and used a blue fluid that turns yellow in CO2 concentrations... you and your engine produce CO2. CAUTION do not let the fluid get contaminated or it wont work.
This, my friends, is not the most comprehensive diagnostic available, but covers much of the ground in this thread. A professional will NEVER disable an engine until the cause has been pinpointed because if he doesn't find it at teardown, it will like return after repairs. I am not an engine builder, but have built a few and repaired many, you wont see or measure a problem you haven't diagnosed beforehand too often for a engine pro to fall into that trap.
Feel free to make corrections or point out omissions... or hell just argue... but maybe this will help
The Horvaths
Young man with a cowboy hat
"Young Mr. Horvaths has recently posted what appears to be an old sitting engine with poor results... his tests look valid and I understand "freshening up" but if it were started and run, it might just straighten itself out (compression wise) by reseating and oiling dry rings."
This is well stated, indeed. We are tearing this engine down more so Dylan can learn from it than any other reason. Doing one for fun is so much better than doing one out of dire necessity.
I think that we should do a leak down test prior to disassembly too. Thank you for bringing this up.
Back to the topic at hand!
- Dad
This is well stated, indeed. We are tearing this engine down more so Dylan can learn from it than any other reason. Doing one for fun is so much better than doing one out of dire necessity.
I think that we should do a leak down test prior to disassembly too. Thank you for bringing this up.
Back to the topic at hand!
- Dad
We are tearing this engine down more so Dylan can learn from it than any other reason. Doing one for fun is so much better than doing one out of dire necessity.
sounds like Dylan is a lucky young man... hope he has a ball...
Now jct, why on earth would you have done all of this other fooling around if you have a scope available? What kind of scope is it? Or better yet what Windows platform does it run? Then I might be able to tell you something... I love oscilloscopes for diagnosis. Since the scope would be giving information on ignition patterns it should show which cylinders really need the above tests. Or do you have a PVT to use with it so you can scope the vacuum waveforms...very handy for diagnosis of "breathing" problems... and could lead you right back to the above tests. Because the scope will help isolate which cylinder and can give some pretty good ideas as to likely cause it would be a great tool to start diagnosis, and it would have cleared up all of the earlier ignition questions like nothing else. Of course someone has to be able to operate it and interpret the information.
I am willing to help you, but if I am to do anything worthwhile... lets restart at the symptoms. Engine pulses... could just be a balance issue, does it happen at idle? In park at 2000rpm, a guess for rpm is ok? In gear(drive) sitting still? In reverse sitting still? For 15 seconds or less put one foot on the brake and one on the gas and do 2000 rpm in drive without moving or spin tires...did it happen? The answers to these questions will help me choose new questions and you don't even have to drive the car.
If my assistance is not required, no harm, no foul.... but you can't accurately diagnose this issue without meaningful information.
sounds like Dylan is a lucky young man... hope he has a ball...
Now jct, why on earth would you have done all of this other fooling around if you have a scope available? What kind of scope is it? Or better yet what Windows platform does it run? Then I might be able to tell you something... I love oscilloscopes for diagnosis. Since the scope would be giving information on ignition patterns it should show which cylinders really need the above tests. Or do you have a PVT to use with it so you can scope the vacuum waveforms...very handy for diagnosis of "breathing" problems... and could lead you right back to the above tests. Because the scope will help isolate which cylinder and can give some pretty good ideas as to likely cause it would be a great tool to start diagnosis, and it would have cleared up all of the earlier ignition questions like nothing else. Of course someone has to be able to operate it and interpret the information.
I am willing to help you, but if I am to do anything worthwhile... lets restart at the symptoms. Engine pulses... could just be a balance issue, does it happen at idle? In park at 2000rpm, a guess for rpm is ok? In gear(drive) sitting still? In reverse sitting still? For 15 seconds or less put one foot on the brake and one on the gas and do 2000 rpm in drive without moving or spin tires...did it happen? The answers to these questions will help me choose new questions and you don't even have to drive the car.
If my assistance is not required, no harm, no foul.... but you can't accurately diagnose this issue without meaningful information.
it's not my scope, it's my friends mechanic's scope which he gave to my friend which he doesn't know to read it and he has to schedule his time to come over to work on his cars and to check mine out...
not a balance issue
getting it on the scope will happen some time soon i just don't know when yet
not a balance issue
getting it on the scope will happen some time soon i just don't know when yet
got it on the scope today. the ign system got a clean bill of health 
i got two ppl says it'll need the heads rebuilt and one of the two ppl is saying could be slop in the timing chain.
just as long as i don't tromp on it, my car is fine. i still haven't done the wet compression test. i will do a cylinder leakage test and the combustion leak detector test
i got two ppl says it'll need the heads rebuilt and one of the two ppl is saying could be slop in the timing chain.
just as long as i don't tromp on it, my car is fine. i still haven't done the wet compression test. i will do a cylinder leakage test and the combustion leak detector test
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