i just realized that the topic of my problem was on someone elses topic. i am truly sorry for this. didn't mean ti do that. i'm still new to this site, so it takes some getting use to. but anyway maybe this will get me better answers on what i need to know. i went to my local machine shop to get some quotes on how much it would cost to have the head of my 4.9 inline engine redone. i tried to explain to the best of my knowlege of what my concerns were. he checked the engine while it was running for any knocks or misfiring. but there wasn't any. i told him that i had used an additive in the rad to seal the head gasket. also i had drained the engine oil and put some lucas oil additive in with the regular oil. he told me it may have been the best thing that i done to save the engine. to be sure of what i was dealing with he wanted me to do a compression test to determin what cylinder was out and also if i had a broken piston ring. he told me to check each cylinder to see if it drops fast or slow. i asked him y and he told me that if the presure drops fast it's most likely the rings if slow then it's the head gasket. i don't quite understand what a compression test has to do with finding out about a head gasket. i just need some advice on what this means. also do anyone know what the compression is on a 4.9 engine so i have something to go by when i do this test thanks T.C.M.

 

There you go, now you have your own thread.

Alright so the machinist was wanting to check the rate at which your cylinders are losing compression. He told you to do a compression test, but that is actually not the right test. Compression test is used to check for peak cylinder pressures, what he meant is a leak-down test. What he was trying to explain was that in his opinion, if you have a cylinder which bleeds compression rapidly, he thinks it's more likely to be worn rings than a blown head gasket. And if it loses compression slowly, that it's more likely to be a blown head gasket.

I don't agree with this.

A head gasket can fail a couple different ways... it can leak into a coolant passage (usually causes white smoke in exhaust) or it can leak into the cylinder next to it, at which point a compression test WOULD show something wrong, because compression would be down on two adjacent cylinders.

So I would start with a compression test, and if nothing shows up, then do a leak-down test and post up the results. Not sure on the numbers for the 4.9 but something like 160+ would be reasonable, with no major variation between cylinders.

 

Neither Chilton nor Haynes give a hard number for compression pressure, but both spec the compression ratio for the 4.9L at 8.8:1. The theoretical maximum pressure you should be able to get is about 135 PSI, depending on intake restrictions, exhaust restrictions, camshaft optimization for specific RPM ranges and, possibly, the phase of the moon.

What is specified is the difference between the highest cylinder and the lowest. Both want the lowest cylinder to be higher than 75% of the highest cylinder -- in other words, if you measure 120 PSI on the highest cylinder then the lowest cylinder should be at or above 90 PSI.

Once you do the dry compression test, squirt a little oil (a little oil -- two or three squirts from an oil can) in the spark plug holes and repeat the test. This will help determine the ring condition, as the oil will help the rings seal against the cylinder wall. If the compression pressure goes up in the "wet" test, the rings are worn. If it doesn't then the problem is in the head.

 

now i know why i joined this site. you guys are so awesome. all of the searching i've done for sound advice and good answers, was all right here all along. i wish i had found this site sooner. but none the less i've learned some great new things that i would have never known. i've always done dry compression tests on engines, but this is first time i heard of a wet test. but it makes perfect sence thankyou. so when i do this compression test, dry first and document the results. then if there's a drastic differance than most likly the rings r worn. in this case then it would be frugel to replace any gaskets without a engine rebuild first right? but if there's not much differance in both test and with the machenist telling me that he couldn't here any knocks, would it be in your opinon ok to just replace the gaskets if the head checks out? i'm using the Lucas heavy oil additive in my engine, sould i be safe to say that this will help slow down the wear in my engine? if i do replace these gaskets only, would it be ok to continue to use this in my engine. keep in mind the knock i did have went away after i added this additive. i'm not sure if i covered up a bigger problem by doing this but maybe you guys can help me on this. remember i trust you guys and girls too don't want to leave anyone out. any advice that you give me, i'll go by that. thanks again ever so much. T.C.M.

 

After you do your head gasket and get it back running, make sure and flush the cooling system REAL good to get all the sealer out. In my experience, those sealers sealed everything but what they were supposed to. However, they worked good enough to get one to the auto auction! A good chemical flush will have you squeaky clean in no time!

 

I agree once you get it fixed flush the system really good. I have had nothing but bad luck with things like stop-leak and I would sooner trust coating a leak in silicone to buy me some time than putting a bottle of that crap in my system.

 

i agree also, but at the time i had no choice. i live in in Newfoundland Canada and rigt now we are in the middle of a cold snap. the shop i normally use is taken up with heavy equipment. so i was trying to buy some time to get by untill i had either another engine or been able to repair mine. i haven't had the chance to do the compression test yet. but as soon as i do i will post the results. that way if you guys don't mind can give me your honest opinon on the best route to take. i hope that there's not much wrong with my engine for i'm thinking about rebuilding it once i get the chance. on the topic of compression testing, i have need of your advice. some people say to remove all the spark plugs at once then test each cylinder. then some say to remove one plug at a time and test the compression. what do u suggest? i want to be able to get the best results that i can so there won't be any doubt on my decision. please except my apology for bugging you guys so much. thanks T.C.M.

 

I would say remove them all. If the gasket is blown between two cylinders you could get pressure from the cylinder next to the one you are testing.

 

I myself belive that the best results are when you have all the plugs out then test each cylinder but thats just me.

 

I'd yank 'em all before I tested. That way the battery would last longer and I'd get a better idea of the engine's condition. Plus it's the perfect excuse, if one is needed, to install new spark plugs.

 

Yup, pull all the plugs out at once when doing a compression test. And don't forget to block the throttle wide open also.

 

yes, pull all the plugs at once, but first, bring engine up to operating temperature. also, hold throttle wide open during the test to allow free intake of air to the cylinders.

if the readings look wierd, don't hesitate to check 2 or 3 times ... i have had trouble more than once, trying to get a screw-in gauge seated properly. if you have compressed air, blow away all the grit and dirt at the base of the plugs (before removing them)... this prevents it from entering the engine and also helps to get proper seating on the compression gauge.

i feel for you easterners ... here, it's just below freezing ... can't imagine what life is like at 20 or more below.

 

Ditto. It's sunny and 75 here. Just like that new Joe Nichols song.

I was in Virginia 3 weeks ago. Wasn't too cold there either, right around 28 before the sun came up and clear for the rest of the day. Sky was beautiful. I figured it would be closer to 15 or so. Guess I brought some down home Southern sunshine along with me.

 

Cold, in a word.

I know, I know . . .

 

Hey.. nice to see another Newfie here.. welcome to the site. I'm from Grand Falls originally.


You're getting good advice here it should help you narrow down your problem. I don't know if I missed it or not but what started all this anyway... why are you looking to have motor work done?