Compression test question

[Guest tagree01]

Hey guys, I am trying to diagnose the engine on my new 1940 Dodge coupe( http://s172.photobucket.com/albums/w27/tagree0/1940%20Dodge/ ) and I would like to do a compression test. The car runs smoothly in 1st and 2nd, but when I got up to speed in 3rd white smoke started pouring into the car. Would that suggest a compression problem?

The problem is that I don't know how to do a compression test. I checked the Tech Tips section of the main site and the Tech Archives and came up empty.

I tried to piece together a protocol based on some of the past threads, but some people say to measure at wide open throttle and others say take out all of the plugs. I don't understand. Google didn't help. Is there a reference I can check? If not, would someone be willing to write out step-by-step how to do a compression check, starting with where to get the gauge (for a 218)? Maybe we could post it in the Tech Archive?

I'd also like to know what a leak-down test is and whatever test you do with a vacuum gauge.

Thanks guys, Tom

[Plymouthy Adams]

if your white smoke has a oil smell sounds like your first order of buisness is to inspect the connect on the right (passenger side of the block) for the draft tube, tight is a must with the porper angle down so it will siphon during forward travel...

If it has a sweet smell like atifreeze..I think I would inverstigate for antifreeze leak in/about the control valve at rear corner of block for leaking and getting on the exhaust manifold...plus at that for the smoke/fume to enter heavily into the vehicle says you have a serious air leak at the firewall and needs to be attended to...

[JIPJOBXX]

You can get a very good compression gage with attachments at Wallmart for around $30.00. Then all you haft to do is take out all the plugs (clean out around plug hole so you don't drop any dirt into cylinder). The just screw the compression gage with adapter into the first cylinder and turn over engine. Your compression gage will set itself at the highest compression for that cylinder. Then go to the next one and follow through all remaining cylinders. Write down all your reading and if your lucky you will have pretty much even compression in all cylinders. I think the low should be around 80 lbs and the high should be around 110 to 120 or so. This is a very easy job to preform and the gage and attachments will make it very professionally done. Remember to clean out all plug holes before you remove the spark plugs and don't leave the key on in the start position (You might just get a little juice from the plug wires. I hope this helps- Jon

[JIPJOBXX]

By the way that is a very cool looking car you have there-Jon

[Guest tagree01]

...plus at that for the smoke/fume to enter heavily into the vehicle says you have a serious air leak at the firewall and needs to be attended to...

Thanks Tim. There is a leak somewhere, but also the exhaust ends under the passenger seat, so that may contribute to the cabin filling with smoke, if that is where it is coming from. I didn't have a fire extinguisher on board yet, so I shut it off immediately and didn't figure out where the smoke was coming from. I could do it again with a fire extinguisher on board to see where the smoke is coming from, but I don't want to do any permanent damage to the engine.

I wasn't sure if it was sweet-smelling smoke or not. I was too busy soiling my pants.

[Plymouthy Adams]

excellent articles on how to do a compression just by going on line and typoing it in...there are those who want the throttle wide open other that say is is not necessary...either way will get you close enough as long as you test all cylinders equally. A leak down test will check the engine to show just where the leak is occuring in your engine..when pressurized and you can hear air rushing into the block by placing your ear/listening device at the oil fill tube..rings..if you can hear it at the edhaust..exhaust valve..it at carb throat..intake not seated...the actual test is a differential test where you measure the air loss compared to air pumped in...this test will also allow you to check for head gaskets as you can hear air leaking into an adjacent cylinder and if it is blown to a water jacket/gasket orifice, then a rise in the water level or presence of bubble in the radiator..then the gasket is blown at that cylinder. The biggest problem with some pressure test of this manner is ability to lock and hold the engine at top dead center compression stroke for the cylinder you are testing..most of thest test can be done just with pressure and not the manifold for the leak down..it is a bit expensive device...that is one item I do not have either...

[Plymouthy Adams]

As your test run was short lived, disconnect the hot feed wire from the input to the ampmeter and tape it off against shorting out..jumper your voltage to the coil for ignition and then use a remote start switch clipped into the starter circuit..start the car..if there is no immediate smoke..then it may be electrical..

If you have smoke yet..try now to determine the odor....it may appear white..but could be a slight blueish tint...then once the odor has determined the source..you have but to trace it back...as long as you have enough oil in the crankcase and water to prevent overheat..you should be ok running the engine finding the cource of the smoke.

[Don Coatney]

Go to the bay and buy yourself a MoTors Manual that covers the year of your car. You can buy a compression tester from any auto parts store for around fifteen bucks. No need to buy a fancy one as it is not required on these engines.

[Guest tagree01]

As your test run was short lived, disconnect the hot feed wire from the input to the ampmeter and tape it off against shorting out..jumper your voltage to the coil for ignition and then use a remote start switch clipped into the starter circuit..start the car..if there is no immediate smoke..then it may be electrical..

If you have smoke yet..try now to determine the odor....it may appear white..but could be a slight blueish tint...then once the odor has determined the source..you have but to trace it back...as long as you have enough oil in the crankcase and water to prevent overheat..you should be ok running the engine finding the cource of the smoke.

The car drove fine for about 20 min before the smoke, but it was in a tight neighborhood where I never got it in 3rd. It didn't smoke until I got a little speed in 3rd. I forgot to mention that, after the smoke died down and the car cooled a bit, I got brave and drove it the 2 blocks home, keeping RPMs down and staying in 2nd gear. It ran as well as it did the first 20 min with no smoke. The smoke didn't originally start until I got up to speed in 3rd.

I'll start it this weekend and find the smoke.

[Don Coatney]

Did you have your radiator filled all the way to the top?

[Guest tagree01]

By the way that is a very cool looking car you have there-Jon

Thanks! I love it. It was the best 3 miles of my life! Some day I'll drive it 10 miles or maybe more.

[Guest tagree01]

Did you have your radiator filled all the way to the top?

Yes, the radiator is full. I don't know if it is going to the engine, but it is full.

[JIPJOBXX]

I bought this after 30 years of fooling around with the old style pressure gage that had a nipple on the bottom and you haft to hold it down in place to work. This as you see has various nipples so you can test almost any engine and not hold it in your hand. I think it is well worth the price to pay a little more and not have the hassel of the hand held type. Jon

[Don Coatney]

Yes, the radiator is full. I don't know if it is going to the engine, but it is full.

Did you have it full all the way to the top? If so that may be the source of your smoke. The radiators in these cars should only be filled to about 1 inch below the neck. If they are filled to the top they will spew the excess water out the overflow tube. If this happened while you were driving down the road the ejected water would turn to steam on the hot engine.

[Don Coatney]

I bought this after 30 years of fooling around with the old style pressure gage that had a nipple on the bottom and you haft to hold it down in place to work. This as you see has various nipples so you can test almost any engine and not hold it in your hand. I think it is well worth the price to pay a little more and not have the hassel of the hand held type. Jon

I see one potential big problem with the compression tester in your picture. The lengh of the threaded fitting that screws into the spark plug hole may be too long and contact a valve. I use the tester with the rubber cone. If the bottom of the cone makes contact with a valve it will simply pop out of the hole. Also there is no hassel with screwing the fittings into the spark plug holes. No wrenching required with the cone type.

[Plymouthy Adams]

Don..I have one similar to the one pictured..looking closer you will see the two different thread sizes and to length for the smaller size thread we use..long and short threaded adapters...I never liked the hand held..for flatheads it is not a prob as access is a breeze..try getting into a tight compartment with close fit manifolds etc..just not very user friendly.

[Guest tagree01]

Did you have it full all the way to the top? If so that may be the source of your smoke. The radiators in these cars should only be filled to about 1 inch below the neck. If they are filled to the top they will spew the excess water out the overflow tube. If this happened while you were driving down the road the ejected water would turn to steam on the hot engine.

I didn't know that. I guess there is no overflow resevior. I did fill it all the way up just before I drove it. That could be the problem. Wouldn't that be nice! I'm not sure my luck is that good.

[bob_amos]

I know that some say that opening the throttle is not necessary but it really is. Two points to make here. One, if the throttle is left closed with a full carb float chamber the venturi effect will pull fuel into the cylinders washing the old film away and give an inaccurate reading as you work your way through the test. Second, all plugs should be removed prior to testing as this will give the engine full and free movement that will be equal during all tests. Leaving the other 5 in with perhaps one or more low cylinder would change the rotation speed giving an inaccurate reading. Also, I like to have the engine rotate the same number of turns for each tested cylinder. Generally one or two rotation more than the total number of cyclinder on the engine. And.. if you find a low cylinder, go back to that cylinder and squirt some oil into it. Retest. If the pressure comes up considerably there may be ring leakage as the oil will fill the ring land areas forcing more compression out of the cylinder. That recommendation on cylinder leak down was a good one. That is the true test for where things are leaking. Harbour Freight has a very good leak down tester for somewhere in the $40 range that works great for this. But injecting regulated air into the cylinder will do just as well.