73 340 Compression Test interpretation help

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jimmyray

jimmyray

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Greetings, All!

I ran a compression test on my 73 340 semi-stock motor (intake & headers). I bought the car used, with 101k original miles, and have no idea about the internals of the engine. It is a numbers matching block, though, with "I" (the letter next to the spark plug, as opposed to the X) heads, don't know the casting number, or their age, but I assume they are stock.

The engine emits blue-white smoke (oil) from a cold start, especially if sitting for a few days. Normally, once warm, it won't smoke, but does smoke under hard acceleration. I warmed up the engine, pulled all spark plugs, grounded the coil, a propped open the throttle. Below are my readings - Dry is normal, wet is with oil poured into the cylinders. I took 2 readings for all but the Dry #2 cylinder, and did not do wet test on #5 or #7, since was too difficult to get around the header!

Please let me know your thoughts.

Compression Test.jpg
 
Smoke sounds like valve guids or old hard seals. I bet a vavle job would not hurt with new guides in the heads. Seems the compression is even for the most part, so no major burned valves and probably has some wear of course on the rings, but the heads are the issues I can see from our info. But not real bad.
 
Another intersting point. It makes 20 psi oil pressure at idle, with only 5-10 inches vacuum, and 60+psi at 3000 rpm. It has a lopy cam, which of course I don't have the specs on. Oil pressure suggests the bottom end is in good shape?
 
jimmyray said:
Another intersting point. It makes 20 psi oil pressure at idle, with only 5-10 inches vacuum, and 60+psi at 3000 rpm. It has a lopy cam, which of course I don't have the specs on. Oil pressure suggests the bottom end is in good shape?
Click to expand...

Yep sounds like the bottom end and bearings etc are in decent shape huh?
 
Valve guides. Block the breather hose coming off the non-PCV valve cover. You should feel the crankcase build vacuum really quick. If the idle speed changes, this confirms. I'm betting those are probably original heads and starting to show their age.

Here's what happens. You get the smoke when starting cold because while the engine sits static, oil runs down the intake valve stems and wets the backside of the valve head. So when you cold start the engine, the extra rich mixture from the choke washes this oil off and you get a puff of smoke when you start. We aren't so used to seeing it these days, but not too many years ago it was considered fairly normal. Now, with the engine warmed up, you trounce the go-pedal. A cool fuel wet wind howls down the ports washing the back of the valves and the sudden raging fires within the chambers cooks the oil out of the saturated deposits making you guessed it. More smoke.

Here's a quickie diagnosis. Do an "Italian Tune-Up" at about 25 mph, floor it until the tranny shifts into 3rd (65-80 mph) lather rinse and repeat. The first time I bet you get a pretty good cloud of smoke, and the engine will ping and rattle pretty good. The next time, the smoke and pinging should be less. By the 3rd or 4th time the smoke should clear up. If the smoke gets worse, your rings are shot or stuck.

How much oil does the engine use? Anything under a quart per 1000 miles in normal driving means something is wrong. More than a quart in 500 miles, something is seriously wrong. Did the car sit for an extended period of time?

Sure, you can put new valve guide seals on, and I would if it were me. BUT start saving your nickels for a top notch valve job.
 
Interstingly enough, the car sat for 7 months at a used musclecar lot, and only had 32 miles put on the odometer. I have put about 800 miles on itsince getting it in May. It still run's very strong, but the smoke it right embarrassing.

Incidentally, it has no PVC valve, only a breather plug on the passnger side of the engine. I have seen smoke coming out of it when the car is hot. The motor has the "Cast Aluminum Valve Covers", and are solid on the driver side, except the filler cap opening. Could this be compounding the problem?

I will try the "Tune Up" you reccommended, since I have noticed a couple of times it did not smoke under acceleration. I never associated it with the phenomenon you suggested.

Do you feel, at this point, that it is unlikely the problem is with the rings?

340.jpg
 
No PCV Valve?!? THERE'S YOUR PROBLEM!!! Or at least some of it.

Really, add one pronto. Less smoke, way less oil leaks, only costs 20 bucks.

A little SeaFoam therapy should free stuck rings, but it won't help worn guides. Here's how: Buy 2 bottles, put 1 in a FULL tank of gas. Pour 1/2 of the second into the crankcase. Dribble the remaining 1/2 bottle evenly between the primaries, ultimately flooding the engine out with the stuff. Wait at least half an hour, or even better until after dark (I think the label mentions "significant vapors" or something, it'll smoke like hell). This will pretty much launch any deposits/carbon/varnish in your engine. Do an oil change 3-5 running hours after adding to the crankcase, but do so within 2 days. Also don't go past half throttle until you have fresh clean oil/filter.
 
C130 Chief said:
No PCV Valve?!? THERE'S YOUR PROBLEM!!! Or at least some of it.

Really, add one. Less smoke, way less oil leaks, only costs 20 bucks.
Click to expand...

YES 100% agree, you must have one and that will take care of a lot of the blowby. The other issues are just some signs of wear.

Also make sure on those valve covers you get the deeper gromet so that it does not suck up oil into the valve, then you will have more smoke all the time. seen that too!
 
C130 Chief said:
Here's a quickie diagnosis. Do an "Italian Tune-Up" at about 25 mph, floor it until the tranny shifts into 3rd (65-80 mph) lather rinse and repeat. The first time I bet you get a pretty good cloud of smoke, and the engine will ping and rattle pretty good. The next time, the smoke and pinging should be less.
Click to expand...

I tried this, and it seemed to smoke less on successive acceleration cycles.

I will try the PVC valve first. I assume I can have a machine shp cut a hole in the cover for it and the grommet? Youda thunk theyda put one on to start with!

Maybe I should put on new umbrella seals as well? Can I do this with compressed air and a regular spring compressor - the common type that clamps on with a dial on the top, or whould I get a specialty tool for chagining them on engine?

Thanks for all of the help!
 
OK a lot of those covers are stamped on the inside for the punch out for the gromets.

2nd yes you can use an adapter for air and get it to TDC, tehn use an on car spring compressor too and put new seals or at least take a spring off and inspect them 1st.
 
Ford trucks in the 60' and 70's used a PCV valve in the breather cap like the one shown. This might help the parts guy find you one.


Chuck
 
I agree, you have good compression, most likely its the valve guide seals. Bottom end is good, oil pressure is good. Seals might fix the problem for a while, but it would be best to replace the guides, and if you are doing that rebuild the heads. And for a few dollars more (if you are looking for more HP) replace them with a set of HD Magnum heads from Hughes engines.
 
You will want to put the PCV valve in the valve cover opposite the breather inlet. Which will be the left side based on the pictures you posted. Buy a regular Mopar style PCV valve and grommet, and measure the grommet to see what size hole you need to make in the valve cover. A low buck hole saw will cut the aluminum just fine. Remove the valve cover and drill from the back.
 
So people don't mis-understand the Ford style pcv breather cap it isn't a convention type breather, it is sealed from outside air and is only open to the valve cover. This type breather cap works quite well if the valve cover doesn't have baffles because this cap acts as the baffle.


Chuck
 
Another point, there must be airflow through the crankcase. The pics jimmyray posted show a breather on the right valve cover and a sealed filler cap on the left. If he were to replace the breather on the right side with the Ford style, and not make other changes, the system would not work properly. The easiest solution would be to put the Ford style on the right side, and replace the sealed filler on the left with the breather currently on the right. This assumes the current and Ford breathers are turn-to-lock and not push-in.
 
If you decide to install the factory type pcv with a grommet thru the valve cover, be sure to install a flat metal plate (baffle) on the underside of the grommet hole where you install it. The stock pcv grommet works fine in the blind holes provided - by the way. There is no baffle, and you do need one in this case. There are bosses there that are either already tapped for small screws or can be tapped. Use some locktite on the threads of the small screws if you put a baffle in.
 
I pulled the cover last night, and the hole will require special grommet (see pic), because of the thickness of the cover at the hole. Of course it is not available from a local supplier...

My Autozone/advance auto/o'brienms auto parts don't seem tpo have a twist in breather cap with a PVC opening, they only have a twist with no opening, or a push in.

Where would I source a twist in ith a PVC opening?

chucker54_1964_5150615.gif
 
All late 60's to mid 70's V8 Ford trucks had them so they should be available. Not like Ford trucks are rare or something. If you asked for one for a 1975 Ford 3/4 ton with a 390 engine any "descent" auto parts store should be able to get you one.

Chuck
 
Well, I have you all know that I found a non-vented breather cap, a grommit, and a stock 340 PVC, got it all installed today, and presto-mundo! Very little if any smoking under hard acceleration! WOW!! It may be smoking a little, but I can't tell through the 34 year old window and rear-view mirrror - so it is a dramatic improvement!

I am going to wait a bit on the valve seals, to see if the smoking at startup is reduced now that the block is adequately vented.

Thanks to all for your EXCELLENT input!

pvc.jpg


pvc2.jpg
 
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