Did a compression test on my truck yesterday. Thr results are as shown


1-155 5-165
2-155. 6-155
3-155. 7-155
4-120. 8-155


the number one cylinder was covered in oil and crust. Not really sure it’s been firing like that. I know the plug gets spark. All 7 other plugs looked either regular or maybe just a tiny bit lean/advanced. I really just say that because I don’t feel like I’ve done enough tuning to get them looking how they should, but comparing to the color pictures in my Haynes manual they look pretty good.

i was gonna squirt some oil into the number 1 and recheck the compression to see if the oil made the rings seal up any more. If so then maybe the oil in the comustion chamber is coming up from the bottom. If the oil squirted in doesn’t make the compression go up I’m gonna assume it’s the valve guides/seals and I’ll pull the cover and inspect 1 but I’ve had it off tons of times I don’t think there’s much of an indicator to something going on with the seals. I could spray the top of the valve seals with some Wd 40 and see if they bubble or spray when turning the motor over.



The reason I was doing a compression check was because the truck keeps blowing the intake valley pan seals out.

Do my compression numbers seem like I have excessive blow by? I don’t think they say that. I think those are pretty regular and not that bad for a 44 year old work horse. I guess a leak down test would be the next bet. That could help me diagnose the valve seals too I guess.

 

not at all bad for a worn low compression engine.

What do you have for a breather set up ?

 

for years the truck used a 10 inch diameter edelbrock chrome 2 piece style with 2 inch tall filter. After getting tired of those gettin dirty constantly I now stack 2 10 inch diameter 2 inch tall filters on top of each other

 

I meant the crankcase breather, but I see you have something hooked up to the rear of the RH valve cover I assume it goes to your bottom air cleaner plate ,are you sure it's unobstructed ?

 

I am amazed they are that high.

Normally around 135-140 in stock form.

Does it have a "straight up" timing set?

Either your gauge is off or it has had some engine work...maybe even milled heads or different pistons or cam.

The valley pan deals can be a hassle - lately I just run a felpro gasket without the pan and RTV on the china walls but I live in the rust belt and rarely see temps above 90-95F. The valley pan thought is to keep the oil from coking up the bottom of the intake manifold on a hot engine.

Gasket surfaces are wire wheeled clean and wiped down with laquer thinner until no gray residue on the paper towel.

Compared to the rest of the cylinders, #4 is down an abnormal amount which would contribute to excessive blow-by.

I assume the PCV is plumbed and working correctly?

Is it pushing oil out of the breather onto your valve cover?

Blue smoke on startup after sitting?

 

the pass side is unobstructed and yes running to the bottom of the air cleaner. Drivers side twist on cap has the foam under it but holding it up to the light without the pcv valve out of it I can see day light through easily. You can actually see what’s on the other side. I guess it’s possible that it is filling with oil vapor after the truck is running. I could pull that off after the trucks been running for awhile and make sure the foam isn’t soaked with oil.

 

If you take off the oil filler cap and hold your palm on the hole you should feel some suction. If not you have a lot of piston blow by, or maybe need a PCV valve. The numbers are a bit high for stock. To get accurate readings remove all plugs and brace the carb open. Be sure to disconnect the igniton. I've been known to plug the inlet PCV filter holein an older engine to develope a vacuum in the crank case. Great for leaks. Pulls them in instead of pushing them out.

squirting oil in the cylinder will tell you if it's the rings. Compression will rise if it seals better. Velve stem seals are often a problem.

 

It's been so long since I touched a 335 series engine I don't remember, is it baffled good under your breather cap ? if oil or anything else plug up your breather you're going to build pressure in any engine.

Remember it's all about the breather, because at WOT is when you have the most bypass and the PCV does virtually nothing when your vacuum drops.

 

forgive me for my ignorance, but what's "wrong" with the numbers you have? I thought we didn't need to worry unless compression testing showed <100psi, or adjacent cylinders w/ low compression.
what will adding oil tell you if we're not dealing with low compression to begin with? I thought the oil test was to see if it was a piston ring issue or a valve seal issue.

 

That things not too tired. My 78 range is 105-115

 

The 1979 Ford Truck Shop Manual says that to be considered within specs the lowest cylinder should be no less than 75% of the highest cylinder. Your #5 is 165 and your #4 is 120. That is lacking only a few pounds to be within specs. You may want to recheck it. Be sure all the plugs are removed and the carburetor is completely open.

 

Don't you mean a little pressure beartracks? What would the suction be from?
Since that would be pulling the PCV off of the engine, seems like it would be natural to see some smoke coming out rather than getting sucked in.

If you do that same test on the passenger side though, you would hope to feel a little suction from the PCV in the driver's side cover.
Maybe I'm missing something and getting that wrong, but that's what I think I've noted on all other engines.

Paul

 

Don't pull the pcv valve. Pull the breather (inlet air) cap and plug. The carb will pull suction throught the valve and crankcase.

 

I do have a straight up timing set, I think other than the carb and exhaust the engine is stock though. Would just #4 cause excess blow by?

I drive the truck all year, it’ll spend time pulling trailers at 100 degrees this summer. I’d like to keep the valley pan if I could. Pcv valve is plumbed to the back of my 2150 motorcraft like it should be. Oil can make its way out from between my drivers side twist on cap and the valve cover. I DONT think there is a negative auction there, I think it probably does a pressure in the valve covers. No smoke on start up or ever.

 

I’m trying to find why my number 1 cylinder isn’t firing and is full of oil, and I’m trying to figure out why my intake seal keeps blowing out. I was checking the compression to see if any of the cylinders were experiencing excess blow by, or if number 1 was. It now seems like I don’t have excess blow by, especially st number 1. So there isn’t really a problem with my data, just that it didn’t answer much.

I guess number 1 is leaking oil from the valve seals. But I still don’t know why my intake seals blow out so easily and frequently. I’ve never done a compression test on anything, so I’m asking you guys if it seems like the data I posted would indicate any kind of excess blow by. But it doesnt seem like it does.


The oil in the cylinder would maybe tell if the rings on 1 were leaking a lot but it it still had that compression. That would indicate that they all should have a lot more I think? And that maybe oil is coming up from the bottom into that cylinder? But the way number 1 isn’t even low, and the others aren’t burning oil I think it’s not the rings.