Thread: FE 390 Blow by problems

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Hi, this is my first time to the forum, lots of good information here. But anyways, Ive got a 70's FE 390, its in a 75 ford f100. The 390 has been bored .040", 10.0:1 pistons, and a highlift cam (i dont know the exact specs cause my dad can't find the papers, but supposively its got .50" lift), the motor also has electronic duraspark ignition and a Holley 4150 650cfm carb.

The problem is that there is some serious engine blow-by, the oil is blown right out of the engine breather tube at the back of the intake and then none is left, and thats just going a couple of miles down the road and back.

I've taken the rocker covers off, and discovered a bent pushrod on the #1 cylinder intake side, so we replaced it and installed new intake gaskets, (didnt help). So now ive just done a compression test, cylinders #6,7,8 have compression that is excessively low, as follows, 90, 50, 70 psi, which is low considering the highest in the engine is 150-160 psi! I believe its head gasket, but it might also be the valves. If anyone has heard of this happening, any help would be appriciated. Thanks

Sounds like you may have some broken rings or maybe a bad piston. Is this a fresh engine, or did it run good till recently?? If it's a fresh engine, I would be suspect of the piston to wall clearance from the bore and hone job. Hard to say without seeing the engine apart, but if it's a fresh motor, something was done wrong on the buildup......

My dad purchased the truck used in 78 from a neighbor, it orinignally had a 360 in it, but it suffered from lifter problems. So he decided to rebuild, and instead of a 360 he found a 390 and had it rebuilt by a good engine shop. He did this in the 80's or something I believe, so the engine isn't that fresh. Sometime in the 90's he sold it to my uncle who said he had to replace the headgaskets once, so it could have a problem there again. My uncle sold it back to us and it sat for a couple years until my buddy and I got it running. We drove it around the farm for awhile and it seemed to work fine but the old carb was shot. So I rebuilt a holley and put it on. It just resently started to have the blowby which isnt a good thing. It could very well be rings or at worst a cracked piston. I believe I'll be taking the heads off to check everything out. But what really bugs me is the bend intake pushrod on the #1 cylinder, it has a 3/4" bend in it at the middle. It was bent so bad it was hitting the intake manifold bolt. So It could be valve related to. Anyways thank you for your help.

P.s When I did the compression test and dropped a teaspoon of engine oil down the plug hole, all cylinder pressures increased 10 psi.

I had this same thing happen to me , I brought an engine with low miles, but it set for a few years. after it was in and running ,it to had a lot of blow by,you could put your thumb over the oil fill and it would build up pressure in the pan and blow out the oil as it ran down past the pan.It had good compression, one cyl was about 10-15 lbs lower then the rest.(135 145 PSI ) I put over 600 miles on it with verious quick fix oil additives and nothing seem to work. I have changed engines scence then and have not tore the old one apart yet but every one i know seem to think that it is stuck ringes. And i might add, it never smoked out the exhaust. so thats my two cents worth.

I'm chasing a suspected blow-by problem on my 406. No smoke out the exhaust, runs strong but I get a burning oil type smell (probably from the breather) when I run at high revs for a while, I also noticed smoke when I was on a clover leaf on ramp the other day. I just pulled the plugs which look perfect ( nice tan color) compression was 160 - 165 with exception of #3 @ 180 and #2 @ 148 (a bit of oil in #2 broght it up to 175). The strange thing about this is that the on-ramp was a right hand loop which would push oil to the left and my low cyl. is on the right , away from where the oil would be pushed ). I had to test the engine cold since its impossible to remove the plugs with out severe injury after its been run . There is no sign of leaking anywhere I'm suspecting a broken ring in #2 but the compression is only marginally low (the engine has 12Kon it)

Thanks,
Mike-monster-

Does the motor have a PCV valve in one valve cover and if it does is it hooked up,if it does not the pressure from the motor will blow oil out anywhere it can.I found out this the hard way years ago,I took a 6 cylinder out of a 67 C-20 and installed a 350,I put aluminum valve covers on it that did not have a place for the PCV valve and it blew oil everywhere under the hood so you need to check the PCV system first.

It could be a wrist pen I am not for sure if it could happen in a 390 but I had a 351 cleveland that did the same thing the oil would spill past the carbarator and when we tore it down the wrist pen had came a lose and rubbed up against the cylinder wall and it was suppost to be a rebuilt engine, but someone must not of did it right.But that could be the problem.

I have a new PCV, I replaced the breather in in one valvecover and found that the PVC was not seating tight into the other valve cover (which I believe was contributing to my problem). I found a twist-lock gromet to replace the loose fitting one. In addition I found that my dipstick does not fit over the tube and that I was getting blowby there. At this time I have removed my DS and have a plug over the end of the tube. I just took a 600 mile drive and no fumes and no oil loss. I'm still concerned about the amount of pressue I may have in the pan so I feel I still need to work on this problem.

Davidm- I would think if I had scored a cyl. that I would have constant blow-by (which I don't).

Thanks all,
Mike

I was talking to crude-is-in

If there's a harsh burnt-oil smell, you've got a scored piston or cylinder wall. It's hard to describe, but once you learn to identify it, it's unmistakeable. Did the engine sit a long time without any coolant? If so, the head gasket could have rusted through. There's an opening in the front that's blocked by the gasket, if it rusts, or if the gasket is put in backwards, the water flows through the front of the head and cooks the rear cylinders.

But what really bugs me is the bend intake pushrod ..........

had the same problems , low compression , oil blown , bend pushrods , and the engine was runing like shitttt , so at last adjestable rocker arms soleved all problems.

I think it may help you .

Hi guys, sorry I haven't been on here for a while. Uhm I still haven't found out what is going on with the 390. The last time I ran it, it flooded & now the oil its very thin and smells like gasoline. I think that its the headgasket since the 3 rear cylinders on the left bank (driver side) have low compression between them. And to R Pope's question, yes the truck sat for a while and it may have been low on coolant. So the headgasket may be wrecked. Also my uncle owned the truck at one time and he had to do head gaskets and he might have screwed it up before he sold it back to me. It could also be the head gasket because of the oil return hole that is right by the # 5 cylinder and #4 on the other bank. I suppose I have to pull the heads off to find out, but my brothers 62 Unibody is in the shop now getting extensive sheetmetal replacement.