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simon_f
23-07-18, 04:49 PM
If I may I would like to pick your brains – tell me to PO if you are too busy.

I have built a 390 engine which ran fine in my Cobra last year. After winter it gave starting problems which I finally solved but now it belches bluish smoke from both exhausts. First thought was valve seals which I changed – still smoked, then I thought the inlet manifold might be sucking oil so I have changed the gaskets now, twice, and it still smokes. Finally I wondered about the amount of oil in it so I’ve drained the oil and filled it with the correct amount according to the book and google (in fact it’s probably a bit low). It still smokes!

After it’s been sitting say overnight it starts first timeish and runs good with no smoke but after a few seconds the smoke starts and builds up until it fills the garage. It’s as though oil is not present in the combustion chambers at first start but gradually leaks in there and gets burned off in the process.
I am now thinking the oil control rings are stuck or broken – but on both banks ??? unusual I would say.
I have checked the compression and all pots are about the same and in line with what is expected.
I have examined the inlet manifold and can’t see any cracks etc. I have lead wired and blued the fit between the manifold and the cylinder heads and whilst there is a variance it is not enough to worry about.

It doesn’t have an oil pan thing under the inlet manifold but it never has and didn’t smoke before.

I would appreciate any advice comments ideas you can give me before I pull the engine, however if you think I am a cheeky get then it’s OK 

I have missed this year’s good weather but I am determined to sort this by whatever means and drive it in the snow.

KevinW
23-07-18, 05:53 PM
maybe a daft question, but do you have any method of venting on the rocker covers?

Bigblock
23-07-18, 05:59 PM
Could be a few things, would be interested to hear what the issue was you sorted just in case it is related somehow.

As you say you have done the valve stem seals and nothing has changed then that could rule out rings getting stuck while not being used, although it does happen.

You could check if you have an engine breather connected to the base of the carb it isn't drawing oil in from the rocker covers etc, just take it off the carb/manifold and into a catch can for a test.

Also the other thing that can happen is that the blow by gasses cant escape from the engine for whatever reason, ie thru the engine breather. Make sure the engine breather is completely clear and disconnected to make sure you are not pressurising the engine block and actually pushing oil up past the oil control rings. Seen this happen on a few cars that came thru the workshop where the breather had either blocked up, kinked etc or been connected incorrectly.

jon1d
23-07-18, 09:23 PM
Just a thought check your pcv is the correct direction if that is fine then open breathing to the air as suggested . At idle you should not have much oil coming from the valve cover vents . Is you oil pressure good ?

simon_f
26-07-18, 03:35 PM
Thanks for the comments guys, I couldn't start it after winter due to carb and timing setting problems - got them sorted now (well at least it starts and idles). I'm not thinking that the oil control rings are stuck mainly due to the smoke coming from both banks in equal amounts. as you say its unlikely anyway and especially unlikely on both banks.
I have no connection between the carb and the rocker covers - I took that off just in case. If I take the breathers off completely it still smokes. The oil pressure sits at around 40 psi I have installed restrictors into the oil feed to the rocker shafts in order to avoid pooling and too much oil submerging the valve stem seals. This was recommended when I changed from Hydraulic lifters to solid lifters.
With the breathers off there is no PCV to worry about.
Just to be 110% sure this weekend I will pressure test each cylinder and do a leak down check. Should confirm what I said earlier that all is good with the head gasket and hopefully the rings. If anybody can think of anything else I would appreciate hearing from you - no matter how far fetched it might seem it could just be the answer.

KevinW
26-07-18, 06:39 PM
smoke from both sides....OK silly question, but have you got an elec choke on the carb and is it working properly?
First fired mine up and didnt realise it was shut - nearly choked me in the garage. I added a 12V feed and all fine now, goes off after about 1/2 a minute.

kdavies3
27-07-18, 11:21 AM
is your pcv connected to the correct port on the carb?

Lloyd Barnes
27-07-18, 11:51 AM
As the others have alluded to, a well designed PCV system is a must. If nothing else its likely to reduce external oil leaks. Its not a passive system, you are actively clear exhaust gases from the block using vacuum.

Couple of questions... Have you pulled the spark plugs? Are they oil fouled?
the right amount of oil in an engine is related mostly to the sump you have chosen to use. Worth checking that again in that context.

Another way to test for a leaking intake is to hook in a vacuum gauge to the inlet manifold. As you can imagine, a leaking intake will usually show as a lower level of vacuum than you would normally expect (17-21" is roughly "normal"). they can also be a great way to check if there is a cylinder specific issue as the readings on the gauge will fluctuate. £20 off ebay and worth every penny when you have a strange problem to diagnose. :-)

simon_f
02-08-18, 09:29 AM
Hi Kevin,
I have a 'spare' Holley Carb I am going to swap this weekend to try and eliminate carburation problems - start at the top again and work down methodically. It may run like a dog but could stop the smoke.

KevinW
02-08-18, 03:55 PM
Hi Kevin,
I have a 'spare' Holley Carb I am going to swap this weekend to try and eliminate carburation problems - start at the top again and work down methodically. It may run like a dog but could stop the smoke.

If it has an elec choke, make sure its wired to a +12V source!

v8pete
05-08-18, 12:02 PM
Just one to throw in here - I remember reading years ago about a mechanical fuel pump failure which would cause fuel to leak into the oil, diluting it and leading to very heavy bore wear. Not suggesting that's whats happened here, but worth knowing about all the same...