Educate me please.

The patient is a '78 F150 4x4 with a "tired" 400 that really runs. It's been three years now since we towed it home, and in that time I've done a TON of work to it and learned a lot. I feel like I'm getting there, with the help of this forum and other online sources, I've refreshed/replaced/upgraded most everything I'm able to work on like carb, ignition, brakes, suspension, and sourced things out like both drivelines rebuilt. The bed was in bad shape to I decided I'd build a flatbed for it, headache rack/gooseneck ball/receiver hitch and all - what a project that turned into but it turned out great! This is a work truck now that I haul firewood with, pull a gooseneck trailer with a horse or two in tow, even run down the highway to town. Truck runs strong.

I'll cut to the chase. Fuel, ignition, charging, cooling, all working great now no complaints. However as good as he runs he uses oil. Only history I have is at some point a "valve job" was done. Idle Vacuum is @ 20 Hg and rock steady. Vacuum behaves exactly as it should, throughout the rpm range, I've done all the "vacuum tests" with a vacuum gauge. There is some blow by, seems like it comes and goes, I think it's worse at highway speeds. The pcv system is working as it should. No smoke out of the tailpipes, they're relatively clean no black soot. Amazingly good oil pressure for a 400, never drops out of "normal" range.

I took a 140 mile road trip today and when I got home was 1/2 quart low. 10W-30 oil. That's about typical, seems like a lot. Doesn't seem to burn as much just tooling around the place as it will on the highway. For a long time I blamed it on a rear main oil seal leak, but since I got that fixed the oil consumption is still there. What a disappointment as he runs so strong. This is my weakest link - engine internals. I've never even had the valve covers off although I'm going to soon. I could use a lot of guidance here, I know it's a high mileage engine but where is the oil going?

 

If it's not leaking it the oil out , then look at the PCV and make sure it has a baffle under it.

Rings, guides, valve seals there's lots of places to burn oil but you should see it on deceleration or on warm start ups for sure if you're burning that much.

If you're looking for something to help the additives can make a big difference, for a while. Rislone or something like that may surprise you.

 

with no external leaks i had the same issue ( using oil ) with my 1977 F150 4x4 351M back in the early 1990's ,,, had to take some crankshafts up to Toronto for balancing about a 61/2 hour round trip ,, before i left topped up the oil -- got home checked the oil level used 1-1/2 quarts round trip . so i decided to look at additives to stop some of the oil consumption ,, not really happy with what i was reading and just happen to pick up a bottle of Mobil 1 synthetic oil and reading the back of the bottle said it would cut down on oil use . crankshafts were ready 2 weeks later and before i left to pick them up i did an oil and filter change to the Mobil 1 synthetic oil . got back home checked the level used only 1/2 quart round trip. i was impressed and have been using Mobil 1 synthetic oil ever since , even after i rebuilt the engine a few years later Mobil 1 synthetic oil is all i have used and still use in this truck and all my other vehicles. might be worth a shot in your situation as well .

 

If you have factory valve covers with a PCV baffle and functioning PCV valve and no external leaks there is only one place for oil to be consumed and that is the piston rings.

most local auto parts stores rent leak down testers. It’s a very easy test to perform. Doing the test will at least verify is the rings are good/bad.

this type of test is standard and a “preventative maintenance check” in aviation to test the condition of non-jet engines.

if you have any air compressor you can perform the test.

It will tell you what cylinders/rings are bad. And, it will also tell you if your valves are sealing properly.

 

Thanks a lot for the replies guys, I'll get back when I have more time

 

The valve guides/seals can be another source of burning oil. It might be worthwhile to plug most of the PCV line so as to draw a vacuum on the crankscase.It will pull leakage back in.

 

440 sixpack, I've never looked into additives, I always considered them a "band-aid" for something like this, but I may give it a try if the alternative ends up looking real spendy. Never heard of Rislone. Can't say if he smokes on deceleration or not but I haven't noticed anything on warm startups.

john-9 WOW that made a huge difference! I've always thought an old worn engine would burn MORE oil with full synthetic, as it is "slippery-er" if that's a word. Definitely something to keep in mind. I've considered going to 15W-40 Rotella diesel engine oil (a little thicker, higher levels of zinc, supposed to have more detergents) but never Mobil 1 synthetic.

atitagain4, I really like the idea of doing a leak-down test. It sounds like that would tell me what's going on, and where. Then I would know what my options are for sure, to actually FIX the problem. Thanks for the great idea.

beartracks, I've heard bad valve seals can cause blow by. Anything I should specifically be looking at when I pull the valve covers? I'm pulling them to paint them, but also want to see how much sludge I find, as I ran quite a bit of Seafoam through it in the beginning, and more recently some Marvels Mystery Oil which I think actually may have quieted a sticky lifter. Also sorry but you kind of lost me "plug most of the pcv line", I've never heard of doing that actually just the opposite that the pcv needs to be up and functioning - but I see what you're getting at, how a "vacuum on the crankcase" could pull leakage back in. Just not sure how you would safely go about this, all I have is a pcv valve in one valve cover going to the back of the carb and a line going into a breather in the air cleaner from the other valve cover. I would hate to blow my new rear main seal! Thanks for the advice.

 

1/2 qt for 140 miles...? If you were burning that, you WOULD notice the blue smoke... my 66 F100 had a 240, valve guides/seals were dunfor, (not sure if seals or guides, haven't torn it down yet) and he would leave a nice trail of blue smoke coming down the hill into town... or any downhill coast for that matter, and he would only be about 1/4 qt low after 6-800 miles if even that low. If it was your rings, and burning that much, you'd notice the blue smoke all the time. Valve guides/seals, smokes on deceleration as it creates a large vacuum, rings, either seized or worn is smoke all the time and possible fouled plugs.

I think you have a leak somewhere you're not seeing... is your tailgate spotted? Have it running and roll underneath and take some pics and really look for an oil trail... my 351 Measly had a bad leak on the oil pressure sending unit canister, I thought it was rear main initially. Out of all the trucks I've owned, if it went through 1 qt. every 300 miles, it would be smokin or leakin somewhere...

And, have someone follow you driving around and watching your tailpipe, uphill/downhill and accelerating.

 

I'm thinking the same thing, that much oil is going to smoke. if it still has the cat and smog junk that would hide it some until it plugs up.

valve seals won't cause blow by that's rings. but bad valve seals and or wiped out guides will cause oil burning that's most noticeable on a warm start up. if the guides are bad even new seals won't stop it.

 

Yeah you're probably right I should have someone follow me around, I really can't tell if he's smoking while driving ( single exhaust - no cat - exits down before axle on passenger side so exhaust sort of vaporizes under vehicle while driving?). On cold startup there is the typical white vapor for awhile when it's cold out, but that clears up to nothing. On warm starts there's nothing that I can see. Drive him around all day, get out anytime and see nothing. Like you say the only way to know if he's smoking blue on deceleration would be to have someone follow me.

As far as leaks go I suspect the oil pan was always leaking a little and I know the rear main was, all new gaskets/seals there and bone dry now (mechanic dropped oil pan to replace rear main). Only wet spot left under the truck is on the bellhousing? where it mates with the engine, just wet no drip. Not sure could be my oil pressure sending unit (I've replaced that), valve cover in the back where I can't see? It just stays a little wet, can't believe I'm losing much oil there - would be lots of oil all over the undercarriage.

Before I replaced the rear main there was oil sprayed all under the truck after a drive like that. It's all dried up now. I thought sure that was it. Like you say smokin or leakin somewhere. I have gotten some good help here and need to look deeper.

 

You could pull a couple plugs, they'll look like crap if you're burning that kind of oil.

 

Right I was thinking the same thing, I think I'll start there. That and a leak down test are next on the list. It's in the single digits here today so I'm in no big hurry.