Howdy from Texas. New guy here that needs a little help.
I have a 1970 f-100 with a 390 that was rebuilt some time in the past but dont know how many miles on the engine. The problem I am having is the oil consumption is way too high, Im talking about a qt every 30 or so miles but the funny thing is it dont smoke and it aint blowing it out from under the engine. I am using a 15/40 oil. Where could all this oil be going?

 

What color is the coolant in your radiator? Possible blown headgasket.

 

Checked that, no blown head gasket. This ones got me stumped.

 

a qt every 30 miles is a large consumption. Gotta be evidence somewhere.



John

 

Check your plugs. If its dumping in the cylinders they should be BLACK. I mean like sooty dirty nasty BLACK

 

Checked the plugs and there was only two that looked to be burning oil but not very bad at all, the rest looked perfect. No oil dripping on the ground at all, The underside of the truck is dry all the way back to the tailgate. Had a bud follow behind me to look for smoke and he said none at all. Like i said the plugs looked good except for two that showed a slight oil burn.

 

Well that slight oil burn is leading way to a huge consumption somewhere lol

 

Air filter dry? Any oil in that area?

 

There are only four ways for oil to get out of the engine:

1) Through the exhaust

2) Into the cooling system

3) Oil leaks

4) Your @$$h0le neighbor next door is sneaking over in the middle of the night and draining it out.

Check your exhaust pipe(s). Are they wet or dry? Black or medium-gray?

Check your PCV valve. Does it rattle when you shake it? (Not a definitive test for condition, but if it doesn't rattle, you've got a problem and should replace it. Heck, replace it anyway, only costs a few bucks!!)

Have you checked under the valve covers to see if the oil returns are clear?

If it's not smoking, leaking, or the cooling system doesn't have oil in it, it's pretty much got to be #4.

 

The exhaust pipes are black with soot but dry, Im pretty sure it must be burning the oil but why wouldnt it be smoking? Its going through a qt every 30 miles or so on the highway so it should look like a crop duster going down the road but it aint, If i stay in town and keep the speed below 50 mph it dont seem to be so bad on oil.

 

I once worked on a Pinto that did basically the same thing. Used a lot of oil, but it didn't smoke (that either the owner or I could see). Also once installed a Ford Reman FE in a 75 F250 and it, too, had similar symptoms.

Yours just sounds more severe, and like you said, it should be smoking like a freight train. Of course, if it's valve seals/guides or a bad PCV valve, it would be worse when decelerating, and may not be quite as apparent as it would be under acceleration.

When I was a teenager, my dad had a 64 Rambler with a flat-head 6. The engine was basically worn out, and it burned a quart about every 75 miles. That one REALLY smoked!!! (Whenever I drove it, my friends accused me of spraying for mosquitoes, and more than once, some smart@$$ behind me turned on his wipers!! )

Make sure the engine is full of oil.

Let it sit and idle for a few minutes after it's hot. You may have to let it idle for 10-15 minutes. Stand back and look to see if you can see any trace of smoke, however light, coming out the exhaust while it's running. That would indicate PCV, valve seals and/or guides, or internal leak where the intake bolts to the heads.

After you've done that, rev it hard, then let it come back to idle. Don't hold it at high RPM's, just hit it hard and let off the throttle. Do this 4-5 times. (You'll need to be able to see the exhaust pipe, so you may need to have someone else rev it for you.) See if you see any smoke, either on acceleration or deceleration. If it smokes when you accelerate, it's rings. If you see the smoke on deceleration, it's valve guides and/or seals.

Take the oil cap off and pull the PCV valve from the valve cover. Rev it up, hard, while holding your hand over the opening for the oil cap. If you feel pressure, you've got blow-by past your compression rings, which would also indicate that you've MOST LIKELY got bad oil rings, also.

Put the PCV valve back in place, but not the oil cap. Hold your hand tight over the opening for the oil cap. Lightly rev (1500 RPM or so) the engine and back off a few times. The engine should develop a vacuum inside the crankcase. If not, you've got a gasket leak somewhere or your PCV valve is bad.

Remove the PCV valve from the hose and plug the end of the hose. Drive it on the highway, at highway speeds, for 30-40 miles. If the oil level doesn't change as you say it's currently doing, replace the PCV valve with a new one and try it again.

You might also want to hook a vacuum gauge to the intake. The vacuum should read around 16-18 in Hg at idle, and the needle should be steady. A visibly fluctuating gauge reading would indicate worn valve guides. Don't worry if it varies some with a change in idle speed, as that would be normal. It's when it jumps back and forth on a regular basis that you have problems.

One other thing. How much oil do you put in it when you change the oil? If, for some reason, you have the wrong dipstick or dipstick tube, you may be overfilling the engine with oil. The engine WILL find a way to bring itself back to the proper level.

If you don't see any smoke when you drive it in the daylight, have someone follow you at night and watch your exhaust. Smoke from the exhaust will show up better at night with lights shining on it. You may even be able to see it in the rear view mirror.

The Pinto mentioned above turned out to be bad rings. The F250 had a bad PCV valve.

I hope, for your sake, it's either the PCV valve or the wrong dipstick.

Try these diagnostics and give us a report back.

 

Will do. Thanks!