I have a 78 f150, 2wd with an inline 300, 3spd +OD trans. in it. My son and I have been working on it since October. It's going to be his first truck when he gets his license in a couple of months. Truck runs great, no smoke, compression check reveals cylinders 1-5 are at 130 to 135 but cylinder 6 is at 120. The engine seems to be under excessive pressure as the pcv valve is not able to keep up with removal of pressure from the valve cover. The breather on the back of the motor (chrome breather with a foam filter in it) is blowing oil-soaked are out of it. The truck seems to have plenty of power driving it around but when challenged with a steep, long hill the motor simply has no power. Up any kind of a hill turns into a 2nd gear, 30 mile per hour climb. My question is this, is the motor shot, even though it runs great? What is causing the excessive pressure build up in the valve cover? I have read that the EGR may have something to do with it. Any advice here would be welcomed. Thanks.

 

Sounds to me like your worn rings are causing excessive blow by.

 

Your compression is within acceptable tolerances: Maximum 15% difference between adjoining cylinders).

These rigs are heavy. I think you're expectations are a tad too high for a lil 300... plus the OD trans is working hard.

What rear gear do you have and what is the final drive ratio?.. that's the rear end gear ratio times 4th gear. For example, a 4.11 rear gear times a 0.80 overdrive gear equals a 3.29 final drive.

 

EGR valve is unrelated; not sure where you would have read that. The EGR valve doesn't need to be part of this discussion.

Those compression numbers seem pretty decent and even though the last cylinder is lower, it's not too far past 10% of your highest reading. I don't see how compression in that range could lead to you throwing that much oil. Something else is going on, like the PCV system routed incorrectly. How do you have it set up?

 

Thanks for the replies. I don't know the rear axel ratio -- I will look it up. I think there is a tag on the rear end, correct? The PCV valve is new so that shouldn't be the problem, it is routed to a fitting on the manifold via a T-connection which also is attached to the power brake assist. Tire size is 31.5 inch BF Goodrich --- not too big, I think its the stock size (265, 75, R15). Any other ideas on why there is so much pressure in the valve cover? Anyone know what the stock rear end is/was on these trucks with this engine/trans set up?

 

120 to me is on the lower end. Acceptable yes, bit on the lower side though.

As stated above check your lines for clogs. Also check and make sure you "new" pcv rattles freely. New parts these days aren't what they used to be.

 

Agreed, make sure vacuum can actually get to the PCV valve. It sounds like you have it set up correctly so long as the breather can actually pull in fresh air when it's not puking the other direction.

Different rear-ends were used. Posting a picture of the axle will get you a pretty quick answer; the axle may have an ID tag as well and finally there should be a letter on your door sticker.

 

Vacuum is good -- actually pulling pretty hard -- and the lines are clear. Still blowing out of the breather which should be pulling air in, correct? Regarding the gear ratio, the door tag says AXLE is 14J. Is that indicative of the gear ratio?

 

Looks like the code 14 in a 1978 f150 is a rear axle ratio of 3.00. Ok, I am a rookie -- what does that mean in my scenario? Thanks again for the help.

 

And what is the ratio of the overdrive tranny gear?

 

transmission is labeled as a B on the door panel. I cannot find what that specific designation means. I know it is a three speed, manual with an overdrive gear.