Need some opinions. I have a nice, not show car, but above average 1971 f100 short box 2x4. Engine hasn't run well for the year that I've owned it and when compression was checked yesterday I have between 105 psi and 120 psi in 7 out of 8 cylinders and 30 psi in one. I'm not familiar with engines as I've only worked on the rest of the truck. How would you guys proceed with this truck if it is something I would think about selling in the next 3 years. ie. I don't plan on having forever as it's a fun truck but not a keeper for me. Would you have someone rebuild the engine? Install a crate motor? Dive into the engine myself and see what I see (this one scares me as I'm not qualified). Sell the truck as is and let someone else mess with it.

I'd love your guys advice as you've been there and done that and I'm still new to this. Thanks in advance.

 

Another option would be to go to a salvage yard and find a used long block with low miles but there is some risk to this as well. i could end up with two bad motors.

 

A lot of pick and pull places will offer some sort of guarantee where if the part you pull is bad they'll allow you to swap it out for another. Only thing that sucks is usually you're going to be pulling the 2nd motor which can be a pain depending on what tools you have available and how confident you are


Does the truck get driven a lot? Low compression can be from a lot of things. Could get lucky and just have a bad head gasket which would be a cheap and relatively easy fix. Personally I'd want to do a little more diagnosis before condemning the engine

 

gettysburg150, thanks for the reply. truck doesn't get drive hardly at all. Maybe 5-10 times a year for less then 20 miles. Not an expensive truck but rather a fun toy for my son and I to work on. Problem is when it comes to engine, I'm lost. What sort of further testing can I do to diagnose a bit more? It does smoke from one exhaust pipe. Once it gets warm you'll be in 3rd gear and running along then it cuts out pretty hard and sputters. no back fire though. Starts every time and runs well for a shot time before it starts to cut out.

 

I'm guessing the smoke is coming from the bank (side) that has lower compression

Cylinder leak down test is a pretty good tool

Another trick is you can put a couple drops in the suspect cylinder and do another compression test, if the compression comes up after that you may have dry rings on the piston or maybe they're stuck due to carbon. A top engine clean followed by an oil change could free the rings up.

Are you losing any coolant? Any oil leaks in the head gasket area? Both could be signs of head gasket issues

 

Thanks again for the advice. I haven't seen any leaks other then a quarter size of oil from the back side of the motor after sitting for a few months.

When you say put a couple drops in the suspect cylinder do you mean of oil? Then do another compression test? If the rings are dry or stuck what is the cure for this? I'll have to google to see what you mean by a top engine clean as I'm not familiar with that.

 

found out what you mean by top end cleaner. Thanks. May try this. This is exactly what I'm looking for, some items I can try at home before I go to the next and drastic step of saying engine needs pulled and redone or replaced. garage I went to just said low compression, it's done. Very frustrating.

 

Yes sorry, that's what I meant. If the rings are sticking that should bring the compression up, its not so much a fix as a test.

Yeah check out google for the top engine clean, I'm sure there is some write ups out there. We have a proprietary chemical that we pour in the cylinders and leave sit over night at work. It breaks up the carbon so the rings seat better. But you have to do an oil change afterwards because all that gunk can get down into the crankcase (oil pan)

 

Thanks. You've given me my weekend schedule. I'll report back after the weekend. Thanks again for the advice.

 

No problem, hope you're able to figure something out

 

You can also pull the valve cover and crank the engine over to make sure all the valves are moving. I'd start with the side that is smoking.

 

I've used "seafoam" as a top engine cleaner. Available at most parts stores. Someone on here a while back mentioned that GM has a Real Good top engine cleaner. Named just that. Whomever it was swore by it.

 

How many miles on the motor?

 

If you are running it 5-10 times a year for less than 20 miles each time, the gas is going rotten in the tank.