I have a 1980 F250 with a 351M engine.
Antifreeze was leaking down the front of the engine above the waterpump and I thought I had a blown head gasket but someone pointed out that it could easily be a leaky intake manifold gasket and that could account for the antifreeze in the oilpan as well.

The leak was slight and I needed to use the truck to pick up some shelving units and as I was driving the truck began to overheat. It never quite reached redline and I kept stopping to fill the radiator with water. It turns out that the radiator also sprung a leak which accelerated the fluid loss.
The engine began running very badly by the time I got it home and I was worried it was damaged but when I opened up the hood to take a better look I found gasoline pooled on top of the intake from a leak on the hose at the fuel filter.
I repaired the leak and looked at the carb which had quite a lot of buildup that I assume is a result of backfiring through the carb though I never heard it. Or pooling gas in the carb just flaming up.
I cleaned the carb and checked the plugs which were pretty bad but the engine is still running poorly.
I bought new plugs and while replacing them did a compression check on each cylinder and found that the rear cylinder on the drivers side (sorry, do not know my cyl numbers) has no compression or at least not enough to register on my tester.
All other cylinders are around 120 save for one that is about 114 and one that is up around 125.

The engine starts easily but is obviously missing on at least one cylinder and will not idle.
Could this compresson loss be a result of a bad intake gasket?
Head gasket? Or am I looking at leaks around the rings in that cylinder?
I have not had this truck for long and have been doing slow repairs to it like fixing the cab corners, heater cable, window seals, etc. I am hoping the damage to the engine is just a matter of new gaskets, preferably just an intake manifold gasket but do not know enough about it to know if that is possible.

If I have to do head gaskets I will pull the whole engine as the truck has a 4" lift and working on the engine is difficult. For just the intake I can manage still in the truck.
If the problem is other than a gasket I will have to look for another inexpensive motor to get the truck going again.

Anyone have an opinion on the likely causes of the low compression in that cylinder? The problem only began at the same time the truck began to overheat and the fuel leak popped up though there was a trickle of antifreeze coming from the corner where the intake and head meet on the driver side front.

 

Sounds like you might have gotten it somewhat hotter than the heat gauge indicated.

Intake manifold gaskets do not cause lack of compression. It is possible that you got it hot enough to warp the head a little and blow the head gasket. It also could be a burned valve.

Water in the oil, low compression readings across the board - this engine needs rebuilding. I'm not gonna say you can't just fix it and drive it for 2 or 3 years, because you just might.

The only way you're going to find out is to pull the top end (manifold, heads) upon which you will immediately see whether the problem with the #8 cylinder is a burned valve or the head gasket.

 

The intake manifold is a dry type. No antifreeze in it. I would pull the valve cover first and look for a broken spring etc. You might get lucky and not have to pull the head.

 

Best way to find where your compression went is a leakage test. Set that cylinder up on TDC and air up the cylinder through the plug hole. Watch and listen for air escaping and you will see your problem. Of course this assumes you have a compressor.

 

Wow, three great responses so quickly. I am a happy man.
If I could only get such responses in the programming newsgroups.

First of all, can anyone tell me what the proper compression should be? It was mentioned that it was low across the board but I do not know the correct compression for the engine.

I do have a compressor and I believe the adaptor for my compression gauge will work to let me put pressure into the cylinder so I will give that a try.

I have never seen a burnt valve. I assume the problem will be readily apparent when I pull the valve cover if there is a burnt valve.

I would be very happy if I could just keep this engine running for a while longer. The truck is only used for it's 4x4 abilities when we get heavy snow and to drive occasional loads of junk to the dump while I am working on the house. It will not see heavy or frequent use in the near future and I am hoping to save the money for those damned ongoing ex-wife legal battles.

Thanks.

 

Just a few thing to keep in mind when airing up the cyl. Make sure to remove the radiator cap as I believe you'll see coolant shooting out indicating a blown head gasket. Open the throttle to listen for intake valve. Listen at tailpipe for exhaust valve, and at breather for rings.(not likely) Also should listen at next cyl in case of gasket. ( also not likely)

Make sure you pull the valve out of the compression tester hose or air will not pass.

Make sure you are TDC on the comp stroke and WATCH OUT FOR SPINNING PARTS ! The piston will want to travel if it is not exactly straight up.

I think 120 isn't really that bad for a used motor. Granted it won't outrun everyone at the strip but, for your needs it should last quite a while yet. I would just put a new head gasket on it if that's what it needs.

 

As beartracks pointed out, there is no coolant in the intake manifold. Since you have coolant in the oil, and you've seen coolant on top of the engine, you most likely have a bad head gasket.

The compression problem may or may not be related to the head gasket, but you have to at least fix that.

When a valve gets burnt, it happens at the valve seat/valve head interface, so it's not visible outside the cylinder. You might be able to see it when you pull the head off, depending on how bad the damage is. If the damage isn't severe, you might not be able to see it without removing the valve.

A simple way to test for valve seat damage is to put some liquid in the chamber, with the head upside down and the valves closed, and see if it leaks out into the port.

 

It will show up in a leakage test too.

 

Fix whatever is causing the oil and water to mix. If you dropped a valve you will know when you pull the valve cover off. One of the valve stems will probably sit lower/higher than the others. If you just blew a head gasket. Then check the head for warpage. You can do this with anything that is long and really, really straight or a machine shop can do this. Either way you really need to pull that head off. You don't have to do both heads at the same time. Although you might as well just in case both gaskets are bad or on the verge of failing. If you have a burnt valve. When you look into the chamber you may see that the valve head has a small pit or a large piece missing, ie the head of the valve is no-longer round. If it didn't burn a hole in the seat ( not likely) then you can replace that one valve and have a machine shop dress up the seat so it doesn't happen again. All of this can be done on the cheap.

Gaskets from PAW are less than 50 bucks a set for pretty good ones. Get detroit or fel-pro not the others.
A valve is about 10 bucks or less from any parts place that carries them.
The machine shop will charge you about 20 bucks to check the heads for straightness. Then it will vary on the valve job. Some shops won't do just one valve if you have only one that is burnt. Others will help you knowing that if they help you out now, they might get you back when you want to do the job right next time.

Whatever you do, figure out why you have oil/water mixing. I've seen industrial engines run for twenty years with a burnt valve. They got used a couple of hours every six months and ran just fine with a dead cylinder. If all you can do is fix the oil/water mixing thing. You can live with a burn't valve until you can afford to fix it right.

Good luck.

 

After months of not having time to investigate I finally got around to pulling the motor on this truck and pulled off one of the heads.
The problem is a bent valve. The valve stuck about 1/4 of the way down and bent the push rod.

What are the odds that I can just replace the one valve?

I really am a novice but as long as I have a good notion of what needs to be done I can accomplish it.
I assume the valve seat is the mating surface that valve seats into and that if it is pitted or warped to a point that the valve will not seat smoothly I have problems.
If the valve seat looks good (I cant tell until I can rent, buy or borrow a valve spring compressor) then I will probably be ok???

I hope to be able to get the one valve, a set of push rods and maybe some new oil seals and put this thing back together hoping for the best.

Is it safe to use a hone or fine mesh to clean up the valve seat before reassembly?

 

You can buy valve grinding compound and a suction cup "driver" to spin by hand to seat a single valve. Find out why the valve got bent tho...

 

I assume the valve bent from overheating.
I was out on the road in hot weather hauling some furniture when the radiator sprung a massive leak from one of the tank seams. The temperature gauge never went into red though it was very close to it. Temperature sender might not be really accurate though.
I let the engine cool and put more water in to get it home but it never ran right after that. I found there was no compression in the rear cylinder and now I know why but I can only assume the overheating caused the valve to stick. Does not seem to have harmed the piston. Not sure if that is a problem on these engines or not.

So the valve seats can be replaced as well?
If it does not seem pitted can I get by without doing it? It sounds like it could be a difficult thing to do properly.

I started looking up replacement valves as well but not certain what I need. There seems to be quite a variety of them.
Also not sure if I need valve guides, valve tips, etc...

This is a 1980 F250 with a 351m engine.
I need to get it back together before the snow starts and the plow truck needs to come through the driveway.

 

Is it the intake or exhaust valve? The big one is the intake. If it's the exhaust I could see that happening from overheating. If you got it that hot you might have cooked the rings out of it too. If so when you get it back together you will know in a couple of weeks.

As for valves a machine shop can order the right one for you. A valve spring compressor can be rented through most places, or you can just let the machine shop do it if you have the dough. As for harming the piston with the valve. It's not likely, but possible. You need to look at the piston very carefully. If it's only got a small dent then it's no problem. But if it hammered the top a lot and made a lot of dents then you might have cracked the piston or worse. Either way I would probably pull the pan and yank that rod/piston out. Then find another piston to replace it. If you don't then it's no big deal. But if it cracked the piston and it falls apart then it will hole the block and you will need a whole new motor.
Valve seats can be replaced by a machine shop. As long as the seat isn't pitted a lot or burned blue then you should be able to put a new valve in it and have the seat dressed up, or do it yourself with the above mentioned tools. But you will have to hone the guide a little with emery paper or have the machine shop do it. Once a valve seizes in the guide it mars the surface and will quickly seize the new valve if you don't smooth out the gouges and such. Don't forget to lube the valve stems when you put it back together. I use motor oil, but some swear by assembly lube or whatever else you can find.
As for guides and tips. The tips come with the valve, In the old days there were replaceable tips on some valves, but I doubt you have any of those in there. The guides have to be put in with a special tool. Best left to machine shop. You should be able to buy the standard 2bbl valve for 351C/400 and put it right in. The only differences were multiple groove keepers or single groove. Single is better for high performance, but multiple works just fine in stockers.

As for pushrods. You only need to replace the bent one. Leave the rest alone until you rebuild it or replace it. Any more questions please ask.

Good Luck.

 

OK, picked up a valve spring compressor and removed the valve.
It is the exhaust valve on the rear cylinder. Looks like a bit of metal build up on the side of the shaft and it did leave some marks inside that look like they are all stuck to the surface rather than gouged out. It looks like foreign material got in there and just spread out between the valve and the mating surface (guide?). Or in an overheating situation can material from the valve shaft itself come lose?

It does not look like there is any scoring inside, only a bit of buildup where the friction took place. I do not even think the valve is bent, more just the added material caused it to bind.

Should I be worried about spun bearings somewhere in the engine? Or maybe just some debris managed to get into that one valve? What is the best way to be sure I do not have more serious problems elsewhere? Disect the oil filter or just drop the pan and look for debris?

No local shops had a valve in stock so I have to wait till monday to see if a local machine shop has any or if I have to order. If I have bad bearings somewhere in the engine though it might not be worth even bother with the head as I just do not have the money, tools, time or knowledge to do much beyond general repairs. Especially since the truck also needs a new radiator and the front wheel u joints are bad and I am told they are a real difficult job to replace. I have neither a torch or a dry garage to work in so...

 

If the valve is bent in the slightest it will leak You can pour some motor oil or ATF in the combustion chamber and see if it leaks into the exhaust port. If it has just some seepage you can probably lap the valve into place with the grinding compound. Clean everything up, slap it back together and pray