1978 Ford F150 - Low Compression in One Cylinder Only

Probably a burned exhaust valve.

Unfortunately, you will need to pull the head to replace the valve. The valve seat will have to be reground also.

Hi,I think as this vehicle is not used often it may have a stuck lifter.Throw some additive in the engine oil(SeaFoam?)Let it idle for an hour.Change engine oil. (HOT!!! let it cool,more beers later).Good Luck!

i think you must chek the cilinder #3, first clean or change the sparkplug, verify clear 0.030" or 0.040", verifiy or change the wire, put seafoam in to # 3cilinder to clean, but i think clean the valves, guides, seat and proof

I would guess a burnt valve also. The overheating could be a contributing factor to the problem. Question, was leaded fuel or unleaded originally required? If it could legally operate on leaded, unleaded doesn't help the problem.

Good luck

I knew a mechanic with an old Ford pickup with a similar problem. He drove the engine until the noise and vibration was really annoying. Upon disassembly he found one exhaust valve had recessed into the seat and took up all available travel in the lifter. Then the valve started to burn due to lack of cooling as it was not seating properly.

As I recall, some early Fords of that vintage don't even have hardened seat inserts as others may have mentioned. I would fix it with 2 rebuilt heads. While it is apart you can look at the overall block, cylinder, pistons, rings, timing chain and crank seals - make sure they are in good condition.

If you really want to nail this down do a cylinder leak down test. Air out the exhaust, exhaust valve problem; out the carb, intake. Bubbles in the radiator head gasket and from the dip stick- piston/rings/cylinder wall.

A couple of hard snaps on the throttle against the brakes may also give you some popping back through the carb (intake valves) or go to the tail pipe and hold your hand against it. If you feel a vacuum (at idle) it's an exhaust valve! A sheet of note book paper might be a little more sensitive.

A high HC reading is indicates raw fuel. A dead or weak cylinder would also show an increase of CO ( incomplete combustion), CO2 would fall due to reduced combustion efficiency and O2 would also rise.

I fail to see the relation ship between ping and a weak cylinder. A weak cylinder is not developing sufficient combustion pressure to pre-ignite anything. I can see the timing being over advanced to try and compensate for the weak cylinder and causing ping in the remaining decent cylinders.

I have known many mechanics in my life but none that could fit in a can. Chemicals do have a place as a maintenance item but you will only waste 6-7 bucks dumping this in the tank or any place else (my personal opinion).

Pour you a little tranny fliud down the crab while it is running have someone rev it up till the smoke clears.

If there is a dirty valuve seat that should clean it and the plug too. You will only needs an oz or 2

I'll let you know how that goes tomorrow. I had a vague memory of something on that nature from my Dad. he's gone now, but he is the one who helped me put this engine in many years ago. A truck's value is not always obvious at first glance. Thanks, CJM

You need to pull the head and find out what is going on if you intend on keeping this truck. While your working on it , you should pull the other one too and get them tested and fixed up at the machine shop. If the lower end is okay, you can enjoy many more years of happy motoring.

When the compression on one cylinder is low, my first impulse is to pull the spark plug, grab an oil can and squirt about two tablespoons of engine oil into the cylinder. If the compression jumps dramatically, say 20 pounds or more improvement, then odds are that you have a broken compression ring.

Viscosity index modifiers like Motor Honey and that goo Andy Granatelli used to sell, have temperature sensitive polymers that can help minimize compression losses from wear. They aren't a cure-all however; far from it.

Foaming of the coolant in the expansion tank might be an indicator too but my experience has been that a blown head gasket will create a dramatic geyser at the radiator cap, if a leaky gasket is where the loss of compression is. That is assuming that the blown gasket at that cylinder is across a water jacket.

If the engine has been badly overheated at some point in its life, than warpage at the gasket joint inter-plane will likely require milling the heads, the block or both with a resulting increase in compression and octane requirements.

A stuck or "hung" lifter could cause the valve to be hung open. After all this time, of that's what happened, that valve is burned, especially if it's the exhaust valve that is stuck open even slightly.

It makes no sense to just do a valve job on one cylinder. If you are going to pull the head for one valve, pull both, reface or replace all the valves and regrind the seats.

Someone made an inference to leaded or unleaded fuel as a possible factor. That was a good point well made.

When the EPA mandated unleaded fuels to protect the catalytic converters, Chrysler Corporation started having terrible problems with valve recession. The deposits of leaded salt from the tetra-ethyl compounds lubricated the valve seats. The EPA gave Chrysler permission to fill the tanks on new cars at the factory with leaded fuel.

Tests have shown that an occasional dose of leaded fuel does NOT pose a threat to the catalytic converters nor does it damage the oxygen sensors. The operative word is "Occasional". My cars, my bikes, my boats, my lawnmowers, etc, all get an occasional diet of leaded fuel purchased at the local airport.

Restoring the compression on your engine with a valve job may prove counter productive. You may find that the improved manifold vacuum which results will prove more than the worn oil control rings can handle. With each new stroke,the engine will be sucking oil from the sump, especially when coasting with a closed throttle.

I suggest you give serious consideration to getting rid of the machine and investing in something more appropriate. Too bad the Cash for Clunkers bonanza is over. Judging from your comments, that machine would have been a good candidate.

L.J.

Restoring the compression on your engine with a valve job may prove counter productive. You may find that the improved manifold vacuum which results will prove more than the worn oil control rings can handle. With each new stroke,the engine will be sucking oil from the sump, especially when coasting with a closed throttle.

Well then replace those rings while the heads are off what do ya say? One could cheap out and try a ridge reamer and pull the pistons or just do it right; no way buying new a truck engine is more cost effective if doing the work righteous yourself.

Fords start having problems around 89,000 miles valve job and timing gears seems like they always will.

Ping of a CA 351M 1978 shouldn't raise eyebrows.

Possible the distributer is not correctly oriented and that gas has washed the cylinder walls lowering PSI.

Good cyl PSI = 150-170

Greater probability there is a crack in the head or valve seat, pull the heads, complete valve job; mill the heads as required and possibly the intake manifold too.

I found it better to do the heads by hand only unless your doing a complete rebuild. Too much pressure from a milled head can cause overheating and blow by around old piston rings. I still have a '71 ford I have done the heads 3 times including once I hand straightening bent valves stems with a hammer and cleaning up the stems with emmory colth. Still runs on those valves today. Drove it recently to Casper to hunt over 1400 miles at 80+ mph and back.

The 351M's out of old Lincoln Continals are great to rebuild and replace truck engines. Seldom have they been abused like truck engines are.

Well ya! But if heavy milling is necessary shimming is required to head gasket or intake manifold leaks often occur.

Never has the overheating scenario occurred on heads I have milled, there is a point beyond which no salvage can be assured, just don't push that envelope.

I had a couple over heat. I had to save buck raising the kids. My Grandfather told me that was the problem and taught me to use a bastard file and a mill file then a peice of 3 in stock I spent hours grinded flat to level heads on.

For some reason I like to keep old trucks around and make them last forever if possible. I have a 1959 Chevy C-60 single axel dump truck with a 427 big block. My wife threatens to sell them all when she gets mad, espically after a big greasey kiss or a hand print on her rump.

I have a '76 300D Mercedes that has never had the heads of it over 900,000 miles lol 3 water pump and 4 injector pumps and 9 alternators.

I don't know what would have become of generations of my family if it had not been repair work on trucks. None of them could step into my grandfathers shoes as the local welder, animal doctor, tractor mechenic, and the guy who solved most of that farming communities problems. He was one of a kind with only a 6th grade education, old people still talk about things he repaired for them today. Some old steam engines are still running he repaired. I can only hope to stand close to his shadow. Sorry this made me think of him.

Don't know the engine, but have you checked the valve lash? Without knowing spec, I'd say you should have at least 3 mils on an intake valve and at least 5 on an exhaust valve. A tight valve (no clearance at TDC) is a dead giveaway that it's hanging open. I've also found that rotating valves in place can affect leakage. Shouldn't be hard to check or adjust, this is routine maintenance on most engines unless the lifters are hydraulic. $40 for a harbor freight leakdown tester would tell you if pulling the head will do you any good or not. You'll need a compressor to do a leakdown test.

After you pull the valve cover if you have a broken spring or weak spring you will notice this. Fortunately these can be changed with out pulling the head. If nothing is noticed hook up your air hose to the cylinder and listen wear the air is going. If you need to change a spring either use compressed air to hold the valves closed or put some small rope in the plug hole and rotate by hand until the valves are closed.

I would suspect a valve job and maybe a short block is needed.

The standard reason for one cylinder this low is a burned exhaust valve. You can check that the valve is on its seat by first removing the valve cover and then positioning the engine at top dead center, compression stroke, on that cylinder. Back off the rocker arm adjuster to the point that the pushrod spins easily with finger pressure. If the engine is at precisely top dead center, it will not spin when you pressurize the cylinder -- but keep hands clear nevertheless. Pressurize the cylinder (with air from an air compressor) and see if it holds pressure. I'm going to guess that it will not. Listen at the exhaust pipe. Chances are good that you will hear air escaping there.

Double check that the valve is on its seat, by ensuring the the pushrod turns easily -- or give the valve a little play if you want. Again, realize that the engine could turn, so you will want to cut off the air supply while your fingers are in places where they could get squashed. If the valve is on it seat and still leaks, then you need to do a valve job, and it is worth doing at least the entire head -- and both heads if you have the time and money. If you are doing the work yourself, and trying to save money in the short term, you could replace just the burned valve, grinding just that seat to match (and to restore the correct seat width). If the seat is badly recessed, then you are may need to replace the head.

If the valve no longer leaks after ensuring that it is on its seat, then the lifter is (was) probably stuck. You should pull that lifter out, and disassemble it, thoroughly clean it and reassemble it, filling it with oil before installing the top piston. If that does not restore its operation, then a new lifter is in order, and if you pre-lube it with moly grease, it will probably seat into the cam fairly well. (New lifters will occasionally damage an already worn cam lobe. Unfortunately, you cannot replace just the innards of a lifter, to retain the match between lifter and cam.)

Like DadW/5, this goes back to my father, and no one will ever fill his shoes. Your advice is exactly on track for what will happen if an exhaust valve is identified. Thanks. CJM

Hello sounds like you did a great job checking miss belive a valve is the promblem might trymisting a little water down carb raise rpm to about 2000 and gently pour about 4 ouncies to check if a piece of carbon on valve , if compression stays low probaly needs valve repaired hope it helps.

Thanks to all, I very much appreciate the factual brainstorming as well as the opinions.

An update from today's efforts:

Added 'Seafoam' to engine oil along with a full pint of detergent and idled for +8 hours. Oil pressure dropped but still at ok level. After a few hours I noted much quieter engine sounds in general but no noticeable improvement in #3 cylinder.

I noticed the engine surge mildly from time to time which lead me to go through the vacuum set-up and check all the hoses. I used a long hose-to-the-ear and determined the need to replace more vacuum lines. Tomorrow.

I then used a vacuum line that enters the intake manifold near the #3 cylinder to introduce about 1/4 can of 'Seafoam' in several small amounts. The first few times, there was noticeable smoke out the exhaust for a while and much less smoke with the last few injections.

This lead to the discovery that the exhaust manifold gasket leaks at the #3 cylinder. I thought I might have dripped some at first, but was careful the next times and confirmed that some smoke was escaping into the engine compartment at the top edge of the exhaust manifold at #3. I'm aware this could very well mean a burned or warped valve if fresh air were able to get back-fed to the exhaust valve.

I used my engine stethoscope to listen to the heads at each cylinder, and there is a distinct tappet sound at this cylinder. Also I confirmed no response when removing the #3 wire at the distributor.

Tomorrow I will use my compressor to see if i can determine where the air escapes the cylinder. There is no sign of cooling system involvement, and I am expecting to find a problem with the exhaust valve,so I will be careful not to prejudice my procedures. This will be the first time attempting an air 'bleed-down test, but I think I can figure it out - cylinder TDC, air through plug hole (compression test hose has air-hose style connector), pressure gauge on hose, adjustable setting.

I don't think there will be too much trouble revealing what i need to know. I was hoping to fix this cheap & easy, but we'll know tomorrow.

My kids call this truck the 'gray ghost' since each time it seems done for and I get her back in service. It's actually silver & rust, but some of you will understand - it was my first truck. Several years ago, just after a new long block, this head sucked valves and got a valve job, out of town on a trip. Anything is possible, but the cause may be known manana.

Again, thanks to all for your help.

The verdict is in.

I removed #3 plug, rotated to TDC, applied air - which went out the exhaust pipe as quickly as I could put it in.

The repairs required are clear now, but I would love to have clear evidence of the reason this one valve failed. The rest of the engine seems fine, and even has enough power to break the wheels loose if I'm not easy on the gas. Go figure.

Thank you all for helping me with this. The air test was just too simple and definitive.

but I would love to have clear evidence of the reason this one valve failed.

Quantum theory.

Almost.

Actually, hydraulic lifters are a confounding issue, but with solid lifters (where you set a particular valve clearance) the process can go like this: some valve seat wear takes up the clearance. That cylinder may be a little leaner than the rest, and running a little hotter, helping to take up clearance. The valve then has lower pressure on its seat, so it can't conduct heat away as well. It gets hotter and therefore longer and loses even more seat pressure, and quickly it is off the seat all the time, and hot combustion gasses are blowing past it. This causes erosion and loss of compression. Eventually the cylinder can't develop enough compression to run, in any meaningful sense, and the condition stabilizes. Occasionally, the condition develops quickly enough that the whole valve head starts to glow red hot and falls off.

Here is a possible answer... A small defect in the one exhaust valve or seat can ultimately lead to leakage of hot gases which gets worse with time. Just after TDC the burning gases in the center of the combustion chamber are at maximum temperature (2000 deg. C or 3600 F) and under reasonably high pressure - they will flow quickly and will progressively "blow torch" a good size hole in the exhaust valve if the leak rate is high enough. The valve material goes soft above 1200 C or 2100 F. Only material in good contact with the valve seat and cooling jacket is spared. Usually, you will see a 1/4" hole (or bigger) on the valve. The hot valve can also cause pre-ignition problems too on that one cylinder. But you still have 7 good cylinders for adequate torque to burn rubber.

The root cause of the leak may be due to material failure (cracks in valve/ seat, flaking of stellite facing or tiny impurities which dislodge). I have seen many material-related failures with exhaust valves on a production 5.6 liter I6 turbocharged engine used for medium-duty vehicles in the 1990's.

Alternatively, you could be near end of the valve life on all valves in the engine due to normal wear and this is the first cylinder to go bad.

CJ, you could have answerer your own question. That leak at the exhaust gasket can do what you feared. You'll be replacing that gasket when you do a valve job on that side. Don't forget to replace all the vacuum lines, they gotta be old and spongy, or brittle.

Something I always recommend is a dash mounted vacuum gauge. Once you get used to seeing it, broken lines, bad valves, and all kinds of problems are easily spotted. They come with a diagnostic chart showing what each change means. On some cars they were called 'economy gauges', and would show you when you were using the most gas. I keep a hand held unit in my toolbox, with the timing lite and circuit tester. The hand held unit will also test fuel pressure, handy when sorting out fuel problems.

I looked at one two weeks earlier & decided not to spend the $25. The bad device is alone on this particular vacuum port, but I now have placed a renewed value on a vacuum gauge and will be adding that too. CJM

the second you learn it "all" they add something else. In the 1850's a cowboy could carry all the information he ever needed in two 8x10 saddlebags from coast to coast, they say. we used to just put our hand over the exhaust pipe and if the valve was bad it would suck up w/ a vacuum. there are additives to put in the tank to pass the emissions test. i wonder if the hed is cracked or there is a leak in the intake manifold. good luck

I am considering leaving the intake valve rocker and push-rod out until I have time to get to the valve job. I believe I will have to remove that lifter also, but will be doing more investigation first.

I like the idea! However, the missing lifter may cause an oil pressure problem. If the pushrod is not there, the lifter will probably cause some sort of mischief if left in. Even if it gets pushed up beyond its ordinary travel (via inertia) and then just sits there, the feed hole from the block to the lifter would likely be exposed.

Probably safer just to minimize the mileage on the truck until you can do the valve job.

Yes Blink,

I am on that same wavelength and have been considering those things. I will approach my idea with great caution. So far I have removed the rocker and push-rod. I attempted to pull the lifter for inspection at least, but since it would not come out with a good magnetic pick-up tool, I think the oil varnish has built up below its travel limit. I think That varnish could be depended on to hold the lifter enough to block the oil pressure feed if I can manage to get it up just past the normal travel of the lifter.

I consider this just a temporary fix, mainly to meet the smog test requirement in the short term, and to be able to clear my driveway. It was due for the smog test last January. Life keeps me distracted from my list of tasks, and it is a substantial list.

Thanks for the words of caution, CJM

As a precaution carry a fire extinguisher...

Thanks a lot Bwire, I've had some trepidation as it is. Today I try it.

Could it really catch fire?

A quick update:

The engine runs just fine with one intake lifter, push-rod and rocker removed. Essentially the same as before.

I found that the vacuum tree at the rear of the engine also was not tight and was also a source of vacuum leakage.

I have replaced the bad vacuum operated choke pull-off and a leaking oil pressure sending unit, cleared up a lot of vacuum hose clutter that was blocking air flow over the engine.

My last issue will be to risk $50 for a new smog test now or go ahead and repair the bad valve only, or have one or both heads re-done at a shop. I'll keep you posted & thanks for the help you have all been. CJM

One new valve, 8 hardened seat inserts, one valve push rod straightened with a hammer, and a success at the smog test place. Just thought it might interest some. CJM

Excellent! Thanks for reporting on the results. Very cool to see your truck's life extended for not too much money.

Send us a picture or two. I am always happy to see what the "bad parts" looked like that caused the original problem.

It would take a trained eye to detect the bad valve. Even with the head removed the only indication was; that valve was slightly deeper seated.

Also I mentioned the bent push rod. It was on a different cylinder, and bent shortly after re-assembly of the head. It seems that in the discussion on additives I have some input from experience to share.

The oil along with engine heat has coated everything with a 'varnish' which began to dissolve apparently due to the additives. This sounds like a great idea, however I would caution that it requires several and consistent treatments followed by removing the oil while still hot.

I hit it pretty harshly with plenty of additives and ended up quitting late in the evening and letting it sit overnight. The results I have discovered so far are, 1. the entire oil mass was a very thick sticky glue-like mass that took some heating up to get fluid again the next day, and 2. some of the 'varnish' came loose from the surfaces as small flakes. This is what bent the push-rod, since the flake that mattered was inside one of my lifters and blocked the pressure release opening, turning it into a 'solid' and not a 'hydraulic' lifter just long enough to bend a push-rod.

I was able to remove and dis-assemble, clean and re-assemble that lifter with excellent results. I will also be changing the oil at much greater intervals than usual for a while.

Laughing Jaguar - I will get to the other head eventually, hopefully soon.

BWire - Almost no cylinder ridge to the feel yet.

A wink to Blink on the lifter cleaning idea, thanks, and a nod to

Dad W/Five Boys about the 'straighten it with a hammer' idea.

Again thanks to all contributors for a collective success. CJM

I took to using straight 10-30 synthetic an I could kick myself for not doing it sooner. I changed filter @ 2500 miles and added a quart at 4000 I'll drop at 5000 hot

Have one cylinder that went from 80 on dry to 125 on wet. What does that tell you?