67 Mustang - Low compression in one cylinder, what does it mean - Vintage Mustang Forums
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post #1 of 25 (permalink) Old 08-14-2019, 06:42 AM Thread Starter
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67 Mustang - Low compression in one cylinder, what does it mean

I have a 67 Mustang with an inline 6 engine. This is my first rehab project. I have been doing projects and then driving the car. This summer, the car has lost power and I started getting blowback through the breather cap. I end up getting oil spilling onto my engine which creates smoke.

I did a compression test 3 months ago and all was OK. I did a new compression test and 5 cylinders were at 150, the 6th cylinder is at 50. I assume this is my problem

The engine has been idling really hard. I adjusted the idle screw, but can't get it to smooth out
I have less power, especially in first gear
There is black smoke that comes from the tail pipe
I removed my spark plugs, which were new a few weeks ago and many of them are charred and covered with oil
I recently rebuilt the single barrel carburetor on the car, but that seems to be working fine.

Again, I this is my first rehab project. I have not done much work inside the engine.
I am assuming my issue is around cylinder 3 which is at 50 compression

When I open up the valve cover and turn the engine, all the springs and rods look to be working properly.

Is this a head issue, rings, seals, ??

Any help would be appreciated, thanks
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post #2 of 25 (permalink) Old 08-14-2019, 07:21 AM
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Compression is about valves and rings. Sudden onset means something broke. Could be a valve spring which would make the valve not close as it should. Less likely is a ring breaking. Black smoke is typically running rich, blue smoke is burning oil, white smoke is leaking coolant. Blow back through the breather is pointing at a valve issue. Oily residue on your spark plugs points to valve stem seal leaks. It sounds like the head really needs to be gone through.
1. How many miles on motor?
2. Check the springs on the valves for the cylinder in question.
3. Valve springs can be replaced with the head on if you can get the valve to fully seat. Otherwise, the head is coming off.
4. To fix your oily plugs you should replace the valve stem seals.
5. Might be the opportunity to just get the head done and make sure it all seals as it should.

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post #3 of 25 (permalink) Old 08-14-2019, 08:18 AM
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Repeat your compression test on the subject cylinder then, yet again, after adding 2oz of fresh engine oil through the spark plug hole. If the reading INCREASES, you have a ring sealing issue. If not, you have a valve sealing issue. Typically, the small six with one cylinder down will have a burnt or stuck valve.
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post #4 of 25 (permalink) Old 08-14-2019, 09:00 AM Thread Starter
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To answer you questions above
1. The odometer reads at 68,000 miles. I don't know if that is accurate. It could be 165, who knows
2. The springs on the questionable valve look OK. When I crank the engine the rods are moving up and down as they should be

I did the compression test when the car was cold. Do I need to put everything to together, let the car run and then do the compression test again ?

I will do the test with fresh oil through the spark plug hole and see what happens. Thanks for that

My bigger question is how hard is it to remove the head and replace the ring or valve seal ? Is that something I can do ? Do I need to pull the engine to do that. There seems to be a lot of room to do it with the engine in. I don't think this car is worth a lot of money. I bought it to learn about working on cars. This would be a great additional project. My concern is that I don't want to put a ton more money into this project

let me know what you think
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post #5 of 25 (permalink) Old 08-14-2019, 09:46 AM
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Yes the head can be removed from the engine while in the car.

Ring replacement means engine out and complete teardown and the using the services of a machine shop.

My concern is that you are at the very beginning of your old car ownership and you don't want to put a ton of money into it.

This hobby can be a money pit, tell us the dollar amount that weighs a ton to you so we can see if your expectations are reasonable or you should be pulling the rip cord.
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post #6 of 25 (permalink) Old 08-14-2019, 10:05 AM Thread Starter
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I bought the car 2 years ago for $3500. Pretty cheap. Attached is a pic. It's in pretty good shape

I have done the following
- added power disc breaks to the front
- rebuilt the front suspension with new parts
- Welded new floor pans into the interior, new carpet, headliner, dash and instrument panel - Inside is in great shape
- New water pump, clean/painted and re-installed the radiator and hoses
- Overall the car looks great.

I am probably in for 8k or so including tools. Not so bad. I don't think the car is worth much more than 8-9k if it were running.
I don't think putting another 2+k into the car makes sense.

It is a hoppy, my 14 year old son and I are working on the car together. We are learning. My hope was to get the car to this point and sell it. I want to buy a nicer car where it's worth the time and money to put into it.

If I need to replace the head, how much would that cost for me to do it
A complete engine teardown seems like a massive job - is it worth it ?
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post #7 of 25 (permalink) Old 08-14-2019, 10:25 AM
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Here's the thing with the small six.... Once you pop the head, you're committed to, at a minimum, replacing the head gasket. The original steel shim head gaskets were about .025" compressed thickness. The new replacement head gaskets are around .050". This means that when you have the head reinstalled that not only is there a larger combustion area volume, due to the added thickness of the head gasket (about 4cc more) which will reduce static compression ratio by 0.4:1. Also, the distance between the camshaft and rocker arm will also increase by .025" potentially affecting valve pre-load.

Normally, when replacing the head gasket, the machinist will resurface the cylinder head, removing the difference between .025" and the new gasket compressed thickness to compensate.

At a minimum, if you don't have a ring sealing issue and don't have to touch the "bottom end", you should anticipate a head resurfacing and inspection of valve guides, valves and valve seats, rocker arms and shafts and replacement of any of these components no longer in spec or serviceable. When reinstalling the head I'd also consider replacing the thermostat.


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post #8 of 25 (permalink) Old 08-14-2019, 11:23 AM
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Yes, definitely follow Woodchuck's diagonstic instructions and:

- If it's a VALVE issue, by all means, pull the cylinder head and have it completely refurbished. Compared to other classic car work, like paint and body, this is a very simple and inexpensive endeavor. If your I-6 received regular oil changes, it will have a lot of life left after you bolt on a fresh cylinder head. Don't skimp. Talk to your machinist, pony up and do everything he recommends. If it needs 12 valves, install them. Have it resurfaced. Install new springs. No point it doing it at all if you don't do it right.

- If its a RING issue, then you're in for a complete engine rebuild. Believe it or not, this is STILL pretty simple and inexpensive compared to other classic car endeavors; like paint and body work. Pulling, rebuilding and reinstalling an I-6 is easy-peasy! If you like twisting wrenches, it's also a lot of fun. Here again, talk to your machinist and do what he recommends. If need be, machine the crank, bore the cylinders and install new pistons. Install a new cam that's a little lumpier. If you do it right, you will get another 150,000 miles or more from your I-6.

Keep asking questions and have fun!

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post #9 of 25 (permalink) Old 08-14-2019, 11:57 AM
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A ring could be stuck. #6 cylinder is where the worst of the carbon and sludge tend to accumulate. I had this issue on a 66 that I bought with a 200 in it. Smoked like crazy and fouled the plugs quickly. I filled the cylinders with diesel and let them soak for 2 or 3 days, cranked the engine until all of the diesel was washed out, changed the oil and it ran pretty darn good. It still used some oil but smoked a lot less. It was a temporary solution until I could rebuild the engine anyway. These engine are super simple and cheap to rebuild. I rented a cherry picker one saturday morning, pulled the engine in my driveway, disassembled it in my garage, loaded the block and head and headed to the car wash armed with several cans of engine degreaser, blew everything out with an air hose, installed new rings, rod and main bearings, valve stem seals, all new gaskets and a timing chain. Painted the engine and stabbed it back in the car that evening.. I had it running that day and it ran great for the next 50K miles until I sold it. I think I had less than $300.00 in the "rebuild". Admittedly, this was a $900.00 daily driver that was beat to hell. My point is that these engines are robust and easy to work on. I would never pay to have one rebuilt.
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post #10 of 25 (permalink) Old 08-14-2019, 03:33 PM
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If you need to remove the head, that's pretty easy and straightforward. You just have to remove everything at the head level and above. It's a long list, but pretty basic once you look at it. You just need to remove:
Antifreeze, Carb, carb linkage, heater hoses, valve cover, spark plugs, vacuum lines, upper radiator hose, exhaust pipe, oil pressure/temp sensor wiring, rocker arm assembly, pushrods, and one bolt holding the power steering pump to the thermostat housing.

You can leave the exhaust manifold attached to the head or take it off. It's ok either way, it just makes it heavier.

I personally like to stand in the engine bay with my feet on the frame rails ahead of the shock towers to lift it up, but there's probably a smarter way to do that. Just be sure you've got the fenders adequately padded beforehand so you've got a place to set it down.

I don't think you can replace valve guides on your own. You could do the umbrella seals, but the guides need to be pressed in. I had my head rebuilt here locally last year and it cost me $550 for resurfacing, pressure testing, 12 new valve guides, 6 new exhaust valves, and modern positive seals instead of those old brittle umbrellas. Not sure how those prices compare for anywhere else, but I had some serious oil leakage going on so it needed it.

If you need to replace the rings, you can do that in the car too. You'll need to take off:
The head, sway bar, (possibly the automatic transmission cooler lines), oil, and the oil pan. You can then access the bolts for the piston in question and unbolt it, remove the bearings and push the piston out the top.

Though really if you're that far in you might as well replace all the rings while you're there. They come in a box with all of them so why not use them?

Nothing really hard about any of it. Just a lot of individual parts to take off and put back on.

Last edited by Magnus; 08-14-2019 at 03:35 PM.
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post #11 of 25 (permalink) Old 08-14-2019, 08:50 PM
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Remove all 6 spark plugs. Rotate the crankshaft so that the offending piston is at TDC (Top Dead Center) on the compression stroke with both valves closed. Using an adapter screwed into the spark plug hole apply compressed air into the cylinder. Make sure that the piston stays at TDC. The compressed air will try to push it down. Now go listen at the tailpipe. If you have a burned exhaust valve you will hear the air escaping past the burned valve into the exhaust system. Listen to the top of the carburetor. If you have a stuck intake valve you will hear the air rushing out. Intake valves stick. Exhaust valves burn. Place your ear near the oil fill tube. If you hear lots of air escaping you have worn/stuck/broken rings.
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post #12 of 25 (permalink) Old 08-15-2019, 08:20 AM Thread Starter
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Thank you everyone for your replies. Here is where I stand. I decided to take the head off the engine with some help. We removed the carburetor and pulled the head with the manifold connected to it. See the pictures attached pictures with everything removed.

This all started with low compression in cylinder #3 and blowback of oil through my breather cap

Upon removing the head, the gasket is in OK shape. Nothing super noticeable.
Two call outs
1. I have a bolt that broke off between the 3rd and 4th cylinder. I have to remove that
2. There is a hole in the manifold which has created a rusted out spot between the 3rd and 4th cylinder

From looking at the cylinders, there are no obvious scratches or damage.

My question, should I go the next step and remove the entire engine and rebuild everything, take out the pistons, etc or should I clean up the head, replace that gasket and put it all back together from here. I am going to remove the manifold since it has a hole in in

Thoughts ?
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post #13 of 25 (permalink) Old 08-15-2019, 08:23 AM
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Bore and Sleeve Only One Cylinder???

This brings up a question I have been thinking about lately. My situation is a little different than OP but similar. I am getting ready to do a leak-down test on the 6 cyl engine in my 1953 IH pickup. Head was rebuilt and I'm still getting a lot of blow-by through exhaust and valve cover breather. When I did compression testing #6 was about 15% less than the other cylinders and I have what I suspect to be piston slap in that cylinder. I'm expecting #6 not to show well on the leak-down.

So to my similar question. Keep in mind a couple things. My parts are hard to find and can get very expensive. My truck is only going to get a few hundred miles per year at most and just around town. If you have a motor with one cylinder having a bore clearance issue but the rest are good, can you bore and sleeve just that one cylinder and reuse the same piston without any issues? If not, why? I can understand why you may not want to oversize one bore compared to the rest but if you are putting the same piston back in what is the problem?

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post #14 of 25 (permalink) Old 08-15-2019, 08:35 AM
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Enlarging the photos it almost appears that #3 was leaking, through the head gasket, into #4 cylinder. Hard to tell without seeing it, physically. Also, is that evidence of a foreign object in the center of #3 piston? Was a #3 spark plug ever removed missing part of the ground electrode? I'd pull the valves from #3 and look at the contact evidence between valves and their seats to see if something may have been stuck or the head of the valve is bent.


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post #15 of 25 (permalink) Old 08-15-2019, 08:47 AM Thread Starter
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Actually if you take a closer look at that picture, It does not appear to be leaking from the cylinder. The hole in the manifold seem to have burned or rusted out a spot from the outside of the head. That is where the bolt rusted and broke. The rust spot does not bleed into the cylinders.

I did not see a foreign object inside cylinder 3. I am not sure what you are seeing.

Again, I am trying to get guidance on pulling the entire engine and rebuilding from the ground up. That doubles the job for sure
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