Interested in your thoughts on one low compression cylinder

[Carmine]

I suppose I haven't posted a whole bunch, but if you read between the lines you may have noticed I am reviving a 78 NYB salon after what I'm sure have been many long slumbers. This car doesn't have much by the way of documented history... I bought it about 7 years ago. It was a mercy purchase. Non-mechanically inclined had just parked it outside and I knew that no one was coming to the rescue in its off-the-beaten path setting in rural Michigan. The reason for being parked was a butcher job electronic ignition conversion. I had him throw in AAA towing to my house as part of the deal. I repaired the ignition system, got it running and then parked it inside for almost a decade. I've started driving it to work out bugs, but I have yet to be satisfied with the smoothness at idle. Whereas my triple white '77 Coupe (also a 400 4bbl) practically Glides along and has pretty decent power, this car feels a lot rougher and lacks pep.

Here's a weird thing about this car and not having any idea of its history.. the odometer indicates 6000ish miles. It was heavily undercoated when new and the body shows zero rust. If you were to judge it from interior condition, 6,000 miles is actually believable. Even the leather wrap steering wheel looks NOS. No elbow splits on the door panels, not even any wrinkles on the dove gray leather. But the exterior is very faded, several dents that look like storage damage and the vinyl top is pretty dead up close.

So yesterday I knuckled down and did a cold, dry compression test. Not so much to obtain the numbers, but to look for the outlier cylinder. I started at cylinder number 8 and worked my way around counterclockwise. By the time I had seven readings very close to each other around 135, it felt like number 7 was a given and that I would be chasing some type of plug wire/cap issue. Nope.. The last one was 90.

I've been pondering the idea of either a bent valve, bent pushrod, or chunk of carbon preventing the valve from closing. I think all of these issues could be connected to long-term storage. I'll be pulling the valve cover later, and in the meantime mulling things over in my head while I do my real estate chores. Just thought I would throw this out there for others to brainstorm about. Here are some photos I took while out running long distance errands on the expressway, hoping that would blow some carbon out, burn off old gas, etc.

 

[Lefty71]

When the motor is not running, some of the valves are open or partially so. I suspect a valve face and head seat took on a little more rust than the others for whatever reason. I wouldnt be surprised if numbers improved with an oil change and a little bit of highway driving. Of course, ultimately some freshening up should be done. Nice car, btw.

I had a two tone grey/silver Grand Marquis back in the 80s. It was a popular look on the deluxe vehicles.

 

[Carmine]

When the motor is not running, some of the valves are open or partially so. I suspect a valve face and head seat took on a little more rust than the others for whatever reason. I wouldnt be surprised if numbers improved with an oil change and a little bit of highway driving. Of course, ultimately some freshening up should be done. Nice car, btw.

I had a two tone grey/silver Grand Marquis back in the 80s. It was a popular look on the deluxe vehicles.

Yes I agree with that probability as well. It will get a chance to be proven out because I am definitely not pulling the head off anytime soon thus tying up my garage with a non runner, but I am willing to go as far as a borescope into the spark plug and or removing the rocker shaft and smacking the number 7 valves with a socket/hammer.

Just looking for other snake oil ideas ike seafoam, Etc I don't suspect this is a burned valve, or I would expect a much lower number than 90 PSI.

 

[bigmoparjeff]

I'd be tempted to do a top engine cleaning with either Seafoam or the GM cleaner (if it's still available).

You could further diagnose things by pumping some air into that cylinder to see where it's coming out. Depending on the brand of your compression tester, you may be able to hook an air hose right to it and pressurize the cylinder. Around 50 psi should work fine, without spinning the engine. If it leaks through the exhaust, unfortunately I would suspect early stages of a burned valve.

A friend's 1974, 400 powered Newport burned a valve at a fairly low mileage, less than 75K miles. He had to pull the head and send it out for a valve job.


Jeff

 

[Carmine]

I'd be tempted to do a top engine cleaning with either Seafoam or the GM cleaner (if it's still available).

You could further diagnose things by pumping some air into that cylinder to see where it's coming out. Depending on the brand of your compression tester, you may be able to hook an air hose right to it and pressurize the cylinder. Around 50 psi should work fine, without spinning the engine. If it leaks through the exhaust, unfortunately I would suspect early stages of a burned valve.

A friend's 1974, 400 powered Newport burned a valve at a fairly low mileage, less than 75K miles. He had to pull the head and send it out for a valve job.


Jeff

I have prepared myself for that possibility, in which case I would probably shop around for a better cam profile and do the timing chain at the same time. But that would be more of a winter project. For the summer I am enjoying my driveway full of C bodies.

 

[Big_John]

I'd run it a bit and then check that cylinder again, first dry and if it's still down, do a squirt of oil and see if it comes up. If it comes up with the oil, it's rings. If it doesn't it's something else.

BTW, I've been using Collinite 855 vinyl "wax" on vinyl tops, truck tonneaus, and convertible tops. After trying other products, it brought back the vinyl top on my 300L quite nicely. Best stuff I've used for that.

 

[Boydsdodge]

I suppose I haven't posted a whole bunch, but if you read between the lines you may have noticed I am reviving a 78 NYB salon after what I'm sure have been many long slumbers. This car doesn't have much by the way of documented history... I bought it about 7 years ago. It was a mercy purchase. Non-mechanically inclined had just parked it outside and I knew that no one was coming to the rescue in its off-the-beaten path setting in rural Michigan. The reason for being parked was a butcher job electronic ignition conversion. I had him throw in AAA towing to my house as part of the deal. I repaired the ignition system, got it running and then parked it inside for almost a decade. I've started driving it to work out bugs, but I have yet to be satisfied with the smoothness at idle. Whereas my triple white '77 Coupe (also a 400 4bbl) practically Glides along and has pretty decent power, this car feels a lot rougher and lacks pep.

Here's a weird thing about this car and not having any idea of its history.. the odometer indicates 6000ish miles. It was heavily undercoated when new and the body shows zero rust. If you were to judge it from interior condition, 6,000 miles is actually believable. Even the leather wrap steering wheel looks NOS. No elbow splits on the door panels, not even any wrinkles on the dove gray leather. But the exterior is very faded, several dents that look like storage damage and the vinyl top is pretty dead up close.

So yesterday I knuckled down and did a cold, dry compression test. Not so much to obtain the numbers, but to look for the outlier cylinder. I started at cylinder number 8 and worked my way around counterclockwise. By the time I had seven readings very close to each other around 135, it felt like number 7 was a given and that I would be chasing some type of plug wire/cap issue. Nope.. The last one was 90.

I've been pondering the idea of either a bent valve, bent pushrod, or chunk of carbon preventing the valve from closing. I think all of these issues could be connected to long-term storage. I'll be pulling the valve cover later, and in the meantime mulling things over in my head while I do my real estate chores. Just thought I would throw this out there for others to brainstorm about. Here are some photos I took while out running long distance errands on the expressway, hoping that would blow some carbon out, burn off old gas, etc.

View attachment 733210

View attachment 733211

View attachment 733212

View attachment 733213

Off topic. Your car is a beauty.

 

[Gerald Morris]

Yes I agree with that probability as well. It will get a chance to be proven out because I am definitely not pulling the head off anytime soon thus tying up my garage with a non runner, but I am willing to go as far as a borescope into the spark plug and or removing the rocker shaft and smacking the number 7 valves with a socket/hammer.

Just looking for other snake oil ideas ike seafoam, Etc I don't suspect this is a burned valve, or I would expect a much lower number than 90 PSI.

It might be a weak valve spring. Anything which prevents the valves from closing completely and well could give that 90 psi compression check. Yes, give #7 a wet check, to verify/eliminate the possibility of a ring seating issue. Could be a ring isn't expanding properly, thus poorly sealing the cylinder. Could also be carbon buildup on a valve. Both of these problems can be solved by steam cleaning your bores. Run it hard on a rainy day, or introduce some water mist into the carb after warming the engine up good, to get some steam in the fuel/air path and the bores, run it hard on the highway again to blow the carbon out, then check if that cylinder still gives sub-par compression.

 

[413]

Have you changed the oil since you had it? How long did istay clean?

Is the choke adjust right, or does it blow black smoke when cold?

Try this, spray a small amount of atomized water spray from a hose nozzle down the carb with it hot and running 1200 rpm. Do it 5 times about 4 seconds at a time. Then go for a drive and make it work.

 

[Gerald Morris]

Have you changed the oil since you had it? How long did istay clean?

Is the choke adjust right, or does it blow black smoke when cold?

Try this, spray a small amount of atomized water spray from a hose nozzle down the carb with it hot and running 1200 rpm. Do it 5 times about 4 seconds at a time. Then go for a drive and make it work.

BIG THANKS for a more precise version of my prescription for mist in the carb. Yes, steaming out valves and bores alike often clears valves and rings of ****, enabling everything to work properly. We all know Chuck's old V8 Ford caught and outran that hateful old Caddy once the rain restored " dat hahway sound" to it!

 

[413]

BIG THANKS for a more precise version of my prescription for mist in the carb. Yes, steaming out valves and bores alike often clears valves and rings of ****, enabling everything to work properly. We all know Chuck's old V8 Ford caught and outran that hateful old Caddy once the rain restored " dat hahway sound" to it!

Yes Sir, don't pour liquid in there. It really helps on a choked up grandma engine that never got warmed up or cleaned out.

 

[Gerald Morris]

Yes Sir, don't pour liquid in there. It really helps on a choked up grandma engine that never got warmed up or cleaned out.

EXACTLY! MIST is the key, and a little goes a LONG way! Big drops WRECK an engine. I pulled apart that 400 when I first got it from a Good Ole Boy, who sold it cheap after ducking it in some damned puddle. THANK THE LORD he didn't duck it MUCH! Everything looked good. I want to rebuild this one, using a forged crank from one of my spare 383s. There's VERY LITTLE carbon in the fuel/air/valve train, and less in the bores, or on the tops of those big lovely 4.36" pistons, so barring damage from a spun bearing or whatnot somewhere I've not yet delved, I think this engine will be rebuildable, and already fairly CLEAN.

 

[413]

Make sure it didn't bend a rod, doesn't take much water to do that.

 

[Gerald Morris]

Make sure it didn't bend a rod, doesn't take much water to do that.

Will do. Mind you, I hand cranked it, and all 8 cylinders went through their entire range of motion. Still, i haven't opened the oilpan to examine that much of the rotating assembly. Be sure, every piston and rod WILL be removed, examined in detail before I spend a cent on new rings or such. I'll have that block sonographed, vatted and magnafluxed also. I agree, the likelihood of something like a bent rod or busted rod bearing is large. Slurping dirty pond water does **** like that to a running engine.

 

[fury fan]

I've started driving it to work out bugs, but I have yet to be satisfied with the smoothness at idle. Whereas my triple white '77 Coupe (also a 400 4bbl) practically Glides along and has pretty decent power, this car feels a lot rougher and lacks pep.


So yesterday I knuckled down and did a cold, dry compression test. Not so much to obtain the numbers, but to look for the outlier cylinder. I started at cylinder number 8 and worked my way around counterclockwise. By the time I had seven readings very close to each other around 135, it felt like number 7 was a given and that I would be chasing some type of plug wire/cap issue. Nope.. The last one was 90.

If 7 are around 135 and 1 and 90, that might give you the roughness, being that 1 cylinder is lower than all the others.
I've had a few engines over the years where all of them were between 90 and 135 (roughly) and the roughness was 'more evenly distributed' -- and they still ran well enough.
In your case, though, the 1 cylinder will be a standout.

I would say your lack of pep vs your other car is not due to one low cylinder, but perhaps due to worn timing chain and/or weak ignition. Initial timing, centrifugal advance, advance rate, and vacuum advance would be on my target list.

 

[Gerald Morris]

If 7 are around 135 and 1 and 90, that might give you the roughness, being that 1 cylinder is lower than all the others.
I've had a few engines over the years where all of them were between 90 and 135 (roughly) and the roughness was 'more evenly distributed' -- and they still ran well enough.
In your case, though, the 1 cylinder will be a standout.

I would say your lack of pep vs your other car is not due to one low cylinder, but perhaps due to worn timing chain and/or weak ignition. Initial timing, centrifugal advance, advance rate, and vacuum advance would be on my target list.

Your advice is quite sound here! I drive an engine with a crack in the deck off the upper headbolt left of the #1 cylinder. This results in a little leakage from #1, and has ever since we got that engine March 2016. The rest of the cylinders test dry from 120 psi to 159, but #1 is around 90 psi also. Still, with some good rebuilt heads, good carburetion, ignition, and yes, a nice double linked timing chain, the engine does quite well. I won't rebuild this block, given that crack and the bores already being .040" over, but I've cared well for it, and it has cared well for us.

 

[Rustyrodknocker]

Any discoveries on this? I once bought a near new 413 that had two badly rust pitted cylinders. This I attributed to being the t we i cylinders with open valves exposed to atmosphere. I have seen engines where mice crawled up the exhaust pipe and in an open exhaust valve and pissed in the cylinder.. ruined.
I had an sbc with some stuck rings I think that was from, well know it was from carbon in the ring grooves that I thought was from not changing the oil but they were stuck hard.

 

[Carmine]

Any discoveries on this? I once bought a near new 413 that had two badly rust pitted cylinders. This I attributed to being the t we i cylinders with open valves exposed to atmosphere. I have seen engines where mice crawled up the exhaust pipe and in an open exhaust valve and pissed in the cylinder.. ruined.
I had an sbc with some stuck rings I think that was from, well know it was from carbon in the ring grooves that I thought was from not changing the oil but they were stuck hard.

Sorry, should have updated but I guess that since nothing really changed I didn't think I had much to add... Building on the thermal shock idea to knock carbon loose, I am aware of what an issue carbon is for Gasoline Direct Injection (GDI) engines. (Fuel doesn't wash the backside of intake valves.) Thus there is now a whole field of fix-inna-can products, all of them designed to be sprayed into the airbox while the engine idles.

Gunk™ offers a version with a 10' tube, theoretically so one person can do it, instead of one keeping a foot on the gas and another operating the can. I decided to take this a step further and take the car on an empty stretch of expressway for some WOT/Italian tune-ups. Once good and warm, I'd floor it and give a good burst of the cleaner. You could defiantly feel a decline in power when the can was activated. Repeated several times, but alas, the power and smoothness aren't any better after this treatment. I even drove it typically lead-footed down to Ohio, but no real change. A boroscope might be my next step, but ultimately I'm sure I'll be doing a top end rebuild. Then the season ended and it got cold. I drove my other 400 4bbl NY'er coupe and it drives like a completely different car (as in much better) so there is definitely an issue with the Salon. Turn in spring of 2026!

 

[Gerald Morris]

Recently read a page on all the Consumerama "fix-n-de-can" **** which actually enriches mechanics and machine shops, sometimes in short order. I can dig the H2O mist treatment, properly administered, but in your case, I think you've indeed done all the good you can from these rites. Time to yank those cylinder heads.... Will Tune In this coming Spring. Best of luck in the meantime!

P.S. LOVELY looking ride there. Damn! If COMFORT was my sole consideration, I'd glom onto these, bigtime.