Just got my first slant six

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Bob Wilkie

Bob Wilkie

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I just picked up a 73 Dart Swinger with a slant six. I noticed the garage floor where it had been parked was black under the tailpipe. I jump started it and drove it home over an hour and it ran fine and didn’t smoke. The oil light flickered at stops and sometimes stayed on. The engine and compartment are covered in what looks like a few years worth of oil. I checked the oil pressure with a mechanical gauge and it idled around 5psi. I checked compression in the cylinders and they were 70, 80, 75, 75, 75, 80. It has electronic ignition. The plugs were all gapped around .040. They were covered in light brown deposits and one of the insulators around the electrode was gone. I’m hoping to flip the car but it could be a keeper too. I have to replace the engine mounts and tranny mount.

My mechanical skills are low but I’ve been learning a lot from you guys and my friend who is a mechanic. With that info, what are your thoughts for the least intrusive and most economical steps to take next?
 
Here are the plugs as I found them

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just a comparison. my 65 dart with 225 slant 6 compression readings were all 140-150. at idle, oil pressure is 50 pounds. when fully warmed up its still in the 45 to 50 range
 
IIRC, the oil pressure light comes on at 6 or 8 psi.... it is clearly low, indicating a worn pump and/or crank/rod bearings. I would put in a fresh oil change with 10W40 oil and see where it ends up. Is there any odor of gasoline in the oil?

If your readings are indeed that low, then the cylinders and/or rings are all well worn; you will typically find around 125 psi new. But, they are all quite consistent and that would be unusual to all be that low AND that consistent....I suspect a test issue first. Soooo.....
- Did you use a thread-in type of compression gauge or one you push on the spark plug holes? If the push-on type, borrow a thread-in type and re-test. And, borrow another anyway as yours just may be inaccurate.
- Also, prop the carb wide open and keep it there for the test.
- Crank the engine over at least 4-5 times per cylinder and then take a reading
There are a few other possibilities for low readings like stuck rings, poor valve seating or a umped timing chain, but first make sure the readings are correct.

The black spot under the exhaust is likely just incompletely burned fuel or carbon in the exhaust pipe. The cold start idle could be too rich or the ignition not strong (and not completely combusting the cold mixture). That issue may just be due to tuning, choke settings, etc. I would not be discouraged over that alone.

The plugs do not look all that abnormal for old plugs. Put in some new ones. There are oil seal rings around the upper ends of the plugs cups and they rot and seep oil; a new set can be had for a few $$ from NAPA.

How did the engine run in general?

BTW, it is very likely overdue to have the valve lash set and the timing set. Start reading up on that.
 
Your first step is to get the three books described in this thread.

Your compression sounds very low—assuming the test was done correctly, that is with all six spark plugs removed, a fully-charged battery, the engine warmed up to operating temperature, and the choke and throttle wide open.

In any event, Tune-up parts and technique suggestions in this thread. The engine requires periodic valve adjustment. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.
 
The car has just over 100,000 miles and I believe that is accurate for the engine too. The oil was 20w50 with a new looking Long filter from last June. I don’t know how old the plugs are. The distributor and wires look like they’re not very old. The engine starts easily and runs smoothly at idle and on the freeway.

I did the compression with a screw in gauge. I had all plugs out and the carb wide open. The battery was fully charged. I don’t remember if it was at operating temp or not. Probably not. I did not have the choke open. Don’t know how. I cranked exactly seven cycles per cylinder test for consistency starting at the front and working my way back. I got readings of 70,80,75,75,75,80.

Today I flushed (additive) the engine for 30 minutes at idle and then changed the oil (10w30) and filter (short). I changed the plugs too and gapped them all at 35. I tried to remove the old brittle Pcv but it broke and parts fell inside the valve cover. The valve cover gasket was leaky so I ordered another one now that I have to pop it off and dig out the Pcv parts anyway. I removed the valve cover. It has layers of flaky burnt grunge layered inside it. The valve train appears pretty clean though in comparison. I still can’t pry the air cleaner off the rear grommet of the valve cover. The grommets are rock hard and need to be replaced.

The motor mounts and tranny mounts are on order too since they’re shot. I can do whatever needs to be done when replacing those mounts.

However I’m trying not to sink a ton of money in this one and I’d like to flip it so a rebuild is not in my wish list.
 
Those compression readings are waaay low. The reason I questioned the readings was that if the cylinders are worn, it would be highly unlikely that they would all wear that evenly as to drop from around 120 psi new to 75-80 psi on all 6 cylinders. Your 7 cranks sounds good, as well as the thread-in tester. But I really suspect something is effecting it in common.... timing chain slipped, exhaust pipe plugged, etc.

I suppose the choke may be effecting it; just open the throttle and at some point you can manually open the choke plate by hand and put something in there like a socket to keep it open.

If you dropped that much in oil weight, the pressure is going to get considerably lower.

The flaky grunge sounds not too uncommon. Again, was there any smell of gas in the oil that your drained out?
 
Welcome. Slants are cool. I also have a small-block and big-block car, and respect them all. Even my M-B diesels, though some of those owners are definitely autistic. One guy added a turbo to his slant and runs 1/4-miles in the 10's (youtube), which thumbs a nose at the continual "replace it" rants.

The low oil pressure might simply be due to gunk on the pickup screen. That is a common problem, and the gunk you see under the valve cover makes it more likely. An easy fix is to drain the oil and fill the oil pan w/ gasoline (safer is diesel or ATF) and leave for a few weeks to dissolve gunk on the screen. Best might be bio-diesel, and since in CA, you can buy a few gallons of Propel Diesel HPR which is bio-based and thus should be a better cleaner, or perhaps even cooking oil. To kill another bird, clean the top end w/ a brush and solvent, which will drain down into the pan. If your pan gasket is suspect, you might later remove the oil pan, and the timing chain while in there (and chain may have slack and need replacing). But do that after verifying the engine is OK. If you do go in deep, replace the O-ring on the oil pump pickup tube, since "sucking air" could also explain low pressure. Worn crank and rod bearings are the main mechanical cause of "low oil pressure", not worn piston rings, so doesn't necessarily go together w/ low compression.

A good sanity test on your compression readings is to turn the engine over by hand. You can just pull on the fan belt w/ 2 hands. If you feel it strongly resist you 3 times per rev, and must wait while you hear the air hiss down in each cylinder, then there is no way your compression readings are correct. With those, you wouldn't feel any resistance from such "air springs". When my 1969 Dart was worn, I measured ~70 psig in 1 cylinder and >100 psig in the other 5. So I agree unlikely that all 6 are evenly worn, so strongly suspect your measurements. I think your spark plugs would also look oily. My bad cylinder showed a perfectly clean spark plug, i.e. raw un-burnt gas flowing thru. If you truly do have low compression, it may simply be the valves need adjusting, which is a common task on these engines (solid lifters, not auto-adjusting hydraulics).
 
Last edited:
nm9stheham said:
Those compression readings are waaay low. The reason I questioned the readings was that if the cylinders are worn, it would be highly unlikely that they would all wear that evenly as to drop from around 120 psi new to 75-80 psi on all 6 cylinders. Your 7 cranks sounds good, as well as the thread-in tester. But I really suspect something is effecting it in common.... timing chain slipped, exhaust pipe plugged, etc.

I suppose the choke may be effecting it; just open the throttle and at some point you can manually open the choke plate by hand and put something in there like a socket to keep it open.

If you dropped that much in oil weight, the pressure is going to get considerably lower.

The flaky grunge sounds not too uncommon. Again, was there any smell of gas in the oil that your drained out?
Click to expand...
No gas smell in the oil. I had the breather cover off and held the butterfly on top of the carb open with a socket extension. Carb is real clean like it was rebuilt.
 
BillGrissom said:
Welcome. Slants are cool. I also have a small-block and big-block car, and respect them all. Even my M-B diesels, though some of those owners are definitely autistic. One guy added a turbo to his slant and runs 1/4-miles in the 10's (youtube), which thumbs a nose at the continual "replace it" rants.

The low oil pressure might simply be due to gunk on the pickup screen. That is a common problem, and the gunk you see under the valve cover makes it more likely. An easy fix is to drain the oil and fill the oil pan w/ gasoline (safer is diesel or ATF) and leave for a few weeks to dissolve gunk on the screen. Best might be bio-diesel, and since in CA, you can buy a few gallons of Propel Diesel HPR which is bio-based and thus should be a better cleaner, or perhaps even cooking oil. To kill another bird, clean the top end w/ a brush and solvent, which will drain down into the pan. If your pan gasket is suspect, you might later remove the oil pan, and the timing chain while in there (and chain may have slack and need replacing). But do that after verifying the engine is OK. If you do go in deep, replace the O-ring on the oil pump pickup tube, since "sucking air" could also explain low pressure. Worn crank and rod bearings are the main mechanical cause of "low oil pressure", not worn piston rings, so doesn't necessarily go together w/ low compression.

A good sanity test on your compression readings is to turn the engine over by hand. You can just pull on the fan belt w/ 2 hands. If you feel it strongly resist you 3 times per rev, and must wait while you hear the air hiss down in each cylinder, then there is no way your compression readings are correct. With those, you wouldn't feel any resistance from such "air springs". When my 1969 Dart was worn, I measured ~70 psig in 1 cylinder and >100 psig in the other 5. So I agree unlikely that all 6 are evenly worn, so strongly suspect your measurements. I think your spark plugs would also look oily. My bad cylinder showed a perfectly clean spark plug, i.e. raw un-burnt gas flowing thru. If you truly do have low compression, it may simply be the valves need adjusting, which is a common task on these engines (solid lifters, not auto-adjusting hydraulics).
Click to expand...
Thanks to all of you for the advice and insights. I WILL GEt back to you after I do a few more things
 
Might want to check in here and/or on slantsix.org and ask questions before jumping ahead and doing stuff that seems like it might be a good idea. Engine "flush" additives such as you put in the crankcase are a great way to finish off (kill) an engine—and you've worsened your odds on that by using a small oil filter instead of a big one. If you have an engine that's very dirty inside and you won't take it apart to clean it properly, do this instead.
 
Dan, where do dispose of 3 gallons of sludge in Canada? Before you do much more, try $7 worth of seafoam and an oil change. Half in the case, let idle for 30 minutes then suck the other half in through the power brake booster line and then stop motor, let it sit overnight then start up and blow out the funk, then change oil.
 
The oil pressure relief valve in the oil pump is known to stick open. This would cause low oil pressure.

Are you sure about the compression tester? If the schrader valve was replaced with a tire schrader valve, you'll lose 25 psi with the tester.
 
Bob Wilkie said:
No gas smell in the oil. I had the breather cover off and held the butterfly on top of the carb open with a socket extension. Carb is real clean like it was rebuilt.
Click to expand...
OK very good. Carb leaks can dilute the oil badly and lead to severe engine wear but that does not seem to be the case.
 
emsvitil said:
The oil pressure relief valve in the oil pump is known to stick open. This would cause low oil pressure.

Are you sure about the compression tester? If the schrader valve was replaced with a tire schrader valve, you'll lose 25 psi with the tester.
Click to expand...
You lost me on that last paragraph
 
Bob Wilkie said:
You lost me on that last paragraph
Click to expand...
The key to Your compression readings is they are within 15%, at that age, that ain't bad. emsvitil pretty much nailed it on the schrader valve, My Craftsman compression
tester ate the original valve, and I didn't know where to get one quickly at the time. So like most gearheads I stuck a wheel valvestem schrader in instead, cars I know
pumped 115-120psi were now reading 90-95psi, the spring in the std valvestem unit is much stiffer than the ones that come in a compression tester.
Another thing to keep in mind is there is a big variance between styles & arrangements of testers. Most good ones have the valve in the end of the hose/adapter, but some
are in the head/gauge assy., this will lower the readings, as will just using the "stepped" threaded end(if it has one) instead of the appropriate long-reach threaded adapter.
Combine a tire valve, the valve at the gauge(basically making the entire hose part of the combustion chamber), and no long-reach adapter, & it'd be no shock to end up
with 35-40psi less than "normal". This of course assumes the gauge is accurate to begin with!!
That's a lot of ash deposit on the plugs, that could be a number of things, but You drove an hour and it didn't smoke. May just need valve stem seals, a valve adj. and a
tune-up. The oil pressure, try an oil change and 10w-40, see what You've got. They've got all of the usual suspects covered for You^^above. BE AWARE, the Slanty fuel
pump is upside down so to speak, if the diaphragm starts leaking, it won't just start leaking/squirting out of the breather on the pump body. It will trickle/run/dribble
right into the crankcase long before it gets bad enough to start hosing out of the breather(or vent) hole. If it's old, it's just a matter of time...................................
 
Ok so I went back and started over. I replaced the spark plugs and the valve cover gasket and I cleaned up the engine. I learned that I didn't properly have the throttle wide open (the choke was) during the first tests and my battery wasn't fully charged either. I checked compression again after fixing those issues. This time I had 85-95 on all six cylinders. I know that is lower than what everyone has said it should be, but it runs great so I am thinking the gauge might be the issue. I then attempted to determine why it has VERY low oil pressure causing the light to come on. I installed a manual gauge and found it idling at about 4psi! I was only able to get it up to around 20psi at higher RPM. I removed the pan and oil pickup and found what appeared to be hard carbon deposits and white nylon gear teeth blocking most of the pickup. I believe the timing gear issue was handled years ago by the previous owner according to the stack of service records that came with the car. I am guessing no one has ever taken the pan off. I thoroughly cleaned the pickup and the pan. The pickup tube had no cracks or tool marks. I put everything back together and the manual gauge still showed about 4psi at idle. I removed the oil pressure relief valve spring and added a washer to the end as suggested by other members. That changed nothing. I ordered a new oil pump and I am waiting to replace the old one. Hopefully that solves the oil pressure problem. Once that is done, the car is ready to sell if anyone is interested.
 
New oil pump can easily create more problems than it solves on a Slant-6. Before you install it, service the oil pressure relief valve to make sure it's doing its job, as described in these two threads: thread 1, thread 2.
 
Ditto on the Schraeder valve. Never replace with a normal one for a bicycle or car tire. Those have a very strong spring (black tire). You want a special one with a very weak spring. I bought some long ago at an auto tools specialty shop. They have white plastic collars, which must identify them (AC ones are green).

If the OP puts solvent in the bottom of the oil pan to de-gunk the pickup screen, then drains it all out, that shouldn't hurt the engine, if the engine is never cranked. But, better to wash down all the top-end crud into the pan, then remove the oil pan to clean the bottom side. You can work on the oil pickup then and even change crank and rod bearings from below, along w/ rear main seal. Most likely, the pan gasket needs changing anyway.
 
Well it turned out to be the pump after all. That was a MAJOR undertaking! At least I now have the satisfaction of knowing everything is clean or new and working well.
 
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