1965 300 383 low compression on 2 cylinder
- Thread starter mathiew
- Start date
First off, burnt valves will not usually cause plugs to foul. Is the engine burning significant amounts of oil? How many miles are on the engine? You have a significant compression drop on two cylinders, Remove the oil filler cap and crank the engine over. Does a lot of smoke (blow by) come out of the valve cover? If so, the rings are worn and the engine needs an overhaul.
Besides worn rings, bad valve seals or worn valve guides can also cause plugs to foul, but that probably is not the cause of low compression. Check for blow by and report back.
Dave
Besides worn rings, bad valve seals or worn valve guides can also cause plugs to foul, but that probably is not the cause of low compression. Check for blow by and report back.
Dave
rkrochen
Senior Member
Valve seals can cause these sort of issues. They dry up and crack allowing oil past the stems into the cylinder. The oil burns and carbonizes. This carbon deposits on the valve face and seats resulting in low and uneven compression. This was very commonplace back in the day. We would replace the valve seals and spark plugs. Then we would run a can of spray carb out through the engine (Chrysler made a very good in). After it sits as per instructions take her for a run and run it hard. If really bad it may take two treatments. A telltale sign of bad seals is a puff of smoke on start up.
I would suggest pulling the valve cover on the side with low compression. Often on an engine that is fouling plugs with bad valve seals, there will be pieces of the dead valve seals at the base of the valve springs. At 98k, depending on they type of service this vehicle was in, it may well need an overhaul as well, but as there is no apparent blow by, it is probably worth trying to replace the valve seals. Another area of concern is that the valve seats may be severely eroded from running unleaded fuels and that could also account for the compression drop. In that case, this engine will need a valve job. If this was one of my cars, I would do the valve seals and see what happens toward reducing the oil consumption. If Mathew is able to do the work himself, he would not be out much expense to try this fix. This car is also due for a timing chain. Paying a shop to do all of this could run into some considerable expense on an engine that might not be worth it in the long run, so there is an element of financial risk here that he will need to evaluate.
Dave
I would pull both the heads and inspect the valves, if they are full of carbon a valve job is probably in order as for sure the valve seals are shot. With the springs removed the valve stems and guides can be checked for excessive wear. The heads should be upgraded to hardened valve seats as part of the valve job. You should also replace the timing chain at the same time. At 95k the engine could also be due for an overhaul. If you know the history of the car, the type of service has a big impact on likely engine wear. A car that was driven on the highway most of the time will probably not show a lot of wear, but an old lady's car that went for a lot of short trips is likely to have a tired engine. If you see lots of sludge under the valve cover, that is usually a good indicator of short trip driving or poor maintainance.
Dave
As a follow a up, if the valve seals are intact and the guides do not show excessive wear, odds are the oil rings in the engine are worn out and the engine should be rebuilt.
Dave
Dave
68PK21 440.6bbl
Old Man with a Hat
Let me get the crystal ball out... it says by page 3 he will have the engine out to do a total rebuild.
Seriously when I did the valve seals on my 1968 Plymouth Fury III 318 in car (no head removal), I found very little of the remnants of the old hard broken valve seals in the head area. Over the years I had to remove the intake to clear out the carbon buildup in the heat riser crossover to get the heat riser/choke working for cold weather operation. Low & behold there are some of the remnants of the valve seals in the lifter valley.
Further on when I junked the body but saved the engine and transmission it was time to clear out the garage and move on so lets tear down this high mileage can't kill it 318.
Hmmmmm whats all this pitting inside the oil pump rotor? Look at the oil pickup screen and find that it has collapsed as it was designed to do so the engine wouldn't starve for oil, so I shake out what's in there clogging the screen, low and behold there are the hardened bits of the valve cover seals!
YMMV
rkrochen
Senior Member
This was a major issue with Chrysler products and you don’t always see pieces of seals, they just get hard and don’t function properly. They all failed at approx 75000 miles give or take some. On the small blocks the intake crossover would plug due to oil contaminated exhaust crossing through it. The cooler temps and redirection of gases was the perfect combination for carbon build up. The other tail tell signs was fouling plugs, improperly functioning choke and poor running. Not well understood problem resulting in unhappy owners. I made a killing replacing valve seals and doing subsequent clean up work. Usually do the timing chain at the same time.
Not sure why you would replace an engine without at least trying to do as I recommended above. To replace the seals you are looking at roughly 6hrs labor. Unless you rebuild for peace of mind knowing you have a good engine.
My personal thoughts are that I see time and again people over diagnosing and over thinking issues of issues like this.
68PK21 440.6bbl
Old Man with a Hat
Well I think on my 318 it was more from the engine being overheated in it's lifetime, I know I cooked it a few times with this $100 DD, heater hose blowing out and 20 miles to go to get home, fug'it IIRC. Cud'int kill that engine, the overheating caused the intake to expand and the gasket area on tear down was all burnt in the middle heat riser area and oil slinging off the cam was getting into the crossover area clogging up the crossover on both sides with carbon.
John Kirby
Active Member
Since adding oil to the cylinders didn't improve the compression it doesn't sound like the rings are bad. I suspect unleaded gas has ruined the valve seats. A valve job with hardened seats and new timing chain is what I would try first. Get a double roller chain. Costs a little more but is worth it.
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