WallyP

Exhaust valves burn.

Intake valves stick.
Intake valves get bent.

Compressed air will sometimes tell you a lot.

One old guideline for compression tests emphasized that you should loosen all of the plugs a couple of turns then reseat them before doing the test. Loosening the plugs can break loose little chunks of deposits that then blow to the exhaust valves during the test. A little chunk that gets stuck between the valve and seat can cause false low compression readings.

Another old guide says that sticking intake valves will cause false low readings. The intake stems get just hot enough to build up coke deposits from the oil, but not hot enough to burn them off. The recommendation was to run butyl cellosolve or a similar solvent thru the carb (I said that it was an old guide) before the test. In fact, there once was a butyl cellosolve product called "The First Step in a Tuneup" for just that purpose. You can often catch sticking intakes by watching the compression gauge. You should see a really big jump, a smaller jump, then a series of progressively smaller jumps in the reading. If you skip any jumps, suspect sticking intakes.

Such sticking intakes can also be a cause for rough idle. Luckily, sticking valves are less common than they were years ago, as today's oils are much better.

Dave928S

You sound like you're thinking what I am Wally ... It would definitely be worth ruling out carbon on the valve face/seat by having another go at cleaning the combustion chambers and intake/exhaust tracts. The sudden onset of the problem seems suspicious. I used to occasionally run high octane racing (aviation) fuel through road car motors to clean out carbon deposits ... a couple of gallons and thrash it to get temps up. BP Avgas (Australian brand) is way more volatile and clean than standard pump gas, and I believe combustion chamber temps are higher (faster cleaner burn) when using it, which tends to get rid of carbon deposits. I used to also use it as a solvent to clean carbs and carbon deposits off pistons and valves .. so it naturally tended to dissolve sticky carbon deposits anyway. If you can get a few gallons it might be worth a try if preliminary tests point to valves ... before you rip the motor out.

928drvr86.5

Update:

With a cold engine, I just conducted a leakdown on cylinder #3, that's one of the 2 that read 60 PSI the other day.

I popped the driver's side dizzy so I could see the rotor and when it's contact patch was roughly starting to align with the "pin" for Cylinder #3 spark. I also had a screwdriver in the sparkplug hole and watched it go TDC before locking the flywheel. From there I screwed the compression tester's adapter into the hole (after removing the schrader type check valve), screwed the hose on and compressed air at ~60PSI into the cylinder.

I know you guys know how to do a leakdown, not so sure I do, that's why I'm outlining what i did. I am somewhat perplexed at the results.

So, cylinder 2 had absolutely no leakage past the intake valves, the exhaust may have but I couldn't listen to the exhaust because the hose from my compressor was a different fitting than the quick-connect fitting on the compression tester's hose, I'll buy the correct one tomorrow. So with that being the case, I had to hold the junction of the two hoses together and bend over to listen to the intake and oil fill. obviously i couldn't reach the exhaust while holding the two hosed together, I was alone with no helper.

I did hear air escaping into the crankcase past the rings but my un-trained ears had a hard time deciding how much air there should be, I know some is ok. Next, I moved to a cylinder with a known good compression reading, cylinder #2. I was hoping to compare the ring blow by and find a difference, same story with cylinder #2, air blowing past the rings at about the same rate. Onward to the best cylinder, #7 .... same story, air in the crankcase and just a very faint hint of air at the intake runner.

Logically speaking, I think the rings can be eliminated, no? To my ears, the cylinders with 160PSI compression have just as much ring blowby ( when listening at the oil fill) as the cylinder with 30PSI. Tomorrow I will have the fitting to connect the two hoses together so i can repeat this and listen for air escaping thru the exhaust.

928drvr86.5

I find this interesting, I was wondering if there might be such a condition that would give low cranking compression but good compression when running. It makes no sense to me that a V8 with 2 cylinders having 1/3 the compression of the others would idle as well as i am seeing.

928drvr86.5

I did watch the compression gage for each cylinder and it did exactly that, big jump, smaller, then dithered in to the final number. The 2 bad compression numbers jumped to 60PSI and then stopped, I let it crank longer with no difference.

Dave928S

Ben

A true leakdown test determines leakage from an engine cylinder as a percentage of what you get through a standard orifice size at the same pressure using a two gauge rig ... but you don't have to use that exact method. You can get pretty much the same information by getting a cylinder up to a set pressure (I use 100psi) and then seeing how many seconds it takes to leak down to a reading above zero (I use 5psi). Check a known good cylinder and you'll find that leaky cylinders for whatever reason will be heaps less. It's then a matter of elimination to determine where it's going. To give you an idea of what you might see, I've had about 7 to 8 second leak down rate from 100psi to 5psi on good cylinders ... and 1 to 2 seconds on a cylinder with a broken compression ring (it had a compression of 110 and an oil shot improved that markedly).

Leaving air feeding to a bad cylinder will then allow you to see/hear where it's going.

If you have air getting past rings you'll know all about it in a big way ... it'll vent at a noticeably different volume out the oil fill. You would definitely notice a difference between good and bad ... so I think you are pretty safe to suspect something else. The fact that you got no improvement with an oil shot on a compression test also goes strongly towards eliminating usual ring issues.

Definitely no pressure build up (or gurgling with the cap off) in coolant when you feed bad cylinders air for a while?

If you have an intake valve leak (stuck or carbon pitting) you should hear the leak through the intake.

If you have leaky exhaust valves you should also hear that through the exhaust ... but it is harder to hear.

There is, of course, the possibility that you have both leaky exhaust and intake valves.

If you just work on it logically and eliminate the possibilities one by one .... you'll get there and it will make sense.

Dave928S

It's amazing how well a V8 can idle with one or two bad cylinders ... it's happened to me too. It seems that the good ones 'carry' the bad ones.

surfridermason

Interesting. and no valve leakage in the one cylinder..... It would be cool if you pull it apart, have no damage other than mildly worn compression rings, that just polished away until there was a poor seal..... Hurry up and pull it out - the suspense is killin me.

surfridermason

These motors especially. I had one here that had the cylinder crack (s4), fall into the crank case, piston with it..... and banged around for miles before they stopped driving. I have the piston with me most of the time- as a good luck charm It's about the size of a golfball.

928drvr86.5

So here is the latest, because I hate it when I search and find a thread relating to an issue I am having only to have it dead end with no follow-up on the outcome.

I came home on Thursday and thought what the hell, why not try a compression test on the bad cylinders again just to make sure. Guess what.... 150 PSI on both that read bad before. So with that I put the "lunch boxes" back on the intake, fire it up, and all this black carbon **** blasts out of the tailpipe and onto the wall and floor of the garage. After a minute or two the carbon cleared out, the car seemed to be running much better, so i took it out for another 30 mile drive on back-country roads. I made quite a few full load 2nd gear excursions to 5K-ish rpm just to make sure I was burning out all the carbon, now the car has just a tad of smoke on start up that clears up right away. Blame it on carboned up or sticking intake valves i guess.

So why is it getting carboned up? My guess is regular trips to work at Deere (which is only 15 miles away) coupled with a few daily excursions to our Proving Grounds which is less than a mile away from my office. I really can't remember the last time I drove the car any longer distance. Also, like a dumbass, I put a low-temp thermostat in to mask a slipping viscous fan clutch problem a few years back, but now that I have installed a new CR radiator and dual electric fan, don't think the engine is getting up to optimum operating temperature. I need to change back to the normal thermostat.

Bill Ball

Yep, just give it periodic Italian tune-ups and you'll be fine!

Dave928S

Good to hear and thanks for letting us know ... glad you didn't go down the pull the motor path. Time to check your plugs and their heat range too ... and give it a good italian tune up What I mentioned before about aviation fuel works like a charm in getting rid of carbon .. and works even better if you give it a good thrashing.

Dave928S

See Bills video for the definition of Italian tune up ... ........ .. in case you were wondering