What's the best way to find a dead cylinder?

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cruiser said:
. Yes, I'm planning on running the test you mentioned, and I really appreciate the technical advice. I just don't want to "throw good money after bad" as the old saying goes.
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a leakdown tester is 80bux at harbor freight, compression tester is 30, and a vacuum gauge is 15. or a lot of parts store "rent them".

i'd consider those tools an investment. and you'd have a lot of your questions answered and be able to move forward with a better idea of which way you want to go.
 
RustyRatRod said:
I think you're on the right track. Regarding the "crate engine" approach. If you plan to start with nothing, in other words, you want a "TOTALLY" complete new engine, just be advised it will not be cheap. Someone will have to come up with an entire long block. That's the easy part. Then, they have to locate all of the tin, the valve cover, timing cover and oil pan. Next is all of the various fasteners. That takes a lot of time and effort. It's not something that's going to happen for say 2500-3500 dollars. Sure, a basic long block can be had for that, but people seem to forget about all the other details. That is of course you plan on using all of that from your present engine. If you want a "plug and play" approach, just be prepared to pay someone for their time, parts and effort.
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oh my lord. yes. all of this. all of the little details and fasteners and all the stuff from your engine that you thought would fit your new engine and some how for some reason don't...

and then the whole: well, now this looks shabby on here... and let's go ahead and paint that too...

all of a sudden you're 4~5K into a rebuild.
 
He's still putting the cart before the horse.
Only 105k? Typically that's nothing for a /6.
 
volaredon said:
He's still putting the cart before the horse.
Only 105k? Typically that's nothing for a /6.
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Okay guys, here's the latest: Just put a vacuum gauge on it by disconnecting the PCV hose from the PCV valve and attaching the gauge to the open end of the hose. Started the engine which immediately went to high idle. The vacuum needle was wobbling quite significantly, perhaps through a 2-3 psi range. It was definitely not still or steady. As the engine warmed up on high idle, the vacuum increased to 18 - 19 psi and still wobbling through a 2-3 psi range. Kicked it down to curb idle, where it read 14 - 17 psi with significant wobbling at 780 RPM. Not steady at all. Momentarily accelerating the engine drove the vacuum down to zero, then returned as the engine settled back into idle. Tried to attach a video to this reply but it wouldn't work. Never at any time saw a steady vacuum reading. Rapidly wobbling 2-3 psi all the time. Thoughts?
volaredon said:
He's still putting the cart before the horse.
Only 105k? Typically that's nothing for a /6.
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volaredon said:
He's still putting the cart before the horse.
Only 105k? Typically that's nothing for a /6.
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cruiser said:
Okay guys, here's the latest: Just put a vacuum gauge on it by disconnecting the PCV hose from the PCV valve and attaching the gauge to the open end of the hose. Started the engine which immediately went to high idle. The vacuum needle was wobbling quite significantly, perhaps through a 2-3 psi range. It was definitely not still or steady. As the engine warmed up on high idle, the vacuum increased to 18 - 19 psi and still wobbling through a 2-3 psi range. Kicked it down to curb idle, where it read 14 - 17 psi with significant wobbling at 780 RPM. Not steady at all. Momentarily accelerating the engine drove the vacuum down to zero, then returned as the engine settled back into idle. Tried to attach a video to this reply but it wouldn't work. Never at any time saw a steady vacuum reading. Rapidly wobbling 2-3 psi all the time. Thoughts?
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More info on engine. In August of 2019, I had a shop replace the incorrect "peanut" head with a correct cylinder head, casting number 2843169. Since then, I've put 31,200 miles on this head. Prior to installation, the machine shop ground the head and valves, installed new valve seals, guides and hardened seats. The new head was installed with a new head gasket (which appears to be leaking engine oil at this time) and then adjusted the valve lash.
 
I would buy the tools needed to check the engine thoroughly and then make a discission. Compression tester is not a money breaker, and some of the kits have the hose needed to apply compressed air for a low budget leak-down test.

vaccum-gauge-readings.jpg
 
Go around hittting each spark plug with a hammer till ya get to the one that doesn't scream...no? tough crowd...I'm here till next Thursday...:lol:
 
bcschief said:
Pull one plug wire at a time with the engine idling the one that doesn't change is not contributing. You may want to do a valve adjustment that may fix your problem. Use insulated pliers or ones designed for this.
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Nah let him get zapped its fun...lets ya know you are alive...:)
 
Vac gauge to pcv hose? Why don't you just disconnect the vac line that goes from base of carb to the air cleaner housing?
That's straight manifold vac and same size as most vac gauge nipples. The pcv hose is twice as big, without the right adapter you'll leak more vacuum than the gauge need to do it's job

And unless you have a museum piece I wouldn't have worried about the peanut head being "incorrect". The peanut head has 6 less potential oil leaks.
 
Connecting the vac gauge to the PCV fitting in the carb will give you the same reading as any other fitting in the carb base that is below the t/blades. Size of the hole does not matter.
 
As long as he was able to allow for it when he hooked up the gauge and it wasn't a loose fit.
Judging by the reading he got im guessing it was probably a good connection. And even if a little bouncey if he got 17" I wouldn't think it's a valve problem. I'm thinking there's still something in that carb causing it.
 
Wait three days and buzzards will start circling over the cylinder
 
RustyRatRod said:
As easy as the plugs are to get to, I'd just run a compression test.
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Rusty, that would be my third test.
1st: pull wire off each plug, one at a time. (this will tell you which one)
2nd: remove valve cover and check rocker arms movement. (possible problem)
3rd: Compression test. (possible problem)
I like the easy-peasy first.

Question was, "how to tell" not "how to fix".
 
cruiser said:
On cold start, the engine goes to high idle like it should, but has a misfire sound from the exhaust pipe. The engine also shakes a bit. When kicked down to curb idle, it idles rough and the engine shakes as if it isn't hitting on all six. When stopped at a red light in gear, the idle is very slow and rough. Idle speed in neutral is set to 750 RPM.
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If you open it up on the highway, does it seem to run ok, check for a VAC leak.
 
Infrared gun told me #4 was pumping @ambient temperature. I had drilled out a jets on a BBD which mad it pig rich on the dyno. I may have wiped the rings on my low mileage 198. It has been sitting since 2009. Driving over the road left me no time to play with it. When I came off the road it was forgotten. Now I am working on keeping my '97 Lesabre, and '05 Lesabre rolling for my 90mi a day commute. My '93 D350 is next. I need a truck to build a house.

My cylinder head had .090 taken off it. Home port job by me, finished by @Dragnmopars from Moparts not sure if he is still alive. Ran the valves set cold to the hot specs on the stock cam for extra lift and duration.
 
cruiser said:
Good question. The answer is no, not yet. Here's what I'm thinking. The motor in my Duster was a replacement engine installed 36 years ago, and now has over 105K miles on it. I'm not sure how much life this engine has left in it. I suspect that it had poor maintenance prior to my ownership. If I can solve the problem I currently have, there's no guarantee that another problem won't pop up shortly thereafter. I plan on keeping this car forever and driving it a lot. There's one school of thought that says to cut your losses and just go with a new engine. That way you can enjoy many years of trouble free driving. Since the current engine now in my car isn't the numbers matching original, it's not that valuable. If it WAS the numbers matching motor, no question - I'd keep it and rebuild it. But this isn't the case unfortunately. Yes, I'm planning on running the test you mentioned, and I really appreciate the technical advice. I just don't want to "throw good money after bad" as the old saying goes. I definitely haven't made the decision to buy a new motor. I'm just in the "thinking about it" phase right now. There's a place called S and J Motors that will sell a rebuilt long engine, balanced and with mild head porting work for about 3K. What's your take on all this, Rusty?
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The 225's are great engines and hard to kill. At 105,000 miles, you are only at a third of its lifetime. I'm sure you have an basic fix, for your problem. I have also heard, as someone said, that the 6 bangers have a rich issue on the ends (1 and 6). I'm not a expert on them, but maybe change the heat range (plugs) on those two.
 

What's the best way to find a dead cylinder?​


See which one is coming up "Short" >

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Usually it's the back cylinder up against the firewall (farthest away from the radiator). That cylinder will run warmer than the rest and those piston rings on that cylinder will wear out sooner, causing lower compression in that hole.

By the looks of the #6 spark plug not burning clean, yep that's your weak cylinder.


☆☆☆☆☆
 
OP has another thread wanting to know what is the best intake gasket and "sandwich gasket" to buy. Maybe a intake vacuum gasket leak? We all pretty much know how much a bear it is to get the intake and exhaust gaskets to seal once the manifolds are divorced. Maybe the OP is on to something.....
 
inertia said:
Check/listen for a vacuum leak, plug any hoses to carb.
Wiggle carb for loose bowl/base, tighten base gskt bolts .
Good luck
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Spray carb cleaner around possible vacuum leak areas like carb and intake. If you notice a change in RPM then look closer for the leak.

Also, have you confirmed your timing and advance?
Good luck.
 
cruiser said:
Having rough idling/driving issues. Suspect that the engine (1974 225 bone stock everything) is missing on one cylinder. So what's the best way to determine which cylinder isn't firing? How about pointing one of those fancy temperature guns at the exhaust manifold adjacent to the suspect cylinder while the engine is running. If it's cooler than the rest, this may indicate a dead cylinder. Any other ideas out there? Thanks, all!
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The infrared temp gun works if you start a cold engine. The dead one will be much cooler and not warm as quickly. Eventually the heat works its way through the manifold iron. You can pull plug wires off and see which one has no effect on idle speed and roughness.
Pull the plugs and check for one that is probably black and possibly oil fouled. Then there is a compression test or cylinder leakdown test.
 
cruiser said:
Hi All: Thanks for all the helpful replies. I went with the recommendation to pull the plug wires individually while the engine was running. As I did so, I observed the engine RPM on my shop tach. Every plug that was pulled showed an RPM drop, then an increase when the plug wire was re-installed. So I was wrong about having a dead cylinder. All the cylinders are firing, which I've confirmed. Something else, probably more serious, is going on as the engine is running just like it has a dead cylinder. I'm puzzled by this and cannot find the cause. The choke plate is fully opened when warmed up. No vacuum leaks. No exhaust leaks that I can find. New plugs. Rebuilt (stock) carb. Maybe I've got something bigger going on. Had the head re-done (grind valves, new valve seats and guides, etc.) about 15K ago. Think I might need some professional help on this one. This motor should drive and idle smoothly and it doesn't. Very frustrating, but I'll keep searching.
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Compression test time.
 
Bewy said:
Connecting the vac gauge to the PCV fitting in the carb will give you the same reading as any other fitting in the carb base that is below the t/blades. Size of the hole does not matter.
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Just had the carb professionally rebuilt, so I'm thinking that its something else.
 
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