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Low Compression #3


Labrauer
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My Plymouth flathead engine # P25-21519 which is in my 48 coupe has started to run around 200 degrees on the temp gauge which is unusual, it has always run 160. The engine started to run rough so I thought new plugs were in order so I change them, I changed the thermostat, flushed the radiator, checked the timing and still no change in the rough running. This morning I did a compression test on the cylinders, this is what I found.  #1 (135), #2 (135), #3 (70), #4 (150), #5 (135) and #6 (140). I put a little oil in the cylinder of #3 and checked compression again and got only 90 pounds this time. My question is where should I start and what may be the cause of low pressure in #3 cylinder. This is not a good time for this to happen.

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37 minutes ago, Labrauer said:

started to run around 200 degrees on the temp gauge which is unusual, it has always run 160. The engine started to run rough

A old trick that might help .... A engine is a air pump ... it sucks air in & spits it out.

If you take a $1 bill & lay it over the tail pipe with the engine running .... The $1 bill should be blown straight out from the air flow.

If the $1 bill starts flapping or fluttering back & forth, you have a valve issue.

 

Not scientific at all, as a air pump it should be pushing air in one direction ... if it sucks the $1 bill back it is a burnt valve .... just a quick test to give you a direction to start looking at.

 

No idea about the higher temps .... I do not think a burnt valve would cause that. .... What do I know?

 

What does the plug look like on the low compression cylinder? .... If you had a blown head gasket leaking coolant into that cylinder, that plug would look so clean as if it was steamed cleaned .... While the others would be coco brown or dirty with carbon.

 

You have something going on, will be basic troubleshooting .... most likely remove the head for a closer inspection .... The compression on all the other cylinders is really good.

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Los_Control I will try the dollar bill trick and see if that will give me some indication of the burnt valve. Yes the other cylinders seem to be great. I just hate to pull the head last time I pulled the head on the 218 I broke three bolts off what a mess that was. The tail pipes have always made a kind of popping sound. 

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I would think the popping sound may be a indication of a bad valve not doing what it should do.

I would also think the last time you pulled the head "fixed" the head bolt issues.

 

In the era these cars were running, it was common to take your car to a Gas station & have a mechanic do a ring or valve job. Sometimes both ... Just depends on the wear of the engine. Often this work was done in 1 or 2 days with the engine left in the car.

 

 

They use to have boring machines they would mount over the engine & bore out the cylinders to add oversize rings/pistons ....

Today we take it to a machine shop & have everything redone .... Back in the day, a valve job was 1 day at the local gas station..

 

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I also suspect a bad valve.   Easy way to confirm, if you have an air compressor:  Buy or make an adapter to attach an air hose to a spark plug hole.  Insert into #3 and attach an air source.   Listen for air escaping at the tail pipe.   You can probably use parts from your compression gauge to make the adapter if it's a screw in type.  

 

Or you can go all in and buy/rent a leak-down tester.  Get the same general info but at greater cost.

 

I like fast, cheap and easy.  

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Since you already have a compression checker, assuming a screw in on, just take the schrader valve out and use it to hook your compressor up to the engine.  The quick connect on the adapter hose should also work with the quick connect on the end of your air compressor hose.

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I'm out of town till Tuesday so when I get back I will try the air compressor but don't the # 3 cylinder have to be in the closed position for the valves in order to see if one of the valves is not celling off? Wouldn't the air just go right through if the valves aren't closed? 

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On 10/13/2022 at 4:53 PM, Labrauer said:

I'm out of town till Tuesday so when I get back I will try the air compressor but don't the # 3 cylinder have to be in the closed position for the valves in order to see if one of the valves is not celling off? Wouldn't the air just go right through if the valves aren't closed? 

yes..   Slight omission on my part.   It also will try to rotate if the air pressure is too high.  so a regulator or blocking it from turning is also needed,  I just use about 10-15psi.

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  • 2 weeks later...

Well people the car is back to running great again. I don't know what happened but I decided I would remove the valve covers and check the valve clearance on all the cylinders. When I got to number three the adjustment was way to tight couldn't get the feeler gauge in. After all were checked went back to the compression test and now 150# all but # three and it has 147#. No more missing when running. I can't figure out why the valves got to tight all of the sudden when the car gets drove every day 

. I'm puzzled. Anyhow thanks for all the great responses from all.  

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  • 1 month later...
On 10/28/2022 at 12:59 AM, Labrauer said:

Well people the car is back to running great again. I don't know what happened but I decided I would remove the valve covers and check the valve clearance on all the cylinders. When I got to number three the adjustment was way to tight couldn't get the feeler gauge in. After all were checked went back to the compression test and now 150# all but # three and it has 147#. No more missing when running. I can't figure out why the valves got to tight all of the sudden when the car gets drove every day 

. I'm puzzled. Anyhow thanks for all the great responses from all.  

I have seen adjusting screws which are no longer self-locking and the lash can change. If you encounter that, instead of replacing the tappet or adjusting screw, you can extend the screw an extra amount and hit the adjusting screw threads with a punch, to upset the threads and restore the interference fit when the screw is run back down into the tappet body. Not pure, but it does save a ton of work

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