Home 
Home Search search Menu menu Not logged in - Login | Register
> Jensen Healey & Jensen GT Tech > Engine & Transmission > Low Compression on #1 cyl.

 Moderated by: Greg Fletcher
New Topic Reply Printer Friendly
Low Compression on #1 cyl.  Rate Topic 
AuthorPost
 Posted: 02-03-2017 01:01 am
  PM Quote Reply
1st Post
Paul Koehler
Member


Joined: 04-06-2005
Location: Malone, New York USA
Posts: 94
Status: 
Offline
Ran a compression check and discovered the following:
cylinders 2,3,&4 were in the 120 range and cylinder #1
came in at 35.
Could all you smart folks suggest a flow chart type series of steps that should be undertaken in order to find the root cause and correction needed to solve the problem?

Back To Top PM Quote Reply  

 Posted: 02-03-2017 04:27 pm
  PM Quote Reply
2nd Post
dwalls1
Member
 

Joined: 04-11-2008
Location: Bloomfield, New Mexico USA
Posts: 207
Status: 
Offline
Guess I'd start with valve clearance, but to have just #1 with tight clearance might be unusual. Maybe first would be another comp check with oil added to cylinder. Might help determine if it's valves or rings but, again on just one cylinder might be unusual. There are lots a guys here that will have a better answer. Good Luck.

Back To Top PM Quote Reply

 Posted: 02-03-2017 11:32 pm
  PM Quote Reply
3rd Post
jomac
Member
 

Joined: 09-16-2016
Location:  
Posts: 71
Status: 
Offline
What did the spark plug look like as compared to the others,

Back To Top PM Quote Reply  

 Posted: 02-04-2017 01:01 am
  PM Quote Reply
4th Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 572
Status: 
Offline
Repeat the compression test on the low cylinder just to make certain it wasn't simply a procedure screw-up on your part. Specifically, make certain the throttle is held wide open during the test. The battery must be fully charged, and capable of spinning the engine over at 200 rpm or better. If not, use jumper cables to a running vehicle.

JH specifies 110-130 psi for a cold engine. Lotus specifies 150-170 psi for an engine that has first been run long enough to bring it up to full operating temperature before doing the test. In either case, pressures for all four cylinders should be within 10 psi. 30 psi is clearly a bit low... like hole-in-the-piston low.

If the pressure re-tests low, then squirt some oil into that cylinder. It's difficult to get even distribution of the oil around a piston in a slant cylinder. So, with the piston at the bottom of it's stroke, aim for the cylinder wall above the high side of the piston. Give it a few squirts, then before the oil all runs down hill, turn the crank through a couple of revolutions, scraping oil into the piston-cylinder gap, and around the piston. Then repeat the compression test for that cylinder. 200+ rpm with the throttle held wide open.

If the new reading is dramatically improved, then the rings were not previously sealing in that cylinder and the oil helped fill the gaps, improving the result. Oil is not a fix, so it's time for a ring job.

With oil added, if the pressure result is pretty much unchanged and still low, then the rings are probably not the problem. In that case, it's most likely a burned or otherwise leaking valve... or several. It could also be a burned piston, collapsed ring lands, or a burned-through crown.

In any case, a persistent 30 psi compression reading is not going to fix itself with medicine poured in, or with anything you can fish into the hole. Having some idea what's wrong before you start is nice, but the fix will involve some degree of engine disassembly. At least removing the cylinder head, and perhaps a complete tear down.

Good Luck,
Tim Engel

Back To Top PM Quote Reply

 Posted: 02-04-2017 03:07 pm
  PM Quote Reply
5th Post
Paul Koehler
Member


Joined: 04-06-2005
Location: Malone, New York USA
Posts: 94
Status: 
Offline
Thanks for the advice. I did the comp. check three times and then the oil to the cylinder procedure with no appreciable improvement.It is a 100,000 plus mileage engine that has had regular attention (oil&filter;plugs; timing belt/bearings; waterpump),but never been opened up. Due to location, it only gets driven 6 months a year and then usually less than 3000 miles.
I think I'll start by switching heads from my spare parts car, so that hopefully I'll have a driver for spring, and then see what attention the original head needs.

Back To Top PM Quote Reply  

 Posted: 02-05-2017 02:14 am
  PM Quote Reply
6th Post
Bfitz241
Member


Joined: 02-05-2015
Location: Saint Augustine, Florida USA
Posts: 169
Status: 
Offline
do you have an air compressor? If you do, adapt the air compressor to the compression tester hose.
Bring #1 to TDC. Trans in gear E brake on. Pressurize the cylinder.
Listen at the carb, if you hear air bad intake valve, listen to the exhaust, if you hear air, bad exhaust valve. Listen at the oil filler cap, if you hear air, holed piston.
you can google "leak down test" for more detail.

Sorry, just saw you've got a spare head....taking the old one off will provide a definitive answer...hope it's something simple.  Good luck

Last edited on 02-05-2017 02:34 am by Bfitz241

Back To Top PM Quote Reply

Current time is 08:02 pm  
> Jensen Healey & Jensen GT Tech > Engine & Transmission > Low Compression on #1 cyl. Top




UltraBB 1.172 Copyright © 2007-2011 Data 1 Systems