• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Effect of bad valves on engine performance?

Share this page

I

ichthos

Darth Vader
Country flag
Offline
What are the effects of bad valves on engine performance? I did a complete rebuild three years ago on my 69 T6 engine, but recently found it has bad compression. I have great oil pressure, but bad readings when I do a compression test. The guy that rebuilt my valves doesn't do very good work (I only found that out after he rebuilt them from friends who had work done there), so that is another reason why I am suspecting the valves. Besides the effects on engine performance, what if any damage can this do to drive with bad valves (it may be summer before I have the money to rebuild the valves)? Any input is greatly appreciated.
Kevin
 
A bad (burned) exhaust valve will make the engine run very rough at low speed. The car will buck when trying to creep along at low speed. On the highway it may be completely unnoticeable, depending on how badly the valve is burned.

A bad intake will frequently result in backfiring into the intake. A bad seal on an intake is much less common than a burned exhaust. It is more common for intake valves to stick open...also causing backfiring into the intake.

Of course, for either the compression check will show low numbers. I guess the short answer is that an engine rarely runs well with a bad valve.
 
Have you tried repeating the compression test with a teaspoon or so of oil added to each cylinder just before testing that cylinder? If the reading comes up sharply, most likely the rings are bad. If it only comes up a bit or not at all, most likely valves or perhaps (if the low readings are adjacent) a blown head gasket.

We're assuming of course that you have checked the valve lash.

Most likely, running with a burned valve won't damage anything but the valve and maybe the seat. You might want to avoid any prolonged full throttle or high rpm operation, though.
 
I am seeing some of the symptoms you are talking about, John. I did try the oil, but it does not go up much, Randall. I did not check the valve lash - would that cause bad compression though?
Kevin
 
ichthos said:
... I did not check the valve lash - would that cause bad compression though?
Kevin
Click to expand...

It would if there is no clearance, causing the valves to not close all the way.
 
Old school...you can tell a bad valve with a vacuum gage. The vacuum will pulse irregularly at idle as the bad cylinder comes around on the stroke. A good running engine with good valves will have a steady and consistent pulsing, but no large pulses.
 
I you can find a good reliable engine rebuild shop, take the TR in now and ask him to run a "Leak-down Test". He will remove the sparkplugs and screw the leak-down tester into one of the sparkplug holes. Then he will add compressed air to that cylinder with the piston at the bottom for maximum chamber volume. Then the air will leak out that cylinder via the burnt valves (if it is the valves). A good touring engine might leak about 3% to 4%. A racing engine aims for 1 or 2 % leakage. He will then move along to do the other cylinders. If you find one (or more) of your cylinders is - for example - 15 or 20 %, then you need to have the problem addressed.

Harbor Fright sell leak-down testers or some TR owners have made their own.
 
Here are two videos how to make your own lesk-down tester and how to use it.
https://www.youtube.com/watch?v=7UcOn8OEt0Y

https://www.youtube.com/watch?v=GvG22YMOzUI

Here is a demo video by a dissatisfied user of a Harbor Freight leak tester. Other videos say the same.

https://www.youtube.com/watch?v=wEYfQbhtNPs


BTW, I stand corrected. The piston should be a the top and the engine should be in gear so that the air pressure doesn't rotate the engine. Readings will be truer if your engine is stll at operating temperature.
 
CJD said:
Old school...you can tell a bad valve with a vacuum gage. The vacuum will pulse irregularly at idle as the bad cylinder comes around on the stroke. A good running engine with good valves will have a steady and consistent pulsing, but no large pulses.
Click to expand...
Hook up the vacuum gauge to your intake manifold , let the engine idle and compare what you see to Scenarios 5 thru 10
https://sbftech.com/index.php?topic=3020.0
 
Back
Top