The 1947 - Present Chevrolet & GMC Truck Message Board Network

Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1947 - 1959 Chevrolet & GMC Pickups Message Board


Thread Tools Display Modes
Old 02-06-2019, 11:50 AM   #1
Registered User
Join Date: Dec 2015
Location: Sweet Home, Oregon
Posts: 13
Exhaust valve not holding pressure?

I've got a 51 3100 with the 216. beautiful restoration about 15 years ago. Has always run very smooth.

A couple months ago i'm driving up the hill and suddenly the truck starts missing/coughing/surging. Looses all power. Idles with a slight miss.

I confirmed that the fuel system is good. I confirmed that i'm getting good spark on all the cylinders. Confirmed that I'm getting good vacuum on the manifold.

while the truck idles, I short out each plug one by one. All plugs, when shorted, cause the engine to run even rougher, except #3.

What is my next step? My hunch is that the exhaust valve on #3 is either stuck open or the seat has gone bad. What is the best way to confirm? I don't want to pull the head until i'm reasonably sure that it must be done.

duramaxguy is offline   Reply With Quote
Old 02-06-2019, 04:00 PM   #2
Divine 59
Registered User
Join Date: Sep 2011
Location: Austin, TX
Posts: 264
Re: Exhaust valve not holding pressure?

I would try a compression test of all cylinders. Could be exhaust valve or a spring issue.

Could also remove the valve cover and observe valve operation, it may make some clean up if you have the valve cover off during engine idling.
My Build Thread
Divine 59 is offline   Reply With Quote
Old 02-06-2019, 04:34 PM   #3
Registered User
Join Date: Nov 2010
Location: calgary alberta
Posts: 3,560
Re: Exhaust valve not holding pressure?

start with a compression test. if you find low on that cylinder pull the valve cover and check the valve lash, if good ensure the valves aren't hanging up in the valve guides or the springs aren't weak or broken. usually an exhaust valve issue. you can also do a leak down test as well. get the engine on top dead center, pull the plug and put air pressure in the cylinder then look to see the the air is going. probably the tailpipe but can also be intake or crankcase. be careful because the engine can rotate with the air pressure if it isn't dead on TDC.
dsraven is offline   Reply With Quote
Old 02-08-2019, 12:08 AM   #4
Registered User
mr48chev's Avatar
Join Date: Jul 2007
Location: Toppenish, WA
Posts: 11,313
Re: Exhaust valve not holding pressure?

Dsraven said real close to word for word what I was going to say.

Pull ALL of the plugs, block the throttle and choke wide open and do a compression test.

Compression obviously should be fairly even within 10 lbs high to low.

If you have access to an air compressor the leak down test will tell you where the leak is by listening to where the air leaks out an how much leaks. I don't know if O'Rilley's or Autozone have those on their loaner program but that would be a good way to go and not a bad way to go if you don't own a good compression gauge. I always figure that you have to factor in how many times you end up borrowing (or renting) any tool or piece of equipment before you need to buy your own.

There is the off chance that your engine is carboned up or has a chunk of carbon under the exhaust valve but I'd say that is a bit of a reach. I used to have to de-carbon my grandfather's car about every few months because he would start it up and sit in it for hours on end with either the heat or ac on after he started getting Alzheimer. He never drove over the speed limit in his life anyhow but with his 35 miles an hour trips to town an all the idling the car would carbon up pretty bad and I'd take it down the road a bit and pull over and pull the air cleaner and run the throttle up with one hand and dribble a quart of ATF down the throat of the carb, put the air cleaner back on and take off up the road with my foot in it and run up the road about ten miles and turn around and run back at a good clip and it was good for a few more months.
Founding member of the too many projects, too little time and money club.

My ongoing truck projects:
48 Chev 3100 that will run a 292 Six.
71 GMC 2500 that is getting a Cad 500 transplant.
77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around.
mr48chev is offline   Reply With Quote
Old 02-10-2019, 10:02 PM   #5
Registered User
Join Date: Nov 2010
Location: calgary alberta
Posts: 3,560
Re: Exhaust valve not holding pressure?

anybody ever used seafoam motor treatment? not recommneding it used in the engine oil, personally, but in the fuel it has helped many vehicles with problems. I use it in the fuel on every oil change in my daily driver. when I had to pull the intake on my 5.3 chevy the back side of the intake valve and the port runner after the fuel injector looked like a new engine. no build up on the back of the vale etc. possibly use a bit of that in the fuel for the long term. for the short term try the your results for us when done.
dsraven is offline   Reply With Quote
Old 02-11-2019, 10:40 AM   #6
Registered User
Join Date: Jan 2018
Location: Knoxville TN
Posts: 69
Re: Exhaust valve not holding pressure?

I had a similar problem on my 49 216. Was going up a hill then suddenly lost most power and limped home.

After making sure it had fuel to the carb and it worked and verified spark at all 6 plugs I then pulled the valve cover and observed the valves 5 & 6 weren't acting right and then noticed the rocker shaft had broken at the bolt hold down hole. Instead of the valves opening when they should the rocker arm was lifting at the break, basically shutting down two cylinders.
I picked up another rocker shaft and it's been running good since.

A compression test would have narrowed it to the dead cylinders but pulling the valve cover was quicker in my case.
Good luck
Volfandt is offline   Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT -4. The time now is 01:27 AM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2019, vBulletin Solutions Inc.
Copyright 1997-2013