Hot Rod Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
1,205 Posts
Discussion Starter #1
All this conversation about scopes and diagnostics brought back a memory of a weird problem I had and never got to figure out, as the customer bailed out on me. One of the few that got away.
It was a 79 Ford 302 with a definate miss. I isolated a dead cylinder with a cylinder kill test. The ignition checked out fine although the firing voltage on that cylinder was a little low. Even with new cap, rotor, wires and plugs it was still low. I did a compression test suspecting a burnt valve but the cranking compression showed all cylinders to be within 10% from high to low which is well within specs. Finally I left the compression guage in the dead hole and fired the car up. When I released the compression that it had pumped during cranking it would only pump back to about 35 psi with the engine running. I checked for a broken valve spring or bad cam lob and found neither. At this point I had three hours into it and the customer didn't want to spend any money on the car and bailed out on me. I never did figure that one out. Any Ideas? BTW I think the low firing voltage was only due to the lack of compression. It doen't take as much juice to fire a plug that isn't in a compressed atmosphere.
 

·
wind & fire = guides to power
Joined
·
1,529 Posts
EXAMPLE 1 - RESTRICTED INTAKE CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 80 If Snap reading is low (much less than 80% cranking compression), look for restricted intake air- severely carboned intake valve , worn lobe on cam, rocker problem,

it may have just needed the "cobwebs blown out"
 

·
Registered
Joined
·
1,205 Posts
Discussion Starter #3
I suspected that it may have had a bad guide on the intake and the resulting coked up valve from oil seepage but never got to tear down the engine ot test any further.
 

·
Registered
Joined
·
222 Posts
Years ago I also had a similar problem with a Mustang II.It had a tunnel ram and two 4s.Cylinders 5 and 6,adjacent,only made partial power.I tried leak-down test,running compression test,replaced the whole intake system and finally replaced the cylinder heads!That one got me!I had three days into the job and the owner quit and sold the car.The worse,the problem was intermittent.
 

·
wind & fire = guides to power
Joined
·
1,529 Posts
Is it possible that a primary jet was clogged?
 

·
Registered
Joined
·
222 Posts
No.I went from the dual quad to a stock manifold with 2 bbl Motorcraft that was in use in my Maverick.When the problem was there you could touch the headers and only fell it slightly warm.I wish I had the car on a Dyno with a labscope to it.
 

·
Registered
Joined
·
393 Posts
I would like to know how you checked for a broken valve spring? Did you remove the springs from the head or just examine them? I had 3 broken springs in my 350 and ran the motor with the valve covers off. Couldnt see it or feel it. I only found it after I removed the springs and examined them. I also had the warm not hot headers on the affected cylinders.

I would like to know how you checked for a broken valve spring? Did you remove the springs from the head or just examine them? I had 3 broken springs in my 350 and ran the motor with the valve covers off. Couldnt see it or feel it. I only found it after I removed the springs and examined them.
 

·
Registered
Joined
·
222 Posts
Yes at the time we took the cylinder heads apart and found out they were quite rare,'69 351W high compression chambers.We were so mad to discover the problem that we took them completely apart thinking there could be cracks,welds or something.Springs were looking good.We did not pressure test them,though.I was so upset that I installed a stock cam/lifters on it and ran the engine without front cover or water pump.After some oil mess,still no change... 5 and 6 only warm.
 
1 - 8 of 8 Posts
Top