English Ian
Newbie
Posts: 6
Reg: 08-27-24
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08-29-24 11:42 AM - Post#2873227
I have a 396 big block engine which misfires on cylinders 5 and 8. Using an infra red thermometer I can see maybe 300 degrees plus on the other cylinder exhausts but around 100 degrees on 5 and 8. If i blank off the CRV valve the temperature of 5 and 8 increases to say 150 degrees. Then if I unblank it it starts to fall again. To me this would indicate the cylinders are at least firing. If I blank off all the suction pipes the temperature remains around half that of the other 6 cylinders. Again, to me this would indicate less fuel going into 5 and 8. What does anyone think could cause this?
Thank you.
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bobb
Super Senior Member
Posts: 6907
Loc: paradise
Reg: 09-05-03
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08-31-24 08:43 AM - Post#2873280
In response to English Ian
welcome to ct. when was the last tune up done? how old are the ignition parts? did you do a leak down or compression test?
| 70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails. |
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English Ian
Newbie
Posts: 6
Reg: 08-27-24
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09-03-24 03:12 AM - Post#2873400
In response to bobb
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 Magnetocheck
"3rd Year" Silver Supporting Member
Posts: 542
Age: 70
Loc: Charlotte, NC, USA
Reg: 09-05-22
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09-03-24 03:26 AM - Post#2873401
In response to English Ian
Is "CRV" British for "PCV" valve? I'm a bit of an Anglophile, even scrubbed my tyres against the kerb, but never heard that one 
If so, it is a bit of a puzzle that blocking it off would affect only 2 cylinders. Where do you have it plugged in? Are you running a stock BBC manifold? Maybe post of pic of your plumbing? How many miles on the engine?
BTW, welcome to the forum!
Bob
'65 Impala SS 396 Convertible
Member, National Impala Association
--It's weird being the same age as old people! |
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English Ian
Newbie
Posts: 6
Reg: 08-27-24
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09-03-24 08:25 AM - Post#2873411
In response to bobb
Thanks for replying. Electric parts all fairly new. No idea when last tune up was! Compression test fine.
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English Ian
Newbie
Posts: 6
Reg: 08-27-24
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09-03-24 08:46 AM - Post#2873415
In response to Magnetocheck
I have no idea why I put CRV valve! Yes, I did mean PCV valve. It’s plugged in to the Edelbrock Performer 2.0 intake manifold near the distributor. I’m not sure how I send photos. Not many miles on engine since overhaul but I don’t think the heads were overhauled.
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Magnetocheck
"3rd Year" Silver Supporting Member
Posts: 542
Age: 70
Loc: Charlotte, NC, USA
Reg: 09-05-22
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09-03-24 09:26 AM - Post#2873420
In response to English Ian
You need to be a supporting member ($) Ian to post pics but you can link to any photos on the web wherever you can store them.
The Edelbrock Performer 2.0 is a dual plane, and #5 and #8 are fed by the right side of the carb if I remember, as are #2 and #3. Is it possible there is some funny business going on with 2 and 3 as well that you are missing? Try using the large 3/8" nipple under the carb for the PCV hose to see if you can't balance things out better, (unless you don't have one there). BTW, I assume the PCV valve passes the "rattle test", You might try replacing it as they are not expensive.
It is possible, but not likely, that 5 and 8 could develop a mechanical problem at the same time. I would recommend a compression check on all 8 just to make sure everything is happy inside those cylinders.
Bob
'65 Impala SS 396 Convertible
Member, National Impala Association
--It's weird being the same age as old people! |
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English Ian
Newbie
Posts: 6
Reg: 08-27-24
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09-03-24 11:45 AM - Post#2873427
In response to Magnetocheck
I’m pretty sure 2 and 3 are as hot as 1,4,6 and 7 but I’ll check tomorrow or Thursday. I have replaced PCV valve. Compression test showed all equal. The chap that’s helping me is much more knowledgeable than I and is quite sure it’s a gasket problem. He can’t see what else it could be. I just wanted to check with people more familiar with large US engines before taking it to bits!
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Magnetocheck
"3rd Year" Silver Supporting Member
Posts: 542
Age: 70
Loc: Charlotte, NC, USA
Reg: 09-05-22
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09-03-24 03:25 PM - Post#2873437
In response to English Ian
Well, pulling the manifold and heads (if necessary) to inspect gaskets is pretty straightforward but not cheap. Keep us posted on what you find out!
Bob
'65 Impala SS 396 Convertible
Member, National Impala Association
--It's weird being the same age as old people! |
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bobb
Super Senior Member
Posts: 6907
Loc: paradise
Reg: 09-05-03
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09-06-24 07:32 AM - Post#2873540
In response to English Ian
at this point i would be looking for a possible carb issue.
| 70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails. |
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aghaga
"4th Year" Silver Supporting Member
Posts: 2391
Age: 70
Loc: Woodlawn, Va.
Reg: 07-05-08
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09-09-24 07:52 AM - Post#2873646
In response to English Ian
PCV should not be connected directly to the intake. Connect it to the base of the carburetor.
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grumpyvette
Performance Subject Matter Expert --
Posts: 17814
Age: 76
Loc: farmersville texas USA
Reg: 03-16-01
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12-10-24 01:30 PM - Post#2876185
In response to English Ian
you can't make logical suggestions without facts, you get fact through testing
personally ID start by very carefully verifying the ignition cap firing order,then, changing out the distributor cap,for a new one,
and swapping to new platinum, spark plugs gap them at about 45 thousands ,
and getting out a quality multimeter , and measuring the ohms resistance in EVERY ignition wire,if they vary by more than about 20% Id suggest they get replaced. you may find the distributor cap, the ignition wires or the spark plugs need replacing.
carefully check for loose vacuum hoses and vacuum leaks
pvc valves are cheap, if in doubt pitch it and get a new replacement?
whats your oil pressure?
fuel pressure"
coolant temp?
have you done a compression test lately?
when was the last time you adjusted the valves?
sounds like a basic tune-ups long over due??
can you number and post clear color pictures of all your spark plugs taken on a clean white typing paper background?
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!! |
Edited by grumpyvette on 12-10-24 01:57 PM. Reason for edit: No reason given.
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English Ian
Newbie
Posts: 6
Reg: 08-27-24
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01-07-25 09:38 AM - Post#2876867
In response to grumpyvette
Thanks for all help. An update:
Inlet gaskets didn’t look good, replaced but still not right.
I blocked the hose to the brake servo and all cylinders are now firing properly. Brakes not so good now though !!!
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