1978 running issue

H

Hardrinseron

Regular user
Hi
Newbie here!
I’ve just commissioned a 78 which I bought in bits and had a full engine rebuild. Initially it ran ok but after about 200 miles it’s developed a noise on startup like a rattle/knock which goes away when you rev slightly and goes away too when the engine has run for a few mins. Any idea what the cause is? It’s also stalling a lot now but I suspect a vacuum leak perhaps?
Thanks
 
Roscobbc

Roscobbc

Moderator
So many things that could be - perhaps if we ask a few questions and try and set some parameters for you.
What is your oil pressure like - on start-up when cold and when hot. Both at idle and when revving (at say 2000 rpm)?
The rattle/knock you describe does it go away completely whe the engine is warmed-up?
Are you headlamps opening/closing quickly?
Does it try to stall when you put it drive (assuming its an auto)?
Have you traced and inspected the vacuum pipe for cracking?
If the engine was re-built, presumably the carb was removed? is the carb to manifold gasket fine? - same with inlet to heads?
Have you tried spraying a water mist over the respective parts to see if its being 'sucked-in' by a vccuum leak?
 
Forrest Gump

Forrest Gump

CCCUK regional rep
Try the old trick of putting your ear to a long screwdriver placed around the engine. You might be able work out roughly where the noise is coming from.
A lazy hydraulic lifter maybe.
 
H

Hardrinseron

Regular user
Roscobbc said:
So many things that could be - perhaps if we ask a few questions and try and set some parameters for you.
What is your oil pressure like - on start-up when cold and when hot. Both at idle and when revving (at say 2000 rpm)?
The rattle/knock you describe does it go away completely whe the engine is warmed-up?
Are you headlamps opening/closing quickly?
Does it try to stall when you put it drive (assuming its an auto)?
Have you traced and inspected the vacuum pipe for cracking?
If the engine was re-built, presumably the carb was removed? is the carb to manifold gasket fine? - same with inlet to heads?
Have you tried spraying a water mist over the respective parts to see if its being 'sucked-in' by a vccuum leak?
Click to expand...
Hi, thanks for the reply!
Oil pressure reads like 3/4 on the guage pretty much all the time (at idle and driving) so i think oil pressure is good
The noise does seem to completely dissapear once either the engine is warm or if you rev to like 2000rpm. Even at cold it goes with some revs
Headlights work but are quite slow
Yes it is an auto. It does seem to stall sometimes when you put it in drive but the most common time to stall is when i go to set off and come off the brake
Vacuum pipes are mostly new though some of the hvac ones might be older. I was thinking about clamping some of the vacumm lines like the headlights to eliminate those leaking.
Yes, the carb was removed. It seems to be ok but i will do the mist test on it and see if anything is drawn in
Will report back! thanks again
 
Roscobbc

Roscobbc

Moderator
It could be a number of things.......even something as simple as adjusting valve lash on the hydraulic lifters - perhaps not set correctly when engine re-assembled following said re-build?
 
Chuffer

Chuffer

CCCUK Member
I am with Ross and Forrest on the hydraulic lifters being a possibility if the lash is not set correctly .It is fiddly job that needs patience and everything done in correct sequence. Could also be a crack or leak in the exhaust manifold that can sometimes sound just like a rattle at start up but diminishes with revs and `takes` up with engine temperature rise . I have just that on my 1980 but it drives and ticks over ok so it will be a winter job to look forward to . 😒
 
Roscobbc

Roscobbc

Moderator
Chuffer said:
I am with Ross and Forrest on the hydraulic lifters being a possibility if the lash is not set correctly .It is fiddly job that needs patience and everything done in correct sequence. Could also be a crack or leak in the exhaust manifold that can sometimes sound just like a rattle at start up but diminishes with revs and `takes` up with engine temperature rise . I have just that on my 1980 but it drives and ticks over ok so it will be a winter job to look forward to . 😒
Click to expand...
Fibre header gaskets can brake-up with age, leak and sound like a rattle.....as engine warms-up and metal components expand the noise can reduce.
 
Chuffer

Chuffer

CCCUK Member
Hardrinseron said:
That is a good question and something i've been thinking i should check. Where is the best place to get documentation on those torque settings?
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You can get Heynes Repair Manuals for C3`s and will probably find one Ebay that might help .Handy for future jobs too . It is my understanding that newly fitted manifolds require re torquing after a couple of run ups to settle in . From what I have read studs should be tightened to 10ft/lbs first after installation starting in the middle and working outwards , then tighten to 20ft/lbs . I recall reading somewhere in the past that GM did not fit gaskets originally and relied on a perfect mating face to give a good seal . I am sure someone on the Forum will correct me if I am wrong on all this .:unsure:
 
johng

johng

CCCUK Member
My 78 didn't have any gaskets on the exhaust manifold and didn't leak. If you've got headers, I believe they are more prone to leaking.
 
H

Hardrinseron

Regular user
Chuffer said:
You can get Heynes Repair Manuals for C3`s and will probably find one Ebay that might help .Handy for future jobs too . It is my understanding that newly fitted manifolds require re torquing after a couple of run ups to settle in . From what I have read studs should be tightened to 10ft/lbs first after installation starting in the middle and working outwards , then tighten to 20ft/lbs . I recall reading somewhere in the past that GM did not fit gaskets originally and relied on a perfect mating face to give a good seal . I am sure someone on the Forum will correct me if I am wrong on all this .:unsure:
Click to expand...
actually i forgot i have one! interesting what you say about new manifolds needing retightening as mine does have new manifolds.Thanks for the info. I've always wondered if it might be the manifolds leaking
 
Chuffer

Chuffer

CCCUK Member
Hardrinseron said:
actually i forgot i have one! interesting what you say about new manifolds needing retightening as mine does have new manifolds.Thanks for the info. I've always wondered if it might be the manifolds leaking
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Are they new cast iron ones or headers ? It will make a difference to the setting up .
 
Roscobbc

Roscobbc

Moderator
johng said:
My 78 didn't have any gaskets on the exhaust manifold and didn't leak. If you've got headers, I believe they are more prone to leaking.
Click to expand...
My experience with fitting header to a number of differing American cars is than some of the cheaper brands can have flanges that do not fit parallel with all of the exhaust ports. I recall a BB Ford with Hooker headers (supposedly industry leaders in term of fit and quality) where they had a one piece continuous flange each side that just wouldn't seal-up across all exhaust ports. I had to do three 'cuts' to the flanges so each exhaust tube would sit 'square' to the exhaust ports.
Header gaskets can be important. I've found the multi-layer pressed all aluminium gaskets seem to work best, are re-usable and last the longest.
 
Roscobbc

Roscobbc

Moderator
Hardrinseron said:
actually i forgot i have one! interesting what you say about new manifolds needing retightening as mine does have new manifolds.Thanks for the info. I've always wondered if it might be the manifolds leaking
Click to expand...
Are they OEM style exhaust manifolds - or tubular headers.
 
H

Hardrinseron

Regular user
Chuffer said:
Getting confused now . Do you have a 4 speed or a TH350 3 speed auto or have you beefed up to a TH400 auto ? The fact that you have headers will rule out what I have said about standard factory cast iron manifold set up .
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It’s a 3 speed auto. I know it’s a TH350 as it has the lockdown cable
 
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