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1979 GMC C1500 oil pressure or pump

Tiny
JRAY1989
  • MEMBER
  • 1979 GMC C1500
  • V8
  • 2WD
  • AUTOMATIC
I have a 1979 gmc truck with a 350. The motor is ike a 74'. Had been over hauled about wanna say 5 years ago lol. By a man I do not know. Ive heard it run about 6 months ago but they didnt spin the tourqe converter. So I snaged the engine for free. Lol. But when I dropped it in I hooked it all up and it runs a little rough the oil pressure is at about 12psi and the vaccum was 0hg changed intake gasket now vaccum is about 10-8hg and oil pressure is about 8 psi. Could I have accenttley clogged the screen with old silicon or just weak pump. If I drop the pan how wold I know. Would timeing effect this. Would missing effect this. Can you help me. Also it has a small new holley carb 4bbl on it do I need to adjust the secondary buttery fly to fix the vac. Thanks
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Sunday, March 8th, 2009 AT 11:00 PM

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Tiny
JAMES W.
  • MECHANIC
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Vacuum is 90% controlled by the intake and exhaust valves and their respective seating. The only other major cause is bad intake gaskets, but according to your post, you changed the intake gaskets so that rules that out
Oil pressure is generaly controlled by bearing clearance. The more bearing wear, the lower the oil pressure. Rarely do GM oil pumps wear to the point where the pressure is to the point you described. You would have had to drop a lot of old silicone and gasket material down the valley to plug the oil screen. Was the valley area around the lifters fairly clean? This is usually a good indication of what's laying in the pan. The rough idle/running, is probably bad valves. A compression test will varify this. Adjusting the secondaries on the carb, I doubt will help unless they are drastically out of adjustment. If you pull the pan, you'll want to remove a couple of rod and main caps (don't mix them up) and look at the crank journals and the bearings. Instincts tell me the motor is just plain tired. There is one possible bright spot here. If your 350 is a 1974 or close to that era, you may have a 4 bolt main block. If this be the case, they are rare, worth money and worth rebuilding. A lot of stockcar racers are looking for these blocks. Hope this helps.
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Sunday, March 8th, 2009 AT 11:46 PM
Tiny
JRAY1989
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  • 8 POSTS
I looked under the valve covers there was a little bit of silcon don on the valve springs and what not. I thought I could just blow out the old silcon with an air compresssor but I guess that backed fired? I going to do that compression test but will the same thing show if the butterflies are out of wack? Thanks jake 74' it is.
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Monday, March 9th, 2009 AT 12:26 AM
Tiny
JAMES W.
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Yes, open butterflies will lower the vacuum. It will also lean out the mixture and make the engine run rough. That's why engine vac is the highest at idle or de-celeration. On acceleration, vacuum, vac drops. This is why they install turbo or superchargers on engines. They turn negative pressure (vacuum), into positive pressure (boost) so it actually "packs" the air/fuel mixture into the cylinders. We can't rule out a plugged oil screen. When an engine is started cold, the oil pressure usually starts out relatively high and gradually drops as the oil warms up. Usually, if an oil screen is partially plugged, the pressure will start out and stay relatively low as it warms up. Also, an engine with worn bearings will usually start out with fairly high pressure, but will drop drastically low as the oil warm up. Hope this helps you.
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Monday, March 9th, 2009 AT 12:03 PM
Tiny
JRAY1989
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Alright I checked the compression all was within 10 lbs. Of 150. Adjusted the buttlerflies and still low vacuum. Called holley they told me that it could be cause of late timing. The thing is that the timing mark that hangs off of the timing cover has never been on there. I was told that if I take the mark on the harmonic balancer and put it straight up at around 12 o'clock that that should be the mark did that but I had to turn the cap all the way BACK to the firewall so can you tell me if I can just put a mark gauge on the timing cover or where a mark is? Thanks again jake
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Saturday, March 14th, 2009 AT 3:15 PM
Tiny
JAMES W.
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You will have two "top dead centers", 12:00 noon. One for intake and one for compression. You will want to be on TDC compression. If you're there, the rotor should point to #1 spark plug terminal on the cap. If you're on TDC intake, the rotor will point in the opposite direction. Without looking it up, I believe the timing setting should be 8 or 10 degrees before TDC. You can kinda' guess at this and adjust a little either way to make it run as you want it. For this, I would use a timing light.
One inportant thing on your vacuum. A worn or streached timing chain can drastically affect it. A badly worn chain causes the valves to run late, thus, reducing the manifold vacuum. You can check chain wear by turning the harmonic balancer back and forth. You will be able to feel when the chain tightens against the cam sprocket. A couple of degrees of slack is acceptable, but if it's 8-10 degrees or more, you need a chain. 350's are known for eating chains. I've seen them worn to the point where they wear a hole through the timing cover, yet the engine still ran.
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Saturday, March 14th, 2009 AT 4:18 PM
Tiny
JRAY1989
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I cant use the rotor to mark tdc because my dad pulled the distributor when he timed it. He used he thumb on #1 spark plug to find compression. Then brought the rotor to #1 on cap I think he was off.I have timing light but I dont know where to line up the line on the harm. Bal. I guess what Im saying is I dont know how I tell if its on intake or compression. Jake
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Saturday, March 14th, 2009 AT 5:53 PM
Tiny
JAMES W.
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If you can't use "the thumb over the #1 hole trick", you'll have to pull the valve cover on the #1 side and watch the valves. While you turn the crank clockwise, and after the #1 INTAKE valve closes, that is the start of the compression stroke. Continue turning until the mark on the balancer is at top dead center (12:00). To reinstall the distributor, you may have to turn the oil pump drive shaft a bit at a time so the dist rotor will line up with #1 term in the cap.
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Saturday, March 14th, 2009 AT 6:11 PM
Tiny
JRAY1989
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Hi sorry to bother you again but I have another question for you. I've been told a bunch of things about this motor that people have just taken there words for. Anyways I dont know the year of this nuber anymore and I do know that it has been over hauled since I have just found this out:
good compression
good heads
new lifters
new timing chain and gears
new cam which just noticed was a HI-proformance cam

now I have ran ito thesse problems:
distribitor turns backweards - instead rpm coming up as cap turned to carb rpms come down and the rpms comes up when turned to firewall on drivers.
Runs to about 50mph and dies down the road- like its out of gas.
FIRE HITS WHEIDSHIELD AS IT COMES OUT OF THE CARB! Like its out of time.
Here what Ive been told and what I've done:
I was told that the fire was cuz its out of time not in the distribitor but the cam. That the cam be wrong for the truck, 180 out, tooth or 2 off, or I need a hi- pro. Fuel pump. I ruled out the fuel pump cause I thought it would not be coughing flame like that if it was just the pump. Plus I dont want that cam. I took the timing cover off and brought it to compression stroke it lined up at 12 and 12 and looked about.5 a tooth off? I was told that a few went 12 and 12 instead off dot to dot? What years went 12 and 12. And how can I figure how this cam goes back? Thanks alot again jake
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Saturday, April 4th, 2009 AT 8:29 PM
Tiny
JAMES W.
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OK, The normal setting for cam/crank is "dot to dot". The reason being this is TDC of the "compression" stroke. This is when #1 cylinder should fire, give or take a few degrees. SO, you line the disrtibutor up to #1 plug wire and the engine should fire up and just need the ign timing "fine tuned". IF, you line the marks up 12 and 12, the cam timing is ok, but you are TDC "intake" stroke so the distributor needs to be installed 180 degrees "away" from #1 plug wire so it spins around to the #1 term when the crank comes to #1 COMPRESSION stroke. SO, bottom line is; either 12 to 12 OR dot to dot is correct. The only difference is how you install the distributor. THE ADVANTAGE of going dot to dot, is, it's easier to make sure the cam/crank timing is correct and not off a notch and easier to see the distributor is pointing at #1 plug wire. Also, remember, when reinstalling the distributor, you may have to turn the oil pump drive shaft a little so the dist drops in where you want it.
If you're blowing fire out to the wind shield, 1- that's got to be pretty impressive. 2- the distributor is firing before the intake valves are closed (cam timing). If the distributor timing was off far enough to do that, Idoubt if you'd even get it started. Hope I've explained this well enough. Let me know how it goes.
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Saturday, April 4th, 2009 AT 10:22 PM
Tiny
JRAY1989
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You was right the cam was out I went stock and put it dot to dot a few trys and she fired right up thank ya. The 79 gmc need a new speedometer cable I replace that and the gauge but kind of didn't work it wobbled up to 30 grinded and popped lol now I cant get to work chhecked the drive gear it seams to work right (took it out spun the plastic gear and the cable turned while out?) So I stuck my finger in the hole And I couldnt feel any teeth or gear? What do you think the problem is and whts the solution to fix it. I really do thank you for all your help man it hasnt been easy fixing my grandads ol truck for the shows you really have made it easier for me thank you again.
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Tuesday, April 28th, 2009 AT 10:37 PM

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