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compression

Discussion in 'Chevy Trucks' started by fireman_31098, Jul 15, 2002.

  1. fireman_31098

    fireman_31098 Junior Member
    Messages: 15

    question. i have a 84 1/2 ton 4x4, with a chevy 400 small block. started to see a drop in oil pressure over the last month. did a compression test and found good #'s in all cylinders, but was only getting about 5 psi tops in the #1 cylinder, tried putting a little oil in there and still didnt get more than 5 psi, what could be the problem? havent heard any knocking. please help!
     
  2. MTCK

    MTCK Senior Member
    Messages: 346

    Maybe a hole in the piston, or a blown head gasket? How does it run?
     
  3. fireman_31098

    fireman_31098 Junior Member
    Messages: 15

    its runs ok, when i get on the gas hard i get a bad clunk clunk feeling, thn it takes off. however if i just ease the gas petal all the way to the floor, i will not get the clunk clunk feeling. it only does this when its under load, wont do it in the drive way when reved up. smokes in the morning, i know the valve seals are probly gone, but i suspect a head gasket also.
     
  4. chevy

    chevy Member
    Messages: 33

    With a drop in oil pressure several gremlins come to mind. One being the bearings are shot, but with no rod knock they can't be to bad. How much oil pressure drop do you have? It also could be that you lanched or lunched a lifter. Some times with not enough oil changes the cam lobes and lifter bases wear. I've seen lifters with 1/4" holes on the bottom of them. This would give you a drop in oil pressure and low compression in your number 1 cyclinder. Either way the rocket must come apart. Mine to I blew a head gasket and I also have a bad valve or two. My choice is to reload a motor that I pulled from a 77 scrapper that I got for parts and do the engine right. Some thing that will give me 300 - 350 h.p. instead of GM'S wopping 185 h.p.
     
  5. fireman_31098

    fireman_31098 Junior Member
    Messages: 15

    yea im not really sure if its the bearings or not, i really dont think so, i pulled the head this morning and found that i had a burned exaust valve on the # 1 about a 1/4" hole right on the edge. this is one of those trucks that i bought form a guy who boght it form a guy and who knows what that guy had done to it. its not all that bad, but the heads are goin to the shop for sure. the pistons look ok, just alot of carbon. with the compression readings i was getting i think the rings are gonna be all right for now, the head gasket looked good. now another question, the small " steam holes" on the 400's , the ones on mine are pluged, is that ok, i t has only overheated on me 1 time cause of a broken fan belt, other than that i havent had any problems with it getting hot. should those plugs be drilled out? oil pressure used to run at 20psi at idle, and 40 psi driving, it now goes to about 55-60 psi when i start it up, then when it warms up i get about 30 psi driving and about 5-10 psi when i come to a stop.
     
  6. chevy

    chevy Member
    Messages: 33

    5 TO 10 PSI idiling thats pretty low. since you have it this far apart, a couple more bolts and the engines out.I pulled my engine when I got my truck 6 years ago and replaced it with a suburban drive train. I then took the engine I pulled and put a reman. crank in it. This came with crank and matching bearings. I put this mess into a grand prix I have and its still running it cost me only approx. 150 dollars, with some elbow grease. Your call, though to go thorough the work of a head job just to lunch the lower end shortly later; might be aggrevating. Any ways Good Luck !
     
  7. Pelican

    Pelican 2000 Club Member
    Messages: 2,075

    A word of caution: If you do a head job and are allready complaining of a drop in oil pressure, your engine is likely to blow soon after the valve job. It sounds like the lower end is marginal allready and by increasing the compression through the valve job you'll be puting a lot more pressure on the bearings.

    I've seen more than one high milage motor blow up 6 months after a valve job for this reason. Better to do the whole engine as has been suggested.
     
  8. fireman_31098

    fireman_31098 Junior Member
    Messages: 15

    yea after more wrenching on the truck today and seeing all the carbon build up in there, a complete rebuild is the only way to go on this one, thanks for all the advise guys, lets hope the block is worthy!!!
     
  9. firebirdude81

    firebirdude81 Junior Member
    Messages: 1

    Fireman,

    I agree that a rebuild is in order however, do you think rebuilding a 400 small block Chevy is a good idea. It seems we are all aware of the 400 weaknesses and if the 400 was any good, GM would have made more of them.

    Purhaps a 350 would be a safer investment, they are after all the flagship engines for GM. If you should dump the 400, check the crank shaft. If it's any good you may be able to sell it to anyone who is making a 383 stroker.

    I see so many engine machinists shake thier heads when they think of all the 400 cranks they scrapped.:realmad:
     
  10. johngus

    johngus Senior Member
    Messages: 117

    Why not go to your local GM dealer and buy a crate 350.It's relatively cheap and comes with warranty!!!!!Probably your best bet: