Unusual Vacuum Gauge Readings
#1
Unusual Vacuum Gauge Readings
Having trouble getting my rebuilt 455 to generate good power. I thought I was having timing issues, then carb issues, but no, looks like engine vacuum readings indicate something else.
Engine is a 70 455 block with lo-comp pistons (.030), C heads with all new big valves, springs, guides, push rods, rocker arms. W31 cam with all new lifters, timing set, oil pump etc. Carb is built to 70 specs.
Engine has good consistent compression (147-150 lbs.), starts if you barely touch the key, and is not backfiring or dieseling. Is not running hot (so far).
My mechanic who put the vac. gauge on it - why did I not do this myself - says that at idle the gauge is rapidly vibrating between 3 inches up to 10 inches. I've looked on line at all the vacuum gauge charts, and I can't find one that indicates this symptom. The closest I've seen is the one indicating a massive intake or carb leak, but mine is a vibrating reading, not constant. I didn't like the way the valley pan fit when I installed it.
Anyone else had this condition? An help would be much appreciated.
Engine is a 70 455 block with lo-comp pistons (.030), C heads with all new big valves, springs, guides, push rods, rocker arms. W31 cam with all new lifters, timing set, oil pump etc. Carb is built to 70 specs.
Engine has good consistent compression (147-150 lbs.), starts if you barely touch the key, and is not backfiring or dieseling. Is not running hot (so far).
My mechanic who put the vac. gauge on it - why did I not do this myself - says that at idle the gauge is rapidly vibrating between 3 inches up to 10 inches. I've looked on line at all the vacuum gauge charts, and I can't find one that indicates this symptom. The closest I've seen is the one indicating a massive intake or carb leak, but mine is a vibrating reading, not constant. I didn't like the way the valley pan fit when I installed it.
Anyone else had this condition? An help would be much appreciated.
#2
maybe this will help
Does your gauge act like any of these senarios????
http://www.secondchancegarage.com/public/186.cfm
go thru each choice and see if your gauge matches the actions of any of
these .... Hope this helps diagnose the problem
Nufo(Fred)
http://www.secondchancegarage.com/public/186.cfm
go thru each choice and see if your gauge matches the actions of any of
these .... Hope this helps diagnose the problem
Nufo(Fred)
#4
Does your gauge act like any of these senarios????
http://www.secondchancegarage.com/public/186.cfm
go thru each choice and see if your gauge matches the actions of any of
these .... Hope this helps diagnose the problem
Nufo(Fred)
http://www.secondchancegarage.com/public/186.cfm
go thru each choice and see if your gauge matches the actions of any of
these .... Hope this helps diagnose the problem
Nufo(Fred)
#5
Thanks for the link Fred. I didnt see a scenario like I detailed above. I'm going to put a vacuum gauge on it myself when I get it home and see what readings I get, and how they change over engine speed. I think it's going to be either a manifold leak issue, or possibly a cam to crank timing issue. Anyone knows what the symptoms are of the cam to crank being one tooth off?
Could the above vacuum readings be caused by the cam to crank being literally one tooth off from being correctly aligned?
#8
When you say the vacuum gauge is sweeping between 3 and 10 inches; do you mean the 3 and 10 inch marks or a 3 to 10 inch variance from whatever the idle vacuum is. If it is the latter it sounds like a vacuum leak. check around the top of the intake with propane of whatever you like. also block both breather holes on both valve covers with your thumbs to make sure there is no vacuum in the crankcase. this would indicate a vacuum leak on the bottom side of the intake.
#9
#10
2. It is a new cam made by the OE supplier to Olds. Have used another one out of the same lot with zero problems.
3. Yes - just rebuilt by Steve Gregori
4. I've pulled all the plugs to check compression. It ranged from a low of 147 to a high of 151.
Thanks for responding. I'm stumped on this one!
#11
When you say the vacuum gauge is sweeping between 3 and 10 inches; do you mean the 3 and 10 inch marks or a 3 to 10 inch variance from whatever the idle vacuum is. If it is the latter it sounds like a vacuum leak. check around the top of the intake with propane of whatever you like. also block both breather holes on both valve covers with your thumbs to make sure there is no vacuum in the crankcase. this would indicate a vacuum leak on the bottom side of the intake.
The reason I keep mentioning the one tooth thing is that I remember when I was putting the motor together that the timing marks did not exactly align with each other no matter what teeth I indexed on. It wasnt a lot off, but it wasn't exact. I used a Melling select set that has the three keyways on the crank. I for sure used the 0 deg. one.
#12
Does it seem to be misfiring? A broken/weak valve spring can give you normal compression readings while cranking and will allow the valve to float with the engine running. I know the valve train components are new, but sometimes new parts fail.
#13
1. Gauge is getting manifold vac from the intake fitting in front of the carb
2. It is a new cam made by the OE supplier to Olds. Have used another one out of the same lot with zero problems.
3. Yes - just rebuilt by Steve Gregori
4. I've pulled all the plugs to check compression. It ranged from a low of 147 to a high of 151.
Thanks for responding. I'm stumped on this one!
2. It is a new cam made by the OE supplier to Olds. Have used another one out of the same lot with zero problems.
3. Yes - just rebuilt by Steve Gregori
4. I've pulled all the plugs to check compression. It ranged from a low of 147 to a high of 151.
Thanks for responding. I'm stumped on this one!
2. From what I understand the W-31 cams are lopey at idle...750 rpm with a high duration cam is pretty low idle and can indicate the readings you are having.
3. See question number 1.
4. Did any ONE plug have more oil on it than the others?
I am inclined to think that the engine is lacking performance because you don't have enough compression for that cam...
I am also inclined to think that the low idle rpm combined with that cam is causing the varying vacuum readings. You said it evens out at a higher idle (1,000 rpm +).
#14
Just an Olds Guy
Join Date: Jul 2008
Location: Edmonton, AB. And "I am Can 'eh' jun - eh"
Posts: 24,525
The W31 cam was for 350 engines, not 455's right? Are you sure you didn't mean W30 cam? Both the 350 W31 (308° cam) and the 455 W30 (328° cam) will run rougher and will not develop the vacuum you need for PB.
#15
An easy way to check for vacuum leaks at the carb and intake is to spray carb cleaner around carb and intake while it's running. If the idle speeds up when you are doing this then you've found one of your problems.
#16
#18
Not yet, but I have eliminated a vacuum leak and the carb as potential culprits. After I got the car back, I hooked it to a GOOD vacuum gauge off a centrally mounted vacuum port and found the reading is still bad, but not what the shop made it out to be. The vacuum level, at idle is more like 10 inches, and the needle flutters irregularly from about 9 inches to 13 inches. I've got one more thing to do before degreeing the cam and that's to pull off the valve cover and see if I have any installed valve stem height issues, or if the pushrods are all the correct, same length.
These "C" heads originally had the small valves. I had the machine shop install the bigger ones, and I'm hoping they botched the seat cutting and I didnt notice it while putting the motor together - this would be much preferable to having to pull the motor to change the cam/crank relationship.
Last edited by costpenn; March 6th, 2013 at 11:15 AM.
#19
I know what you mean! I'm pretty sure if the cam timing was off a tooth in either direction the vacuum reading would be steady but low, If the engine started at all. I think you are on the right track looking at the valvetrain. Usually large needle movments on the vacuum gauge indicate an intake valve problem, a small nibble on the needle indicates exhaust valve trouble.
#20
I know what you mean! I'm pretty sure if the cam timing was off a tooth in either direction the vacuum reading would be steady but low, If the engine started at all. I think you are on the right track looking at the valvetrain. Usually large needle movments on the vacuum gauge indicate an intake valve problem, a small nibble on the needle indicates exhaust valve trouble.
#22
come here come here go away go away
sounds like the vac gauge doesn't know what to do. most cylinders creating a vaccuum and possibly two cylinders applying pressure to the gauge. follow the bouncing ball. Poor gauge.... it's gotta be really confused... you get my point Costpenn. I think you found your culprit. Two valves not closing all the way....
Nufo(Fred)
P.S. this is an almost totally unprofessional assesment.
Nufo(Fred)
P.S. this is an almost totally unprofessional assesment.
#23
Trying one last thing before pulling the motor. Does anyone have the spec let's say for a 68 W-30 using the 402194 cam (308 duration) that is detailed in the CSM for when you are checking valve timing without pulling the cover? It's the procedure you do on the #4 cylinder by measuring the height of the intake pushrods at two different places in the cycle.
#25
Latest update:
Bought the Mondello installed valve height gauge and all valves check within spec. Did not mallet test the springs, but didn't see anything obviously wrong. Did the CSM test on the intake pushrod travel length and
Travel difference between two readings should be .210
Travel difference between two readings actual was .075
Even allowing for the +/- .030 tolerance specified in the CSM, there's a problem.
Will pull the fan and pump off and attempt to degree the cam to confirm, but I think it's pretty certain to be a timing set issue.
Is it worth it to pull the radiator and front end off the motor and drop the pan to fix this without pulling the engine out of the car? I really dont want to pull the whole motor.
Bought the Mondello installed valve height gauge and all valves check within spec. Did not mallet test the springs, but didn't see anything obviously wrong. Did the CSM test on the intake pushrod travel length and
Travel difference between two readings should be .210
Travel difference between two readings actual was .075
Even allowing for the +/- .030 tolerance specified in the CSM, there's a problem.
Will pull the fan and pump off and attempt to degree the cam to confirm, but I think it's pretty certain to be a timing set issue.
Is it worth it to pull the radiator and front end off the motor and drop the pan to fix this without pulling the engine out of the car? I really dont want to pull the whole motor.
#27
I'm going to degree the cam as a check to the test I did on the pushrod travel. I'm pretty sure the issue is the cam is not opening and closing the valves at the right times because they are not timed correctly due to crappy timing gear mark placement.
#31
costpenn--- buy a new chain & gears from BTR Performance(Bill Travoto ).
Having trouble getting my rebuilt 455 to generate good power. I thought I was having timing issues, then carb issues, but no, looks like engine vacuum readings indicate something else.
Engine is a 70 455 block with lo-comp pistons (.030), C heads with all new big valves, springs, guides, push rods, rocker arms. W31 cam with all new lifters, timing set, oil pump etc. Carb is built to 70 specs.
Engine has good consistent compression (147-150 lbs.), starts if you barely touch the key, and is not backfiring or dieseling. Is not running hot (so far).
My mechanic who put the vac. gauge on it - why did I not do this myself - says that at idle the gauge is rapidly vibrating between 3 inches up to 10 inches. I've looked on line at all the vacuum gauge charts, and I can't find one that indicates this symptom. The closest I've seen is the one indicating a massive intake or carb leak, but mine is a vibrating reading, not constant. I didn't like the way the valley pan fit when I installed it.
Anyone else had this condition? An help would be much appreciated.
Engine is a 70 455 block with lo-comp pistons (.030), C heads with all new big valves, springs, guides, push rods, rocker arms. W31 cam with all new lifters, timing set, oil pump etc. Carb is built to 70 specs.
Engine has good consistent compression (147-150 lbs.), starts if you barely touch the key, and is not backfiring or dieseling. Is not running hot (so far).
My mechanic who put the vac. gauge on it - why did I not do this myself - says that at idle the gauge is rapidly vibrating between 3 inches up to 10 inches. I've looked on line at all the vacuum gauge charts, and I can't find one that indicates this symptom. The closest I've seen is the one indicating a massive intake or carb leak, but mine is a vibrating reading, not constant. I didn't like the way the valley pan fit when I installed it.
Anyone else had this condition? An help would be much appreciated.
#32
Gary,
A few weeks ago, we did the CSM test on #4 intake pushrod. The travel should have been around .200 between the measuring points. Turned out to be only .075. It might be a flat cam issue, but with all the other clues listed above I'm pretty sure it is a cam to crank timing issue. Anyway, I'm going to pull the front of the motor off when I get back from our Zone show and do an in depth degreeing of the cam and will advise as to the results.
A few weeks ago, we did the CSM test on #4 intake pushrod. The travel should have been around .200 between the measuring points. Turned out to be only .075. It might be a flat cam issue, but with all the other clues listed above I'm pretty sure it is a cam to crank timing issue. Anyway, I'm going to pull the front of the motor off when I get back from our Zone show and do an in depth degreeing of the cam and will advise as to the results.
#33
Degreeing the cam should always be done during engine assembly.
Before going thru all that, though maybe try 2 things
1) retorque all the intake to head bolts. Go 'round 3-4 times.
2) advance the timing a little. Worked wonders on the high compression 403 I was working on last year.
"I am also inclined to think that the low idle rpm combined with that cam is causing the varying vacuum readings. You said it evens out at a higher idle (1,000 rpm +)."
This leads me to believe you need more timing. When the idle is higher, the MA system is advancing the timing and the engine is much happier.
I ended up with VA hooked to the manifold vacuum, so the initial was set back for easy starting, then when it starts, the timing advances quite a lot, and the MA then adds more with RPM increases. The MA had to be modified to suit, and the VA canister replaced with travel-limited aftermarket unit dialed down to ~10 degrees.
Non-stock compression and cam and whatnot required non-stock timing curves to match.
Much happier now. With stock curves and settings, below ~800 rpm, the idle would just deteriorate to crap and then stall. Low vacuum was allowing the power piston to richen the mixture, which only made things worse.
You may find that securing the intake and advancing the timing will cure your ills.
Before going thru all that, though maybe try 2 things
1) retorque all the intake to head bolts. Go 'round 3-4 times.
2) advance the timing a little. Worked wonders on the high compression 403 I was working on last year.
"I am also inclined to think that the low idle rpm combined with that cam is causing the varying vacuum readings. You said it evens out at a higher idle (1,000 rpm +)."
This leads me to believe you need more timing. When the idle is higher, the MA system is advancing the timing and the engine is much happier.
I ended up with VA hooked to the manifold vacuum, so the initial was set back for easy starting, then when it starts, the timing advances quite a lot, and the MA then adds more with RPM increases. The MA had to be modified to suit, and the VA canister replaced with travel-limited aftermarket unit dialed down to ~10 degrees.
Non-stock compression and cam and whatnot required non-stock timing curves to match.
Much happier now. With stock curves and settings, below ~800 rpm, the idle would just deteriorate to crap and then stall. Low vacuum was allowing the power piston to richen the mixture, which only made things worse.
You may find that securing the intake and advancing the timing will cure your ills.
#35
Degreeing the cam should always be done during engine assembly.
Before going thru all that, though maybe try 2 things
1) retorque all the intake to head bolts. Go 'round 3-4 times.
2) advance the timing a little. Worked wonders on the high compression 403 I was working on last year.
"I am also inclined to think that the low idle rpm combined with that cam is causing the varying vacuum readings. You said it evens out at a higher idle (1,000 rpm +)."
This leads me to believe you need more timing. When the idle is higher, the MA system is advancing the timing and the engine is much happier.
I ended up with VA hooked to the manifold vacuum, so the initial was set back for easy starting, then when it starts, the timing advances quite a lot, and the MA then adds more with RPM increases. The MA had to be modified to suit, and the VA canister replaced with travel-limited aftermarket unit dialed down to ~10 degrees.
Non-stock compression and cam and whatnot required non-stock timing curves to match.
Much happier now. With stock curves and settings, below ~800 rpm, the idle would just deteriorate to crap and then stall. Low vacuum was allowing the power piston to richen the mixture, which only made things worse.
You may find that securing the intake and advancing the timing will cure your ills.
Before going thru all that, though maybe try 2 things
1) retorque all the intake to head bolts. Go 'round 3-4 times.
2) advance the timing a little. Worked wonders on the high compression 403 I was working on last year.
"I am also inclined to think that the low idle rpm combined with that cam is causing the varying vacuum readings. You said it evens out at a higher idle (1,000 rpm +)."
This leads me to believe you need more timing. When the idle is higher, the MA system is advancing the timing and the engine is much happier.
I ended up with VA hooked to the manifold vacuum, so the initial was set back for easy starting, then when it starts, the timing advances quite a lot, and the MA then adds more with RPM increases. The MA had to be modified to suit, and the VA canister replaced with travel-limited aftermarket unit dialed down to ~10 degrees.
Non-stock compression and cam and whatnot required non-stock timing curves to match.
Much happier now. With stock curves and settings, below ~800 rpm, the idle would just deteriorate to crap and then stall. Low vacuum was allowing the power piston to richen the mixture, which only made things worse.
You may find that securing the intake and advancing the timing will cure your ills.
By the way, that intake nipple you sold me for converting my iron intake over to a correct 72 455 MT design worked out great!!!
#38