68' idle problems
Melting Slicks
Joined: Jan 2012
Posts: 3,033
Likes: 503
From: myrtle beach sc
Army
This happened about eleven or twelve years ago to me. The L-79 was stock, except that I had changed to a Holly spread bore carburetor. I had to add an AC breather cap to the one valve cover that had that hose going up to the air cleaner base. The Holly carb was in the way and I had to block off that hose hole in the air cleaner base and use the breather cap, insted.
I changed my engine to a ZZ4 crate motor a little after that. I still have my original motor, but it is wrapped up in plastic.
What happened to me was one day I started the car and the idle was so low that it would stall out. I think that I tried to adjust the idle on the linkage and it did not help that much. I am sorry but I really can't remember, being an old critter, now. I happened to bump my oil fill cap and I noticed that it was a little loose. I think that I removed the cap and just closed the hole off with the palm of my hand, and the idle settled down! I really couldn't believe it at the time. I had a new NOS chrome L-79 cap and I put it on and everything was fine. Really strange, but it was a vacuum leak. Lou.
I changed my engine to a ZZ4 crate motor a little after that. I still have my original motor, but it is wrapped up in plastic.
What happened to me was one day I started the car and the idle was so low that it would stall out. I think that I tried to adjust the idle on the linkage and it did not help that much. I am sorry but I really can't remember, being an old critter, now. I happened to bump my oil fill cap and I noticed that it was a little loose. I think that I removed the cap and just closed the hole off with the palm of my hand, and the idle settled down! I really couldn't believe it at the time. I had a new NOS chrome L-79 cap and I put it on and everything was fine. Really strange, but it was a vacuum leak. Lou.
Race Director
Joined: May 2006
Posts: 18,663
Likes: 2,563
A PCV is a controlled vacuum leak. The oil fill and the PCV being on the same valve cover, if you remove the oil fill the PCV will suck directly through the open oil filler as opposed to the normal route which is through the crankcase and the breather on the opposite valve cover.
But back to the original problem where I asked if the OP is causing his mechanical fuel pump to suck fuel through an electric pump that may have a partially clogged strainer?
But back to the original problem where I asked if the OP is causing his mechanical fuel pump to suck fuel through an electric pump that may have a partially clogged strainer?
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,111
Likes: 9,240
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I'm going to rebuild my L79 this fall and winter, its in storage now, I'll keep that in mind as I want to keep it original looking but with a beefier cam, RPM intake with the oil fill tube in place and my world heads that are on my LT1 motor now. Thanks Loup
Melting Slicks
Joined: Jan 2012
Posts: 3,033
Likes: 503
From: myrtle beach sc
Army
Melwff, The 68 327's had a separate, front of the intake manifold, oil fill tube. You did not have the oil fill in either valve cover, like on a 350. Lou.
Last edited by loup68; Apr 28, 2017 at 06:22 PM.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
...
really, wow. That makes no sense at all. Very surprising. And you didnt have a hose going to your carb that would affect the vacuum from the fill tube. I still dont see how positive crankcase pressure would affect vacuum inside the carb and intake. Can you post pics of your motor please. Thanks
Last edited by greggome; Apr 29, 2017 at 08:56 PM.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
I bypassed the electric pump and got the same results so it does not appear that the electric is restricting fuel to the mechanical pump. I also replaced the mechanical pump just to see. Still having the same symptoms. With The car runs fairly well with on electric. running just the mechanical the car idle fluctuates and will climb to 1000+ Rpm then intermittently drop back down to 600s. Without the electric on It hesitates under acceleration and will without warning lose power sputtering until it stalls. Put the electric on and the cars idle will drop from the 1000 range on the mechanical and hold a low but steady idle at 600. At this point Im out of ideas. guess I'll try blocking the oil fill tube and see what happens.
really, wow. That makes no sense at all. Very surprising. And you didnt have a hose going to your carb that would affect the vacuum from the fill tube. I still dont see how positive crankcase pressure would affect vacuum inside the carb and intake. Can you post pics of your motor please. Thanks
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,111
Likes: 9,240
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I bypassed the electric pump and got the same results so it does not appear that the electric is restricting fuel to the mechanical pump. I also replaced the mechanical pump just to see. Still having the same symptoms. With The car runs fairly well with on electric. running just the mechanical the car idle fluctuates and will climb to 1000+ Rpm then intermittently drop back down to 600s. Without the electric on It hesitates under acceleration and will without warning lose power sputtering until it stalls. Put the electric on and the cars idle will drop from the 1000 range on the mechanical and hold a low but steady idle at 600. At this point Im out of ideas. guess I'll try blocking the oil fill tube and see what happens.
Last edited by Rescue Rogers; Apr 29, 2017 at 10:55 PM.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
Well my gas cap is the locking type and it fits pretty loose as the gaskets old and it also vented. I have the original fuel filter with the return to the fuel tank.
it does sound like the motor isnt getting enough gas and the electric pump is pushing the fuel past the mechanical like its starving. Have you tried opening your gas tank cap. Its supposed to be vented, maybe the vent in the cap got clogged some how. If you heard a wooosh then its plugged. Either that or there is a vacuum leak and your carb needs the added pressure to get the fuel into it. Very weird . Are you running the original style filter and fuel return line or is your car a big block, which didnt use the return?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
All looks fine in the tank. I just use my vacuum/fuel pump gauge to check fuel pump pressure at the pump . I was thinking maybe the fuel filter was the problem but testing the output line of the filter showed 4.5-> 5 psi when cranking the motor but the reading didnt hold as it should at which point I closed off the filters overflow tube going back to the fuel tank and got a reading of 8 psi which held for > then 1 minute on the gague. In either case the fuel pressure should be enough to feed the carburetor unless something changes while driving the car. I have traced all the fuel delivery possibilities from the tank to the carb with no solution. I continue to wonder if its a vacuum problem which I just cant locate but why a vacuum problem would not be consistent from mechanical to electric pump is a puzzle.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,111
Likes: 9,240
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
whats the pressure look like when the electric pump is running? I don't think the electric pump is the problem but part of the solution. If the pressure is dramatically higher maybe theres a blockage in the carb that the higher pessure is forcing by the clog
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
The pressure with the electric and the overflow tube closed off is 10+ psi from the fuel filter to the gauge ( the gauge tests the pump pressure. the carb is disconnected from the fuel source on the tests.) The holley carb I had on the car was when this problem started so i had my original Q Jet rebuilt and installed because I thought it was a problem with the holley but i am having the same issue with the Q jet so i dont think its a carb problem.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,111
Likes: 9,240
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
have you replaced the mechanical pump yet? If it was a vacuum leak I cant see how adding fuel pressure would drop the idle, especially with 2 different carbs.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,111
Likes: 9,240
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Wow, that is a weird mystery..... I would try replacing the fuel line from the pump to the carb in case it has an obstruction, then put in a temporary line from the tank to the pump and see what happens. If all that fails to help then you may have to drop your tank and pull the fuel sender to clean that out. We're running out of possibilities here. We are assuming at this point the carbs are good and theres no vacuum leak
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
yes I'm pretty much out of ideas. i will do a fuel volume test to see how much fuel is actually flowing to the carb. I also thought i will try to bypass the fuel filter to see if that has any effect. as a last resort I'll try getting a T fitting and hook my gauge up to the fuel line allowing it to stay connected to the carb, run the car and watch the pressure gauge as the problem develops.I'd be curious to confirm if the pressure stays constant when the idle RPMs fluctuate and during times of sputtering/stalling. That should confirm if fuels being delivered properly.
as
as
Wow, that is a weird mystery..... I would try replacing the fuel line from the pump to the carb in case it has an obstruction, then put in a temporary line from the tank to the pump and see what happens. If all that fails to help then you may have to drop your tank and pull the fuel sender to clean that out. We're running out of possibilities here. We are assuming at this point the carbs are good and theres no vacuum leak
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,111
Likes: 9,240
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
good idea, Good Luck!!
Melting Slicks
Joined: Mar 2010
Posts: 2,268
Likes: 374
From: Randolph nj
just a shot in the dark ,, how old are the fuel lines, I have seen them collapse or delaminate from the inside out it happens more often due to the crappy garbage gas we run these days ,, have you checked for ware on your fuel pump rod ends or the lobe for the fuel pump , these are just some things that come to mind ,, good luck with it i really hate dumb stuff like this 
also i know its dumb but the check the fuel filter direction
also i know its dumb but the check the fuel filter direction
Last edited by forman; May 2, 2017 at 10:33 PM.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
Fuel hoses were replaced all around 2 years ago. I didnt pull the fuel pump rod out but the end I could see seemed to look ok. carb fuel filter in ok. I had bypassed the external fuel filter and all seemed improved I finally thought I found the problem until I parked the car in the driveway , came back later and started it up to put in the garage and it stalled out on me 3 times. on the 4th try it stayed running and I could pull it in. The more I read about this type of problem online the more probable causes come up with opinions on possible ignition issues such as unstable timing, excess distributor play, and the reoccurring vacuum leak theory. Crazy situation. I'm going to continue messing with the car tomorrow in hopes of finding some clue.
Last edited by greggome; May 6, 2017 at 11:01 PM.