73-87 Chevy _ GMC Trucks => Performance => Topic started by: 78ScottsdaleK20 on November 14, 2007, 05:28:23 PM

Title: Vacuum Leak
Post by: 78ScottsdaleK20 on November 14, 2007, 05:28:23 PM
I have seen this topic come up before, but I hate to piggy back.

I suspect I have a vacuum leak. I have taken vacuum readings and have 15 hg with a very slight oscillating needle. The needle moves no more than 1 hg either way. Edelebrock tech support says this is not uncommon with a mild cam. My cam is .448 lift 214 duration @50 and 112 degree lobe seperation.

I have tried everything to find the leak. WD-40 and carb cleaner around all suspecting areas. No change in RPM's. Place the hand over carb and RPM's raise. This transmits to vacuum leak. Where, is the big ?

I did run the motor for about 8 months with only glass packs on the header collectors for the exhaust system. Wonder if I did damage to the valves. That could cause the vac. leak. Short of pulling the junk 882 heads from my daily driver, I am hoping that I find the leak somewhere else. Unless this is a blessing in disguise, and the valves are sticking, then I can replace them with some better castings.

I did some research on which heads are good, so I have a list of what to look for and what to stay away from. Anyone else have suggestions on good flowing heads, and am not willing to pay $1,000 for a set. I think I want to stay with 1.94 for the intake valves, as long as they flow really well. The way gas prices are going, I heard the 2.02 setup just get you to the gas pump faster, especially if I am not racing the motor, just pulling, since it is a 3/4 T truck.

Title: Re: Vacuum Leak
Post by: team39763 on November 16, 2007, 03:47:02 PM
I'm not sure about your vacuum problem, it sounds frustrating.  I think I'm about to go through the same thing with my truck.  As far as heads, I hear a bunch of good about the vortec setup.  Then you just need an intake(pretty cheap on ebay) and possibly some new springs.  Good luck with it man, I hope you get it figured out.
Title: Re: Vacuum Leak
Post by: VileZambonie on November 16, 2007, 06:41:53 PM
Did you set your ignition timing? As far as pinpointing your vacuum leak, if you can't find it go to a shop that has a smoke machine and you'll find your leak that way. As far as the needle fluctuating it sounds like a valve sticking or valve spring is weak/broken. I agree that the Vortec heads are a cheap nice upgrade.
Title: Re: Vacuum Leak
Post by: 78ScottsdaleK20 on November 17, 2007, 09:48:27 AM
I have my timing set at 12 BTDC. The guy that built the motor recommended somewhere in the 8-12 BTDC, I have found 12 has been the best.

I have not set the total timing since I only have a basic timing gun. I should either get timing tape for the balancer or new gun with advance (preferable). HEI has 50,000 volt and it was new with plugs and wires when motor was swapped.

The valves sticking is what I am afraid of since,
1. I ran glass packs straight from the header collectors for about 8 months including during the winter months during the first part of engine life. Since then I have 2 1/2 dual exhaust with mufflers.
2. The heads are 882's and knowingly not the best around.

I have an air fuel ratio gauge installed in the cab so I can monitor it at all times. Pulled plugs two days ago (originals since new motor) and #1, 8, 2, 4 & 6 are black sooty caused from rich fuel mixture. Not wet though. The other plugs look ok, but show signs of richness too just not as bad as others.

I'll keep ya posted as to findings. Thanks
Title: Re: Vacuum Leak
Post by: team39763 on November 17, 2007, 10:55:37 AM
Hey Vile, how would I be able to tell for sure if I have a bad/weak valvespring rather than a sticking valve?  Would the sticking valve cause the exhaust to suck in every now and then?
Title: Re: Vacuum Leak
Post by: 78ScottsdaleK20 on November 17, 2007, 11:29:07 AM
I am not Vile obviously, but I heard if you suspect a valve sucking in, you can take a dollar bill at the tail pipe and stick it over it, if you have a problem then it would start to suck the bill into the pipe...Is this correct?
Title: Re: Vacuum Leak
Post by: VileZambonie on November 17, 2007, 11:54:00 AM
No that won't work. But it does if you use a hundred dollar bill  ;D

The only proper way to test a valve spring is on a valve spring tester checking it's pressure vs installed height and compressed (valve open)

A visual inspection would pinpoint a broken spring. Look carefully and at the inner spring also.

Perform a dry compression test...Do a compression test like so:

Install a battery charger to maintain consistent cranking speeds.

Pull out all the plugs after you warm up the engine.

disable the coil by unhooking the B+ lead

Crank the engine for 4 complete compression strokes at WOT (Wide Open throttle) You'll see it pulse 4 times on the gauge or hear the starter strain 4 times at each compression stroke.

Record your results for each cylinder on paper and post your results.
Title: Re: Vacuum Leak
Post by: 78ScottsdaleK20 on November 17, 2007, 05:42:34 PM
Silly ?

I have an air/fuel ratio gauge mounted in the cab and hooked up to O2 sensors installed in the header collector. Why would the engine run WAY better in the morning or at night? Of all the tuning and re-tuning I have done with the Edelbrock 1406, my air/fuel ratio is perfect in the morning and night, but runs rich during the day, and even erratic mixture-sometimes rich sometimes lean but never in between?

This must be due to the atmospheric pressure...I do suspect a vacuum leak as I have posted before. I think I am starting to answer my questions here...

Anyone else...??? ???
Title: Re: Vacuum Leak
Post by: VileZambonie on November 18, 2007, 07:41:22 AM
Well 1st off your a/f ratio is invalid when it's cold. O2 sensors don't even wake up until they reach an inner core temp of about 600F so don't believe your O2 readings until you've reached operating temp.

Sounds like you have a vacuum leak.
Title: Re: Vacuum Leak
Post by: 78ScottsdaleK20 on November 18, 2007, 09:41:44 AM
I agree that the O2 sensors are not reading correctly when cold. Once warmed up, the A/F is still better at night and in the morning than the day. Correct me if Im wrong, but if there is a vacuum leak this could happen too as the engine heats up and expands...
Title: Re: Vacuum Leak
Post by: VileZambonie on November 18, 2007, 10:07:01 AM
I wouldn't concentrate so much on what your a/f ratio gauge reads because if there is a vacuum leak your readings are false anyway. Remember an O2 sensor is effective as an a/f ratio calculator only if the engine is running normal. It reacts to oxygen on the inside of the exhaust and on the outside of the exhaust. It cannot detect high HC's (unburned fuel which does not indicate rich contrary to what most people think) or CO which is essenstially your rich / lean indicator but only reacts oxygen. So if you have a vacuum leak as you suspect it is never going to indicate correctly. You can use a gas analzyer to accurately determine what's going on though but I doubt you have one readily available?
Title: Re: Vacuum Leak
Post by: 78ScottsdaleK20 on November 19, 2007, 03:20:42 PM
No I dont have a gas analyzer available. Dont think thats necessary. Arent they a couple of hundred bucks...

Drove the truck about 175-200 miles rounds trip for some wood haulin, and the a/f ratio was spot on all day, with the exception of the part throttle stumbles a little. The temp was about 35-40 degrees.

Got in it this morning to go to work at 1 AM and the a/ f was ok half way through the trip (10 miles) then started running erratic. Drove it home and it was rich all the time. Sometimes lean at cruise, and the part throttle was terrible. Temp was around 35-40 degrees.

I would suspect the erratic engine operation and a/f is from a vacuum leak. So I will do some more work on it and post updates.

Thanks for all the suggestions, ideas and comments.
Title: Re: Vacuum Leak
Post by: Timber on November 24, 2007, 11:41:34 PM
I guess I'm lucky.  I have a vacuum leak that started a few weeks ago.  It's behind the dash somewhere in the AC/vent/heater controls.  Whistles while driving, it winds down sort of like a jet engine winding down when I turn the engine off.  None of the controls work (just bought the truck) and I'm not crazy about chasing it down and fixing all that stuff, but at least I know where it is.

Don't forget that stuff, yours might not be whistling.

Good luck finding it.
Title: Re: Vacuum Leak
Post by: team39763 on September 30, 2009, 05:52:31 PM
Sorry...had to bring this one back from the dead.  My dually makes a whistling noise like it needs to expel some air.  It's kinda broken up like whoo,whoo,whoo,whoo,whoo(it pulses with the idle).  Then when I shut it off it sounds like it just relieved some pressure...it goes whooooooooo(long drawn out whistle).  I've been looking over my hoses and don't see anything broken or out of place. The engine idles fine.  I noticed that when I remove the oil cap, it stop the whistle, so I'm leaning towards something in the vacuum system just not sure what.  My PCV valve is working.  I have hydroboost brakes.  Any advice or suggestions?
Title: Re: Vacuum Leak
Post by: VileZambonie on September 30, 2009, 06:57:16 PM
Do you have air injection or did you once upon a time? Tell us more about your emissions and crankcase ventilation if that's where you suspect it.
Title: Re: Vacuum Leak
Post by: team39763 on September 30, 2009, 08:43:38 PM
No air injection.  Doesn't look like it ever did.  I don't see a EGR valve or anything. I have both valvecovers coming together at one T-fitting and then venting to the throttlebody.   
Title: Re: Vacuum Leak
Post by: team39763 on October 01, 2009, 09:53:29 AM
I think I'm gonna try adding a valvecover breather/filter.  When I took off the oil cap, the whistle went away, so maybe the motor needs better crankcase breathing?  I'll give it a try and see what happens.
Title: Re: Vacuum Leak
Post by: VileZambonie on October 01, 2009, 10:23:19 AM
Wait, you have both valve covers going to a vacuum source? That's a no no. One should be a PCV valve with a hose going to the large vac port the other should just be a filter vented to the air.
Title: Re: Vacuum Leak
Post by: team39763 on October 01, 2009, 07:08:07 PM
That's how the truck was when I bought it, but I just fixed that with a filter.  I'll see how it sounds in a bit.