Diagnosing a vacuum leak questions/ symptoms
02-22-2011, 01:27 PM
#1
Diagnosing a vacuum leak questions/ symptoms So this may be stupid/ignorant but I've been doing some questioning.
I know on a carbed car, a vacuum leak is very apparent by the high idle, surging, etc. My question is, how do EFI cars normally respond to vacuum leaks more importantly if the car might have been tuned while a minor leak was present.
What would be the obvious stand out symptoms and could they be hidden by IAC, tuning, etc?
For instance:
I installed my used Fast intake quite some time ago, on first start up the car idled very high and I had to keep backing the blade off and plug a hole in the tb blade someone had drilled before it was possible to make the car settle down to desired idle.
Used the gaskets that came with the intake.
The car was then tuned.
1) Even after tuning the idle still likes to surge and fluctuate when slowing down. It's mild so I don't usually pay it much mind.
2) The car is maxing out 28.5# injectors while only making 350-360rwhp according to EFI Live. Base tune loaded was lean to 14:1 and after tuning to 13:1 the injectors see 94-96% IDC at 6000rpm.
3) Often when rolling with no throttle applied the idle will hang above 1000rpm until the car has been at rest for several seconds.
4) 1.8th mile MPH seems legit to mods on the car, but 1/4 mile seems 3-4 mph short.
5) Lots of noise from the engine that sounds a lot like what one would expect is the sound of valves opening and closing in the heads (not a tick, but a hollow echoing almost plunger like sound)
6) Sometimes at idle there is a hissing sound (could be coming from airflow in cone filter or over MAF screen, but mentioning it anyway)
7) Car often smells like fuel even with the lean tune, but mpg on highway is a decent 18-19mpg @70mph
8) I have hunted for vacuum leaks in the past using the ether/carb cleaner/starting fluid method around orifices, but supposing the intake gaskets were to blame would it not be possible the ether does not get close enough to the ports?
Like I said just some brainstorming and I could be way off and the issues above may just be a combination of part throttle/idle tuning and nature of the cam. The 2 that throw me off are the somewhat low power numbers and the top end laziness. Car traps the same as a header/intake/lid/tune z28 6 speed and a stock 6 speed 2011 5.0.
A 2003 cobra with CAI/exhaust traps the same in the 1/8 but 5mph higher in the 1/4 at my same weight.
I know on a carbed car, a vacuum leak is very apparent by the high idle, surging, etc. My question is, how do EFI cars normally respond to vacuum leaks more importantly if the car might have been tuned while a minor leak was present.
What would be the obvious stand out symptoms and could they be hidden by IAC, tuning, etc?
For instance:
I installed my used Fast intake quite some time ago, on first start up the car idled very high and I had to keep backing the blade off and plug a hole in the tb blade someone had drilled before it was possible to make the car settle down to desired idle.
Used the gaskets that came with the intake.
The car was then tuned.
1) Even after tuning the idle still likes to surge and fluctuate when slowing down. It's mild so I don't usually pay it much mind.
2) The car is maxing out 28.5# injectors while only making 350-360rwhp according to EFI Live. Base tune loaded was lean to 14:1 and after tuning to 13:1 the injectors see 94-96% IDC at 6000rpm.
3) Often when rolling with no throttle applied the idle will hang above 1000rpm until the car has been at rest for several seconds.
4) 1.8th mile MPH seems legit to mods on the car, but 1/4 mile seems 3-4 mph short.
5) Lots of noise from the engine that sounds a lot like what one would expect is the sound of valves opening and closing in the heads (not a tick, but a hollow echoing almost plunger like sound)
6) Sometimes at idle there is a hissing sound (could be coming from airflow in cone filter or over MAF screen, but mentioning it anyway)
7) Car often smells like fuel even with the lean tune, but mpg on highway is a decent 18-19mpg @70mph
8) I have hunted for vacuum leaks in the past using the ether/carb cleaner/starting fluid method around orifices, but supposing the intake gaskets were to blame would it not be possible the ether does not get close enough to the ports?
Like I said just some brainstorming and I could be way off and the issues above may just be a combination of part throttle/idle tuning and nature of the cam. The 2 that throw me off are the somewhat low power numbers and the top end laziness. Car traps the same as a header/intake/lid/tune z28 6 speed and a stock 6 speed 2011 5.0.
A 2003 cobra with CAI/exhaust traps the same in the 1/8 but 5mph higher in the 1/4 at my same weight.
02-22-2011, 01:51 PM
#2
I will tell you when i am diagnosing intake leaks on the LS engines, I dont typically get much an audible response with crab/brake clean, what i end up doing is wathcing my Fuel trims while i spray the brake clean, when you find the vac leak to will see the numbers change drastically.
Mark
Mark
02-22-2011, 03:15 PM
#3
That's a great tip! I can log while spraying.
Are there any sensor scans that would make it painfully obvious or even hint to vacuum leaks assuming the car was tuned to compensate a leak? I mean it seems very strange my injectors are working so hard for a mild cam, stock heads, at an elevation of over 1000ft.
I might be doing myself a favor to just pick up a intake gasket set and pull the intake and put them on. Could be done in around an hour and I might even be able to see if the old ones were sealing based on dust and debris around the ports?
Are there any sensor scans that would make it painfully obvious or even hint to vacuum leaks assuming the car was tuned to compensate a leak? I mean it seems very strange my injectors are working so hard for a mild cam, stock heads, at an elevation of over 1000ft.
I might be doing myself a favor to just pick up a intake gasket set and pull the intake and put them on. Could be done in around an hour and I might even be able to see if the old ones were sealing based on dust and debris around the ports?
02-22-2011, 04:06 PM
#5
Yes I did replace the valley cover bolts. Should have mentioned that since it's the common newb mistake.
I could be barking up the wrong tree entirely.
I'm just trying to determine why a 360rwhp car is maxing 28.5# injectors when other people get away with well over 400rhp before hitting the IDC I am seeing in my logs.
I am about to start trying my own tuning and don't want to be working on a tune for a handicapped car.
Any and all suggestions are welcome, I can scan and post logs of whichever PIDs would help. Might be obvious but theoretically I could compare Barometric pressure and MAP readings at idle for vacuum correct? If so what would be a good estimate for my cam at 850rpm? This could be a dumb idea too if so let me know.
I could be barking up the wrong tree entirely.
I'm just trying to determine why a 360rwhp car is maxing 28.5# injectors when other people get away with well over 400rhp before hitting the IDC I am seeing in my logs.
I am about to start trying my own tuning and don't want to be working on a tune for a handicapped car.
Any and all suggestions are welcome, I can scan and post logs of whichever PIDs would help. Might be obvious but theoretically I could compare Barometric pressure and MAP readings at idle for vacuum correct? If so what would be a good estimate for my cam at 850rpm? This could be a dumb idea too if so let me know.
02-22-2011, 06:12 PM
#7
Don't know anyone with one...I have a hookah and could blow some fairly thick clouds of smoke into the intake if that would work.
I tried my best to spray under the intake with carb cleaner. Didn't seem to have any effect.
Holds about 13" vacuum. STFT were around +7-8% at idle this startup, go normal if I rev and takes them a while to come back.
Would an idle log help?
I could be wrong about it being a vacuum leak...could just be poor tuning and the low numbers could just be poor setup. Might be tracking a non-existent demon.
I tried my best to spray under the intake with carb cleaner. Didn't seem to have any effect.
Holds about 13" vacuum. STFT were around +7-8% at idle this startup, go normal if I rev and takes them a while to come back.
Would an idle log help?
I could be wrong about it being a vacuum leak...could just be poor tuning and the low numbers could just be poor setup. Might be tracking a non-existent demon.
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02-22-2011, 06:58 PM
#9
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I also had a problem with high idle (like 1500rpm) when the tune is set at 800. I did find that when I put a piece of duct tape over the iac hole my idle was 800 and no rolling idle either. This is bypassing the iac obviously. Although I want to know do you all think plugging the iac hole has pinpointed that the intake has a leak? I ordered the new gaskets for the intake and I'm in the process of installing them. I will get back and report if my idle problems go away with the new gaskets. Oh and removing the duct tape so the iac will be reading again. I don't think the iac motor is bad, but I guess this might tell me. Any input is appreciated guys. OP please keep us updated.
Brian
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02-22-2011, 10:12 PM
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Subscribed same question, keep it up yall seems like alot of good info. Anyone have more to say about the symptoms of a vacuum leak on a FI (not carbed) car? Are they pretty much the same?
02-22-2011, 11:52 PM
#13
I mainly deal with unmoded cars, but typically i see no idle or drivability in the cars till well after the CEL is on. I recently worked on a yukon with a 5.3L and know to check the LTFT on it at cold start because they are notorious for intake gaskets... It was at 23 on both banks, car ran great, started spraying carb clean and no change in idle, but LTFT dropped to 8ish till it sucked in all the spray them climbed back up. Tip-if you ever change the intake gaskets on a LS based truck motor spend the extra $30 and get the H/D Fel-pro upgrade gaskets... well worth the $80
Mark
Mark
03-07-2011, 12:40 PM
#14
I have a very similar problem. I had the car Dyno tuned and a few days later Istarted getting a front KN code, so I removed the intake only to find 1 wire completely splayed with only a few wire strand still intact(pinched between intake and passenger side head). So I fixed the KS wire harness(an VAC leak), and varified the KS's an harness were good and reinstalled the intake,the engine has been running smoother, but my MPG took a big dive (under 10 mpg).
So my MPG took a dive after fixing a slight vacuum leak.
So my MPG took a dive after fixing a slight vacuum leak.
03-09-2011, 02:00 AM
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I also have a similar problem with a fast 92/92 setup of which I got the car with it already installed so couldnt give much info on how or if the valley cover bolts was replaced. My check engine light is on and reads lean on both banks and also knock voltage low I believe and I also get terible gas mileage for mostly stock setup. Ive tried spraying carb cleaner, brake cleaner and ethenol but never notice a change in the idle. When the weather is dry it only seems to idle high once in a while but when its raining or wet out it idles around 1500 to 2000 for about 5 sec every time I come to a stop. Sorry to have made such a long post.
03-09-2011, 06:10 PM
#16
I'll tell you my symptoms when i had a loose intake, It would start cold with a much higher than normal idle then idle down too low and misfire and shake. Then as it got warm enough to get into closed loop it woooood INSTANTLY stop shaking and run ok but it still had a funny idle. Was at the right rpm but was unsteady. It also had poor throttle response. All the intake bolts were very loose, after tightening them 20 times! they finally stayed tight. Runs nice now.
These computers can hide vacuum leaks in closed loop very well. I've had good luck using propane to check for leaks. Unplug the IAC before you try checking for vacuum leaks. You may want to take it out of closed loop too.
These computers can hide vacuum leaks in closed loop very well. I've had good luck using propane to check for leaks. Unplug the IAC before you try checking for vacuum leaks. You may want to take it out of closed loop too.
Last edited by O2Form; 03-09-2011 at 08:11 PM.
06-13-2015, 12:01 PM
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So this may be stupid/ignorant but I've been doing some questioning.
I know on a carbed car, a vacuum leak is very apparent by the high idle, surging, etc. My question is, how do EFI cars normally respond to vacuum leaks more importantly if the car might have been tuned while a minor leak was present.
What would be the obvious stand out symptoms and could they be hidden by IAC, tuning, etc?
For instance:
I installed my used Fast intake quite some time ago, on first start up the car idled very high and I had to keep backing the blade off and plug a hole in the tb blade someone had drilled before it was possible to make the car settle down to desired idle.
Used the gaskets that came with the intake.
The car was then tuned.
1) Even after tuning the idle still likes to surge and fluctuate when slowing down. It's mild so I don't usually pay it much mind.
2) The car is maxing out 28.5# injectors while only making 350-360rwhp according to EFI Live. Base tune loaded was lean to 14:1 and after tuning to 13:1 the injectors see 94-96% IDC at 6000rpm.
3) Often when rolling with no throttle applied the idle will hang above 1000rpm until the car has been at rest for several seconds.
4) 1.8th mile MPH seems legit to mods on the car, but 1/4 mile seems 3-4 mph short.
5) Lots of noise from the engine that sounds a lot like what one would expect is the sound of valves opening and closing in the heads (not a tick, but a hollow echoing almost plunger like sound)
6) Sometimes at idle there is a hissing sound (could be coming from airflow in cone filter or over MAF screen, but mentioning it anyway)
7) Car often smells like fuel even with the lean tune, but mpg on highway is a decent 18-19mpg @70mph
8) I have hunted for vacuum leaks in the past using the ether/carb cleaner/starting fluid method around orifices, but supposing the intake gaskets were to blame would it not be possible the ether does not get close enough to the ports?
Like I said just some brainstorming and I could be way off and the issues above may just be a combination of part throttle/idle tuning and nature of the cam. The 2 that throw me off are the somewhat low power numbers and the top end laziness. Car traps the same as a header/intake/lid/tune z28 6 speed and a stock 6 speed 2011 5.0.
A 2003 cobra with CAI/exhaust traps the same in the 1/8 but 5mph higher in the 1/4 at my same weight.
I know on a carbed car, a vacuum leak is very apparent by the high idle, surging, etc. My question is, how do EFI cars normally respond to vacuum leaks more importantly if the car might have been tuned while a minor leak was present.
What would be the obvious stand out symptoms and could they be hidden by IAC, tuning, etc?
For instance:
I installed my used Fast intake quite some time ago, on first start up the car idled very high and I had to keep backing the blade off and plug a hole in the tb blade someone had drilled before it was possible to make the car settle down to desired idle.
Used the gaskets that came with the intake.
The car was then tuned.
1) Even after tuning the idle still likes to surge and fluctuate when slowing down. It's mild so I don't usually pay it much mind.
2) The car is maxing out 28.5# injectors while only making 350-360rwhp according to EFI Live. Base tune loaded was lean to 14:1 and after tuning to 13:1 the injectors see 94-96% IDC at 6000rpm.
3) Often when rolling with no throttle applied the idle will hang above 1000rpm until the car has been at rest for several seconds.
4) 1.8th mile MPH seems legit to mods on the car, but 1/4 mile seems 3-4 mph short.
5) Lots of noise from the engine that sounds a lot like what one would expect is the sound of valves opening and closing in the heads (not a tick, but a hollow echoing almost plunger like sound)
6) Sometimes at idle there is a hissing sound (could be coming from airflow in cone filter or over MAF screen, but mentioning it anyway)
7) Car often smells like fuel even with the lean tune, but mpg on highway is a decent 18-19mpg @70mph
8) I have hunted for vacuum leaks in the past using the ether/carb cleaner/starting fluid method around orifices, but supposing the intake gaskets were to blame would it not be possible the ether does not get close enough to the ports?
Like I said just some brainstorming and I could be way off and the issues above may just be a combination of part throttle/idle tuning and nature of the cam. The 2 that throw me off are the somewhat low power numbers and the top end laziness. Car traps the same as a header/intake/lid/tune z28 6 speed and a stock 6 speed 2011 5.0.
A 2003 cobra with CAI/exhaust traps the same in the 1/8 but 5mph higher in the 1/4 at my same weight.
