Finding a vac leak?
- Thread starter Stang|ess
- Start date
LaserRed01GT
Banned
To check for a possible vacuum leak, use your scan tool to look at long-term fuel trim (LONGFT) and short-term fuel trim (SHRTFT). With the engine warm and idling, add the long-term and short-term fuel trim numbers together. If the total correction is greater than plus 20% at idle, it probably indicates a vacuum leak that is making the fuel mixture run lean. To confirm the leak, rev the engine to 2,500 rpm and hold. Again, note the LONGFT and SHRTFT numbers. Add the numbers together. If the total correction at 2,500 rpm drops 15% or more, it confirms a vacuum leak at idle.
To find the vacuum leak, try pinching off individual vacuum hoses one by one while watching the SHRTFT number on your scan tool. When the SHRTFT number drops, you’ve found the leak.
To find the vacuum leak, try pinching off individual vacuum hoses one by one while watching the SHRTFT number on your scan tool. When the SHRTFT number drops, you’ve found the leak.
LaserRed01GT
Banned
I like the carb cleaner idea, but thanks for the wicked awesome tech response
.
The reason it's better to diagnose a vacuum leak with a scanner is because even though there are now carb/brake cleaners that are advertised to be O2 sensor safe, they are not. They still contain chemicals that can hose electrical sensors, namely the O2's.
blacksheep-1
Member
- Oct 21, 2006
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In my little lo-tech world, I used to like to hunt them with a can of CRC, it's non flammable unlike other products. i wouldn't know if it's sensor safe or not.
And yes Brakeclean is hell on cockroaches, If you get the can that has a trigger on it you can entertain yourself for hours. We found this out when somebody developed the neighborhood wetland across the street. Some of those dudes are like 2inches long down here. It's a lot more fun than watching the bugzapper.
And yes Brakeclean is hell on cockroaches, If you get the can that has a trigger on it you can entertain yourself for hours. We found this out when somebody developed the neighborhood wetland across the street. Some of those dudes are like 2inches long down here. It's a lot more fun than watching the bugzapper.
speedfreak98r
Banned
brakeclean non flammable???????????? WHAT? did I read that right.
I have personally had a windstar engine bay shoot a 5 foot fireball out of the hood after I sprayed brake clean to clean the mess after a large P/S leak.. brakeclean only needs a small spark to go up in flames. just about every tech I know has had this happen to them or has seen it before.
At work I use a combination of a scan tool (IDS
) and then I use carb clean to check if the intake manifold gaskets are leaking.you really need a scan tool so you can watch the long term and short terms to tell what you are looking at. Because some vac leaks are small enough that the engine will not shoot up in rpm or almost stall ( sometimes the stall instead of shoot up in RPM's) plus it may not be a vac leak at all. You really need a scan tool with live data to do any type of EEC diagnostics.
trust me on this I do this for a living, I spend all day fixing electrical systems and driveability problems get a scan tool with live data if you are going to try and fix check engine lights and driveability problems.
katshot
Member
brakeclean non flammable???????????? WHAT? did I read that right.
I have personally had a windstar engine bay shoot a 5 foot fireball out of the hood after I sprayed brake clean to clean the mess after a large P/S leak.. brakeclean only needs a small spark to go up in flames. just about every tech I know has had this happen to them or has seen it before.
At work I use a combination of a scan tool (IDS
you really need a scan tool so you can watch the long term and short terms to tell what you are looking at. Because some vac leaks are small enough that the engine will not shoot up in rpm or almost stall ( sometimes the stall instead of shoot up in RPM's) plus it may not be a vac leak at all. You really need a scan tool with live data to do any type of EEC diagnostics.
trust me on this I do this for a living, I spend all day fixing electrical systems and driveability problems get a scan tool with live data if you are going to try and fix check engine lights and driveability problems.
Yes, some brake cleaners are non-flammable. Chlorinated CRC for instance (the old red cans) will not burn, while the newer non-chlorinated stuff (green can I think) is quite flammable. So you need to be careful.
LaserRed01GT
Banned
Soapy water can be used to look for a small leak in a tire, but it would do nothing for a vacuum leak. Bubbles are made by pressure not vacuum.
blacksheep-1
Member
- Oct 21, 2006
- 322
- 4
- 18
I should've been clearer, the CRC lubricant is non flammable (unlike WD 40) that's what I hunt down the vacuum leaks with. What I hunt down cockroaches with is the Brakeclean product, sorry for the misunderstanding.
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