You scan tool can help you find a vacuum leak.
Thread Starter
|
Elder User
Joined: Jan 2008
Posts: 605
Likes: 1
You scan tool can help you find a vacuum leak.
Since these engines seems to have a lot of vacuum leaks it helps to be able to verify the problem quickly. One way is to hook up your scan tool and monitor fuel trims at IDLE when vacuum is high(normally). If your STFTs and LTFTs are HIGH(adding fuel because you're lean do to a vacuum leak) at idle and your IAC counts are LOW(IAC doesn't need to add more air since the vacuum leak is already adding too much) you have a vacuum leak. You can pinpoint the leak by monitoring your STFTs(they react more quickly than do LTFTs or RPM) while spraying a flameable liquid at the suspected area. If you see fuel being pulled while spraying you know you've found the vacuum leak.
Postmaster
Joined: Oct 2009
Posts: 3,432
Likes: 7
I've very glad you posted this. The fuel trims are very important values that can help you pinpoint lots of problems. The LTFT is of little value here other than isolating the range the problem occurs in. STFT tells you what the computer is doing now, while the LTFT is the average trim over multiple drive cycles. There are different LTFT values for each RPM range.
I do have to admit though, trying to spray around the area of the leak while monitoring the scanner seems to be a handful.
I do have to admit though, trying to spray around the area of the leak while monitoring the scanner seems to be a handful.
Posting Guru
Joined: Dec 2005
Posts: 1,550
Likes: 6
From: Mount Juliet, TN
Got a easier way and you don't have to get out of the driver seat.
At idle read the short term fuel trim and long term, add these together for each bank. Now rev up to 2500 rpm and record each bank again, as long as you don't have any more than 15% gain there is no vacuum leak.
The problem is most people don't have scanners that read STFT and LTFT
At idle read the short term fuel trim and long term, add these together for each bank. Now rev up to 2500 rpm and record each bank again, as long as you don't have any more than 15% gain there is no vacuum leak.
The problem is most people don't have scanners that read STFT and LTFT
Thread Starter
|
Elder User
Joined: Jan 2008
Posts: 605
Likes: 1
You need to have an assistant do the spraying while you watch the scan tool. The STFTs are going to react instanteously when you hit the bad spot so you can't really do both things at once. Don't down play the value of the LTFTs. The more you study fuel trims the more you'll realize you need both STFTs AND LTFTs to arrive at TOTAL Fuel Trim in order to properly control the adaptive fuel strategy. Don't forget about the post cat sensor either. Even though the manufacturers don't turn their cards face up on the table we know the post cat 02s are being used for fuel trim as well. The manufacturers will deny this and tell you they are only used for cat efficiency, but you must remeber what Aretha Franklin said: "Believe half of what you see and none of what you hear". Most guys on this site are into fixing their own trucks and since 96(advent of OBDII) I don't see how you can fix anything without a decent scan tool. These days you can get a decent scan tool(interface and software) for around $200. I'm talking PC based NOT handheld garbage.
Thread
Thread Starter
Forum
Replies
Last Post
NotMatt
1987 - 1996 F150 & Larger F-Series Trucks
16
Jan 8, 2025 05:58 PM
CTDpower
1997 - 2006 Expedition & Navigator
11
Dec 16, 2016 02:28 AM
88XLTLariat
1987 - 1996 F150 & Larger F-Series Trucks
2
May 26, 2016 05:28 PM
Preuett
1987 - 1996 F150 & Larger F-Series Trucks
12
Jan 11, 2016 04:57 AM
V8SHO
Explorer, Sport Trac, Mountaineer & Aviator
4
Jan 8, 2016 03:54 PM
