My idle is ranging between 643-659 rpm per my code scanner. Will an IAC cause low mpg?

 

You must pay more attention to what you read if your to benifit from info presented.
No, the IAC will not change fuel mileage except by your 'foot' as you drive. It controls air bypass only.
Find the air leak, then remove the IAC and clean it.
Do a reboot of the computer and all will be cleared.
The codes 171/174 are the key to your issue.
You cannot say there is no air leak because the computer knows there is!
It has caused the fuel tables to shift rich, as said before.
If you can read idle rpm, you should be able to look at the fuel tables if using a good Scanner.

Good luck.

 

I monitored my fuel trims on my way home tonight. Not sure what LT and ST mean, but I was between -5% and 10% depending on whether I was accelerating or coasting. At idle, in park (~650RPM) I was ~25%
If there was a vac leak, wouldn't I always be high because the detected lean condition?

 

Spent some time looking into the issue yesterday.

1. IACV removed and cleaned. Resistance across terminals in spec. RPM still ~650.
2. Rich condition induced via carb cleaner in vac line. Both O2 voltage above 0.9mV. Ok
3. MAF recleaned
4. Sprayed carb cleaner on vac lines to detect leak by rpm jump. Spraying line on upper part of throttle body elbow that goes by brake booster showed increase in rpm. Line did not have cracks or holes, but rubber is deteriorated. LT fuel trims within +/- 10% spec after replacement and short drive.

Driving into work today I noticed P0171/P0174 as pending codes and LT fuel trims ~23% at idle.

 

NEVER do what this guy did to his vacuum harness. You WILL destroy EVERYTHING that carb cleaner comes into contact with, plastic or rubber. VERY foolish thing to do. Read the back of the can. Carb cleaner is for carburetors that pre-dates this generation of vehicle.

 

The codes still tell you there is an air leak.
The fuel trims are to far out of line and cause the codes.
You cannot see a worthwhile change in fuel trims from a short drive cycle.
Having a Scanner and being able to see the data is good but miss or non-interpetation of what you see and the background of it missing, is not.
.
A lesson in idle:
The program has an idle table with inputs from the CHT, the IAT, A/C, power steering loading etc that can shift the table to meet the the engine load.
The dynamic hot running idle is 750 +/- 50 rpm.
This is arrived at by the input from the crank sensor as a tach.
The tach speed is compared to the table.
If it doesn't match, corrections are made by modulating continiously the IAC until it does and keeping the match within the 50 rpm tolerence..
.
When an 'unaccounted for leak' is present the system will try to readjust by changing the IAC opening.
The OX sensors detect the excess air and richen the fuel tables.
It's the fuel table changes that go out of limts that generates the codes for both banks because the trouble is common to both banks.
The only two ways the fuel tables will change is 1. Wait for corrections after a repair or 2. Reboot the PCM to set the tables back to there original configuration and let them re-ajust again to the operating conditions.
.
Bottom line is the Scanner shows the data but you must be able relate all possible causes just like a doctor uses symptoms and tests to determine the treatments to clear health issues.
Good luck.

 

Sucks to be that guy! Oh wait, that's me! I'll have to rinse this off tonight to do a little damage control. Luckily I only sprayed one hose as I had already suspected it and I used the carb spray straw. Just want to clean up any overspray.

What other options do I have for detecting a vac leak? My drive to work is 50mi one way @ 70mph.

 

lol, - Hey, being that guy can happen to anyone. A cheap "Mighty Vac" pump works. Harbor Freight was selling a nicer brass type, real cheap at one time. It looked to be a nicer one anyway. I use a plastic type you can find anywhere.

 

Is there a way to check the DPFE sensor?

 

Just wanted to give a quick overview of my truck and what I've been measuring.

Stock 2002 5.4L with 185k mi. I haven't done much engine work on it other than rebuilt alternator, New AC clutch & bearing, new idler & tensioner pulley in the last 20k mi.

Recent work includes:
Cleaned IACV (11.9ohm across terminals) 650RPM idle speed
Cleaned MAF
New air filter
New fuel filter
New PCV & inspected line including rear boot
Verified O2 sensor function
Checked Intake tube for leaks, none detected.
Max operating temp ~180-185F (likely a worn thermostat)

With the above work I unplugged the battery over night to reset the computer. I drove the truck 50mi on highway each way to work. While driving on the highway my ST fuel trims fluctuate between +5% and -5% depending on accelerating or coasting. The LT fuel trims are always under +10% while driving around. When I got to work I checked these and they were +23%. After driving home once I got in my garage they were +7%, but the P0171 and P0174 codes were pending on my scanner.

I'm going to take another look at my vacuum lines tonight and see if I can find anything with a mityvac. How do I check for a vac leak on my intake manifold?

 

Don't waist your time on the DPFE because it has nothing to do with the 171/174 codes.
It's hoses are accross a calibrated orface in a metal tube off the exhaust manifold.
If the hoses or DPFE has a problem you would see a 4xx code for that system malfunction.
Good luck.

 

I'm checking vac.

PCV has 20in
Red line from fuel rail 20in
Lower line on dpfe 20 in
Upper green line that goes to egr 0. Should this have vac on the egr?

Tried the propane, no noticeable change.

 

Brake booster can hold vac
Vac applied to egr and engine dies

Been idling for 30 min. Lt trim under 10%

 

Two hoses enter intake duct just before tb. Upper has strong vacuum. Lower has none?

 

You cannot read any vacuum on the DPFE.
It is only an electical pressure sensor that is used to detect small changes in pressure accross the two small hoses.
You likely mean the EVR solenoid.
No there should be no vacuum to the EGR.
If there was the motor would stumble and die from the exhaust gas being let in the intake through the open EGR valve.
It is only opened during light throttle in third gear or overdrive at light throttle between 45 and 60 mph.
Good luck.