97 xlt 4.0

[Extremealpine7]

Is there anyway to raise the idle on a 97 xlt with 4.0 SOHC occasionally it will stall while pulling out of a parking space while switching gears could this be a worn IAC any help would be appreciated

 

[koda2000]

Start by cleaning your IAC, throttle body and MAF. When's the last time you changed your spark plugs and ignition wires? If that doesn't help, there is a throttle body plate stop screw that you can raised a little bit. Doing this will increase the idle.

 

[imp]

Start by cleaning your IAC, throttle body and MAF. When's the last time you changed your spark plugs and ignition wires? If that doesn't help, there is a throttle body plate stop screw that you can raised a little bit. Doing this will increase the idle.

@koda2000

Wasn't it nice when we had that option available? Even Drive by Wire has an adjustment (factory only, locked once set, forcing movement breaks off the adjustor!), but it's purpose and operation are entirely different. After months of troubling events with the damned system, I tore the TB apart, to figure out how they worked. FYI, if you haven't been exposed to this: a spring-loaded plunger holds the TP slightly open when power is off. That plunger opposes the mainspring, which forces the TP closed. Key-on causes PCM to force TP fully closed for about a second, then releases it. Watch one do it once, with the rubber duct removed.

During that quick TP movement, PCM evaluates TPS output, TP angle change vs. output. If not within acceptable limits, it immediately throws Limp Mode, or worse, in my case "Forced Engine Idle". imp

 

[96eb96]

There is nothing really officially adjustable. Assuming it is reasonably clean the PCM opens/closes the IAC enough to keep it from stalling. Once it does all it could the vehicle may stall. If it stalls there is likely some vacuum leak (maybe intake gaskets) or a bad Iac. The throttle screw can be adjusted but is not really recommended.

 

[imp]

There is nothing really officially adjustable. Assuming it is reasonably clean the PCM opens/closes the IAC enough to keep it from stalling. Once it does all it could the vehicle may stall. If it stalls there is likely some vacuum leak (maybe intake gaskets) or a bad Iac. The throttle screw can be adjusted but is not really recommended.

@96eb96
True that repair manuals mostly state do not adjust the stop. The PCM has control over idle speed, and attempts to keep it constant, regardless of changing loads on the engine, those being electrical, like lights, blower, and mechanical, such as the A/C Compressor.

Owners Manuals state starting the engine is to be done with no throttle advancement, foot off the gas pedal. Upon cranking, IAC is opened fully, to admit maximum air, then quickly closes down as engine fires and commences running. Personally, I like the IAC system better than the Drive by Wire, for two reasons: DBW complicates things more, and takes control of the throttle away from the driver more readily, for certain faults, one being ABS failure. For my part, if my ABS fails, the brakes still work otherwise normally, I certainly don't want my engine shut down, yet that's exactly what it did! imp

 

[koda2000]

I've adjusted the throttle plate stop screw a tiny bit to increase the idle on my '97 SOHC (probably raised the idle around 100 RPM). I achieved the required result with no ill effects.

NEVER attempt to lower the idle via the stop screw as the throttle plate can become stuck in the throttle body bore. I lowered the idle speed on my 5.0L XLT (which must have vacuum leak somewhere, but I was unsuccessful in finding it and didn't want to invest any more of my time and energy trying to locate the leak) by plugging the hole in the throttle plate. Worked out perfectly.

 

[imp]

I've adjusted the throttle plate stop screw a tiny bit to increase the idle on my '97 SOHC (probably raised the idle around 100 RPM). I achieved the required result with no ill effects.

NEVER attempt to lower the idle via the stop screw as the throttle plate can become stuck in the throttle body bore. I lowered the idle speed on my 5.0L XLT (which must have vacuum leak somewhere, but I was unsuccessful in finding it and didn't want to invest any more of my time and energy trying to locate the leak) by plugging the hole in the throttle plate. Worked out perfectly.

@koda2000
Never realized there was a hole in the throttle plate on Gen. 2s. Only thing not "kosher", seems the air flow through that hole is measured, while that from the "leak" is not. Must be a small vacuum leak. What led to believe there was one? imp

 

[koda2000]

@koda2000
Never realized there was a hole in the throttle plate on Gen. 2s. Only thing not "kosher", seems the air flow through that hole is measured, while that from the "leak" is not. Must be a small vacuum leak. What led to believe there was one? imp

@imp - When I first bought the truck (2 years ago for $1500) it had a CEL for PCM not able to control idle speed (too high). I installed a new IAC and it did not help. The idle, especially when cold was quite high and even after the engine warmed up it was too high. It should have been around 650 RPM, but as I recall it was more like 850 RPM. This was enough to make it necessary to put extra pressure on the brake pedal while stopped in traffic, and had a negative impact on the vehicle's general driveability. I found this annoying, and I figured it was hard on the transmission as well.

I checked all the vacuum lines and connections and found no leaks. I even swapped the throttle body/TPS with one of my other 2000 5.0L's to no avail. I suspect a there's a small vacuum leak in an intake gasket. At any rate, I was tired of screwing with it. Then I noticed the hole in the throttle body plate and I figured that if too much air was getting in and increasing the idle, by leaning the air/fuel ratio, maybe covering that hole would offset the extra air. I put a piece of tape over the hole, and the idle dropped right down to where it's supposed to be. Then I removed the tape, tapped the hole in the throttle plate and installed a small brass machine screw in the hole using red Loctite. This is the family's "spare vehicle" and is in the roughest shape of all our Ex's/Monty's aesthetically (though not terrible). So good enough is good enough.

 

[drdoom]

I suspect a vacuum leak such as intake manifold o rings, PCV elbow, rotten small lines etc. On that model there is a three way rubber elbow at the IAC valve, major leak if that's loose.

 

[Extremealpine7]

thanks for all the reply's guys I did just do a tune up not more than a thousand miles ago and it got better but still does it sometimes ill have to clean the IAC better or it could just be worn out

 

[imp]

@imp - When I first bought the truck (2 years ago for $1500) it had a CEL for PCM not able to control idle speed (too high). I installed a new IAC and it did not help. The idle, especially when cold was quite high and even after the engine warmed up it was too high. It should have been around 650 RPM, but as I recall it was more like 850 RPM. This was enough to make it necessary to put extra pressure on the brake pedal while stopped in traffic, and had a negative impact on the vehicle's general driveability. I found this annoying, and I figured it was hard on the transmission as well.

I checked all the vacuum lines and connections and found no leaks. I even swapped the throttle body/TPS with one of my other 2000 5.0L's to no avail. I suspect a there's a small vacuum leak in an intake gasket. At any rate, I was tired of screwing with it. Then I noticed the hole in the throttle body plate and I figured that if too much air was getting in and increasing the idle, by leaning the air/fuel ratio, maybe covering that hole would offset the extra air. I put a piece of tape over the hole, and the idle dropped right down to where it's supposed to be. Then I removed the tape, tapped the hole in the throttle plate and installed a small brass machine screw in the hole using red Loctite. This is the family's "spare vehicle" and is in the roughest shape of all our Ex's/Monty's aesthetically (though not terrible). So good enough is good enough.

@koda2000
You are made of the same "stuff" as I; for that I salute you. Have always hated to follow "protocol" when it interfered with my own impressions of some problem. Certain minor issues with EFI I have decided do not warrant 100% fixation. imp

 

[96eb96]

There is actually a PCM pid called "IAC" I believe. It will tell you the duty cycle commanded to the IAC. I believe it should be in the high 20s or 30s at idle. If it is over 50 or under 20 there is some issue - vac leak, throttle body, screw out of adjustment, etc.When they say the PCM adjusts for aging components, this is what they mean.... It may be bidirectional too where you can increase/decrease it to test your IAC.
This would require a professional type scantool like Forscan or the VCM/IDS.

 

[imp]

@96eb96
Geez, I think I understand some of what you say, but not much. I'm "old school", trying to catch up, but not likely to happen.

Still, the guys who lounge in la-la-land today, not understanding what a thousandth of an inch of crankshaft bearing clearance is, will never successfully assemble an engine.