Engine dies low idle after sitting

[PaulZ]

Non Ford, hope that's OK here. I have a '97 Dodge dump truck that sits for long periods of time, I don't keep a battery in it. When I do run it, often it will not idle right away. If I rev it up and slowly back of the throttle down to an idle that usually works, but just letting off the pedal quickly it will die. No big deal, but then yesterday a friend called from the smog testing shop, his '97 Rover Defender that had been sitting a long time also would not stay running. I drove over there, had him pop the hood and start it up. I did the same thing, revved it up and slowly backed off the throttle a couple times. It worked, idled fine, he passed the smog test even. I was a hero, but I have not idea why this happens. Is it a common thing? My guess is it's either some kind of computer thing or the injectors needing to be exercised.

 

[Dirtman]

Few things could be the issue.

Sitting for long periods means the fuel pressure can bleed off so it'll be hard to start or run crappy for a moment but usually that goes away in literally 10 seconds.

Second would be the IAC valve getting stuck in the closed position. The IAC valve should open at startup to increase the RPMs for a short period since cold engines need a higher idle. If they get stuck closed the idle will be low and possibly stall the engine until the engine warms up. Putting your foot on the throttle does the same thing the IAC valve does, lets air into the throttle body to increase RPMs.

Third. Fuel injected vehicles do not like the battery being removed. The computer stores all the fancy data about how the engine operates over it's life so everytime you take the battery out for more than a few minutes you are essentially rebooting the computer and erasing all that data. It will then take a while for the computer to "figure things out" again the next time you start the engine. So this could cause it to run like crap for a while as well.

 

[RonD]

+1 to what Dirtman said

Assuming gasoline engines

But can also be an issue with the Choke if it only happens when engine is cold started
All gasoline engines need to be Choked on Cold Start, fuel injection doesn't change that, just changes the way its done

All fuel injected engines have an ECT(engine coolant temp) sensor, and an IAT(ACT) air temp sensor
The computer uses these to set the right amount of Choke to use on cold start
Choke in this case means how much extra fuel to add to get gasoline Vapor level high enough so spark can ignite it reliably

All fuel injected engines also have an IAC(idle air control) valve, the computer use this valve to let in more or less air(like a throttle plate does) to set the idle higher or lower
Idle screws don't work with fuel injection because there are no Jets to suck extra fuel from

On Cold start computer reads engine and air temp and sets Choke accordingly and it also opens IAC valve a bit more to set High Idle until engine warms up a bit

Since the ECT sensor is the more important and less costly sensor, lol, I would replace that just on speculation
If you have or can get an OBD2 reader then you can look at what the ECT sensor is reporting to computer BEFORE cold start, should be outside temp, if its higher then computer won't set enough choke or idle high enough so engine would stall
(ECT sensor has no connection to dash Temp Gauge, just FYI, its only connected to computer)

To test IAC Valve warm up engine FIRST then let it idle and unplug the IAC Valve's 2 wire connector
Engine RPMs should drop to 500 or so, or engine may stall, either is GOOD, it means IAC Valve is working and no vacuum leaks either
If RPMs do not change then IAC Valve could be stuck at the one position or "dead" not responding to voltage changes, so computer can set the high idle needed on cold start

 

[PaulZ]

Thanks guys! Interesting stuff, mostly above my 65 year old carburetor trained brain. The fuel pressure and computer retraining theories could certainly apply to my dump truck, but I should have mentioned my friend had driven 20 miles to get to the smog shop. The truck (Range rover Defender with a v8, Buick maybe?) had not been started in a year he said, but started and ran well until he got there. So maybe that's the IAC valve. It was at full temp when I arrived, and again all I did was throttle it a couple times from under the hood without letting it die and then slowly let it back down to idle. Ran fine after that, went for a test drive and then let the smog guy test it.

 

[Dirtman]

Easy to check, just pull the IAC valve off and with it still plugged in turn the key to run and watch the piston inside it. It should open fully then close some. If it doesn't move at all either take it all the way off and clean it good to see if it starts moving again, or replace it. They often get full of carbon so sometimes cleaning is all they need.

 

[PaulZ]

Thanks again Dirtman, I'll be doing this.