"Hunting idle condition"

[rford]

I have a 1989 F-250 5.8L, I have replaced or tested with "none good" all sensors and computer. Truck does not seem to have any vaccum leaks, no codes are present. If truck idles for more than 61 seconds (throttle untouched) a surging starts.
Cat is unobstructed, timing is correct, base idle is correct,fuel pressure correct(38lbs), distributor pick-up was replaced, e-coil swapped out, injectors have been rebuilt & tested, fuel filters are new. Has eeciv ignition. Truck runs great with exception of this condition. I think it may be related to the computer control circuit somehow. If I connect a timing light and observe timing marks they move with this surging. Timing chain is new as rebuilt engine has about 5k on it.
Any helpful information on this would be appreciated.

Thanks,
rford
Still scratching my head!

 

[rford]

I have a 1989 F-250 5.8L, I have replaced or tested with "none good" all sensors and computer. Truck does not seem to have any vaccum leaks, no codes are present. If truck idles for more than 61 seconds (throttle untouched) a surging starts.
Cat is unobstructed, timing is correct, base idle is correct,fuel pressure correct(38lbs), distributor pick-up was replaced, e-coil swapped out, injectors have been rebuilt & tested, fuel filters are new. Has eeciv ignition. Truck runs great with exception of this condition. I think it may be related to the computer control circuit somehow. If I connect a timing light and observe timing marks they move with this surging. Timing chain is new as rebuilt engine has about 5k on it.
Any helpful information on this would be appreciated.

Thanks,
rford
Still scratching my head!

IAC & throttle body are clean & clear, Idle Air Control is new.

 

[d-kuzmen]

I may have some info for you. I'll get back to you lemme look.

 

[eco]

I have a 1989 F-250 5.8L, I have replaced or tested with "none good" all sensors and computer. Truck does not seem to have any vaccum leaks, no codes are present. If truck idles for more than 61 seconds (throttle untouched) a surging starts.
Cat is unobstructed, timing is correct, base idle is correct,fuel pressure correct(38lbs), distributor pick-up was replaced, e-coil swapped out, injectors have been rebuilt & tested, fuel filters are new. Has eeciv ignition. Truck runs great with exception of this condition. I think it may be related to the computer control circuit somehow. If I connect a timing light and observe timing marks they move with this surging. Timing chain is new as rebuilt engine has about 5k on it.
Any helpful information on this would be appreciated.

Thanks,
rford
Still scratching my head!

What do you mean when you say it seems that there are no vacuum leaks?

Another thing that would really help is a brakedown of the emissions. Got an exhaust gas analyzer?

When it surges, from what RPM to what RPM does it surge?

 

[Skandocious]

If it starts to surge after a certain amount of time has passed after starting it then I would suspect one of the sensors that only starts to be used after the truck has entered closed loop. While the truck is warming up, said sensor is not giving readings to the computer and thus not making it surge. Just an idea

 

[F 150Cobra]

mine is doing this also.. it surges from 700 to 800 only when its recently started and is warming up.. after it warms up i can idle for hours without surging..

is yours diong the same? from 700 to 800 ?

 

[ufpddirector2]

kam reset?

 

[bowtiehatr]

the gasket where the manifolds meet is notorious for leaking. spray some carb cleaner at the split and see if the idle goes wacko more than usual.

 

[Mr. Stroker]

Mine will sit there idling, then it will pick up and come back down to normal. Couldnt figure out wth was goin on. Happened when i was around a bunch of chevy guys to lol. Well i remember powerwashing under my hood the day before and the clip that goes to the IAC was broke, so it slid out a little bit. If i pulled it all the way out the motor would die, so i used some good ol electrical tape and so far so good. Not sure if this will help but check it out anyways.

 

[Old_Paint]

the gasket where the manifolds meet is notorious for leaking. spray some carb cleaner at the split and see if the idle goes wacko more than usual.

Propane gas is better for finding leaks, and a hell of a lot safer. What's going to happen if you have flammable liquids vaporizing on a hot engine, and one plug wire decides to leak a little? You're gonna have one hot engine bay fire. If the flex portions (plastic) of the fuel rail give way from the heat before you can stop the fuel pump, you're up the creek without a boat, let alone a paddle. If you use propane, you can use the nozzle of the little propane torch to point the propane where you want to check for the leak, in a relatively confined space. It's a whole lot easier to poke that torch under the plenum than it is to try to spray liquids under there. Even if that does ignite, you get a quick flash, and maybe a lit torch and a little bit of singed hair. It's already a gas, so, cannot soak into and damage anything that might not already be damaged. Carb/injector cleaners are all solvents, and will tend to soften any sealant that might have been used on a gasket (unless it's RTV, but that's a whole other story on intake sealing). When the sealant softens, the gasket could move. This is generally a bad thing. Most non-metal gaskets are impregnated with some sort of organic (read petroleum based) sealer or varnish. Solvents do gaskets NO GOOD. Even the metal ones tend to have some kind of soft coating to help them seal. Neoprene (VC/lifter valley) gaskets REALLY don't like solvents.

My whole negativity about using liquid solvents to find leaks is that if you don't already have one, you stand a pretty good chance of creating one. I say this everywhere I post, and mean it. I'd much rather read about how a problem was found and resolved, or even how it turned out to NOT be the problem, than I would to read about how many more weeks someone is going to be in a burn ward. A bottle of propane and a torch can be had from Wally World for less than 15 bucks, which is about what 2 cans of Carb cleaner costs. It'll be useful for a lot more things, and for a lot longer than the carb cleaner. Use carb cleaner for what it was intended. Clean carbs with it. Keep it off hot engines.

Oh, and by the way, if the propane goes in through a leak, you'll find the idle gets nice and smooth, not wackier. I know, I used this technique for this VERY problem, and did indeed find a blown plenum gasket. I think my truck likes propane more than it does gasoline, because it sure ran sweet as long as I fed it propane.