1987 Suburban Idle Issues

[shotrod69]

‘87 R20 Suburban with 5.7and TH400.

Having an issue with truck idle.
Starts up fine in the morning or after work.
If I start it up to roughly 1-3 hours after (inconsistent) it’ll only occasionally not start, most of the time it’ll fire right up. Then when it tries to idle down it’ll go to 300-500 and start stumbling, missing, and shaking hard. Clears up immediately with throttle. Runs fine if I keep it above 600-700. Drive just fine as well.

New parts and other info:
IAC
MAF
Cap and Rotor
Coolant sensor
Rebuilt fueling side of carb
injectors
regulator
fuel sending unit and tank
water pump
battery basically new
Ignition coil
4 degrees advanced (it’s been like that long before this issue started)
PCV
Sprayed down for vacuum leaks, as well as spraying lines to check for pin hole.

I’m sure I’ve done other things I can’t think of.

Another thing that happened recently was that a bunch of air rushed in to/out of the filler neck when I fueled. A solid 3 seconds. Sounded like a drain valve on an air compressor minus the water sounds.

 

[gmbellew]

make sure the fuel charcoal cannister is installed correctly. the tank should not be pressurized.

 

[shotrod69]

make sure the fuel charcoal cannister is installed correctly. the tank should not be pressurized.

I haven’t touched it at all, and it was working without issue before.
I’m also not sure how that’d affect the truck the way it’s acting up.

Any ways to test it?

 

[gmbellew]

I haven’t touched it at all, and it was working without issue before.
I’m also not sure how that’d affect the truck the way it’s acting up.

Any ways to test it?

I'd just make sure all the lines are hooked up correctly and that it isn't full of fuel. if the tank has been getting pressurized, it may have forced it full of fuel.

if it is fuel saturated, it could be pushing fuel into the TBI, even at idle, and causing it to be so rich it almost floods.

 

[fast 99]

Take the charcoal can off, turn upside down or shake it. Shouldn't be any liquid fuel inside. It's common for some pressure inside the tank but yours sounds excessive.

No charcoal pieces should shake out either.

 

[shotrod69]

Take the charcoal can off, turn upside down or shake it. Shouldn't be any liquid fuel inside. It's common for some pressure inside the tank but yours sounds excessive.

No charcoal pieces

Take the charcoal can off, turn upside down or shake it. Shouldn't be any liquid fuel inside. It's common for some pressure inside the tank but yours sounds excessive.

No charcoal pieces should shake out either.

I’ll check that when I get to the shop.
Was told that it could be the purge valve, but I couldn’t find it. All I’ve got is the lint from the tank and a line to the tbi

 

[RecklessWOT]

While not ideal (just for the integrity of the tank) my '87 burb has always had that pressure in the gas tank thing going on, sometimes it'll blow the cap right out of my hand when unscrewing it. On my list of things to fix one day, but just for the sake of figuring out your issue I'd say I can almost guarantee that has nothing to do with it.

Also fwiw, I don't even have a charcoal can on my truck at all anymore, so that also doesn't affect idle...

 

[fast 99]

Also fwiw, I don't even have a charcoal can on my truck at all anymore, so that also doesn't affect idle...

It will if full of fuel. During restarts ECM primarily uses MAP and algorithm for fuel control. OBD1 vehicles generally do not have preheated O2 sensors. Doesn't use live feedback sometimes for several minutes, depending on conditions.

 

[shotrod69]

Take the charcoal can off, turn upside down or shake it. Shouldn't be any liquid fuel inside. It's common for some pressure inside the tank but yours sounds excessive.

No charcoal pieces should shake out either.

Just checked it, sorry for the long wait. But I did shake it and there wasn’t anything moving inside, and nothing came out.

Any way I can “delete” it to see it maybe it’s plugged up or otherwise bad?

Edit: it did absolutely reek of gas when I took the hoses off

 

[gmbellew]

Just checked it, sorry for the long wait. But I did shake it and there wasn’t anything moving inside, and nothing came out.

Any way I can “delete” it to see it maybe it’s plugged up or otherwise bad?

Edit: it did absolutely reek of gas when I took the hoses off

did you take it out of the bracket, or just wiggle it around in the bracket? I've had my line off that goes to the TBI and I don't remember getting a very strong fuel smell.

if you didnt remove it from the bracket, I guess you could take off both hoses, hook up a plastic hose to the gas tank port and blow and see if fuel comes out the TBI port or the vent port.

 

[shotrod69]

I took it out of the bracket, mines the tank to canister to tbi style. Shock the hell out of it, no rattling or anything. An air gun seems to push air through without an issue

 

[gmbellew]

the cannister port in the TBI is ported vacuum and only gets vacuum off idle. so if it isn't filled with fuel, you are probably OK on that part of the system.

 

[gmbellew]

did this issue occur before or after you replaced the IAC. there is a chance you need to let the ECM relearn IAC position. you can jumper A and B with a paper clip on the OBD1 plug and then key to on for about 30 sec. you should hear the stepper motor start clicking once it is all the way shut. then turn key off, remove the jumper, and see if that helps.

 

[shotrod69]

did this issue occur before or after you replaced the IAC. there is a chance you need to let the ECM relearn IAC position. you can jumper A and B with a paper clip on the OBD1 plug and then key to on for about 30 sec. you should hear the stepper motor start clicking once it is all the way shut. then turn key off, remove the jumper, and see if that helps.

Months after. Did the iac in September. But I will give that a shot tomorrow morning and post an update

 

[gmbellew]

if you have a vacuum gauge, you may hook that up to the EGR port on the TBI and see if your vacuum is steady at idle.

and ALDLDroid with a blue tooth OBD1 cable goes a long way when diagnosing TBI issues. it's a slow computer system that only reports data every second or so, but it is still a lot of help.