87 350 TBI High Idle

[TheGolem]

First of all, Im a new square body owner, I just picked up a 1987 V10 Suburban Silverado a week ago, and love it! The truck is in great shape for its age, though it does need paint/bodywork and tires. My brother named him The Golem, hence my username

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Anyway, ive recently had an issue where he idles extremely high. I dont have a tachometer, but I know its high, cuz I almost have to yell at my passengers. This Only happens when I put it in park, after its been in another gear. This causes the transmission to slam into gear, which is not good. I know this means that the engine is getting too much air, and I have no clue where my IAC is, can someone tell me? I took pics of my engine bay. Also; there is a silver steel wool like hose that runs from the air cleaner to the bottom of the engine on the right side (last picture), and its not affixed very well to either, could that be the culprit? and what is it called? The fan is always on, making it sound like a jet. Is that normal??
THANKS!

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[1987 GMC Jimmy]

I'm having a similar problem on mine. It's not a constant high idle, though, and mine's a bit rich. Not horrible, though. Anyways, here's a picture of the IAC. It's the black thing on the left that can be removed with an adjustable wrench. My TPS is off in this picture so don't worry about that. I don't know what that tube is. Mine has the same setup minus the tube, and it it runs pretty good. Before you start thinking about the IAC, though, make sure you have no vacuum leaks, as that can cause a consistently higher idle, and for reference, can you see a screw on the driver's side front corner of the TBI, or is it smooth? There's a minimum idle air screw there that was sealed from a factory that many a ding **** crack open and play with.
Edit: The image uploaded on its side. The IAC is on the left of that green connector.

 

[Georgeb]

I would first check for a vacuum leak. Does it ever idle down? Is it effected if the brakes are applied or not?

 

[TheGolem]

I'm having a similar problem on mine. It's not a constant high idle, though, and mine's a bit rich. Not horrible, though. Anyways, here's a picture of the IAC. It's the black thing on the left that can be removed with an adjustable wrench. My TPS is off in this picture so don't worry about that. I don't know what that tube is. Mine has the same setup minus the tube, and it it runs pretty good. Before you start thinking about the IAC, though, make sure you have no vacuum leaks, as that can cause a consistently higher idle, and for reference, can you see a screw on the driver's side front corner of the TBI, or is it smooth? There's a minimum idle air screw there that was sealed from a factory that many a ding **** crack open and play with.
Edit: The image uploaded on its side. The IAC is on the left of that green connector.

How do I check for vacuum leaks? I'll have to check on that throttle body screw when I get off work. I bought this truck as a toy basically to help me learn more about working on cars. But if it's too difficult, I'll take it to my mechanic

 

[TheGolem]

I would first check for a vacuum leak. Does it ever idle down? Is it effected if the brakes are applied or not?

When I start it up cold, it idles high (which is normal) then idles down, but as soon as I put it in gear, and back to park, it sounds like a freight truck. The brakes don't affect the idle, but the truck is very jumpy while at a stop sign/light, I just assumed that was normal with its age and all that torque. It does idle low in reverse though. I always feel like it's going to die, but it hasn't yet. It has a cheap battery in it that doesn't have enough power for that truck, so I'm going to replace that tomorrow, but any other help is greatly appreciated. Runs beautifully otherwise

 

[TheGolem]

I'll get a video of it tonight, Ive never had a car older than an 05 (sadly) so maybe it's normal and I'm just not familiar with it? It doesn't have all that engine sound proofing that late model suburbans do. Like I said, I don't have a tachometer, but it is LOUD, and I can feel the revolutions....if that makes sense

 

[adamj]

Don't worry about that hose coming from the air cleaner - that hose just directs warm air from the exhaust manifold into the air intake to bring the engine up to operating temp faster, and it won't cause a vacuum leak or idle issues. Many trucks this old have long lost that tube, your truck will run fine with or without it.

But as others have stated, I would begin by checking all vacuum lines for any leaks. You can spray throttle body cleaner over the vacuum lines and if you observe a change in idle after you spray on a particular section, that indicates a leak there, then narrow it down and replace the hose or cut and splice in a new piece of vacuum tubing.
But it sounds like you could have a large leak, so just feeling the lines by hand and visually inspecting you might be able to find it, or something is not connected all the way.

Once you've ruled out the possibility of a vacuum leak, I would then move on to that Idle Air Control (IAC) and check that the throttle position sensor (TPS) and throttle plate is calibrated correctly. There's a procedure for doing this with a multimeter, but check out those vacuum lines first. (vacuum leaks can be a real pain to find, so really check it over)

 

[1987 GMC Jimmy]

The catalyst sticker on the top middle of the core support is a good reference if you have it. It shows the whole vacuum network. Just make sure all the connections are tight according to that diagram. I think I have one in case yours is gone for some reason. People say that spraying carb cleaner or WD40 around the base of the TBI are good indicators of a failed TBI base gasket, which is arguably the most common vacuum leak. That's the best place to start. The screw tells you if someone's messed with the idle or tried to. If not, you don't have to go clean up someone's likely mistake.

 

[TheGolem]

Don't worry about that hose coming from the air cleaner - that hose just directs warm air from the exhaust manifold into the air intake to bring the engine up to operating temp faster, and it won't cause a vacuum leak or idle issues. Many trucks this old have long lost that tube, your truck will run fine with or without it.

But as others have stated, I would begin by checking all vacuum lines for any leaks. You can spray throttle body cleaner over the vacuum lines and if you observe a change in idle after you spray on a particular section, that indicates a leak there, then narrow it down and replace the hose or cut and splice in a new piece of vacuum tubing.
But it sounds like you could have a large leak, so just feeling the lines by hand and visually inspecting you might be able to find it, or something is not connected all the way.

Once you've ruled out the possibility of a vacuum leak, I would then move on to that Idle Air Control (IAC) and check that the throttle position sensor (TPS) and throttle plate is calibrated correctly. There's a procedure for doing this with a multimeter, but check out those vacuum lines first. (vacuum leaks can be a real pain to find, so really check it over)

Thanks, that's very good information. I always wondered why it got up to operating temperature so quick, with it being so old. I'll inspect those vacuum lines, and let y'all know

 

[adamj]

And as for the fan sounding loud, you more than likely need a new fan clutch - an excessive roar is one of the signs the fan clutch is bad. Once everything is warmed up, the fan sound should drop, it's normal for the fan to sound louder when starting up cold, but should be relatively quiet at operating temperature. I replaced mine not too long ago and was amazed at how quiet it got.

 

[glockholiday]

Damn! I definitely have to paint my grill black now.

 

[TheGolem]

And as for the fan sounding loud, you more than likely need a new fan clutch - an excessive roar is one of the signs the fan clutch is bad. Once everything is warmed up, the fan sound should drop, it's normal for the fan to sound louder when starting up cold, but should be relatively quiet at operating temperature. I replaced mine not too long ago and was amazed at how quiet it got.

That may be what I need to do. The sound is more fan than anything, though the idle is still high.

 

[TheGolem]

Ok Im having a difficult time getting videos of it. I looked at the vacuum lines and see no obvious issue, but Ill test it with the fluid this weekend (sunday/monday for me). Thanks y'all, you're certainly more help than the folks on GM fullsize

 

[TheGolem]

Damn! I definitely have to paint my grill black now.

TBH I thought that was stock until you pointed it out lol