TBI Idling Waaay Too High

[1987 GMC Jimmy]

I've gotten back to working on my Jimmy. I replaced the old IAC valve a while back to see if that would get the idle down to a lower stepper count than it was. Well, after fixing a bear of a transmission leak, I'm trying to run the truck. It's stinky rich, it's idling at like highway RPM, and it runs pretty rough. I tracked down the engine vacuum leaks. The TBI was leaking vacuum, and I had it rebushed. The PCV seal was leaking, and I replaced it. There shouldn't be anything else, right? I can hear a constant hiss under the dash when sitting in the car. I'm scared the brakes have some kind of vacuum leak. Do I need to drive it around to set the IAC right? I did the reset procedure correctly. I cannot figure out how to get this thing running right. I've come close but never right on the money.

 

[Georgeb]

You should try to find the source of the hiss. Try disconnecting and plugging the vacuum line to the brake boostter

 

[yevgenievich]

Constant hiss from the dash can be ac controls(vacuum selector switch). You can disconnect it and cap for testing on passenger side of the engine bay near a firewall.

 

[1987 GMC Jimmy]

You should try to find the source of the hiss. Try disconnecting and plugging the vacuum line to the brake boostter

Constant hiss from the dash can be ac controls(vacuum selector switch). You can disconnect it and cap for testing on passenger side of the engine bay near a firewall.

Alright, I tried both. There was a vacuum leak between that actuator ball and the firewall, and I fixed it. Definitely something wrong with the brakes; one side of the reservoir lost all its fluid and none leaked on the ground. I still hear the hiss inside. Whatever I did, it idles good for about three to five seconds, and then it idles up and I hear the hiss. This is with the brake line plugged, by the way.

 

[yevgenievich]

Alright, I tried both. There was a vacuum leak between that actuator ball and the firewall, and I fixed it. Definitely something wrong with the brakes; one side of the reservoir lost all its fluid and none leaked on the ground. I still hear the hiss inside. Whatever I did, it idles good for about three to five seconds, and then it idles up and I hear the hiss. This is with the brake line plugged, by the way.

check the back of the ac control panel or cap it by the firewall on the engine side

 

[1987 GMC Jimmy]

check the back of the ac control panel or cap it by the firewall on the engine side

Which one specifically? There are several.

 

[yevgenievich]

it will come through firewall center/passenger area

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[1987 GMC Jimmy]

it will come through firewall center/passenger area

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Negative on that hose. I think what I'm hearing is the vacuum leak noise coming from the engine because it's just a softer version of what's outside. It's really really aggressive. That's with the breather off. With it on, it's muffled, but the idle still sucks as much. It's like a normal startup for a split second, and then it feels like someone presses on the gas. Throttle is good, not binding. I noticed when you turn it off, it sounds like someone's opening a soda. I've had the breather off, and I it's never been that loud unless it's just the TBI sucking in more air. I really don't know here.

 

[yevgenievich]

Start spraying carb cleaner around if you have not done so yet. Can also go back with a computer and check what mapp ssensor is showing. Have you also eliminated egr as a possible issue?

 

[1987 GMC Jimmy]

Start spraying carb cleaner around if you have not done so yet. Can also go back with a computer and check what mapp ssensor is showing. Have you also eliminated egr as a possible issue?

Yeah, I guess I'll have to do the carb cleaner test. I haven't done that yet, though. I did the old cigar smoke into the vacuum hoses to check for leaks, and I'd patch up the leaks where I saw puffs coming out. I'll check the MAP sensor with the computer. When I first bought the car, I was getting some EGR codes and funky behavior so I just replaced the valve, and it seemed to run fine since. I did recently replace the IAC valve. The old one was on there really loose, but I snugged this one up pretty good. Is it supposed to be turnable by hand or something? It was idling high before I replaced the IAC because it wouldn't step down quite as far as it should, but it's been like this since I put the new one on.