I have an 87 RS with the 2.8L tuned port injection. The problem is that the engine does not idle down almost as if the throttle cable is stuck. I've checked the cable and it is not sticking. I am wondering if the IAC is malfunctioning. How can I tell if it is bad or just needs adjusting?


I've checked the following: throttle cable for sticking, TPS, vacuum leaks and all are good. Any suggestions on how to get the engine to throttle down and get the idle set correctly are welcome.

I can't live with the idle at 2000 rpm.
Thanks.

 

You can take the IAC out and check it for buildup and clean it up. Don't be too rough in cleaning it, then use this procedure to correctly set minimum air.

https://www.thirdgen.org/tpimod2

I battled an idle issue and it turned out my TPS was bad. I had it set to the correct voltage initially and could never figure out why it would just change. I hooked my scanner up and watched the voltage hang when I let off the throttle. A new one cost me $15 or so, best money I've spent to-date.

Good luck,
Eric

 

Thanks Eric. When I set the TPS using the volt meter, it seemed like it was smooth and did not hang up anywhere, but for 15 bucks I might just put in a new one and eliminate it. I'll try the adjustment on the IAC tonight and see what happens.
Thanks,
Jeff

 

Ok, I've replaced the TPS and still have the same problem. The TPS for this lists for $73.58 from GM. I also went through the procedure to to set the IAC. All seemed fine until its driven and then the idle is back to 1500 or higher.

When this problem got really bad is when the radiator blew out the side tank and I replaced it, put in a 180 thermostat and 200/185 fan switch. At the same time, I put in bushings on the throttle body shaft to fix a sticking throttle. I am now wondering if the temp is not hot enough for the coolent sensor and the ECM is using the IAC to kick up the idle.

To test this, I am going to plug in the old fan switch to keep the circuit and let it get hotter and see what happens.

 

You need to disassemble the IAC, retension the spring to at least 2¼" free length, and clean/lightly oil the pintle shaft:

 

I guess my TPS was so cheap because I got the GP Sorenson version from Adavnce..maybe it was $30, I don't remember.

I noticed you mentioned your TB. I had a "sticky throttle" issue a couple weeks ago and it turns out my problem was that I had cleaned my TB really well with carb cleaner and it removed the lubrication on the shaft/blades not letting them close to the proper level. I wonder if when you replaced the bushings, you did some cleaning and this may have been the result. My solution was to WD40 the shaft. Didn't get anywhere near any sensors.

Hope this is more helpful than my last attempt.
Eric

 

Ok, I decided to put in a new IAC since I have 170K miles on the engine and its probably the original. I can get everything working fine in the driveway, but as soon as I take it out for a test drive, the idle is up to 1200-1400 rpm. Its better than before, but not down to 700 as it should be.

The only error code set is 24 VSS which is caused from the high idle when the vehicle is stopped.

Oh, one other thing. I disconnected the battery to erase any previous settings in the ECM.

So, any other suggestions?? I've run out of ideas.

 

I think you may be looking at it backward. The lack of a reliable VSS signal will create an idle problem, since the VSS input is what drives the DFCO routine in the ECM. It's not the idle causing the "24" but the "24" causing the idle problem.

 

Yeah, I hear what you're saying and I've given it a lot of thought. Code 24 is set under multiple conditions which all seem to come together when I sit at a red light. One of them is if the idle is above 1400. There is also a note in the GM manual to disregard a code 24 set when drive wheels are not turning.

Somehow the ECM thinks it needs to kick up the idle. As I mentioned, i can set it in the driveway perfectly but it resets itself after reaching speeds above 35mph. (another note from the GM manual)

 

Ok, I'm at a point where I believe I really need a scanning tool to see what is going on with the rest of the sensors. A drive to Los Angeles went well until climbing up the grapevine. The idiot light came on about half way up and then went off just after peaking the top. Haven't checked the error code yet, but while caught in traffic on the downhill, it actually idled correctly. Now its back to high idle again. The only thing I can gather from this is that the temp guage was just under 220. Not sure if the 180 thermostat is causing my problems with the coolent sensor and the ECM.