3400 idle problem, 97 Trans Sport

[redmetrix]

I have a 1997 Trans Sport with 209,000. At about 190,000 miles, a high idle problem showed up. There are a few variations to the problem. You can be driving the van with the idle being normal (always going back to 600 rpm), and then it will start to idle at 1000 and stay there. It will idle at 1000 right from starting the engine also. This will occasionally happen, but it may go for a month with no issue at all. Other times, I go to start it and the idle will climb to 1500 and stay there, but if I turn it off and restart, the idle will be right at 600 where it should be. Then again, restarting has not corrected the idle at times and it would climb back to 1500. That has happened about a dozon times over about a year. The SES does not come on.

There has also been two or three instances, were I would start the engine, and the idle would climb to 2000 and stay there. Then there was once or twice when the idle climbed to 2500 from a start. Restarting would correct this.

I know there have been others with this issue on the GM minivans specifically. I have read the other post in the forum about the high idle problem on a 3400 Grand Am, it ended up being the TPS. I am not sure what to check or where to start.

This may be irrelevant, but I had a 93 GTP with the LQ1 3.4 which developed a high idle. Replacing a few leaking vacuum hoses and the upper and lower intake gaskets fixed this. That was straightfoward to diagnose because the problem was consistent with every cold start. The 3400 in this Trans Sport however is a different story since the problem is NOT consistent.

[pocket-rocket]

I would start simple with cleaning the IAC (and to rule it out). It's possible it could be sticking some.

[redmetrix]

What would be the best way to go about cleaning the IAC? I've heard throttle body cleaner, carb cleaner, and even electrical parts cleaner. Would I take out the IAC and spray it with one of these? I suppose I should clean out the hole on the TB also where the IAC attaches.

[pocket-rocket]

Yeah. take it out and spray it down. I wouldn't go sticking anything like a wire brush in the IAC bore or on the pintle that way you avoid scratches that could throw off the airflow in there. If you have to use anything to scrub in there I would use a Q-tip with some carb & choke cleaner.

[Azrael]

Idle speed is also dependant on readings from the coolant temp sensor. A bad CTS is usually accompanied by really bad fuel economy though. It's still worth checking the connection on the sensor since it's intermittant.

[redmetrix]

Fuel economy is right where it is supposed to be, but I still do believe the issue could be the coolant temp sensor. This is a basic question but does the temp gauge on the dash get its signal from the CTS? The reason I ask is because sometimes, not every time, when this high idle condition would show up, the gauge would read a little higher than normal, meaning closer to 210 than the normal 190.

[Azrael]

Rockauto shows one sensor with 3 terminals (2 sensors in 1). You can use an ohmmeter to compare resistances. One sensor will use 2 terminals and the other will use the 3rd and complete the circuit through the sensor's body.

The 2-wire circuit is used for the PCM. The 1-wire one (usually green) will be for the gauges.

[redmetrix]

I took out the IAC and cleaned it with TB cleaner along with the IAC bore on the TB. This didn't fix it. Today the van was driven making 4 different stops. Idle stayed normal for the first three starts, on the fourth start, the idle came up to 1500. I turned off the engine while it was at 1500 and restarted and the idle was normal, and it stayed normal for the 12 mile trip back home. Does this rule out the IAC?

This is also worth mentioning. For example, say the engine is warm at normal idle and I hit the throttle so it revs to 3000, it will settle right back down quickly to 600 like it should. Now, if the idle is up at 1500 and I hit the throttle, the rpms don't settle out at 1500 real quick, they will come down real slowly back to 1500, or they won't even go back to 1500 but instead settle at 2000 or 2500 and stay there.

[redmetrix]

I am still having issues with the idle. If it is not at the normal idle it seems to be at 1100 RPM half the time. All I have done so far is clean the IAC. I went to RockAuto to check out the coolant temp sensor. Now, the website shows a coolant temp [I]sensor which had two terminals and the site also shows a coolant temp sender which has three terminals; I believe this is what Azrael is refering to; both of them look very similar; does the engine have both of these?. Where is the coolant temp sender located? Is it possible the connection could be shaky between the plug and connector on the sender causing the idle problem? As far as checking resistances what am I looking for?, do I need to see a certain amount of resistance between two of the terminals? I assume I would see infinite resistance with one of the three terminals compared to another since it is a separate sensor(two sensors in one).

Other than this, could I check anything else such as the TPS? Any other ideas or input is really appreciated.

[2000GAGT]

im also having a problem like this,
over the years it has come and gone but its been more consistent as of late. i kinda have the same thing where the idle will hang higher. If i come to a complete stop in drive with the foot on the brake the car will be idling around 800 rpms and if i shift to neutral it will climb to about 1100 rpms and hang there. I can also notice it when im coasting at low speeds when i take my foot off the gas the rpms dont drop like they should and it feels like the car is not slowing down as fast as it should while coasting
my IAC is only a few months old so i dont think its that but maybe something to do with the TPS and causing a bad signal to make the IAC open more? I also cleaned out the MAF about a month ago with the MAF spray cleaner.

[redmetrix]

The idle problem seems to be occuring more often now and more consistantly, something's going on here. The most basic way to explain the issue is that the idle settles at 1100 from a start up, after ANY input on the gas pedal, say after driving a short distance, the idle is now at 1500. Restarting may or may not correct the idle.

I am wondering if there is any other things I could diagnose under the hood. Otherwise I am going to start replacing parts, Most likely the TPS first, then possibly the coolant temp sensor. If there was an intermittent open to a sensor's electrical connection, I would think that would cause the SES light, but I am not having the light at all. Could the MAF contibute to a high idle? It seems to me like the TPS or coolant temp sensor is sending out an incorrect signal causing the problem.

[2000GAGT]

m thinking TPS. I have a 2000 Grand Am GT with a 3500 LX9 and pretty much the only sensor i've never replaced is the TPS since im using my old throttle cable from the old engine. and those wires have been spliced before when i use to have a jet module tapped into it.

[redmetrix]

To me it seems there are enough people out there that have or have had this issue that there would be a clear solution. For example, type "venture high idle" on google and I find multiple postings on different sites. People say they get a high idle condition, they turn off the engine and restart and idle is normal. Even though There are others with the exact same thing happening, I have not found any definite answer. Most of the posts are like: it could be this or could be this. It's like no one gives any feedback saying "this was the problem."

I suppose it would help me if I had an OBD2 scanner on hand so I could see what the coolant temp sensor is reading when this idle is high. I've read on one site it could even be the ECM that is causing the problem.

[pocket-rocket]

ELM327 based OBD2 interfaces can be had on ebay for less than $20. It just takes a while to get here from Hong Kong, lol.

[2000GAGT]

i was just wondering with my car personally since i live in MN and it does seem to be a lot more frequent in winter that maybe it has something to do with the fact i have the TB coolant bypass done. There's always discussions if that does anything and maybe heating the TB really does do something like thaw out the IAC? but does anyone else have this "mod" done? could just be my car but its weird my girlfriends 2005 G6 GT does this too once in a whileand she has an electric TB so maybe its something in GMs sensors or programming.