84 Sport High Idle (Page 1/1)
maxalator JUL 30, 08:05 PM
I bought an 84 sport that worked almost flawlessly when I bought it but I left the car sitting for 3 days after finishing the front suspension and now the car refuses to idle any lower than around 3000rpms. Me and my father looked everywhere for a vacuum leak and have found there to be none. I tried replacing the IAC valve to no avail and even replaced the fuel injector which wasn't working properly but the car is still stuck with the high idle. What else could be causing it to idle so high?
Xenoblast JUL 31, 09:50 AM

quote
Originally posted by maxalator:

I bought an 84 sport that worked almost flawlessly when I bought it but I left the car sitting for 3 days after finishing the front suspension and now the car refuses to idle any lower than around 3000rpms. Me and my father looked everywhere for a vacuum leak and have found there to be none. I tried replacing the IAC valve to no avail and even replaced the fuel injector which wasn't working properly but the car is still stuck with the high idle. What else could be causing it to idle so high?



Maybe the throttle is stuck? see if the throttle switch is seated all the way in the throttle body.
maxalator JUL 31, 11:26 AM

quote
Originally posted by Xenoblast:


Maybe the throttle is stuck? see if the throttle switch is seated all the way in the throttle body.



The throttle is definitely not stuck I made sure to check that
AsaBergman AUG 02, 06:22 PM
First thing, the 84 idles higher from a cold start than later years. My 84 manual runs perfect with no vacuum leaks. On a cold start it will fast idle between 1800 and 2700 depending on the temperature that day. After it warms up it'll idle down to around 800 after a couple of minutes. It can't see the temperature of the air, only the coolant which there's a lot of so it may fast idle for several minutes even if it's 80 out. If you're in 60 degrees and above and it doesnt idle down after 5 minutes then there's a problem. I'll list what I can think of:

The 2.5 engine has three temperature sensors: one in the thermostat housing(this goes to the ECM), one on the top of the cylinder head(temp gauge), and one on the back(radiator fan). If the one in the thermostat housing is old it's likely reading wrong. if it has the circular plug rather than a newer plug with two pins it is likely ancient. As different sensors are for the ECM and the dash gauge you can see the correct temp on the dash but the ECM sees a very different reading and idles wrong. Even if it's a new sensor make sure the connector isn't corroded.

If your thermostat is stuck open(very common) it'll take forever for the coolant to reach proper temperature and it will idle high.

Next if the battery has been disconnected the IAC will not fully close(and thus 'zero') until you drive up to around 35 mph. This can cause a high idle that won't go away until you bring it up to that speed and restart the car.

Vacuum leaks. Any vacuum component can leak internally. I disconnect every vacuum device and cap off their lines to see if it changes the idle and then reconnect one at a time. These are:
EGR valve - saucer thing between valve cover and throttle body. These can start to leak internally and can be tested with a vacuum pump. This can pretty commonly leak at it's gasket too.
Evap charcoal canister - driver side engine bay on trunk wall. The valve can get stuck open or the plastic can crack.
MAP sensor - hard vacuum line you can inspect with a flashlight.
Thermac valve - this is a circular device on the air filter housing duct. It's connected to a thermal sensor(non-electric) inside the air cleaner which then has a vacuum line to the throttle body, I'd suggest permanently capping this off as they rarely work by now and can cause heating issues if the valve stays shut.
Cruise control - these can leak internally and should just be disconnected and capped until you solve the idle.

The PCV valve hose, elbow, and grommet can leak.

The throttle body gasket can leak if the bolts are loose. Gasket kit for throttle body is pretty cheap and is easy to do.

The throttle position sensor can be bad and be telling the ECM the wrong value. You can test these by checking resistance with a multimeter. If you buy a replacement get a quality one.

An unlikely but possible issue would be incorrect ECM. When I got my 84 manual the previous owner replaced the ECM with one from an 85 AUTO Fiero. It would drive perfect but idle at 3k at a stop and turn on the check engine light. Took me awhile to figure that one out. There's a list online of the correct ECM and PROM codes for each year and drivetrain.

I also had the intake manifold gasket leak and the engine would never drop below 3500 rpm. You might be able to check for this by spraying something into the engine bay. I used the felpro intake gasket and installed it dry with no sealer.

Let me know if you need further info or any pictures of anything in the engine bay.
maxalator AUG 03, 10:25 AM
Thanks for the info when I heard there was 3 sensors in the car I immediately suspected it to be the ECM sensor. I already got the replacement for it and the cooling fan so hopefully this can fix the 3000 RPM idle on the car. I'll take the car on a drive if this doesn't fix the issue to see if its just the ECM acting up and post a reply when its figured out.
Dennis LaGrua AUG 03, 10:59 AM
Try checking for a leak at the EGR valve or for a general vacuum leak.
I just fixed a high idle problem on a friends car and it was a bad O2 sensor. The scan voltage with the key on/engine off of the O2 sensor should read about 47 mv. If it reads much higher than this; when the engine is started it sends a rich condition signal to the ECM, then the IAC tries to compensate by opening and letting in more air and the idle goes up. If it is the O2 sensor only replace with a quality unit like a Denso. Most of the Chinese made brands are junk.

[This message has been edited by Dennis LaGrua (edited 08-07-2021).]