- Joined
- Jun 2, 2012
- Messages
- 25,363
- Reaction score
- 8,370
- Points
- 113
- Location
- canada
- Vehicle Year
- 1994
- Make / Model
- Ford
- Transmission
- Manual
900rpms is a bit high for warm idle
Manual trans should be about 650, automatic 750(in Park, 800 in gear)
After engine is fully warmed up, let it idle and unplug the IAC Valve
RPMs should drop to about 500-600, engine barely running
If it does not drop then you have an air leak, vacuum leak
With IAC Valve still unplugged, pull off one vacuum hose at a time from intake and plug its port with finger
When RPMs drop you found the system with the leak, follow that vacuum hose
Start with power brake booster hose and PCV hose
If RPMs do drop to 500-600 then no vacuum leaks, and its the computer(PCM) that is causing the higher idle
Hard to fix that, if you want you can pull out the PCM, open it up and have a look at the 3 blue capacitors, they leak after 20+ years and can cause odd issues, under $5 to replace them just to be sure, and free to have a look
It's also good to have a look at the PCM connector, for corrosion on pins, and unplugging and plugging back in ANY connection cleans it off and can make for a better connection, that applies to all connections in the vehicle
All the sensors run on 5volts, and 0-5v is the Return signal to the PCM, so a dirty connection will cause a lower voltage and incorrect sensor data
Manual trans should be about 650, automatic 750(in Park, 800 in gear)
After engine is fully warmed up, let it idle and unplug the IAC Valve
RPMs should drop to about 500-600, engine barely running
If it does not drop then you have an air leak, vacuum leak
With IAC Valve still unplugged, pull off one vacuum hose at a time from intake and plug its port with finger
When RPMs drop you found the system with the leak, follow that vacuum hose
Start with power brake booster hose and PCV hose
If RPMs do drop to 500-600 then no vacuum leaks, and its the computer(PCM) that is causing the higher idle
Hard to fix that, if you want you can pull out the PCM, open it up and have a look at the 3 blue capacitors, they leak after 20+ years and can cause odd issues, under $5 to replace them just to be sure, and free to have a look
It's also good to have a look at the PCM connector, for corrosion on pins, and unplugging and plugging back in ANY connection cleans it off and can make for a better connection, that applies to all connections in the vehicle
All the sensors run on 5volts, and 0-5v is the Return signal to the PCM, so a dirty connection will cause a lower voltage and incorrect sensor data
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