RPM problem

[zinomori]

I have 89 Bronco Full Size and I love it so much. Here is the problem: when idle the RPM's jump up to about 2000 RPM and stay there for 3-5 seconds, then drop to 400-500 RPM and sometimes quitting completely. Still I don't try anything to fix the problem. Just wondering where to start from? Take your time for the best advice/help, but please don't say worthless comments.

Thanks in advance.

 

[budster 95]

We have some very knowledgeable people on this sight, unlike yourself. With that kind of attitude, especially on your first post, you may be better off taking your truck to the local mechanic.

 

[miesk5]

Get it tested for Diagnostic Trouble Codes (DTCs)

or DIY using a pretty good self-taught mechanic and pal's Self-Test, BroncoJoe19

do the KOEO = Key On Engine Off portion first & Remember to have engine @ Normal Operating Temperature before doing the KEY ON ENGINE RUNNING (KOER) portion

report back with Code(s)

Want a guess w/basic diy test?

ok,

here is one;

Sensor, Idle Air Control (IAC), Idle or Noise Problems & Troubleshooting;

For high idle problems, try unplugging the IAC motor.

If the idle drops, its usually due to an input signal to the computer which results in the computer raising the idle speed. If this is the case, look at all of the computer inputs to make sure everything is up to par.

If the engine RPM does not drop when the IAC motor is disconnected, then either the IAC motor is not functioning properly, or, there is a vacuum leak in the engine.

 

[bobstrat]

If the engine RPM does not drop when the IAC motor is disconnected, then either the IAC motor is not functioning properly, or, there is a vacuum leak in the engine.

from personal experience, the rpm drop does not rule out an intake leak.