odd one.....

[SS Spoiler]

got an odd problem with my car. car starts ok, runs well
but when it gets warm these strange issues pops up.
1... stepping on the brake the idle rises to 2500-3000 rpm.
let up on brake idle drops to normal.
2....turn headlights on idle rises.. shut lights off...normal
3....turn heater fan on idle rises.. shut fan off...normal

this is a total mystery to me for a mechanical fuel injection system.
motor is a DPI with cam and headers....no turbo....5 yrs ago.
replaced plugs, wires, cap and rotor...
replaced throttle cable...had slight burr on it...
adjusted valve lash...
serial #2691, stick shift. 160,000 miles
it all relates to a warmed up engine, acts up after 20/30 minutes of driving..


any ideas?.....

[Bitsyncmaster]

My guess would be a bad ground which upsets the idle ECU when you run other loads. So it puts voltage on the ground circuits.

I would first check that ground feed through bolt which is behind the rear vertical board (need to drop that board).

[82DMC12]

got an odd problem with my car. car starts ok, runs well
but when it gets warm these strange issues pops up.
1... stepping on the brake the idle rises to 2500-3000 rpm.
let up on brake idle drops to normal.
2....turn headlights on idle rises.. shut lights off...normal
3....turn heater fan on idle rises.. shut fan off...normal

this is a total mystery to me for a mechanical fuel injection system.
motor is a DPI with cam and headers....no turbo....5 yrs ago.
replaced plugs, wires, cap and rotor...
replaced throttle cable...had slight burr on it...
adjusted valve lash...
serial #2691, stick shift. 160,000 miles
it all relates to a warmed up engine, acts up after 20/30 minutes of driving..


any ideas?.....

Hey Paul,

Just a quick thought, the thermistor located in the Y-pipe, if it's showing open circuit between the black/slate and black/yellow wires at the idle ECU, will result in an idle speed around 2500 RPM. Perhaps it's faulty and going open when hot. You could make a jumper and jump those two terminals on the back side of the white plug at the idle ECU to temporarily fix the condition.

Can't explain why it would do it with headlights/ heater but it's worth a look. Cold, the thermistor should read around 10,000 ohms (check between those two wires I mentioned) and hot the thermistor should be significantly less than that, maybe 3000 ohms or less.

Andy

[David T]

All of the actions you mentioned caused an increase in the electrical load and when you remove the load the idle goes back to normal. With that as a working theory the problem is electrical. Start with the battery. Check it's age, if it is over 7 years old it should be replaced or at the very least load tested. After checking the battery, check the system voltage and test the alternator. If the alternator is not working as it should (maintaining at least 12.4 volts) it should be replaced. Now that we have checked the major components of the electrical system, if you still have an idle problem, do some voltage checks at the idle ECU. Make sure it is getting full system voltage and it has a good ground. If that checks out, check each sensor that connects to the ECU and the wiring. It is possible the idle ECU has a problem internally and doesn't like a drop in the voltage or the idle motor. As a quick check to verify it is a problem in the idle system, disconnect the idle motor and see if the idle remains steady when you turn on the fan.