'88 300TE, high idle speed

Apr 24, 2007 2 Replies
T

Folks,



I have been troubleshooting high idle speed (~1200 warn engine) on an '88



300TE. I repaired the rubber hoses to and from the idle speed motor (may not be proper name) and a number of vacuum lines, then I checked the engine codes since "check engine" light is on and found code #5. Also, when engine is warm if I unplug the idle speed motor, RPMs drop to 750.

My questions are:


- what signals the idle motor to shut off when the engine is warm?


- what is the function of the vacuum temperature switch located on the driver side of the head next to the valve cover?


- what is the procedure for resetting the check engine light?



Thanks in advance


saeed


One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors. - Plato [427-347 BC]


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T

Idle speed control unit (computer) does go bad so this is a common culprit. You can buy rebuilt unit online or direct from rebuilder... mbcoupes.com under part suppliers.

To reset the code... press and hold that diagnostic button for like 20 seconds or so.

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T

I have done additional work to isolate the cause of high idle speed to no avail:

- Removed the idle speed motor, bench tested OK

- Cleared the engine codes

- Swapped the following -one at a time- with parts from an '89 300TE: over volt relay, ignition control and the ECU units behind it (located behind the battery area)

- repaired possible vacuum leaks

Also noticed that when engine reaches operation temperature, there is noticeable hesitation when I attempt to increase RPMs.

Any ideas? thanks saeed

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