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  #1  
Old 08-21-2007, 11:46 PM
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Join Date: May 2007
Posts: 114
rough idle - trick the system?

I got the car to start (87 300E) and I'm trying to track down the rough idle now. What's been done:

cap/rotor
fuel pump relay
ovp relay
determined there is spark
eha 1/4 turn clockwise
icv swap w/ known good unit
checked breather lines - all look good
swapped little button thingy on throttle linkage
enriched mixture - no help
leaned mixture - no help

Symptoms:
under idle (eg in P, R, N, D, 3, 2 w/ no gas pedal applied), the engine runs at around 500rpm (used to run at 700-800 rpm) with the vac gauge (economy gauge) above the lowest point. Engine sounds like it is going to stall, and sputters out when throttle is first applied.

Question:
Can I trick the system, as the idle is fine when slight gas pedal is applied, by manually increasing the throttle's starting position to have some throttle applied, or does that little click box thing need to be applied in order to start the car? Are there any other vac lines that should be inspected that could pos be related to this problem outside of the 4 breather hoses? What else can I check to determine the source of the rough idle - fuel delivery is fine, spark is fine, air is fine, and the car runs fine under load?

Thanks for your help.

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  #2  
Old 08-21-2007, 11:52 PM
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Location: Evansville, Indiana
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Fix the vacuum leaks. New boot between air meter horn and throttle body (check the clamp at the throttle body, likely it's loose and tightening it will temporarily improve things), idle control valve hoses, injector seals, injector holder seals. Until you get the vac leaks fixed, you cannot set the idle mixture. Only change the setting on the EHA if you are measuring the duty cycle and have verified that it actually needs adjustment, else you are going to make things worse, not better.

You should have instant strong throttle response -- if you have throttle "lag", you have vac leaks.

Check the fuel pump relay, too -- if it gets hot, replace it. Intermittant fuel pump operation will cause all sorts of idle problems.

Peter
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  #3  
Old 08-22-2007, 12:22 PM
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Join Date: Aug 2001
Location: Houston, Texas
Posts: 393
Air sensor pot ? Check the voltage on the center terminal should be between 0.5 and 1V, ideally 0.7V. Note that the wire connector must be plugged into the sensor pot to do this test.

Coolant temp sensor ? It is the last sensor on the engine, towards the windshield. The resistance should be between 2.5K and 3.5K when cold and between 250 and 350 ohms when engine temp is 80 degC. This sensor is very important, it sends signals to both the fuel compuiter and the ignition computer.

Injector seals ? Spray some carburettor cleaner around the seals and see if rpms get affected. In general, any other fuel leaks ?

Intake manifold seals ? Again try spraying carburettor cleaner at this and various places where you suspect vaccum leaks. Did you measure vaccum at the intake manifold ? It should be between 16-22 Hg on the vaccum gauge.
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  #4  
Old 08-23-2007, 09:52 AM
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Location: SoCal
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If idle is low, I would not suspect vacuum leaks. Also, pot problems usually cause the EHA and ICV to ramp idle up, then down. Make sure throttle linkage is allowing a return to idle - if pushing linkage to rest stop causes idle to increase, look there for problem.

See if unplugging connector to ICV changes the idle. If not, then likely you have no ECU function. Check the OVP relay and fuse.

Steve
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  #5  
Old 08-23-2007, 01:38 PM
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what do you mean by: "if pushing linkage to rest stop causes idle to increase, look there for problem." - if it makes that little box w/ the clicker depress?
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  #6  
Old 08-23-2007, 09:07 PM
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The throttle shaft actually actuates two more switches - one switches in the idle circuit, and the other indicates full open throttle to the ECU. These switches aren't on the linkage - the microswitch you see is part of the emission controls.

Steve
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  #7  
Old 09-21-2007, 06:12 PM
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Quote:
Originally Posted by sbourg View Post
If idle is low, I would not suspect vacuum leaks.
I want to correct this - unavoidable since a vacuum leak in ours DID cause a rough idle, after posting the above. Anyway, the input port of the vacuum check valve in the line from the manifold to the brake booster broke, causing both rough idle, power brake problems, and a lot of hissing under the hood.

Steve

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