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Well, '85 Turbo has the LH-Jetronic... (long)
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Posted by Micah in NC [Email] (more from Micah in NC) on Thu, 7 Feb 2002 11:36:06 Share Post by Email
In Reply to: Replaced the black tipped with red or black tipped?, Charles, Wed, 6 Feb 2002 18:48:13
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system. I know nothing about them. Idle problems on them can have completely different causes/fixes because the 1985 cars with Turbo was the first year of the use of the 16valve engine and also the LH-Jetronic fuel injection (non-continuous) system.

The reason you don't want to put the red-tipped switch back in is that the black switch is the updated design. It works better. Bear with me and I'll tell you why.

The black switch lets the engine (and therefore, exhaust and O2 sensor) warm up a little longer than the red-tipped switch (100 deg. vs. 70 deg. F.). On these older cars, the O2 sensor is not pre-heated, it relies only on exhaust temperature to warm it up and it must be hot to send a signal to the K-Jetronic computer (under the back seat).

If (in the case of the red switch) the car is started when exhaust & O2 sensor are cold but ambient temps are close to or higher than 60-70 degrees F., then the computer will "listen" to the O2 sensor nearly right off the bat after startup. The problem with that is the O2 sensor, which isn't funtioning correctly yet because it is not yet heated by the exhaust. The process of the O2 sensor achieving proper temp takes probably 5 minutes or less, and, during that time is when the bad idling may occur.

However, if the outside temps are really cold (way below 60 deg. F.), then the red-tipped sensor will work fine. You see, the coolant temps are thus very far below the 60-70 deg. point, and it takes much longer for the engine coolant to warm up to the switching "on" point (60-70 deg.) By that time, the exhaust and O2 sensor are most always sufficiently hot, and O2 sensor is operating correctly then.

So, to review, the red-tipped switch is the old design and may work fine, depending on the climate you live in. The black-tipped switch, though, is the newer design and works better if your car has been sitting "cold" even if ambient temps are up to 100 degrees F. I don't know how warm Sweden gets, but red-tipped switch probably always worked fine over there. In the US, with warm spring & summer temps in some states, the red-tipped switch just caused driveability problems too often.

That's my take on the situation. Hope this helps, as I'm only a Saturday/shadetree mechanic at best.

Micah in NC
'84 900S, 201K mi.

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