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Re: yet another turbo question...I must seem to be an idiot
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Posted by Eric Law [Email] (more from Eric Law) on Tue, 28 May 2002 13:10:57 Share Post by Email
In Reply to: yet another turbo question...I must seem to be an idiot, steve, Tue, 28 May 2002 11:32:48
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Steve,

I'm 90% sure there was only one turbocharger in '88: the Garrett T3. But I don't think it matters for the problem you describe. If disconnecting the W hose yields a lot of boost, the turbocharger's probably OK.

You can get more info by searching the board, but there are a multitude of problems that can cause the APC system to produce base boost. Here are the ones that pop to mind:

- Bad boost-control valve (a/k/a APC valve). Check this first since it's easy! Turn ignition on (engine not running), and remove the plug from the valve, then plug it back in. Valve should click once when you insert the plug, indicating valve is energized. If this does not happen, check resistance of the valve. IIRC, resistance should be somewhere in the tens of ohms - if open or shorted the valve's junk.

- Brake-light switch problems. APC sets base boost if it thinks the brakes are applied.

- Noise in engine (sounds like this is what you have in mind). Unless the motor is especially noisy, this usually won't cause a base-boost condition. Symptoms tend to be more along the lines of varying reduced boost under different conditions.

- Pressure sensor problems. The system has a pressure sensor that's mounted under the dash - note this is NOT the overboost fuel-cutoff switch, it's different. A bad sensor can cause reduced or base boost. If the APC valve clicks continuously when the engine's not running, the pressure sensor is often the culprit.

- Pressure *switch* problems. Not the same as the above! There's a pressure switch that detects when the cruise control is engaged (CC is pneumatically operated) to reduce the boost. Bad switch = base boost. Think this one's under the dash too - follow plumbing for cruise control system to find it.

- Connection/moisture problems in the control unit. Not sure where this is mounted in your car, but if it's exposed to moisture this is a potential problem. The unit is sealed with O-rings that deteriorate as they age, and let water in. This corrodes the circuit board and causes all sorts of strange problems. Also have seen solder joints fail from vibration where the connector attaches to the PC board.

Good luck!

Eric Law

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