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Re: low or no boost Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 12 Jun 2001 16:34:39
In Reply to: low or no boost, nathan1133, Tue, 12 Jun 2001 13:58:59 Members do not see ads below this line. - Help Keep This Site Online - Signup |
If the needle goes into anywhere into the yellow, then you are getting boost. If you consistently get half-yellow, then your car is stuck in base boost. Normally you should get full boost (right up to the red line). If the APC system thinks the engine is knocking, it will reduce the boost down to base boost (half-yellow) until the knock stops.
The system fails safe - that is, for most failures, you get base boost.
First off, the easy stuff. If you aren't running premium fuel, try a tank. Your engine may be knocking. If a tank of premium doesn't do the trick, then it's time to troubleshoot.
First off, why was the APC box replaced? They usually don't fail, and they ARE the most expensive part of the system. I usually start with the cheap stuff first.
Make sure there are no vacuum leaks. That can cause lean running and knock. Make sure the timing is correct - too advanced, and the engine will knock. Pull the spark plugs. They should look a light grey. If they're a harsh white, you're running lean, and you've got to fix that problem. Once the lean burning is fixed, then see if you don't get full boost.
Next, with a COLD engine, reach under the turbo and grab the wastegate actuator arm. It's the metal rod that runs to the wastegate actuator. You should be able to move it back and forth along it's length a little. Not much, maybe 1/4 to 1/2 inch, but it should move. If it doesn't move, find out why not. If it's stuck, it could be stuck in base boost.
The findings will be the same with a hot engine, you'll just lose most of the skin off your hands.
You can check the APC solenoid - it's the plastic cylinder mounted on the radiator with an electrical connector and three hoses. First, with the engine at idle, pull the connector off and stick it back on. You should hear a click when you do this. No click? Stick a voltmeter into the connector and make sure you get 12 volts (or so) and ground. If so, suspect the solenoid.
To check the solenoid - at idle, the solenoid should be energized. That means it has power to it. Pull the hoses off the solenoid one at a time and mark them. Get a piece of hose and connect it to each leg of the solenoid in turn. With the solenoid energized, you should be able to blow air into all of the three legs. Now, unplug the connector. You will be able to blow air into only two of the three legs. If I were really good, I'd tell you which ones, but I can't remember. I'm pretty sure the hose that goes to the intake of the turbo is the one you can't blow into.
If you can only blow into two of the three legs with the solenoid energized, then you probably have a bad APC solenoid.
There are other things that can keep you in base boost. If the APC thinks the brake pedal is pressed, or the Cruise Control is ON, it will keep you in base boost. Check the wiring.
Go back through the archives. Many folks have some pretty good writeups on base boost problems. Search on 'base boost'.
, Tue, 12 Jun 2001 16:34:39 <-- Viewing This Message
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