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Re: periodic base boost Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 7 Aug 2002 09:14:06
In Reply to: periodic base boost, nick  [Profile/Gallery]
, Tue, 6 Aug 2002 23:20:18
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If the boost gauge is only going to about half-yellow, then for all intents and purposes, call it base boost.
You get base boost if the APC solenoid is de-engerized. Two reasons why the APC solenoid could be de-engergized - the APC system commands it, or something in the wiring between the APC box and the APC solenoid.
Between the APC box and the solenoid are two things - the connector on the box, and a pressure switch in the cruise control system. So reseat the connector on the APC box. I've seen base boost caused by a loose connection there. The CC pressure switch opens the voltage line to the APC solenoid if the CC is operating. Yeah, I know the CC isn't on when you're looking for full boost. But the APC solenoid is an inductive load, and inductive loads eat up electrical contacts. So it's possible that the contacts internal to the pressure switch a pitted and worn, and don't always make the best contact. As an experiment, wire the switch out. Just don't use the CC while it's wired out.
The other reason is the APC box is commanding base boost. It does this when it detects knock. Again, two possiblities - your engine is knocking, or the knock sensor THINKS it's knocking.
Engine knock is caused by too advanced timing, low fuel octane, lean running, hot spots in the cylinders, engine load, or too high compression (usually from carbon buildup in the cylinders). It's affected a lot by air temperature. So your engine may be on the hairy edge of knock, so sometimes it does, and sometimes it doesn't, depending on outside air temperature, or engine load, or phase of the moon.
Check your timing, and make sure it's spot-on. Make sure you have exactly the right spark plugs, gapped per the manual. Make sure the plugs are fresh. Make sure you don't have any vacuum leaks (lean running), the fuel filter isn't filled with 8 years of crud (lean running). Look at the plugs to see if you're running lean, and if so, fix it - it might mean the AMM needs adjustment, or the O2 sensor is tired.
If you aren't running high octane fuel, run a tank or two of the highest octane you can find (typically 93 in the US) and see if that helps. If it does, you know you've got a knock problem to chase down. And your engine might just need high octane to get full performance regularly.
The other part was the APC THINKS the engine is knocking. The knock sensor isn't that sophisticated. Some vibration can sound like knock to the sensor under some conditions. So if you have a valve rap (hear that at idle), the sensor might pick that up on occasion as knock. If your motor mounts are shot, that puts a lot of extra vibration into the engine, and that can sometimes trick the sensor into knock.
So, for the base boost condition, it can be a lot of things. Start with the easy stuff first (better gas, a tune up) and go from there.
posted by 140.157.4...
, Tue, 6 Aug 2002 23:20:18, Wed, 7 Aug 2002 09:14:06 <-- Viewing This Message, Wed, 7 Aug 2002 22:40:55
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