Sporadic limp mode with no CEL

[KZCool]

I have a 99 OBS with 290K miles and AT.  The car has been running great as I take good care of it.  Then one day after adding oil the CEL came on with "Faulty Knock Sensor". It would randomly retard the timing, then run normally.  It's a recently new sensor but I changed it with a cheapo anyway.  Same thing, so I tried a different one.  Swapped the ECM, sea foamed the engine, new spark plugs, oil change, new pcv valve, adjusted my valves, even moved the knock sensor to the hole back and to the right of the original knock spot.  There's no CEL now but it will still randomly go into limp mode and then come out of it strong, and then lose some of that extra power.  I really want to get this fixed for sentimental reasons and because the engine is still very strong.

[1 Lucky Texan]

How old is the battery? MY06 WRX would go into limp mode and code for odd stuff - fixed with a new battery.

 

I THINK, looking back on it, the key was, it would happen immediately at starting - maybe a 'race' condition between the ECU booting up and signals from some sensors still out of range?

 

just a thought.

[bstone]

No CEL but there might be a pending code. Scan it and see what comes up.

[1 Lucky Texan]

any signs of wiring damage from rodents or???

 

also, you could try refreshing some ground connections. Take 'em loose, wire-brush the various surfaces, reconnect and smear with dieletric grease.

[KZCool]

any signs of wiring damage from rodents or???

 

also, you could try refreshing some ground connections. Take 'em loose, wire-brush the various surfaces, reconnect and smear with dieletric grease.

Which connections do you suggest? The battery is pretty old, but is strong.  Ill also go get the OBD checked. When the car is first started it runs fine. When it does happen its almost always after a few miles.  Makes me think faulty sensor somewhere the degrades from the heat.

[1 Lucky Texan]

every ground connection you can find - usually a few near the top of the 'bog bone' pitch stop at the firewall, sometimes some on some 'bosses' cast into the intake manifold. usualy on the fenders, etc.

sometimes corrosion 'wicks' down the large battery cable under the insulation - you might check those wires. Honestly, sounds like the battery may be OK. You could check the voltage at the battery while someone else starts the car - probably shouldn't drop below 10 volts. Or, consider having it load tested at the part store - where I live, 3 years is 'old' for a battery but cooler parts of the country can get 6-8 years from a battery


here's an example picture;

 

 

 

Edited August 2, 2016 by 1 Lucky Texan

[KZCool]

ok thanks,  I'll go through them.

[1 Lucky Texan]

check the wire to the knock sensor - and look at the connector pins to see if one is pushed backwards or corroded.

[KZCool]

I went through the grounds, and. Decided to take it for a long haul on the interstate. It seemed to have helped. Im hoping the computer just had to learn more about the car since it's a new one to this car, and my knock sensor only had one wire but the leads are fine on it. I ended up putting a thick rubber gasket under it and running a wire under the gasket to on to to ground the sensor haha.