Engine Warning Light - No obvious fault
Discussion
Just did a three-hour trip to collect my Dad for Christmas in my 03 plate 9-5 Aero. In the last ten minutes of the journey the orange engine warning light came on and has remained on. Performance is unaffected and a visual inspection of the engine doesn't reveal any serious (or even new) oil leaks.
A quick peek at the forums suggests pulling fuse 17 for 15 minutes to reset the ecu and carry on regardless hoping the error was a blip, or getting a fault code at a mechanics. Option two is unlikely on Christmas Eve in the depths of rural Suffolk. Would I be crazy to try option one? I do have to do the three-hour return leg. Bearing in mind I spent Saturday driving a rental 7.5 tonne curtain-side with three warning lights on from Newbury to ExCeL and back with no issues - am I invincible? Or have I used up my share of luck?
A quick peek at the forums suggests pulling fuse 17 for 15 minutes to reset the ecu and carry on regardless hoping the error was a blip, or getting a fault code at a mechanics. Option two is unlikely on Christmas Eve in the depths of rural Suffolk. Would I be crazy to try option one? I do have to do the three-hour return leg. Bearing in mind I spent Saturday driving a rental 7.5 tonne curtain-side with three warning lights on from Newbury to ExCeL and back with no issues - am I invincible? Or have I used up my share of luck?
The right answer is "reason unknown" of course. A lot of the time CEL (and CGL) "coming ons" are just sensor failures. If it was me and I was sure water/oil/turbo-boost were all ok and the car was running normally, I'd risk it - BUT dont blame me if it ends badly - I'm just saying the odds are in your favour. I'm assuming no other lights are being displayed? Is tick-over normal? I had a lengthy bout of BOTH CEL and CGL coming on at once in my 02 9-3 which turned out to be a series of sensors around the slush box needing replacing, and a CEL plus ABS light problem (coupled with bad tick-over) in our 01 9-5 which was related to throttle body replacement being required (not a cheap job!)
Despite having no 'mechanical sympathy' at all, I found the fault. A vacuum tube, leading from the front of the engine bay to a box mounted on the centre of the bulkhead, had come adrift. I then had to pull fuse 17 to get the 'engine light' to reset. I then tried pulling the hose again, but the light didn't come on. It was only with revs applied (about 3500) that the light came back on. I reattached the hose and cable-tied it as it seemed a bit saggy, then reset the engine light. Have since driven 250 miles. - no light!
I've now been told that the hose activates a pump that returns unburnt fuel back into the combustion cycle. It is only at higher revs that the sensor notices something wrong with the exhaust gas mixture and throws up a warning. The engine light / ecu / fuse 17 reset is the only way of getting the light to go out, but it does reprogram the ecu in some way. I am finding that boost comes on at higher revs than previously - apparently the ecu learns your driving style and adapts (clever) - when resetting, the car unlearns the behaviour that it has learned to adapt to the individual's driving style.
I've now been told that the hose activates a pump that returns unburnt fuel back into the combustion cycle. It is only at higher revs that the sensor notices something wrong with the exhaust gas mixture and throws up a warning. The engine light / ecu / fuse 17 reset is the only way of getting the light to go out, but it does reprogram the ecu in some way. I am finding that boost comes on at higher revs than previously - apparently the ecu learns your driving style and adapts (clever) - when resetting, the car unlearns the behaviour that it has learned to adapt to the individual's driving style.
The only vacuum tube from the front of the car to the 'box' on the bulkhead is the one operating the boost by-pass valve; usually when it's disconnected you get an annoying whistling/dying seal sort of sound. You don't get a turbo whoosh-type of dump valve on a 9-5 as the air intake is a closed loop - hence the dump valve ricirculates the non-required boosted air back into the intake side of the turbo.
The reason you had a light come on was because unmetered air was being sucked in through the disconnected pipe, the mixture is too lean (right amount of fuel, too much air) and the O2 sensor triggers a fault.
Explainatory diagram:
The vacuum pipes also control the fuel pressure regulator (FPR), which keeps a certain pressure on the fuel rail, and excess fuel under pressure returns to the tank (i.e. the injectors take so much, the rest is returned to sender). A vacuum on the FPR means it starves the injectors of fuel, and pressure means it delivers more fuel. However, the by-pass control valve means the FPR is on a separate vacuum circuit from the by-pass valve (dump valve). Usually a split vacuum pipe to the FPR will cause overfuelling (right amount of air, too much fuel), emptying a tank in record time!!!
Two different symptoms caused by adjacent bits of vacuum pipes! I (and so many other 9-5 owners) have replaced the vacuum pipes with silicone aftermarket products that don't go as brittle as the factory stuff. The underbonnet temperatures are huge, mostly thanks to a glowing red turbo most of the time, so those standard hoses bake hard. If you do replace them, try Samco 3mm hose (available from Demon Tweeks amongst others), and only replace one length under your bonnet at a time - otherwise you lose track of what goes where. Note, some people recommend 3mm pipe, some go for 4mm. it's up to you - I went for 3mm, it fits!
HTH
The reason you had a light come on was because unmetered air was being sucked in through the disconnected pipe, the mixture is too lean (right amount of fuel, too much air) and the O2 sensor triggers a fault.
Explainatory diagram:
The vacuum pipes also control the fuel pressure regulator (FPR), which keeps a certain pressure on the fuel rail, and excess fuel under pressure returns to the tank (i.e. the injectors take so much, the rest is returned to sender). A vacuum on the FPR means it starves the injectors of fuel, and pressure means it delivers more fuel. However, the by-pass control valve means the FPR is on a separate vacuum circuit from the by-pass valve (dump valve). Usually a split vacuum pipe to the FPR will cause overfuelling (right amount of air, too much fuel), emptying a tank in record time!!!
Two different symptoms caused by adjacent bits of vacuum pipes! I (and so many other 9-5 owners) have replaced the vacuum pipes with silicone aftermarket products that don't go as brittle as the factory stuff. The underbonnet temperatures are huge, mostly thanks to a glowing red turbo most of the time, so those standard hoses bake hard. If you do replace them, try Samco 3mm hose (available from Demon Tweeks amongst others), and only replace one length under your bonnet at a time - otherwise you lose track of what goes where. Note, some people recommend 3mm pipe, some go for 4mm. it's up to you - I went for 3mm, it fits!
HTH
Edited by P2BS on Thursday 27th December 23:43
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