Boxster 986 misfire problem
Discussion
I have a 2000my Boxster 3.2S which hasn't been used very much over the last year or so, something I decided to remedy earlier this year with the plan to use it as a semi-daily driver. Its been well serviced and I quickly covered a good few hundred miles in it. Then one day after a journey of perhaps 120 miles, it started to lose power and what started off as a slight misfire got progressively worse and it struggled to rev much over 2500-3000rpm, I was close to home so tried to nurse it slowly back to base but as I stopped at a roundabout, it cut out completely. To my surprise, it re-started and then ran perfectly with no more misfiring. During this time, there was no CEL light on (which I wold have expected if it was a MAF problem).
However, next day, it started with difficulty and continued missing badly - again no CEL. I plugged in a cheapo generic OBD reader which simply gave codes indicating that all six cylinder had suffered from misfiring - but nothing else.
So the question is, what would cause all six cylinders to misfire? New spark plugs and coil packs were fitted late last year so I'm ruling those out (for now). The fact that it re-started and ran perfectly after cutting out at the roundabout just confuses things too. My local mechanic reckons that there's not much that would cause all six cylinders to misfire but his best guess would be a fuel related issue (bad fuel, water in fuel or a tired fuel filter being the favourites).
I've yet to look at the car properly but does anyone have any ideas?
Any thoughts most welcome.
However, next day, it started with difficulty and continued missing badly - again no CEL. I plugged in a cheapo generic OBD reader which simply gave codes indicating that all six cylinder had suffered from misfiring - but nothing else.
So the question is, what would cause all six cylinders to misfire? New spark plugs and coil packs were fitted late last year so I'm ruling those out (for now). The fact that it re-started and ran perfectly after cutting out at the roundabout just confuses things too. My local mechanic reckons that there's not much that would cause all six cylinders to misfire but his best guess would be a fuel related issue (bad fuel, water in fuel or a tired fuel filter being the favourites).
I've yet to look at the car properly but does anyone have any ideas?
Any thoughts most welcome.
Back in the day when I had a 986 I had the MAF fail with similar symptoms to yours and no CEL. It would run better by disconnecting the plug from the MAF (goes into a default mode) so I then replaced the MAF and all was cured.
If you do change the MAF make sure you get the right one (986 part number) because it's different to the one on the earlier 2.5 model which has a 996 part number....I got the wrong one for my 2.7 initially and it didn't work!
If you do change the MAF make sure you get the right one (986 part number) because it's different to the one on the earlier 2.5 model which has a 996 part number....I got the wrong one for my 2.7 initially and it didn't work!
A bad MAF could certainly account for all 6 cylinders misfiring.
That the engine was ok after being turned off then restarted has me liking the MAF for this. A power reset often fixes a problem (at least for a while) that is with an electronic component. Bad fuel would not be affected -- obviously -- by turning off the engine.
Less likely would be a low fuel supply/pressure problem. Because the misbehavior was absent after turning off the engine this suggests a bad fuel pump and not a fuel filter.
As an aside: I don't know if the 2000 MY has the non-serviceable fuel filter. ( I know my 2002 does. The fuel filter is part of the in fuel tank fuel pump and its enclosure/housing and has so much filtering capacity that when the fuel pump in my 2002 Boxster quit at around 172K miles the filter medium -- a fine plastic mesh -- had oodles of mesh area for the fuel to flow through.) (My 996 Turbo has a serviceable fuel filter and a small one at that and I have this replaced every 20K/25K miles.)
Anyhow if the fuel filter is a serviceable item this could need replacement but this smacks of throwing parts at the symptom. Still if the filter is due to be changed it should be changed.
Another possible explanation is the car's alternator is not working properly. After a while the alternator's output drops and the battery has to make up the electrical power deficit. It can do this for a while, a brief while, but as the battery voltage gets low the engine misfires. (The coils need a goodly amount of voltage to create a powerful enough spark when the electrical power to the coil is removed. If this voltage is not enough the spark is weak and misfires occur. In newer models not only does the engine begin to run poorly other systems turn on warning lights.)
And as with the MAF turning the engine off for a bit "fixes" the alternator and the battery is able to start the engine and the alternator then generates sufficient electrical power until the next time.
That the engine was ok after being turned off then restarted has me liking the MAF for this. A power reset often fixes a problem (at least for a while) that is with an electronic component. Bad fuel would not be affected -- obviously -- by turning off the engine.
Less likely would be a low fuel supply/pressure problem. Because the misbehavior was absent after turning off the engine this suggests a bad fuel pump and not a fuel filter.
As an aside: I don't know if the 2000 MY has the non-serviceable fuel filter. ( I know my 2002 does. The fuel filter is part of the in fuel tank fuel pump and its enclosure/housing and has so much filtering capacity that when the fuel pump in my 2002 Boxster quit at around 172K miles the filter medium -- a fine plastic mesh -- had oodles of mesh area for the fuel to flow through.) (My 996 Turbo has a serviceable fuel filter and a small one at that and I have this replaced every 20K/25K miles.)
Anyhow if the fuel filter is a serviceable item this could need replacement but this smacks of throwing parts at the symptom. Still if the filter is due to be changed it should be changed.
Another possible explanation is the car's alternator is not working properly. After a while the alternator's output drops and the battery has to make up the electrical power deficit. It can do this for a while, a brief while, but as the battery voltage gets low the engine misfires. (The coils need a goodly amount of voltage to create a powerful enough spark when the electrical power to the coil is removed. If this voltage is not enough the spark is weak and misfires occur. In newer models not only does the engine begin to run poorly other systems turn on warning lights.)
And as with the MAF turning the engine off for a bit "fixes" the alternator and the battery is able to start the engine and the alternator then generates sufficient electrical power until the next time.
Thanks for your advice everyone. It does indeed look like a MAF problem: when I unplugged it, it ran perfectly. However the problem has snowballed into a bigger issue...
The two Torx screws holding the MAF sensor in place are rusted really badly and there's no chance whatsoever of getting them out. I initially thought about removing the complete airbox and intake pipework but it appears that this is a major operation with loosening of the inlet manifold required in order to wriggle the air box out of the car. If I'm going to go to the trouble of that, its probably worth thinking about junking the air box completely and fitting a decent air intake kit complete with new MAF housing. If I go that route, I could literally cut or break up the original air intake housing in order to remove it as many others on the web seem to have done.
Any thoughts on this or am I missing something?
The two Torx screws holding the MAF sensor in place are rusted really badly and there's no chance whatsoever of getting them out. I initially thought about removing the complete airbox and intake pipework but it appears that this is a major operation with loosening of the inlet manifold required in order to wriggle the air box out of the car. If I'm going to go to the trouble of that, its probably worth thinking about junking the air box completely and fitting a decent air intake kit complete with new MAF housing. If I go that route, I could literally cut or break up the original air intake housing in order to remove it as many others on the web seem to have done.
Any thoughts on this or am I missing something?
I must admit that I'm not particularly keen on the idea of fitting an aftermarket kit as they can possibly introduce more issues. It looks as though getting sufficient space to drill out the rusted screws is going to be tricky to say the least - but I've got nothing to lose by trying.
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