Misfiring above 2200RPM but only when hot... need help
07-21-2013, 06:50 PM
#16
Voltage was at the battery, gauges are just like the 928 ones and not very accurate as I'm sure you know. The A/C was blowing full, cooling fans on full etc. It is a tad low but nothing that would cause something like we are experiencing.
951 110 198 02 has a busted tube from the BOV to the part. While it could potentially be allowing unmetered air in to the engine, it's not the issue as it is sealed up at the moment.
TPS connector is fine, the switch is operating as it should. Tested before. Has Idle contact, cruise and WOT.
Done did the TPS/DME test. All checks fine.
Vacuum hoses/elbows replaced last week, all sealed and new as are the FPR and dampers.
Checked for differences in idle by pulling each spark plug wire and each injector wire, obvious changes were preset for each and every pull.
Caps and rotors are in good condition as are the plug wires.
At this point I'm leaning towards the Temp II or AFM but am open for recommendations.
951 110 198 02 has a busted tube from the BOV to the part. While it could potentially be allowing unmetered air in to the engine, it's not the issue as it is sealed up at the moment.
TPS connector is fine, the switch is operating as it should. Tested before. Has Idle contact, cruise and WOT.
Done did the TPS/DME test. All checks fine.
Vacuum hoses/elbows replaced last week, all sealed and new as are the FPR and dampers.
Checked for differences in idle by pulling each spark plug wire and each injector wire, obvious changes were preset for each and every pull.
Caps and rotors are in good condition as are the plug wires.
At this point I'm leaning towards the Temp II or AFM but am open for recommendations.
07-21-2013, 06:52 PM
#17
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[QUOTE=SeanR:10626720] voltage from the alt stayed at a constant 12.55v through out cold and hot operation while everything was running.
12.5 -12.7 volts is normal battery voltage. If charging system was working, it should be 13-14 volts. Either a bad wire off of the alternator or a defective alternator.
12.5 -12.7 volts is normal battery voltage. If charging system was working, it should be 13-14 volts. Either a bad wire off of the alternator or a defective alternator.
07-21-2013, 06:55 PM
#18
voltage from the alt stayed at a constant 12.55v through out cold and hot operation while everything was running.
12.5 -12.7 volts is normal battery voltage. If charging system was working, it should be 13-14 volts. Either a bad wire off of the alternator or a defective alternator.[/QUOTE]
Already addressed that. Yes it is low, but not going to cause what we are seeing.
12.5 -12.7 volts is normal battery voltage. If charging system was working, it should be 13-14 volts. Either a bad wire off of the alternator or a defective alternator.[/QUOTE]
Already addressed that. Yes it is low, but not going to cause what we are seeing.
07-21-2013, 06:55 PM
#19
If it was the voltage, it would be occurring all the time.
07-21-2013, 07:04 PM
#20
Rennlist Member
Voltage was at the battery, gauges are just like the 928 ones and not very accurate as I'm sure you know. The A/C was blowing full, cooling fans on full etc. It is a tad low but nothing that would cause something like we are experiencing.
951 110 198 02 has a busted tube from the BOV to the part. While it could potentially be allowing unmetered air in to the engine, it's not the issue as it is sealed up at the moment.
TPS connector is fine, the switch is operating as it should. Tested before. Has Idle contact, cruise and WOT.
Done did the TPS/DME test. All checks fine.
Vacuum hoses/elbows replaced last week, all sealed and new as are the FPR and dampers.
Checked for differences in idle by pulling each spark plug wire and each injector wire, obvious changes were preset for each and every pull.
Caps and rotors are in good condition as are the plug wires.
At this point I'm leaning towards the Temp II or AFM but am open for recommendations.
951 110 198 02 has a busted tube from the BOV to the part. While it could potentially be allowing unmetered air in to the engine, it's not the issue as it is sealed up at the moment.
TPS connector is fine, the switch is operating as it should. Tested before. Has Idle contact, cruise and WOT.
Done did the TPS/DME test. All checks fine.
Vacuum hoses/elbows replaced last week, all sealed and new as are the FPR and dampers.
Checked for differences in idle by pulling each spark plug wire and each injector wire, obvious changes were preset for each and every pull.
Caps and rotors are in good condition as are the plug wires.
At this point I'm leaning towards the Temp II or AFM but am open for recommendations.
If you suspect NTCII, just pull the connector off and/or check the resistance on the sensor and/or put a 1k radio shack resistor in in the connector. Neither will detect a bad harness, but if the sensor is bad it should be easy to find.
AFM could well be the problem too -- clarks has a good diagnostic. Don't rule our cracked solder joints in the DME/KLR either....
07-21-2013, 07:08 PM
#21
You haven't listed what non-stock equipment is on the car. Might help to diagnose the problem.
I have driven a track car that gave similar problems. We chased the problem for many race weekends. The car worked perfectly for 30-45 minutes and then it would stumble and misfire if driven longer. Turns out the low impedance after market injectors were heating up the injector drivers in the DME and causing the issues. When cool they worked fine but after being driven for 30-45 minutes at high duty cycle would cause the driver circuit to fail. Probably would have eventually burned up the drivers, but intermittent problems suck. Sure points to something heat related. Good luck. Still.....charging circuit at 11 volts is bound to cause some problems sooner or later. That needs to be fixed.
I have driven a track car that gave similar problems. We chased the problem for many race weekends. The car worked perfectly for 30-45 minutes and then it would stumble and misfire if driven longer. Turns out the low impedance after market injectors were heating up the injector drivers in the DME and causing the issues. When cool they worked fine but after being driven for 30-45 minutes at high duty cycle would cause the driver circuit to fail. Probably would have eventually burned up the drivers, but intermittent problems suck. Sure points to something heat related. Good luck. Still.....charging circuit at 11 volts is bound to cause some problems sooner or later. That needs to be fixed.
07-21-2013, 07:24 PM
#22
If you feather the throttle or just slam your foot down it will eventually get up there, back fires are exhaust so it's not lean. Will check out further when I get the car back on Wed or Thursday.
Thanks again.
ETA: Car is 100% stock as far as I can tell. Brad may be able to say if anything else has been done.
Thanks again.
ETA: Car is 100% stock as far as I can tell. Brad may be able to say if anything else has been done.
07-21-2013, 07:28 PM
#23
You can get the charging system tested at any auto part stores. You may just have to replace the alt regulator module. At least you can fix that problem.
07-21-2013, 07:31 PM
#24
Nope, it can be related to a bad alt getting worse with temp. It not just the voltage output, but the amount of current it can supply and you don't have a amp meter so you can't see what the alt is supplying. At that low a voltage, it isn't working correctly.
You can get the charging system tested at any auto part stores. You may just have to replace the alt regulator module. At least you can fix that problem.
You can get the charging system tested at any auto part stores. You may just have to replace the alt regulator module. At least you can fix that problem.
07-21-2013, 08:46 PM
#25
07-21-2013, 08:59 PM
#26
Rennlist Member
That's not "necessarily" true, but probably true enough for first pass diagnostics. The conductivity of things change as the temperature changes, so a circuit that's working ok on low voltage when cold might not work so well when hot. That said, I'd still play the odds and check the usual suspects first discussed above, but just wouldn't cross anything off the list until you find the cause. The minute you mentally dismiss a potential cause, you increase the chance of that being it. 
07-21-2013, 09:00 PM
#27
07-22-2013, 01:08 PM
#28
Racer
Thread Starter
The car has an after market chip. I don't know what brand as it was installed in the 90's. Could that be part of the problem?
Thank you guys for all the help. For those of you that don't know SeanR he is one of the best 928 Mechanics in the country. People bring their cars to him from around the country. He doesn't get to work on 944/951/968's nearly as much so I started this post to get some input from guys that may know the car better.
Thank you guys for all the help. For those of you that don't know SeanR he is one of the best 928 Mechanics in the country. People bring their cars to him from around the country. He doesn't get to work on 944/951/968's nearly as much so I started this post to get some input from guys that may know the car better.
07-22-2013, 01:55 PM
#29
Drifting
YMMV,
Similar car - but many differences (if you don't want to read the whole story -- try swapping your DME)
Our ChumpCar is a 924S NA that runs Ethanol, that being said -- I will describe our issues and
eventual fix and you can decide if it applies.
When the car was running hot (as in hot outside) -- it would hesitate and back fire -- both exhaust and intake. Basically the car was drowning in fuel. Sometimes we noticed what we thought was RPM cut-out (as the tach would bounce). Car generally idled fine. We replaced the coil, wires, plugs, Ref and Speed sensors. We swapped the fuel injectors, tried with and without a chip. Nothing seemed to make it disappear. Fought this off and on for a couple years. It progressively got worse.
It never had issues in the garage -- only on track.
One day we grabbed the DME and about burned off all my finger prints. This was mid-race
so we just ducted some air to the DME and it seemed to be happier -- didn't go away,
but it was driveable (for awhile). When we raced at night -- it got better.
After the race I pulled the DME and "fixed" some solder joints -- put it back on and it still seemed
to have issues during a track session.
In a last ditch effort -- I swapped DME's -- and it's been purring like a kitten ever since.
Mike
Similar car - but many differences (if you don't want to read the whole story -- try swapping your DME)
Our ChumpCar is a 924S NA that runs Ethanol, that being said -- I will describe our issues and
eventual fix and you can decide if it applies.
When the car was running hot (as in hot outside) -- it would hesitate and back fire -- both exhaust and intake. Basically the car was drowning in fuel. Sometimes we noticed what we thought was RPM cut-out (as the tach would bounce). Car generally idled fine. We replaced the coil, wires, plugs, Ref and Speed sensors. We swapped the fuel injectors, tried with and without a chip. Nothing seemed to make it disappear. Fought this off and on for a couple years. It progressively got worse.
It never had issues in the garage -- only on track.
One day we grabbed the DME and about burned off all my finger prints. This was mid-race
so we just ducted some air to the DME and it seemed to be happier -- didn't go away,
but it was driveable (for awhile). When we raced at night -- it got better.
After the race I pulled the DME and "fixed" some solder joints -- put it back on and it still seemed
to have issues during a track session.
In a last ditch effort -- I swapped DME's -- and it's been purring like a kitten ever since.
Mike
07-22-2013, 04:21 PM
#30
Rennlist Member
That's not "necessarily" true, but probably true enough for first pass diagnostics. The conductivity of things change as the temperature changes, so a circuit that's working ok on low voltage when cold might not work so well when hot. That said, I'd still play the odds and check the usual suspects first discussed above, but just wouldn't cross anything off the list until you find the cause. The minute you mentally dismiss a potential cause, you increase the chance of that being it.