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what item might cause misfire when coolant gets hotter than usual?
Coming home from work yesterday got stuck in 45 minutes of stop and go traffic. A/C on, coolant temp went to 110 C. (It used to read a rock-steady 90 C before the timing belt job I did last month, during which I replaced the thermostat with a fail-safe 91 degrees C model from Motorrad. After the timing belt job, it usually wanders around between 90 and 100 C.) Turned off the A/C and the temp slowly crept down to around 95 C.
When the traffic jam resolved and I was able to speed up, the car started shaking mightily under load (1200 - 2000 rpm), had less than the usual power, and the check engine light came on (but not flashing). After I got home, I read codes P0327 (knock sensor 1 signal too low), P0300 (random/multiple cylinder misfire), P0301 (cylinder 1 misfire), P0302 (cylinder 2 misfire), and P0307 (cylinder 7 misfire). After several hours of the car sitting, I swapped the 2 Power Output Stages (POS) N122 and N192 (also sometimes called ignition control modules, ignition amplifiers; these fire the coils when the ECU sends them juice) with each other, and took the car for a test drive. I was hoping that the misfires would move to different cylinders because of the POS swap. I had done that successfully a few years ago to positively ID the POSs as culprits in a no-start condition - https://forums.ross-tech.com/showthr...udi-4-2)/page2 But the car ran fine. I took out the POSs and they felt warm. I think they're probably supposed to, since they have heat sinks on them.
This morning I tested the POSs by turning on each of their 4 channels with DC voltage (1.5 up to 12 V) and measuring their resistance. There's a procedure for such testing at https://www.audiworld.com/tech/elec181.shtml. None were open (infinite resistance) while turned on, which I expected because the car ran fine, but the resistances measured were all up in the 50K to 85K ohms range, much more than written up in the testing procedure. (Flipping the ohmmeter leads made them read higher, for instance 130K ohms.) I then used a hot air gun to heat one of them while testing again. The resistances all dropped, such as to 30K ohms. But none went open.
I've just ordered new ones, since I hear they commonly go bad anyway. But the car runs fine now. Any ideas on what other items could cause a misfire when things get hot?
I suspect the coils themselves went intermittent due to the heat.
With regard to your cooling system, does the mechanical fan clutch pass the pencil test? When cold the mechanical fan freewheels, but as warm air flows over the clutch in the middle, it firms up its grip, so when the engine is hot, the clutch is spinning the fan with authority. The fan clutch fails eventually in either limp noodle mode (most common) moving no air or locked tight (rare) blowing hard all the time.
Does the other fan, the electric fan, come on with the A/C? It should always power up when you turn on the A/C even if the engine is cold. It should also kick in when the mechanical fan isn't enough, like in hot summer stop-and-go traffic. When cold, check the fan blade for wobble indicating worn fan motor bearings. There is also a Cooling Fan Control, which switches on the electric fan at different speeds. It has three blade connectors for the wiring which get loose and corroded. It is hiding below the driver's headlight accessible from underneath. It's an awkward one, or maybe that's just me.
Is it possible you missed the power plug for the fans after doing your timing belt job? Or the wire connector locking tab broke? If so, tiny zip ties to the rescue although you might have to daisy chain them together to double the length.
In an overheat situation, you could also crank up the cabin heater and run that fan on MAX.
Regarding the ignition, your suspicion of the coils has me thinking maybe I should get a new set. But I'm slightly worried about fitment. When I did the timing belt job, I found dried brown stuff in the spark plug boots; must be some kind of boot grease from a long time ago (previous owner). So I ordered a set of boots from Rockauto, but then I found that mine wouldn't come off the coils with any reasonable amount of effort. Even if they would have come off, the new boots looked about 3/4" shorter than the old ones. There were a number of changes made to various things early in the 1997 model year, and this must be one of them. Rockauto refunded me the money but would only cover half the cost of return shipping, because, according to them, they should have fit. I ended up cleaning the spring contacts in the old boots with a little brush.
The electric fan works; I know because I was watching it when I recently recharged the A/C. I did not check all the speeds, but it did sound like it was blowing harder at times. (I had issues getting refrigerant in because of the adapter I used at the low pressure switch port, which wouldn't let refrigerant in if I tightened the adapter onto the port too much, so I had the car running for quite a while while I messed with it.) The fan speed resistors and their wiring look OK.
The fail-safe thermostat did not have an air bleed hole in it, but after a couple of months of operation I'd imagine that any trapped air at the stat would be gone by now.
I have not done the pencil test; I'm fuzzy on the details. (Somewhere I read that a carrot could also be used; a guy was telling his wife about how versatile his Audi was.) I'll have to learn about testing the clutch; hate to have a flying piece of pencil poke a hole in the radiator.
I guess it is the pencil test because everyone can find a pencil. Or a rolled up newspaper or magazine or paint stirring stick or your hand like this guy
I suspect the coils themselves went intermittent due to the heat.
With regard to your cooling system, does the mechanical fan clutch pass the pencil test? When cold the mechanical fan freewheels, but as warm air flows over the clutch in the middle, it firms up its grip, so when the engine is hot, the clutch is spinning the fan with authority. The fan clutch fails eventually in either limp noodle mode (most common) moving no air or locked tight (rare) blowing hard all the time.
Does the other fan, the electric fan, come on with the A/C? It should always power up when you turn on the A/C even if the engine is cold. It should also kick in when the mechanical fan isn't enough, like in hot summer stop-and-go traffic. When cold, check the fan blade for wobble indicating worn fan motor bearings. There is also a Cooling Fan Control, which switches on the electric fan at different speeds. It has three blade connectors for the wiring which get loose and corroded. It is hiding below the driver's headlight accessible from underneath. It's an awkward one, or maybe that's just me.
Is it possible you missed the power plug for the fans after doing your timing belt job? Or the wire connector locking tab broke? If so, tiny zip ties to the rescue although you might have to daisy chain them together to double the length.
In an overheat situation, you could also crank up the cabin heater and run that fan on MAX.
My D2 had the mechanical fan swapped out for a electrical fan but it wasn't wired up to automatically turn on. Can it be wired up to turn on automatically when it reaches a certain temp? If so, how do I go about doing that? Also my stock small electric fan runs all the time. I'm guessing this isn't suppose to?
Electric fans are often sold with or retrofitted with temperature-controlled switches or relays that make them come on and off automatically with temp. Coolant temp would be better than air temp through the radiator, I'd think. There are loads of them out there.
The original electric (auxiliary) fan has wiring that runs it at various speeds and off, as BrianC72gt points out - it's not supposed to run all the time. Maybe whoever installed your electric fan as a replacement for the original "viscous" fan messed with the auxiliary electric fan's actuation. The main belt driven fan is supposed to run all the time, with the fan blade speed varying with air temp causing more or less mechanical lockup in its clutch. Maybe you could tie in the original electric fan switching system to your now electric main fan. It could be figured out from the wiring diagram, but you'd have to make sure the original system can handle the current draw of the big main electrical fan.
But if, as you say, the mechanical fan was swapped out for an electrical one, what happened to the pulley and serpentine belt length?
06-08-2019, 12:09 PM
what item might cause misfire when coolant gets hotter than usual?
