Sudden complete power loss while cruising -- ideas what happened?

[KansasQuattro]

I'm at a complete loss to explain some bizarre behavior yesterday.

Background, 6MT Stage 3 B5 S4 with off-the-shelf 034 tune on their stage 3 kit, 172k miles.

102 degrees ambient air temp in the middle of a 300 mile drive, cruising in 6th at 80 mph. I had previously towed down a trailer but was driving back without towing anything (towing on this stage 3 tune is insane, btw, fuel economy drops to 9mpg)

Last year, I had a lot of trouble with the rear diff after a prop shaft bearing failure I replaced the prop shaft and the rear diff x 2 and added the JXB center bearing. It made a lot of noise, which is why the diff was replaced twice. Ultimately decided the noise was just due to the bearing. This noise sounded worse, and I noticed more droning from the rear yesterday. But I'm unclear how a rear diff issue could basically cause the car to act like the clutch was suddenly disengaged (except RPMs didn't decrease).

Incident:

While cruising with A/C running pretty hard set to 65 to make it tolerable since it was so hot. Coolant temp in the middle. Oil temp 240ish, not quite to middle. Cruise control on. Suddenly there's a mild to moderate jolt and the cruise shuts off and there is complete power loss. The engine is still running, but giving any amount of gas does nothing. It's like the clutch became disconnected from the engine suddenly. Shift down to 5th gear, same thing. Coast to a stop and engine stalls eventually before it stops.

Get out and check everything. Assumed timing belt snapped. Everything looks ok. Open hood and let cool off for a few mins. Get back in, cranks fine, sputters, then starts and runs normally. So I start driving again and it's driving ok, then the exact same thing happens again about 5 miles later.

Now I am thinking it's a clutch failure. I let it cool off again for a few minutes, then restart it, and now I try flooring it to test the clutch. It does not feel smooth at all. Very jerky power delivery. I then pull into a parking lot and let it cool off for about 30 minutes this time. I restart, fill up with new fuel in case the fuel was bad, then I get back on the highway. I do some hard pulls and it's fine. I wonder if it's A/C related and overheating something in the engine bay so I drive the rest of the way home without the A/C. No issues. When I get close to home I turn the A/C now that I know I've made it. It's fine. Pull hard, smooth power delivery, not jerking like it was before. Clutch doesn't feel like it's slipping.

I scan for codes when I get home and the only code is the misfire on cyl 5 which has been intermittent since the stage 3 tune was installed (and I was just told it's par for the course with the tune).

I am at a loss but now feel that this car is not reliable for long trips, especially in hot weather. Any ideas what would have caused behavior like this? It literally felt like the car just got knocked out of gear even though it it was still in gear, and putting it in other gears made no difference. Throttle did absolutely nothing, the engine would rev but ZERO power transfer to the axles.

???

[RMode]

Many potential things

First? Check your cruise switch. Remove, inspect he 2 contact points. If you see any signs of arcing, you found your issue, and it’s a cheap solution. Have seen this multiple times.

Even a mediocre tune should not have any misfire codes. But that’s not your issue ultimately.


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[AvantGuard]

Check your fuel pressure-could be an intermittent fuel pump iossue.

[CELison]

Maybe an issue with the throttle body or low voltage. Can't really say. Maybe run a throttle body adaptation. I have the one off my car still. You can have it if you want it.

[KansasQuattro]

Interesting. There is a problem with the cruise switch. Sometimes you have to click it multiple times and sometimes it disengages when you hit the accelerate button. I have looked up why this happens and the fix for it on youtube.

But the weird thing here was that when it happened, power delivery to the wheels went to zero even though the engine was still running. I would give it throttle manually, the RPMs would increase (I'm not sure if I could hear the engine actually rev or if it was just the needle moving), and nothing would happen, like the clutch was suddenly disengaged. I shifted to another gear and same thing, throttle would blip the needle and nothing. Car coasted untll it stalled then coasted to a stop. I am not sure if it was on cruise the next time it happened.

I can't think why a defective cruise stalk switch would do this. I figure it would just do the behaviour I was already experiencing. I can't figure out a bad switch would knock it out of cruise then ignore all throttle input (the throttle is drive by wire, correct?). The RPM needle increased so my input to the pedal was going somewhere, just not to the ECU to tell it to fuel and open the TB and get on going.

This makes me think an electrical issue (ECU overheating?) but I can't think what.

Fuel pump maybe? Would an overheating fuel pump do this? Although I would expect the engine to sputter or immediately cut out, not keep on going without throttle doing anything.

The other weird thing is that when the engine finally stalled and RPMs went to zero when it was at about 20-30 mph or so in 6th, I didn't feel anything. No jerk or anything. It was literally like the clutch was disengaged or the transmission somehow popped out of gear even though it was stiill in 6th. Which makes me wonder if this is still all related to the prop shaft bearing and rear diff failure last year and the persistent droning from the replacement, which did seem worse on that drive. But again, I can't come up with a hypothetical failure model.

[Silverex]

When you have throttle body faults car will still drive/rev fine but have zero power. I’d try doing a throttle body adaptation like mention to make sure it’s good.

[GrumpyBear]

I'd suggest you figure out 100% whether or not RPMs are really increasing when you give it gas after power cuts out, or is it a false reading on the tach. If RPM is really increasing, and speed is not, then the problem has to be at the clutch or downstream from there.

[RMode]

Interesting. There is a problem with the cruise switch. Sometimes you have to click it multiple times and sometimes it disengages when you hit the accelerate button. I have looked up why this happens and the fix for it on youtube.

But the weird thing here was that when it happened, power delivery to the wheels went to zero even though the engine was still running. I would give it throttle manually, the RPMs would increase (I'm not sure if I could hear the engine actually rev or if it was just the needle moving), and nothing would happen, like the clutch was suddenly disengaged. I shifted to another gear and same thing, throttle would blip the needle and nothing. Car coasted untll it stalled then coasted to a stop. I am not sure if it was on cruise the next time it happened.

I can't think why a defective cruise stalk switch would do this. I figure it would just do the behaviour I was already experiencing. I can't figure out a bad switch would knock it out of cruise then ignore all throttle input (the throttle is drive by wire, correct?). The RPM needle increased so my input to the pedal was going somewhere, just not to the ECU to tell it to fuel and open the TB and get on going.

This makes me think an electrical issue (ECU overheating?) but I can't think what.

Fuel pump maybe? Would an overheating fuel pump do this? Although I would expect the engine to sputter or immediately cut out, not keep on going without throttle doing anything.

The other weird thing is that when the engine finally stalled and RPMs went to zero when it was at about 20-30 mph or so in 6th, I didn't feel anything. No jerk or anything. It was literally like the clutch was disengaged or the transmission somehow popped out of gear even though it was stiill in 6th. Which makes me wonder if this is still all related to the prop shaft bearing and rear diff failure last year and the persistent droning from the replacement, which did seem worse on that drive. But again, I can't come up with a hypothetical failure model.

I suggested it because the exact thing happened to me. The cause was the cruise switch, and it solved it. I was able to immediately pull over, car started right back up, and so long as cruise wasn’t used, it was fine. Replaced the switch, and it all worked the way it should .


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