I took my shark ('88 S4) for a long-ish drive yesterday, coming back to the Bay Area from the hills outside Downieville. That's about 100 miles, and the first half of the drive is great "spirited" windy mountain highway. The weather in the mountains was beautiful and slightly warm. As soon as you got down into the valley, it was over 100 degrees. My temp gauge on the dash was running noticeably hot. I stopped in Sacramento for lunch and smelled a little smoke. I looked under the hood and saw a very tiny wisp coming from the top/rear of the engine. Didn't see anything wrong in there with the flashlight, so I ate my lunch and then started driving again.




About 15 minutes later, the car started running poorly, and very soon it would sputter and die at idle and had no power. I pulled over on the side of the highway, and this time there was a noticeable amount of smoke coming from the engine compartment. I opened the hood and now there was smoke coming from the same spot behind the engine, under the intake plenum. No visible fire, but lots of smoke. I waited for it to clear, and looked in there with the flashlight. The rubber boot attached to the MAF, as well as the plastic Y that attaches to the boot and sends air to the idle speed valve were almost completely gone, with charred nubs in their place. What was left is brittle and blackened with a few white ashy spots.

Now, the weird part is that nothing else was damaged. All the wiring and vacuum hoses and whatnot in that area look perfectly fine. There are no burned electrical connections or wires, no leaking fuel fittings, and there isn't anything else in that area that should ever get hot enough to start a fire.

That means whatever burned it must have come from inside the engine, maybe through the crankcase vent? Or, maybe I got one hellacious cough of unburned fuel pushed up through the intake that then somehow got mysteriously ignited? Is it possible the cams are off-timing or something, and have been roasting that hose slowly from the inside ever since I replaced the timing belt?

Fortunately, it's only about $150-ish worth in damage, but I'd sure like to prevent it from happening again! Anyone else ever heard of this happening? I'm really not sure what to even check!


Edit: I just went out to take pictures and thought of something else. This car hasn't idled very well since I got it, and it died a couple times on me when I stopped at red lights. That means I had to restart my stalled engine after waiting at a light. It's likely the 4 second delay had (barely) passed, and the burnoff filament in the MAF could have been a source of ignition if there was fuel and air hanging out in there. I pulled out the MAF and it's looking rough- the plastic cover on the side is warped and melted. That _really_ sucks. So much for a $150 repair.




So, that possibly answers the question about ignition...now how do you suppose fuel got in there?

-Andrew

 

I wonder if it wasn't fuel, but rather a reasonable amount of oil being sent back to the intake from the previous driving. Perhaps there was enough in there during the burnoff cycle to start the badness.

That situation sucks but, given how much worse things tend to go when there's fire in the engine compartment, I'm glad you got off relatively easy.

Good luck

 

There is a troublesome fuel line behind the MAF. I had a case where it was dripping just a little bit (maybe a few drops a minute) and eventually started a little fire that was small enough to put out with a can of pepsi.

 

I think that fuel can get in there when the diaphragm in your fuel regulator or one of the dampers ruptures. Then the vacuum sucks raw fuel into the intake system.

 

There's no vacuum system that can feed fuel there...

But..OP..what does your CE panel look like, any browned/burnt fuses or relay locations??

If the MAF internals blew up...a fuse somewhere didnt blow, or it was fuel external to the MAF electronics..but...wow.

 

@Speedtoys No, I don't see anything wrong in the fuse panel.

Also, I checked the fuel lines back there and don't see any signs of leakage. No smells, no unusual damp or clean areas. I just replaced all the fuel lines back there 6 months ago, and checked them again for tightness when I had it apart a couple months ago.

I like @Zirconocene 's idea about it being oil that got ignited. It has been using a lot of oil, maybe that's where it's been going? Also, I suspect it has the wrong dipstick. The one that's in there looks much newer than the rest of the engine, and it doesn't fit very well. It's possible it's been chronically overfilled. Maybe some oil vapor got pushed up into the MAF during a panicked re-start at a traffic light, at just the right moment, and lit up a nice 5 liter blowtorch in my intake?

I'll bet the temp switch on top of the intake is trashed now. Maybe the IAC valve too. Oh well, I was wanting to take the intake manifold apart anyway. :-P

 

Na..I especially don't like the oil idea.
Oil is -hard- to ignite, and it's not in a 'consumable' form for an ignition point up there.

Sure, there may be a very passive pool at the bottom of the intake elbow there if driven excessively hard for a long enough time, but a pool of oil needs quite a lot of help to ignite...usually something already ON fire that can initially wick the oil, or in a fine mist. And that's not happening there..on THAT side of the throttle body.

 

what kind of air filter do you have?

 

Absolutely!
And probably why the car stalls and runs poorly.
The vacuum source for these pieces is about 1" from the butterfly, thus 2" from the boot that burned.
Pull off the 90 degree vacuum connectors to both dampers and the regulator. Smell for fuel, if it doesn't run out. Use a small probe (toothpick) into open metal vacuum tube to check for fuel.
New Porsche S4 dipsticks have a "green" O-ring at the bottom of the plastic. Good luck getting that into the dipstick tube, even with grease. I change every one of these to the "correct" O-ring....which needs to be tight, but not insanely tight, like the "green" O-ring.

 

Greg, I follow what you and Jerry are saying..but..how does any vapor going the wrong way in the intake get near a possible ign source?

I know the hot wire gets..hot..but..the AMOUNT of fuel vapor that would have to be at that point, at the rather narrowly allowable (somewhat heavy 1.4% by volume) mixture for the <1s that the wire is actually hot...sounds highly improbable.

A few inches away might as well be a spark plug gap given the time the hot wire is 'hot' (if ours do this like Mercedes MAFs did) and the glacial (in relative terms) amount of time it takes for any fuel vapors that may be present, to get concentrated enough to achieve a possible ignition.

Just sayin it would take rather large amount of fuel ahead at the throttle body where the two vacuum reference ports are..to be present to reach ignition point..immediately after you turn off the key, for a short # of milliseconds.

At the lower flammable limit of gasoline, assuming the volume between the hotwire and the TB vacuum port is a eight cubic inches (which its not, its larger)..that's a massive two cubic cm of raw fuel that must first gather down there and instantly vaporize and THEN make it's way to the hotwire to ignite..again, all within one second.

Something would have to be quite literally spraying fuel at the hot wire at that time...which is why I asked Jerry where any possible vacuum source that MAY have tons of raw fuel in it exists ahead of the hot wire in the airflow.

This looks like an electrical fire escaped from the MAF body...whats it look like INSIDE the maf electronics area??

 

The heavier concentration of ash appears to be low, at valley level..

 

Can you take a look at your spark plugs to see if they are black and moist—rich. Did you have any issues with backfiring?

 

And to Stan’s question were you using a k&n air filter or similar? If so did you “Re-oil” it recently?

 

A more basic observation is that upon noticing the very first whiff of smoke creeping up from under the air box, that would have been an immediate red flag for me to stop everything, and with a fire extinguisher at the ready, remove the air box and see what was happening. I wouldn’t have started it up again. I think you are very lucky that you didn’t have a carbeque

 

@Mrmerlin : I have a paper filter, nothing fancy. Plain old orange Fram one. Seems like I would have had to really oil the heck out of a K&N one to make something like this happen.
Strangely, the filter looks absolutely fine. No sign of soot on it at all.

@Speedtoys : I know motor oil is pretty hard to ignite, but maybe it was vapor from the hot day / hard run that made it up the tube? If that wasn't it, then it was fuel. Maybe a regulator issue caused fuel to pool up in the intake, which then evaporated and floated up into the MAF in the heat? I don't know though- seems like that would have ignited -fast- and made a nice backfire, especially if it was enough fuel to burn straight through the intake tube.

There were at least two occasions where the car stalled at a light, I waited for the light to turn green (at least 30 seconds or a minute), and then I started it back up. Possibly enough time for very hot fuel vapors to make it up to the hot wire, if I was sitting there playing with the go pedal while I was waiting.

I can't think of many ways that combustible materials could have made it backwards through the intake and up to the inside of the MAF.

I suppose it's possibly a failure of the electronics in the MAF, but... that seems like a LOT of heat generated by a sensor, especially considering it didn't burn out the fuse. I'm actually curious now exactly how flame-resistant that MAF tube is. I'm sure they thought of safely handling a backfire when they spec'd it out. If it was an electronics failure, it seems more likely that flaming hot sparks or pieces from the electronics made it down into the intake and ignited some other fuel source.

@GregBBRD : I'll check out the vacuum line going to the regulator. I can pop the MityVac on there too, and see if I get any fuel out of it. It seems like this is a big enough problem that it should be pretty obvious.

My dipstick does not have a green O-ring, and it's really hard to reach because the loop/handle is too short and the plug wires are directly in the way. It seems like the plastic part is too large of a diameter to properly fit the dipstick tube. It's very tight, and I can't even have an o-ring on it at all. I'm guessing it's supposed to look like this one on 928srus.com:


But it doesn't, it looks just like the "New Style" one in this photo:


Looks like I should get a different dipstick. Anyone got one they'd like to part with?