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Sudden loud bang and smoke on top of engine...was doing front lamps wiring!!!


Dinsdale

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Rather nervous here. I just had a big shock!

 

The car is the 91 Regal Coupe I've posted about recently. It has a 3.1L multiport F engine, as written on what I understand to be the rear valve cover on top of the engine.

 

I was tidying up the front lamp wiring, and was turning on the ignition (not running the engine) and the lamps to check stuff as I proceeded. Then I'd turn everything off and disconnect the battery.

 

During one check (ignition on/parking lamps activated) there was a very loud bang in front of me, followed by smoke coming up from under the rear valve cover. I disconnected the battery very quickly, and just stood there in shock. It's not my car.

 

When my heart rate had settled down, I connected the battery. The ignition and parking lamps were still on, just as before the bang. I went to start the engine.........it fired up no problem, running as sweet as ever.

 

I left it it that and came in here for help. I haven't tested anything else yet.

 

If anyone can suggest what might have happened, I'd be extremely grateful. I have no idea at all why this happened, or what might now be toast. There are some wires which run under the valve cover, so I checked the Haynes manual for enlightenment.........no joy there.

 

Mainly, I'd like to know what electronic component(s) is/are under that valve cover. If I have to remove it I will, but would really rather not :).

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If you didn't have the battery tight there may have been a crack when you started it. That happens to me when I use jumper cables once in a while.

 

I'm not a 3.1 guy but there shouldn't be anything under the valve cover on any engine.

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If you didn't have the battery tight there may have been a crack when you started it. That happens to me when I use jumper cables once in a while.

 

I'm not a 3.1 guy but there shouldn't be anything under the valve cover on any engine.

 

It just happened, out of the blue, while I was checking the lamps with the engine not running, but ignition on for checking the turn lamps.

 

I've never known anything like this, and I've been doing cars for more than forty years. The bang was loud, as in an exhaust backfire. What on earth could cause that, with just the parking lights and ignition on? There was a fair amount of smoke, which hung around due to the absence of wind.

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If you didn't have the battery tight there may have been a crack when you started it. That happens to me when I use jumper cables once in a while.

 

I'm not a 3.1 guy but there shouldn't be anything under the valve cover on any engine.

 

I think he meant the intake plenum.

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I think he meant the intake plenum.

 

Could be that....'plenum' is a new word for me, though. There's a "something" on top of the engine with a capital F stamped on it. Maybe something else too, but definitely an F there. The smoke drifted up from under that, after the bang.

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Was the smoke smelling of wires/plastic, or something else?

 

No, and when I put my nose over the area after the smoke had dispersed, it just smelt like an engine. No hint of anything burned.

 

Totally weird event.

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Well, melted wires or electronics would have a very noticeable odour.

 

Did the bang/smoke happen as soon as you turned the key to on (ignition)?

 

No. I'd turned on the ignition and parking lights maybe two or three minutes earlier. So I was standing at the front of the car, having figured that all looked well and planning my next move and.....BANG!!!! Totally out of the blue. I wasn't touching anything at the time, just standing, looking and thinking.

 

Maybe a short or bad earth I hadn't noticed had caused something to get hot....but what? You'd expect a blown fuse, or some smoke, maybe a smell. Not a deafening BANG.

 

Some wires run under the plenum....to what? I'm certain I had the wiring safely rigged at the time. As I wrote, all the lamps worked after the bang as they had before it.

Edited by Dinsdale
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You're likely going to have to pull that plenum and look around.

 

Here are my latest thoughts.......

 

Mr Idiot Dickhead (that's me) had checked the air filter and is awaiting a new one. So he hadn't bothered to put the bolts back in, just left the cover loose on the top of the filter box.

 

On a totally windless day, the car was started several times within the space of half an hour.

 

He had fiddled with a temporary earth just to the rear of the driver-side headlamp, very close to the air filter box actually, after noticing that it was sparking.

 

 

At the moment, this course of events would explain the bang and the lack of a smell from burned electronics. Not sure about the smoke, where it came from to get under the plenum, it's trajectory etc. It lingered a while in the unwindy conditions, and looked spookily pretty as it very slowly wisped away :).

 

The more I think about it, the more I feel sure that petrol vapor exploded. It was that kind of bang. I'll be checking further when I wake up in the morning, of course.

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Could be that....'plenum' is a new word for me, though. There's a "something" on top of the engine with a capital F stamped on it. Maybe something else too, but definitely an F there. The smoke drifted up from under that, after the bang.

If its on the center of the engine and it says 3.1 mpfi or multiport, its the intake plenum. The throttle body goes attached to it

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There technically should be no reason for petrol vapors to be under the plenum. However, there is a fuel pressure regulator there, and if it goes bad, it could certainly leak fuel into the plenum itself and that possibly could cause some vapors in that area that could ignite.

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There technically should be no reason for petrol vapors to be under the plenum. However, there is a fuel pressure regulator there, and if it goes bad, it could certainly leak fuel into the plenum itself and that possibly could cause some vapors in that area that could ignite.

 

Thanks.

 

I'm still hanging on before pulling the plenum to check out things further. Maybe something will come to mind that makes sense.........not too optimistic about that :).

 

There is absolutely no smell of petrol anywhere around the engine bay, and certainly not from under the plenum. Maybe I should cover the plenum for a while tomorrow morning and see if any smell builds up around that fuel pressure regulator you mentioned.

 

I had an idea today - I stuck my hoovering attachment under the plenum. I have a short length of garden hose taped to the hoover pipe and I use this for getting into enclosed box sections to clean up after welding preparation. I emptied the hoover first and managed to get the pipe pretty much everywhere under the plenum.......all I got was a half teaspoonful of dust and sand. It seems that no component has exploded to bits, then.

 

The engine fires up and runs fine, which is perhaps the most baffling thing considering how loud that bang was and the subsequent smoke.

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You can check the fuel pressure regulator without pulling the plenum. I think you can get to its vacuum line on the right side of the plenum not too far from the throttle-body. It runs to the vacuum harness on top of the throttle body. Apply a vacuum to its vacuum line and make sure no fuel comes out. If no fuel comes out, then it is probably fine and I can't think of any other thing in that area that would be producing fuel vapors.

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