94 Ls400 Firecracker Pop From Engine - Help

[yotoy82]

1. One day noticed that the radiator fan shoroud next to the engine fan was loose, all the screws at the top had been broken off the plastic. Thought this was odd, attributed it to heat and old age. Rather than buy an entire new shroud that was 139 plus shipping, got some L-Steel Brackets, had it drilled in and got put back into place.

2. I have had the car for a week now, starts just fine. Two days ago in the morning, after the car sat overnight, went to start it and BOOM. There was a firecracker sound from the engine, and could hear something shake, I would say/guess that the firfilter housing was the exit for this miniexplostion which lifted it up so much maybe that it hit the hood. Car turned off.

Second start. Just fine, no issues. Throughout the entire day, car starts just fine. Next morning, turn the car on, BOOM.. Its like a firecracker just went off.

3. I have now concluded that this mini explosion is what lifted the air filter up along with the other plastic covers in effect pulling on the fan shroud.

I don't know what to call this, engine runs perfect once on, so it's not really a "misfire"

I'm not too familiar with the 94, and before randomly start changing parts, ie wires, plugs, caps, and rotors, thought would get everyones input here.

It's got 120,000 miles on it roughly, almost no service history with me on it because just bought it, but it would be safe to say it is on its original timing belt, original spark plugs, etc.

[SRK]

What you have there is a true "backfire" or a spit-back. It means flame and combustion gases in the intake tract. Normally such things are restrained by the throttle plate, but not always.

So you're looking for something that would cause a reversion. Lean starts will do it - delayed ignition of the fuel and the intake valve opens just when it gets going after a long pause. Another is a sticky intake valve hanging open a bit. But in your case I'd suspect a crossfire between two ignition wires. Pull the covers off and have a good look at them to see if you can find any carbon tracks or spots where the wires are close to one another.

[yotoy82]

Thanks for the reply! backfire is really the word I was looking for. A look at the spark plug wires?

[SRK]

Any place that the wires nearly touch can be a source of a cross-fire - in other words the energy jumps as a spark to the adjacent wire and causes the wrong plug to fire. They can also arc to ground. Normally there will be a tiny pinhole in the wire insulation, or carbon marks around it. It was more common on older engines, but as Lexus routes the wires close to each other, and over long distance, it's worth checking.

[yotoy82]

I have Leaky valve cover gaskets maybe that has corroded the spark plug wires a bit. To inspect the wires completely a lot of things have to come off, so at that point might as well replace them?

[SRK]

That's expensive and may not be needed. For sure pull the plug connectors - one of the wells may be full of oil if the gaskets leak.

[yotoy82]

Thank you for your time, will have a look!

[landar]

Wait a minute. I need some context/clarification here. Did you buy that 95 that you were looking at and mentioned in your other post? http://us.lexusowner...y-a-used-ls400/

[yotoy82]

Nope, ended up buying a pretty well kept 1994 LS400. 120k ish miles, paint gorgeous. Interior great except drivers seat. Overall drives 85% as good as my 96' but needs a few small things. First things first diagnosing the backfire!

[landar]

Well congrats on your 'new' ride. Backfires are typically caused by timing issues and as SRK mentioned, it could be several wires arcing over which essentially introduces an errant spark timing. Perhaps a distributor cap is arcing internally and doing the same thing. You might try replacing the caps and rotors and cleaning and spraying the wires with a drying agent to see if that helps. And perhaps the timing belt is worn but I can't imagine it being so bad as to throw timing off that much, especially since it runs so smoothly. In any case, you need to get this resolved soon because a backfire can potentially cause some serious damage including a fire. That would ruin your whole day. And engine compartment.

[yotoy82]

Well said lander, would totally ruin my day! It's only an issue on startup 1 out of three cold starts. Going to have it checked. Wanting to do complete timing service, complete tune up, and valve cover gaskets anyway soon, after that should be good to go another 120,000 miles.