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Misfire And Audible Spark (ticking) Under The Hood


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I am hoping someone will know what to look for or what to replace.

I still love my LS400, its from 1991.

It hasn't had any proper maintenance on it for 15 years when I got it, and now I am a year along, a couple of issues solved, but one thing remains.

When it is warmed up, it starts misfiring. Today I had a brilliant idea, and popped the hood and started listening.

Any time it misfires, i can heer a ticking sound coming from the top of the engine, somewhere around the cover where the leads are.

The only thing I have not replaced yet are the rotors and caps. I did replace the plugs and leads.

Did I do something wrong somewhere. Help !?!



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Thanks for responding, but I finally found what may be the trouble.

After a new test drive to heat it all up, I popped the hood and for the first time I heard a hissing sound - i.e. a vacuum leak, right under the intake, next to the power steering pump. But I cannot localize it. Hissing changes correspond to the misfires.

It will probably be the Idle Up Air Control Valve hoses, but I cannot be sure.

Even if I clamp them off I can hear hissing. Are there any other vacuum lines running under there ?

I cannot seem to find any diagram of vacuum lines anywhere.


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You have two lines to that valve. Clamp them both. I did the bypass on mine but that didn't fix my hesitation and misfire at 55-60 mph. I did have a vacuum leak from the intake box flex hose and a split hose from the iac to the intake box that cleared up the off idle hesitation.

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Damon, you the man.

Allright, I thought so. I am actually thinking of replacing the lines and the Valve itself. But the clamping off will be the quick fix and to check if those are the culprits.

I will be checking all intake hoses as well.

Thanks for the info.

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No problem. I've been hunting my issue for a couple weeks now. Went out and installed the new intake hose and no change. I played with the TPS and got it to idle up to 800 in park but as soon as you pull it down to drive or reverse it goes straight down to 400. I'm wondering if it is a transmission issue putting a load on the motor.

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Ha, I had an issue with the TPS as well.

I talked to a mechanic, an he told me that the TPS is there to register 3 things :

1. Foot of the throttle

2. Foot on the throttle

3. Kick-down

So the notion that you can regulate the rpm is false.

The trouble is that when the computers register that your foot is on the throttle whlie in fact it is not, the switching of gears, i.e. going from Park to Reverse, will go a bit more violent than it is supposed to. Same goes for going from Neutral to Drive, it will go in with a bang, because is registers that your foot is on the pedal.

The only way is to do it the official way, which you can find on lexls.com. You will need feelers of 0,40 mm and 0,85 mm, and the most useful thing someone sent me with a purchase of another TPS : the plug that fits on there, with wires attached, so you can use an ohmmeter to measure the TPS.

That's the only way to measure the resistance and continuity.

The TPS is simply there to register what your foot is doing, and when it is off the pedal, the gearbox will be muchsmoother and relaxed, not with a bang.

Hope this helps.


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I read that article and will be trying that out this weekend. The car is a third car that sits in the garage so I can afford to do that. I can manipulate the rpms with that no problem but I know it is not right. I keep reading different things about where the rpm's are supposed to be. I hear that in park normal it should be around 800 to 850, then I read that it is supposed to at 600-650. But I don't see what it is supposed to be in drive at idle. Right now mine likes 400, a little lower when warm in gear at idle.

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Well, when mine is warm it will go as low as 400 rpm, but when you turn the radio, ac and lights off, it will go up. It depends on how much you're asking of the system. But I believe that when all vacuum hoses are tight, the ECU will regulate the rpm, so I do not think you have to adjust anything. It was all good when it left the factory when it was sold to it's first owner, so why change it ?

The only issue I can think off is the leaky capacitors thing with the ecu itself, but I haven;t seen that over here. I have another good ECU that I bought off ebay, and it is good as well.

I also did a throttle body clean, which is a !Removed!, because the coolant and air hoses are fused on the pipes, but that might help as well.

I found that it is a standard maintenance for this vehicle after the first 10 years and maybe 200.000 km. Mine is a 1991 and it was done once, so I did it again. It was a HUGE improvement on fuel economy, no more fuel smell after a cold start.

Cleaning the MAF sensor after all that will probably help as well. Just use a MAF cleaner spray can, it will do the trick.

So good luck with you efforts.


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Well, every time the leak hissing gets louder, one clearly audible click or tick is there, when it misfires. I think it is just a side issue.

When the vacuum is resolved, then all will be fine I hope.

Or else, maybe the loose connector I found, the bottom one on the ignition coil, that may have damaged the coil. That was a mistake I made when I just got the car and started replacing things myself lol.

The old ones were still good, so I can put one back if needed.

The MAF actually has been cleaned, I got a special spray and is is still ok.

Just don't touch it. That's what it says on the can.

Thanks for all the help guys.


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  • 1 month later...

Just to conclude this issue :

I had replaced the plugs and leads, but some of the short leads were too long, forcing me to put them in with a bit of a bend in the wire.

After inspection of the wires, i found a little melting spot on the no.5 wire, making me believe it had arced, so I put the old wire back.

Also the two leads that were too long, I suspected them, so I also put the old leads back, no.2 and no.8.

Problem solved.

No more arcing, just a spark where it needs to go.

Next problem, the MAF.

Solution: IT IS NOT A MAF !!!

It is a Karman Vortex sensor, and any attempt to clean it (other then the MAF-way with a spray, without touching it), will KILL it.

Got a 2nd hand one from ebay, 70 dollar, put it in, running very smootly. Better than ever !

Only it smells like gas with a cold start, so next step I will be putting a new Coolant temp sensor, maybe that will help.

I learn something every day.

But now i need to find a good source for a set of new leads that are the correct length, maybe even heve numbers on it, so maybe anyone can advise me what to buy ?



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