Idle Missing

[Se7enRed7]

I just bought a 1994 GS300 with 131,000 miles. I love the car, but when i pulled into my driveway, i was talking to my friend and didnt turn the car off for a few minutes. I noticed when at idle, the engine occasionally skips a beat, like a cylinder doesnt fire or something. Has anyone had this problem? Anyone have any ideas of what it might be?

thanks

[SKperformance]

Don't you have a distributer and cap?

Change them

[SRK]

Don't you have a distributer and cap?

Change them

So on the basis of a minor symptom, which may be caused by nothing more than a set of worn out spark plugs, you are advising this person to change the entire distributor and cap? Don't you think that may be a very expensive, and futile "repair"? Diagnosis is the correct approach, not hanging a bunch of parts on the engine to see what happens.

To the original poster - check the last time the spark plugs were changed. Have the ignition system analyzed on a scope to see the secondary wave form.

Like the ING guy says "Save your money".

[Se7enRed7]

To the original poster - check the last time the spark plugs were changed. Have the ignition system analyzed on a scope to see the secondary wave form.

Well, today was an interesting day. I returned the car to the owner and put a "stop payment" on the check i wrote him because there was a large spot of transmission fluid on my driveway. I took the car back to the owner and when he repairs it, i'll take the hold off the check. (it was a 500 mile round trip) <_<

Is there an external triggering source when testing the ignition system? I just so happen to be an electronics technician, and have an oscilloscope sitting in my bedroom. If i knew what the secondary waveform was supposed to be, i could diagnose and fix it myself with relative ease (if it is an ignition problem). Thanks for the replies !

[SKperformance]

I didn't mean to change the distributer but the rotor and cap. I meant you have a distibuter ,right.

Change your cap/ rotor as it is a cheap and usually needed maintance part.

my bad explaing it.

[SRK]

Is there an external triggering source when testing the ignition system? I just so happen to be an electronics technician, and have an oscilloscope sitting in my bedroom.  If i knew what the secondary waveform was supposed to be, i could diagnose and fix it myself with relative ease (if it is an ignition problem). Thanks for the replies !

The secondary wave form is picked up inductively, usually by an iron ring placed around the high tension ignition wires. The trace is then analysed for faults, although doing so requires a fair bit of experience in knowing what a good trace looks like. Usually the coil wire, and the number one plug wire are used to raster all of the ingition pulses in firing order. It may be easiest to watch someone using an actual ignition scope first - but it sounds like you may be on to something.

This is assuming that your car has a separate coil for all cylinders, and not coil on plug distributorless ignition. If it is like the LS400, then it can be scoped.

[Se7enRed7]

The secondary wave form is picked up inductively, usually by an iron ring placed around the high tension ignition wires. The trace is then analysed for faults, although doing requires a fair bit of experience in know what a good trace looks like. Usually the coil wire, and the number one plug wire are used to raster all of the ingition pulses in firing order. It may be easiest to watch someone using an actual ignition scope first - but it sounds like you may be on to something.

This is assuming that your car has a separate coil for all cylinders, and not coil on plug distributorless ignition. If it is like the LS400, then it can be scoped.

Ignition systems work by switching a coil and some sort of resistance in series. There is a switch that rapidly switches between high and low resistances. The coil stores the current in the circuit. Ohm's law being v=IR. When the coil stores the current from the low resistance the current will be high. In example :

12V=(2ohms)(6amps)

Then when the resistance is changed to say, 2000ohms, and the coil stores the current from the first switching cycle:

(2000ohms)(6amps)=12000V

Notice how the current remains the same, but the resitance determines the voltage.

The coil cannot store the current for very long, and the current drops off very quickly. The time can be found by:

5(inductance\resistance) = T

With the different resistance, the charge and discharge times will be immensely different.

Say you have a .1Henry coil and the same resistances listed above :

5(.1H\2 ohms) = .25S

When the resistance is changed to 2000ohms

5(.1H\2000) = 250µS

So if you have an abnormal pulse width either long or short, you can then measure the inducance or resistance and diagnose the problem from there. If everything is fine with the inductor and resistors, then check the output voltage, and if everything is fine there, the plugs are probably bad.

So a short question at the end of a long and boring post...

What are the values of the inductor and the resistances?

[SRK]

The inductive (I think I am using the right term) pickup is like those for timing lights. The scope shows the voltage sensed, but it is the trace that reveals the problems. In short, the first spike shows the firing voltage, that required to establish the spark across the gap at the plug. The next area is the spark line, which is the kV required to maintain the spark. The oscillations that follow show the coil discharging after the spark is extinguished. Just what an appropriate shape is to these portions takes some experience, simply because one has to be familiar with the particular ignition system, and then know what is an abnormal trace. All manner of things can be determined (spark plugs fouled, excessive gap, low compression, high resistance in the wiring etc.) which is why this method is so usefull, at least on older engines. But even modern ones can show things which would otherwise not produce a trouble code.

[nismO to TRD]

how about your vacum hoses?

but it dosen't explain the trani fluid...

[Se7enRed7]

how about your vacum hoses?

but it dosen't explain the trani fluid...

Didnt check the vacuum hoses. I dont have access to the car right now, as it is 200 miles away. The guy i'm buying the car from is going to fix the transmission leak, so i'm not worried about the leak. What i am worried about is if it had been driven with low transmission fluid levels. When i picked up the car, the fluid was full, and when i dropped it off it was still about the same level. Should i be worried about the transmission going bad? The plugs are being changed today, and the mechanic is also doing a compression check.

BTW, the reason i'm so anal about things being wrong with my car, is i also own a mitsubishi 3000GT, and have dumped thousands of dollars into repairs.

[Se7enRed7]

Just taked to the seller's mechanic. (A honda mechanic) He told me there was no leak, or ignition problem. Well, i beg to differ, because there was a spot of tranny fluid on my driveway and every so often when at idle, the rpm's take a dive and quickly come back up to normal. Given the information in this and previous posts, if you were in my shoes would you keep the vehicle, or let the owner find another buyer?

BTW, I paid 7,000USD