MKIII Idle problems; CPS?

Hey guys,

I have an 87 MKIII turbo, and right now the car is consistently idling bad; "missing". It never quite stalls but it's definitely not firing properly.

I keep the car garaged in the winter and drive it in the spring and summer.

I went ahead and did the ohm and continuity checks to the coils. There is a 12V supply coming through each of the three connectors going into the coils. Everything checked out according to specs.

The plugs are brand new and the plug wires are brand new. I did resistance checks on the wires too; they're fine.

I decided to pull each spark plug and I noticed #2 doesn't have any burn on it like the other 5 plugs; it's shiney, like new. The plug did smell strongly of gas too. So obviously that's the culprit cylinder that isn't firing and is receiving fuel.

Now, I checked the CPS (cam position sensor) and after pulling the things that were in front of it--I noticed the dust cover was badly cracked. After removing the cover (it broke into several pieces) there was: oil, carbon, dust, water, and corrosion all over inside of the CPS.

I checked the ohms on the CPS connector and it checked out properly according to specs.

My question is: What's the most likely cause for a single cylinder to consistently not fire properly?

And, is replacing the CPS recommended since the interworkings have been gulping all this garbage (moisture, oil, dust, etc.) for a while?

Thanks guys.

I dont think the CPS is to blame.. You should get a new dust cover though.. I recall someone saying they were only a couple dollars at the toyota dealership..
First i'd check the plug.. its easy, and quite common to crack the plug when putting it in.. sometimes its a crack you cant see right away, check your wire.. maybe replace it with another wire to test...

Pull the number 2 plug and wire out.. put the wire on the plug and ground it out good..(there are alittle devices you can buy that will hold the plug and wire, and ground it out on the car for a couple dollars.. have someone turn the car over to see if the plug is sparking..

I dont think the CPS is to blame.. You should get a new dust cover though.. I recall someone saying they were only a couple dollars at the toyota dealership..
First i'd check the plug.. its easy, and quite common to crack the plug when putting it in.. sometimes its a crack you cant see right away, check your wire.. maybe replace it with another wire to test...

Pull the number 2 plug and wire out.. put the wire on the plug and ground it out good..(there are alittle devices you can buy that will hold the plug and wire, and ground it out on the car for a couple dollars.. have someone turn the car over to see if the plug is sparking..


Thanks for the reply. I'm going to try that and hopefully get this problem straightened out. I have a suspicion it's probably somethings else, but, it'd be nice if all this is, is a simple spark plug problem.

Okay guys,

Here's my results from some testing.

I took out the #2 spark plug, a Bosch copper plug (it was looking shiny new with no cracks or damage), and connected it back to the #2 spark plug wire. I grounded the plug properly to the engine using a clip and a wire (used a similar clip shown in a Haynes repair manual).

I cranked the engine and let the engine run for a few seconds, and, as expected, the #2 didn't fire at all, whatsoever. I checked and made sure it was grounded, and, the wire was grounded tight on three separate occasions and the plug still wouldn't fire at all.

Next, as an experiment, I hooked up an old Denso platinum plug that I had laying around to the #2 spark plug wire and cranked the engine, and, I got sparks this time. Now, I let the engine run for a little while and I noticed the sparks were very intermittent. For example, I wouldn't get any spark for 2 to 3 seconds then I would get a brief burst (it seemed weak) of a spark for a fraction of a second, then, a second later of no spark I would get a constant spark for 2 to 3 seconds, then nothing for several seconds. The plug was grounded tight, so I don't believe the plug was becoming ungrounded during engine idle.

Let me be clear, there was no clear pattern to the sparking--it was basically very random with good periods of no sparking. I suppose I'm expecting a strong spark and a very reliable pattern of sparking--I'm not getting this at all on #2. I've tried some other Denso plugs I had lying around and got the same seemingly weak and very random pattern of sparks.

Lastly, I put the Denso plug into the engine and let the engine run for a while--it still has quite a rough idle.

Anyway, I'll keep experimenting. I believe I'll try a different wire for #2 and see what happens; the #2 wire currently on there is brand new.

If it isnt the wire, then you probably have a problem with your coil, or the 2 wires going to the coil.. (i had to replace mine, they were all cracked up)
But i would put another wire on there first to make sure

Also, its a good idea to use die-electric grease or plug boot grease when installin the wires.. Keeps the wires from leaking, and makes them easier to take off

Hey hey, good news!

I pulled the coil off to give it another inspection. The coil seemed to have deep cracks around near its base and even on the sides of it. The plastic ends on the wires just seemed to fall apart when I did the electrical tests outlined in the repair manual.

So, I went ahead and replaced the whole coil w/ wire with another (used) one that I bought recently.

Then, I went down to the garage that I have the car stored at and hooked the new coil up. I started the car and the engine idle was firm; it ran beautifully! The car doesn't shake any more. And no more erratic idle noise coming from the exhaust!

But, I have to say I'm unhappy that I went through all those electrical tests in the book, and, the coil passed them all, so, for a time I disregarded the coil as being the source of the problem while I wasted time doing other things. And, I still haven't ascertained if the problem was the wire or the coil module(s), maybe I'll play around and find out.

Anyway, I'm happy the car is running great now. I have a feeling since the car is so old that I'll be back in here with a new problem after a while--ah well.

Thanks for all the help!

Awesome work! You're on your way to becoming a Supra know-it-all already. :)