Not running right, SD light and rosy cat, diagnoses

For the first time in 33 years my 308 developed a problem this week. Earlier this year I restarted the car, 85 3008 GTB QV, after taking it out of winter storage. All was fine, as usual. I hadn't had a chance to drive it yet with the poor weekend weather in New England and an extended vacation. But a couple of days ago I decided to get it out. When I started it, it was obviously not running correctly. Sounded funny and slow to respond to the throttle. I reved it a bit to see if it would clean out, but no. Then the slow down light came on and I shut it off. Sure enough, the cat was just starting to glow a rosy red. Time to diagnose the problem. So I let it cool down and then got out my pocket spark detector. Started the car and laid it on the 1-4 coil wire. Nothing. Laid it on 5-8 and it flashed away. No spark at all on bank 1. Funny thing, these flat plane crank V8s will run smooth as a top on one bank.

OK, no spark form the coil, where to start. First thing I did was to measure the impedance of the crank sensors. They were all about 740 ohms, so that didn't appear to be the fault. Next up, swap the digiplexes. No change. Looking like a coil. Time to swap coils. Still no change. No spark on bank 1, bank 2 was fine. Maybe a bad connection or broken wire. Checked and compared the bank1 harness to bank 2. Checked grounds, checked power. All good. So, maybe the crank sensor is bad even though impedance is ok. It can happen. Get out the scope and look at the pulses from bank 1 and bank 2.

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Identical! Sensors are definitely not the problem. So now I know that the crank sensors are good, the wires and connections are good, both digis are good, and both coils are good. I decided to look at the coil trigger signal sent to the coil - terminal by the digi to see if it would tell me anything.

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Bank 2 looks like what is expected. The initial part shows the drop in voltage from 12V at the negative terminal as the digi grounds the negative side. It looks OK on both banks. The bank 2 trace then shows a sharp positive peak followed by a return to the 12V level as the coil discharges across the spark plug gap. But bank 1 shows and exponential decay back to 12 V. This is indicative of the coil discharging through a finite resistance, not a spark discharge. Thus, it appears there is a problem on the high tension side of the ignition. To verify this I took a single spark plug wire and connected it directly to the bank 1 coil with a grounded spark plug inserted in the other end. When the car was started it sparked as normal. Since it would be unlikely that all 4 plugs would short out at the same time it appeared the problem was in the dizy. Popped it open and check it for cracks etc. It appeared good. Rotor appeared good on visual inspection too. So I pulled out my ohm meter again and measure the impedance between the rotor tip and the engine block. 92k ohms. Not a dead short but certainly much less than the infinite impedance of a spark plug gap, and it would account for the exponential discharge of the bank 1 coil. Problem solved.

 

Yes. Well not shorted but something in the rotor is resulting in a path to ground through a resistance of 72k ohms (92 was a type). This is an OEM rotor, not after market, but old. I expect to have more to say about this tomorrow if my suspicions are correct.

 

OK, I couldn't wait until tomorrow. I wanted to know how the rotor failed so first thing I did was clean it up. Then I pulled the spring clip out and cleaned out the mounting hole and the spring clip. There was a lot of black crap in there. When cleaning I notice what looked like a small hole in the plastic at the bottom of the mounting hole which would be right under where the rotor contacts the carbon in the dizy cap. I tried to stick a pin in the hole to see if it would contact the rotor. It would not. Either the hole didn't go all the way through or it was too small for the pin. I put the clean spring clip back in the rotor and measured the impedance. With all the crap removed from the hole it showed an open circuit. I reinstalled the rotor in the car and though maybe it would run now. When I started the car it kicked right over, obviously running on all 8, but then quickly went back to running on four. I pulled the rotor off and took the spring clip out. What I found was a new carbon deposit, evidence of the failure mode.

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I ordered 2 new ones fro Superfromance. $13 each plus shipping tot he US, what ever that comes out to be. I've bought many items from them over the years. Always been happy with the stuff. Yes, the OEM ones are Marelli. You may fine some new old stock if you are lucky. But most are after market and I would not be surprised if many venders are selling the same part at different prices. I believe Ricambi sells them for $44, AWItalian wants $30, Eurospares wants $16.. Why is anyone paying more than what Ricambi wants? I'm wondering if 308 Milano ordered the right rotors. Mr Fiat listed the 308 QV rotors for $105.

 

My rotors are (were) original (33 years old/33k miles) and one cap is original. I had to replace the other cap, back in 2010, because the spark plug wires needed to be replaced and I could not get the screws that penetrate the wire out. Aside from that there was nothing wrong with that cap. I know a lot of guys would have replaced both caps but 1) I don't believe in replacing parts that aren't broken just because they are old, and 2) I'm cheap. I will, however, replace both rotors. Less than a tank of gas these days. And I run regular gas in the car because that's what the manual say to use.

 

I was questioning because you said you got them for $65 but Mr. Fiat lists then for $105. ????? So they gave you a deal on bad parts? Your invoice says Italian Auto Parts. Is that Mr. Fiat too?

 

I did measured the resistance when I took it out. It was 72k ohms. mentioned that a couple of times above. And I did clean it and soak in in vinegar. That an old trick to remove rust and oxidation. After that it measured open circuit. Put it back on the car to see what would happen. It started on all 8 but quickly dropped the 1-4 bank again. Pulled the rotor and measured resistance and is was about 0.8 meg ohm. Pulled out the metal spring clip and there was the tell tale carbon track, as shown in post #5. I cleaned the rotor again and put a couple of drops of epoxy in the mounting hole and was going to see what that did, just for fun, but then figured, why bother. I'm just going to wait until the new parts arrive and be done with it.

I was thinking of selling the car since I really don't drive it much since I got the 355 and the Porsche. But it's hard to part with it after 33 years. I was offered more for it from a dealer than most GTSs retail for. And really, how many GTB QVs are there around? I've heard 29 for the US in 85. Also heard from 83-85 there were 38 for the US (which I don't believe). But I believe the number was around 50. By the late 80's everyone was suffering from Magnum syndrome and wanted a GTS. I bought a B because I thought the GTS was ugly and a flexible flier.