Are my plug wires bad and if so would that cause stalling after the car is hot? - Page 3 - Alfa Romeo Bulletin Board & Forums

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post #31 of 301 (permalink) Old 02-16-2017, 08:38 PM
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Unless the coil wire itself is faulty, I doubt that faulty spark plug wires would cause your engine to completely stall after coming to a stop when warm and then not restart. Bad plug wires tend to cause misfiring under load, not a complete engine stall and no-start (again, unless it's specifically the coil wire and/or connectors). The fact that the engine will eventually stumble and die after merely idling in the garage points to a temperature-related issue with a sensor and/or its wiring, or perhaps something in the fuel pump circuit (the dual relay comes to mind; has this been replaced? The fuel pump wiring is also a known problem with these cars, check voltage directly at the pump when the engine won't start).

First of all, you state that your "digital rev meter was freaking out and bouncing all over the range" before the engine stalled in your garage. What is this, exactly, and what is it connected to? This certainly smacks of an ignition related issue, but a failed ignition component usually causes the engine to instantly stall, not stumble and die . . . .

Which leads me to your statement that after removing the spark plugs, they are "brown," but not wet. Is this immediately after the engine stalls, or after cranking the engine in an attempt to restart? If you are getting fuel but no spark while cranking, the plugs would certainly be wet after a while.

As far as resistance checks are concerned, ohmmeter bench tests of room temperature components are almost completely useless in my experience, especially when chasing an intermittent failure. Electrical components tend to fail when hot and under stress, so check them during those conditions! Luckily for you, the symptom seems to be easily reproduced, so let the engine warm up and stall in your garage with a fuel pressure gauge hooked up. This way, it will be obvious if it is a fuel pressure-related issue. If not, check for spark after the engine stalls either directly by grounding a plug connector or by using a timing light. If all seems well, it is either a fuel mixture issue or something is causing the injectors to not fire.

If so, unplug the coolant temp sensor wires and measure resistance of the sensor. If it is within specs for the current temperature, plug the CTS wires back in and unplug the injection control unit connector and make sure the CTS resistance value is the same as at the sensor. If so, move on. If not, you have a wiring problem. Do the same for the airflow meter, first directly at the sensor at then at the injection control unit connector. If a wiring problem is suspected, check the fuel injection control unit grounds at the bank 1 valve cover, and the main engine ground at the front left of the engine compartment. Otherwise you will have to trace through the above circuits until a problem is found.

If absolutely everything checks out OK *WHILE HOT* (dual relay included), I would try to borrow a (known good) spare injection control unit and see if that works . . . . or kick yours and see if the engine starts .

Good luck,
Chris
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Chris A.

'81 GTV6 rescued from junkyard, "GT" car
'86 Porsche 944 Turbo track/street car
'73 BMW 2002tii fun street car
'87 Porsche 944 race car project (stalled)

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post #32 of 301 (permalink) Old 02-19-2017, 02:49 PM Thread Starter
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Here's a good video of the situation. You can watch the revs go nuts on the multi-meter beginning around :40. It stalls out eventually, with more pronounced misfires starting around the 2:00 mark. Injectors check out, fuel pressure is good. I also swapped in my backup combo relay with no effect. It has to be a problem with the ignition electrics, right? Or maybe something as simple as bad gas?

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post #33 of 301 (permalink) Old 02-19-2017, 03:28 PM
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You need to check the connectors at the T-Stat housing. Not that they are plugged in; but they are plugged into the right sensors. I had mixed two up on a Milano a long time ago and it caused all sorts of haywire idling and dying.
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post #34 of 301 (permalink) Old 02-19-2017, 03:53 PM Thread Starter
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Will do. The sensors are all new with new connectors, but I'll check everything again.

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post #35 of 301 (permalink) Old 02-19-2017, 04:35 PM
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That is an ignition problem. I had the same fault it was a heat fault in the ignition amplifier. You say you have already changed the ignition amplifier. There are not many places left - the pickup in the distributor, the green wire that runs from the distributor to the ignition amplifier or the harness that connects the amplifier. I do not have a wiring diagram to hand but as I recall the tachometer signal is provided to both the ECU and the tachometer separately. By that I mean the tachometer signal you are seeing does not have anything to do with ECU. If it is downstream of the ECU then you cannot rule out a bad connection in the ECU as being the cause - worth looking at a wiring diagram to confirm.

If it was me the first place I would look is the green wire from the distributor to the ignition amplifier and the connectors at both ends of it. If that doesn't help then I'd look at the pickup in the distributor.
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post #36 of 301 (permalink) Old 02-19-2017, 04:57 PM Thread Starter
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The green wire is intact and doesn't appear to be arcing.One thing to note is that it runs with no misfires until the idle starts to slow as the AAV closes.

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post #37 of 301 (permalink) Old 02-19-2017, 05:42 PM
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The AAV is mechanical and has nothing to do with the tachometer signal. Causation and correlation are two different things, the AAV closes when the engine is hot, the tachometer signal goes bad when the engine is hot - the common factor is that the engine is hot, not that the AAV closes.

The green wire and the plug at the distributor can be deceptive - they look good but can be bad - bad wires and poor connectors are known to have heat faults. They get hot and the connection becomes intermittent - especially bad solder joints. The green wire has two cores an internal wire and shield and it can be bad without too much obvious external damage.

Anyway your tachometer signal is bad - this will cause your issue - the ECU is also getting a bad tachometer signal which will cause misfires. The tachometer circuitry is pretty much isolated and it shouldn't be too hard to track down where it goes bad. The clue to the root cause of your problem is the tachometer signal - there is no way it should jump around unless the circuit is bad.
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post #38 of 301 (permalink) Old 02-19-2017, 06:00 PM Thread Starter
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I know I've read elsewhere that a jumpy tach needle inside the car is indicative of distributor issues, but mine is rock steady. Does that fact have any bearing on the solution?

As for the AAV, I was just noting that the poor running seems to happens precisely when the engine is warm enough for the RPMs to begin dropping, that's all.

I'll do some further investigation into the distributor wiring after I get the kids to bed. The CTS checks out, as does the wiring. When I replaced it recently, I checked the wiring to the ECU at the harness and it was also fine.

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post #39 of 301 (permalink) Old 02-19-2017, 08:12 PM
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Quote:
Originally Posted by chairmankaga View Post
Here's a good video of the situation. You can watch the revs go nuts on the multi-meter beginning around :40. It stalls out eventually, with more pronounced misfires starting around the 2:00 mark. Injectors check out, fuel pressure is good. I also swapped in my backup combo relay with no effect. It has to be a problem with the ignition electrics, right? Or maybe something as simple as bad gas?
https://youtu.be/3sW-PujDMJs
Not bad gas, you would have trouble starting it if that were the case . . . .

Quote:
Originally Posted by tpalfa View Post
That is an ignition problem. I had the same fault it was a heat fault in the ignition amplifier. You say you have already changed the ignition amplifier. There are not many places left - the pickup in the distributor, the green wire that runs from the distributor to the ignition amplifier or the harness that connects the amplifier. I do not have a wiring diagram to hand but as I recall the tachometer signal is provided to both the ECU and the tachometer separately. By that I mean the tachometer signal you are seeing does not have anything to do with ECU. If it is downstream of the ECU then you cannot rule out a bad connection in the ECU as being the cause - worth looking at a wiring diagram to confirm.

If it was me the first place I would look is the green wire from the distributor to the ignition amplifier and the connectors at both ends of it. If that doesn't help then I'd look at the pickup in the distributor.
I tend to agree as I said in my previous post, but again, where exactly are the multimeter leads connected? You'll need to get your hands on a wiring diagram and test the primary ignition circuit point by point while the engine is running well so you know what the readings should be when all is well. That way, you will be able to pinpoint the area that is acting up.

Though it sure seems like an ignition issue, it wouldn't hurt to have a fuel pressure gauge connected during testing just to rule that out. Several others have mentioned fuel tank vent issues, and if it weren't for the jumpy voltmeter I would have guessed that your problem lies in that area. I have had a few customer cars (a couple of Porsches and an Audi) with clogged or pinched fuel tank vent hoses, and what happens is that because the fuel cannot be displaced by incoming air, the suction of the fuel pump actually creates a strong vacuum within the fuel tank, and at some point the pump can no longer draw fuel from the tank and the engine stalls (a friend who also has a Porsche shop had a 944 where the plastic fuel tank actually imploded!). After sitting, the vacuum in the tank slowly subsides and the fuel pump is once again able to transfer fuel to the engine.

Luckily for you, your symptom seems to be easily reproducible in the garage and the fuel tank vent issue is even easier to test and rule out before delving into wiring diagrams. Run the engine and let it warm up, and when it begins to stumble, unscrew the gas cap. If you hear a rush of air into the tank and the stumbling ceases, you have your answer. If not, and the engine continues to stumble and die with the gas cap off AND the fuel pressure is OK, move on to the ignition system . . . .

Chris A.

'81 GTV6 rescued from junkyard, "GT" car
'86 Porsche 944 Turbo track/street car
'73 BMW 2002tii fun street car
'87 Porsche 944 race car project (stalled)

Last edited by cda951; 02-19-2017 at 08:38 PM.
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post #40 of 301 (permalink) Old 02-19-2017, 09:06 PM Thread Starter
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I did replace the vent valve a few years ago, which is why I'm maybe overconfident that's not the issue. So I'll absolutely check it first thing tomorrow!

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post #41 of 301 (permalink) Old 02-20-2017, 05:58 AM
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Remove the fuel cap when the engine is misbehaving and see if it fixes the problem. If it does not then you do not have a tank pressure problem.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #42 of 301 (permalink) Old 02-20-2017, 07:06 AM Thread Starter
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The car idles perfectly for 20+ minutes this morning, and all I did was tinker with the coil wire, CTS connector, and AFM connector. Pull, inspect, reseat. No weirdness with fuel tank/cap. I was still getting intermittent flashes of anomalous numbers on both the revs and dwell (it jumps around regardless but still averages out to about 950 RPM and 64 degrees) but it never stumbled. I figure it has to be some janky old wiring or a bad connector, possibly a bad coil wire.

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post #43 of 301 (permalink) Old 02-20-2017, 03:44 PM Thread Starter
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Plug wires from Centerline showed up today. Initial impressions - the car runs MUCH more smoothly. I was able to back the idle down a bit more with no stumbling and most importantly, it never stalled. Not sure if it was repairing the coaxial wire to the distributor, unmuddling the CTS connector, the plug wires, or all three. Hopefully I'll have a chance to get it on the road soon,,
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post #44 of 301 (permalink) Old 02-20-2017, 04:49 PM
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Tank pressure would be additive to pump output pressure because they are positive displacement fuel pumps. And if that's the case, then the fuel pressure regulator could be seeing higher primary pressure than normal, and may be putting out a correspondingly higher secondary pressure to your injectors. In effect you are boosting the system pressure with an over pressurized fuel tank. Until I fixed it, my Spiders idle was erratic and it would even die. There's a diagram of the venting system on our CD, but Mike has it with his GTV 6 at college. Maybe I can find it with a search...
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post #45 of 301 (permalink) Old 02-20-2017, 07:36 PM
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Quote:
Tank pressure would be additive to pump output pressure because they are positive displacement fuel pumps. And if that's the case, then the fuel pressure regulator could be seeing higher primary pressure than normal, and may be putting out a correspondingly higher secondary pressure to your injectors.
That is exactly what was happening to my son's Verde which I referred to in an earlier post. I have had a similar problem with my Spider which has Webers. I was data logging WOT runs and adjusting the jetting and no matter what I did it got steadily richer. The fuel tank vent was insufficient for the rising pressure in the fuel tank as it warmed up. It now vents directly under the spare wheel well, like a race car.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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