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Old 05-06-2012, 04:46 AM
thcardoc thcardoc is offline
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Stalls Randomly DC-V

This is an older car today, but at the time it was less than two years old. The owner reported that the vehicle cuts out or stalls. It can happen anytime, at idle, cruising on the highway,or accelerating. It may do it repeatedly through the day or it could go two or three days without the problem occurring at all. When it happens it might restart easily, or be difficult to get it restarted and it may have to sit and then everything is fine again. Sometimes it cuts out on the highway, but starts running again without it ever slowing down.

There have been four previous attempts to fix this, now it's up to you to figure it out and fix it once and for all. How do you start your testing to first narrow down the list of possible causes to at least the system causing the shutdown? We have to do that first, then we have to figure out how to prove exactly what the failure is and fix it.

The car is a 1985 Chevrolet Blazer with a 5.0l. Carbureted.
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Old 05-06-2012, 02:58 PM
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At first I would assume that it would be a fuel pump going bad because that same problem happened to me over a period of month before it finally quit.

But it sounds like an ignition problem. I would check the pickup coil and then the coil itself and all connections as well. Also, a fusible link could be loose at the starter or just acting up (going bad) which is unlikely. Because when they go bad, you're dead in the water.

A lot of people swear that they can check a module and if it shows good, they believe it. I myself have checked many modules with a tester and it would show that it was good, but be bad. I've also checked good modules (new ones straight out of the box) and the tester would show that it's bad and it would be good. Modules can be very sporadic and it's the only ignition part that I don't mind changing regardless of test results.

Now that being said, unless the module testers were changed (make them better) I have absolutely no faith in them.

A coil wire could have a bare spot on it that's barely visible shorting out against something.

If I couldn't find an ignition problem, I would check my fuel (carburetor, fuel lines, gas filters, sending unit in the gas tank (pickup tube and sending unit are together) etc. etc.

Here's one more thing that hasn't happened in a long time. Vapor locking! Fuel lines running to close to exhaust. Does this vehicle have dual gas tanks? The switch or the splitter separating the two tanks could be going bad not letting fuel to the carburetor.
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Old 05-06-2012, 03:18 PM
thcardoc thcardoc is offline
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Hi Dan.

OK that's a great start, you have a good idea of what the failure could be. So you hop in and go for a ride. About fifteen minutes later your driving and the symptom occurs. You notice the engine abruptly cuts off, but resumes running almost instantly. It occurs one or two more times as you head back towards the garage leaving you hoping you make it back as much as wishing it would simply die and stay dead long enough to get to prove what is going on.

Based on your initial considerations, which ones are still in play, and which ones are ruled out?

What could you do to narrow the possibilites further the next time out the door?
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Old 05-06-2012, 03:23 PM
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I forgot the ignition switch at the bottom of the steering column. And, might want to check the neutral safety switch.

*Edit* It sounds like the hot wire (12 volt) going to the distributor has a loose connection or a naked spot on the wire itself touching metal causing the distributor to loose fire. If all I've mentioned above checks out, I would start tracing all wires that has anything to do with ignition and then I would start electrical wires such as battery cables making sure they're not shortening out against metal.

When the engine stops, did he still have current going to the accessories?
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Last edited by Daniel Wood; 05-06-2012 at 04:39 PM.
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Old 05-06-2012, 05:01 PM
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Yes all the accessories still have power while the engine cuts out.

For those following along, feel free to jump in anytime with your try at what could be wrong, and/or how you would test from here to possibly narrow down and identify the cause of the problem.

Last edited by thcardoc; 05-06-2012 at 06:51 PM.
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Old 05-06-2012, 07:06 PM
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Does this vehicle have an external electric fuel pump?
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Old 05-06-2012, 07:41 PM
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When it cuts out, hold it to the floor, as the engine keeps turning. When it cuts back in again, one of two things will happen. 1) it sputters and starts running (fuel problem). 2) when it starts running again, it lights all the fuel pumped into the exhaust causing a very load explosion, possibly splitting the muffler in two (ignition problem).

As soon as it cuts out, toss it into neutral and switch off the key. Pull off road. Open hood. Open air cleaner. Pump throttle and see if carb is empty (no squirt). If accelerator pump squirts fuel out, its most likely an ignition problem. If there is no fuel it is definitely a fuel problem.
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Old 05-07-2012, 06:15 AM
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Originally Posted by olddog View Post
When it cuts out, hold it to the floor, as the engine keeps turning. When it cuts back in again, one of two things will happen. 1) it sputters and starts running (fuel problem). 2) when it starts running again, it lights all the fuel pumped into the exhaust causing a very load explosion, possibly splitting the muffler in two (ignition problem).
LOL, well that's not the kind of testing I'd like to promote but it would be accurate and the result if it occurred would be #2, hopefully without the car now requiring a new muffler. If that happened, you'd be paying for the muffler and it's bad enough that as a technician working in a shop your already putting in un-billable tiime.

Quote:
As soon as it cuts out, toss it into neutral and switch off the key. Pull off road. Open hood. Open air cleaner. Pump throttle and see if carb is empty (no squirt). If accelerator pump squirts fuel out, its most likely an ignition problem. If there is no fuel it is definitely a fuel problem.
Along with your response here the symptom is identifying itself as an ignition problem, but as Dan was considering, is it on the power side, the tach side, a component issue of some type, or where? Today we typically have equipment that could make proving this much more efficient, back in '87 a lot of the toys we use today simply didn't exist. So the next step is to start proving what is failing that is causing the shutting down the ignition. Who has a plan and what are you looking for as you set it up?

One of the biggest traps is you could search and find an issue visually and fix "A Problem", but the random nature of the failure could leave you without proof that you fixed "The Problem" with the exception of it occurrig again which would only prove that it's not fixed yet.
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Old 05-07-2012, 06:46 PM
olddog olddog is offline
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I'm thinking Daniel most likely already guessed it, but there is no fun in that.

So ignition is the problem. It could be loosing the 12V positive to the coil (I doubt it) or the switching side (negative side of coil to ground) is not working.

Put a meter on the positive side of the coil and see if you still have 12 volts when it quits. This will tell you which direction to look.

PS:
Just in case it is the coil, it wouldn't hurt to measure the primary and secondary with an ohm meter. Hot and cold. Wonder where we could find a spec on that coil to compare it to? Oh that's OK, I know you have a manual.

Last edited by olddog; 05-07-2012 at 06:51 PM.
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Old 05-08-2012, 06:07 AM
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I'm thinking Daniel most likely already guessed it, but there is no fun in that.
In fact he did, but like you say there's no fun in that and just because you find "a problem", that doesn't always guarantee that your have repaired "the problem".

Quote:
So ignition is the problem. It could be loosing the 12V positive to the coil (I doubt it) or the switching side (negative side of coil to ground) is not working.

Put a meter on the positive side of the coil and see if you still have 12 volts when it quits. This will tell you which direction to look.
The meter can show you the voltage, but does it update fast enough to show you if it quickly drops off but comes right back? Sometimes the good old testlight is still the right tool. Today we'd be headed out the door with an oscilloscope connected to the positive side of the coil, the tach or negative side, and a low amps probe monitoring the current through the whole ignition circuit.

During a failure you would see the voltage to the circuit drop. If you could do four traces you would in fact see drops in the voltage to the coil while system (battery) voltage stayed at it's normal 14.5v range.

Quote:
PS:
Just in case it is the coil, it wouldn't hurt to measure the primary and secondary with an ohm meter. Hot and cold. Wonder where we could find a spec on that coil to compare it to? Oh that's OK, I know you have a manual.
Measuring primary and secondary winding resistance is how this was taught, and that can show you if you have a completly dead circuit, it cannot however reliably show you if the circuit is good.

Resistance checks are static in nature and are generally time poorly spent.

OK, your losing voltage to the coil, what's next?
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Old 05-08-2012, 02:26 PM
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Originally Posted by thcardoc View Post
In fact he did, but like you say there's no fun in that and just because you find "a problem", that doesn't always guarantee that your have repaired "the problem".
Actually, I just went through the process of limitations (not replacing parts just to see if it would fix the problem with the exception of the module). You never did say what the problem was.
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Old 05-08-2012, 07:29 PM
olddog olddog is offline
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Voltage to the coil changing is likely a bad contact on a switch or a loose wire. I suppose there could be some safety interlock system. I had a car that low oil pressure shut off the fuel pump, so a bad oil pressure sending unit caused a loss of fuel. So a electrical diagram should be consulted at this point. Then check all the connections and switches.

As for the meter response, I have been pretty spoiled with good ones. You can set them to remember (trap) the highest and lowest voltage. Not sure what the update rate is but I think they are around 100 times a second. I'm certain they are at least 10 times a second.
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Old 05-08-2012, 07:47 PM
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I hold the solution back as long as possible to give people time to think through the steps. Diagnostics is a combination of observation, investigation, research, and then test and prove. Sometimes the steps take place in seconds, and there are others that can really test someones patience and committment.

This car had an aftermarket alarm system installed in it that disabled the ignition system by opening up the power feed to the coil. However, they had trouble with the system and went back and had it removed. The person that removed the alarm used a crimp connector to put the coil feed back together and tucked it back into the harness. It took a year and then it started losing it's connection causing the symptom as reported. It made for a real challenging diagnosis because we didn't get paid diagnostic time. (Most techs still don't get paid properly but I'll leave that for another day) Three other techs took a shot at this thing and had already replaced the usual suspects, (ignition module, pick up coil etc.) But when things lik this ended up in my stall, I kept them until they were fixed. This thing didn't even act up the first two times that I drove it, and might only throw a hint of the problem once or twice when it did. Today I'd have an oscilloscope on the key signals but we didn't have those kinds of tools back in the 80's at least not the kind we could hook up and drive down the highway with.

Anyway, your first impression of the symptom had you question the possibility that the coil was losing power, and that of course was the solution. The one trap we can sometimes fall into is you could have made that assumption, investigated and found the crimp connector in the harness and replaced it without ever having experienced the problem or measuring the loss of voltage. Then should the problem not actually have been fixed you'd be treated as if you did something wrong.

It's great to see some more people take a crack at these, again everyone is welcome to try. Watch for the next one in a week or two.
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Old 05-08-2012, 08:26 PM
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Electrical problems can drive you nuts to begin with. But when someone jerryrigge's something like that. You really want to take them out and beat the living daylights out of them. That's what a soldering gun, solder and heat shrinking tube is for. Not these cheap connectors!

That really chaps me when someone does something like that.

I'm retired now and I no longer have the luxury of the modern-day equipment that everyone uses. I was a mechanic before I change careers to do body work and that was a passion that I truly love.

I heard this saying once and then I heard it again the other night on TV. If you find a job (profession) that you really like or love, you'll never work a day in your life. That may sound corny to some people, but I was like a kid in a toy factory. I missed it to.
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Old 05-08-2012, 08:34 PM
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Quote:
Originally Posted by Daniel Wood View Post
Electrical problems can drive you nuts to begin with. But when someone jerryrigge's something like that. You really want to take them out and beat the living daylights out of them. That's what a soldering gun, solder and heat shrinking tube is for. Not these cheap connectors!
Ditto......

Quote:

I heard this saying once and then I heard it again the other night on TV. If you find a job (profession) that you really like or love, you'll never work a day in your life. That may sound corny to some people, but I was like a kid in a toy factory. I missed it to.
I live by that, I darn sure ain't doing this for the money.... (two jobs, the shop and teaching, seven days a week, never gonna retire till they close the lid)
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