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Emergency - Can a mechanical fuel pump fail intermittently?

23396 Views 119 Replies 5 Participants Last post by  av8r115
Hey guys,

My '87 3.9L Dakota just quit on me and I've got to get it fixed TODAY. I think it's the fuel pump, but let me know if you think it's something else.

Going down the road the engine just died - no sputtering or anything. BUT when I pumped the gas it ran briefly, less than a second. That was while I was still coasting in Drive. It did that the first 2 or 3 times I pumped the gas, but not again.

After sitting about 45 minutes, it started after cranking for a couple of seconds.

I think the carb is being starved of fuel, the brief seconds it ran were when the accelerator pump shot fuel from the bowl (assuming this carb has a bowl), but then ran the bowl dry. I think that's also why, when it did start again, it took a few seconds of cranking.

But I don't understand why it would start again after sitting a few minutes. That's what makes me unsure if its the fuel pump or not.

The fuel filter was replaced in August.

If I was flush with cash I'd just slap a new pump and filter on it. Right now I've got be careful what I spend the $$ on.

I thought about water in the gas, but wouldn't that make it sputter for a while before it just quit completely?
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You really would have to have a fuel pressure gauge spliced in to measure the pump pressure to diagnose it. Or disconnect the fuel line and crank the engine (with primary coil disabled) with the fuel line inside a glass jar, and observe whether it delivers constant fuel supply.

Very possible that it's flooding, and that pumping the gas gave it more air to compensate by opening the throttle plate. And that's why it would take 45 minutes for it all to evaporate before it would start.

Flooding vs fuel starvation should be easy to tell, by removing the air cleaner lid and smelling for gas. If none, starvation. If heavy odor, flooding.
That's running way too rich. I suspect a carb rebuild is necessary.

Autozone and other parts stores can read your fault codes for free, with a code scanner; so the Check Engine light doesn't have to be working.
I just realized, with the 3.9L V-6, we have the old problems of the worn-out timing chain guide, and also the bad distributor bushing. Have you replaced either one of these? How many miles on the truck? Normally they're shot between 100K and 150k miles.
av8r115 said:
Looks like I need to go ahead and replace the pickup and coil. Sounds like that would not only fix the problem w/ the engine quiting, but would improve my MPG as well. Money well spent.

The truck just crossed 220K miles on it. Given the condition of the truck when I got it, I'm willing to bet neither the timing chain or distributor bushing have been replaced.
Won't do any good to replace the pickup if you don't also replace the timing chain and guide, and the distributor bushing. There will be slop and wobble that will make the pickup intermittent or make it hit the shutter. It's possible that the pickup itself is still good, along with the coil.
1992 is the start of the Magnum, my truck died because of this, at less than 100K. True, there is no fuel injector sync, but the ignition can cut out. And timing chain wear on these engines does pre-date the Magnum. Regardless, at that mileage, there WILL be slop.
Problem is, it's not only the rotational slop, but the side play that will kill your ignition. As that bushing wears, the distributor shaft wobbles. This in turn causes the gap between the reluctor teeth and the pickup coil to vary, which changes the field strength and makes the ignition triggering intermittent. Back in the 1970s it was not uncommon for the reluctor to loose a tooth or shear off the pickup, then the car would just die.
Primary coil resistance should be less than about 1.3 ohms, typically about 0.8 to 1.3 ohms. That's across the primary - the positive and negative terminals.

Secondary coil resistance is from center tower to the primary positive terminal. Should be between about 11K ohms and 13 K ohms.
No. Do you have the old cylindrical shape coil, or the newer square one that bolts up to the front passenger side of the engine? The older has ring tongue terminals to the primary, the newer one has a modular 2-wire plug. The terminals on the older one are much easier to probe, but you should be able to get in to the newer one. Is this a digital or analog multimeter? Is it set on the right range for resistance?
Does no reading mean the display is blank, or does it mean infinity (open circuit)? If the latter, then the coil is open and is bad.
Simple household test. Take an incandescent bulb that has cooled off and measure across its contacts (between the threads and the center contact). When cold, should be about 0.5 ohms or so.
Sounds likely.
There is a special O2 sensor socket that you can essentially rent at Autozone - they have a policy where you can return it for full refund when done. The socket has a slot up the side to accommodate the wiring. I would NOT use a universal sensor for several reasons:

1) You have to splice the wiring, and can't ever use an OEM fit again.
2) The brands are often inferior. Bosch invented the O2 sensor, and I've had good luck, but it's 1/8" longer than OEM, and the socket won't reach completely over it, and will round off the hex. Others report poor performance with Bosch. Bosch uses four long slots in the sensor tip to sense the exhaust gas, where other brands such as NTK and Denso use 16 holes placed all around the tip for more uniform exposure to the gases.

Denso for this car is only about $47, well worth the price. At one time they were $70 to $90.

Critical: Do NOT get the anti-seize compound that is supplied with it on the tip of the sensor - make sure it's on the threads only. The compound contains sulfur compounds that will destroy the sensor quickly.

Be careful removing it. If it's stuck, run the car until warm, then as it's cooling down, spray PB Blaster or similar penetrating oil around the base of the sensor while the car is cooling down. It will be drawn into the threads while the sensor body contracts. Repeat a few times if necessary.

Having said all that, with no O2 sensor fault code, I'd check other things first. Did you check all of the ground connections?
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Coil is within spec.

O.E. Replacement

Resistance in Ohms:
Primary 200 Scale 1.2 - 1.9
Secondary 200 Scale 6.5k - 12.5k
Actually, federal law prohibits doing away with the catcon, even if you don't have emissions testing. And the loss of backpressure may adversely affect performance. Certainly it will affect the noise of the exhaust. I realize they are expensive, but they do serve a purpose. And they cut emissions levels of hydrocarbons as much as 10x.
Umm....NOTHING in my post has anything to do with global warming. Or freon. Or greenhouses. Or ozone.

Excessive hydrocarbon emissions are proven to cause smog, which in turn causes and aggravates asthma and other lung diseases, as well. Gasoline contains benzene, which is a known carcinogen. If you are over 45 years old, you'd know what a nightmare Los Angeles and other cities were until the Clean Air Act was passed in 1970. It's orders of magnitude better than it was then.

The muffler shop may know the law, and they could be fined if caught, but IF is the operative word. Here in MA, if you sell a car that does not comply, you are legally responsible for the cost of restoring it. Other states may have similar laws.
I'm just advising you that eliminating the catalytic converter is illegal everywhere in the US. What you choose to do is at your own risk; $25,00 fine for the shop, $2,500 for the vehicle owner. Page 237, Section 203 (a)(3)(A) and (3)(B)

Penalties under page 239, Section 205.
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