misfire under load?

1993 DODGE RAM
162,000 MILES • 5.2L • V8 • 2WD • AUTOMATIC
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THEROG
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Just recently after replacing the fuel pump the truck developed an occasional misfire; it will smooth back out.
After going at expressway speeds, then going at normal intersectional road speeds the truck will begin to misfire, sometimes more than 1 cylinder, can feel it being worse at times than others.
This happens even if the truck has been turned off and/or fueled up in between the changes in load.
but only from fast to slow not from slow to fast.

over the past year it has had developed a die off after going uphill at very low speeds and stopping at top.
i have replaced all cylinder ignition electrical items: plugs, coil, wires, rotor pin and cap; sensors: throttle, map, idle-air, mani-temp, fuel pressure. have checked and replaced or stopped all vac breaks. most recently within the past 3 months the fuel filter, and within the last 2 weeks the fuel pump. it does take a second to get the truck to turn over like the fuel is not getting there immediately.
May 19, 2023 at 10:31 PM
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JACOBANDNICKOLAS
  • Experienced Mechanic
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Hi,

If there is a delayed start, you may have a fuel injector or injectors leaking and causing the pressure to drop. That could cause a misfire.

I realize the fuel pump has been replaced, but did you check the pressure to make sure it is within the manufacturer's specifications? If not, do it and pay attention to how quickly it drops off.

Here is a link you may find helpful; it explains how to test pressure.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Also, if you haven't checked for diagnostic trouble codes, do this as well.

Here is a link that explains how it's done:

https://www.2carpros.com/articles/retrieve-trouble-codes-for-chrysler-dodge-plymouth-odb1-1995-and-earlier-car-mini-van-and-light-trucks

This truck has an OBD1 system, so you won't need a scan tool.

Let me know what you find or if you have questions.

Take care,

joe

May 20, 2023 at 2:56 PM
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THEROG
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i was leaning towards the injectors, next. It is the only thing i haven't, yet. The delayed start is what made me go ahead and replace the pump, thinking it would fix the delayed start and choke out at hilltop stops. but that caused it to not fix either and develop the miss.
May 21, 2023 at 12:01 AM
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THEROG
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got the codes, so there is an air intake voltage code and a fuel injector electrical malfunction code.
May 21, 2023 at 7:12 AM
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THEROG
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intake air temperature sensor voltage code is what i was trying to say.
May 21, 2023 at 7:15 AM
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JACOBANDNICKOLAS
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Hi,

Do me a favor. Let me know what the codes are you found. That way, I can dig through the manuals and see exactly what is happening.

Let me know.

Joe
May 21, 2023 at 12:58 PM
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THEROG
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17, I don't have a thermostat
21, o2 sensor
23, air temp sensor
27, injector circuit
May 21, 2023 at 11:08 PM
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THEROG
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i also don't have o2 sensor, i guess I'll grab one, but this would be my beater that actually had issues w/o.
May 21, 2023 at 11:10 PM
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JACOBANDNICKOLAS
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Hi,

The misfire is likely from the cylinder 4 fuel injector. (Code 27) Code 23 is related to the idle air control. I suspect code 27 is where we need to focus if there is a misfire.

I attached the diagnostics below specific to the code (27). Take a look through them and let me know if you are comfortable performing the tests.

Let me know.

Joe

See pics below. Also, look at pic 5. Where did you get the info related to the air temperature sensor? I want to make sure I'm not missing something.

May 22, 2023 at 12:29 PM
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THEROG
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haynes Manual list of codes from flash of mil.
May 22, 2023 at 11:21 PM
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THEROG
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i replaced 1 air temperature sensor on front of the manifold last year, but i think I noticed 1 back of EGR valve the other day.
May 22, 2023 at 11:22 PM
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THEROG
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yes i can do those test.
May 23, 2023 at 7:29 AM
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THEROG
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There is power to injector and the ohms are good.
May 23, 2023 at 10:27 AM
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THEROG
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voltage is full strength.
May 23, 2023 at 10:28 AM
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JACOBANDNICKOLAS
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Is there an injector pulse if you check for ground via the opposite wire than power?

Joe
May 23, 2023 at 2:17 PM
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THEROG
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No but i can hear the tick via stethoscope.
i will check again when it starts missing on drive to work,
it is happening now after about 15 minutes driving but still only after slower speeds.
May 23, 2023 at 8:50 PM
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THEROG
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so after driving to work, there was no occurrence of misfire.
i did leave the air filter assembly off.
i rechecked the mil codes and got a 27 twice.
i do get a 12 and it has been more than 50 times since the last time i disconnected the battery, but i read that it was just the sequence start code, is that true?
May 23, 2023 at 10:01 PM
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JACOBANDNICKOLAS
  • Experienced Mechanic
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Hi,

Something is happening with the cylinder 4 injector or circuit. When you did the tests above, did you have the PCM disconnected and checked the resistance in the wiring as indicated? If you did and it was within spec, the PCM itself is suspect.

Let me know.

Joe
May 24, 2023 at 11:53 AM
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THEROG
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no, i didn't disconnect PCM as of unfamiliarity with where to do so, i will readdress it this evening.
May 24, 2023 at 10:36 PM
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JACOBANDNICKOLAS
  • Experienced Mechanic
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Hi,

Let me know what you find. If it is connected, that could skew the results.

Take care,

Joe
May 25, 2023 at 12:36 PM
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