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Misfire under load?

Tiny
THEROG
  • MEMBER
  • 1993 DODGE RAM
  • 5.2L
  • V8
  • 2WD
  • AUTOMATIC
  • 162,000 MILES
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.
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Saturday, May 20th, 2023 AT 5:31 AM

19 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
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

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Saturday, May 20th, 2023 AT 9:56 PM
Tiny
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  • MEMBER
  • 17 POSTS
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.
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Sunday, May 21st, 2023 AT 7:01 AM
Tiny
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  • 17 POSTS
Got the codes, so there is an air intake voltage code and a fuel injector electrical malfunction code.
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Sunday, May 21st, 2023 AT 2:12 PM
Tiny
THEROG
  • MEMBER
  • 17 POSTS
Intake air temperature sensor voltage code is what I was trying to say.
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Sunday, May 21st, 2023 AT 2:15 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
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
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Sunday, May 21st, 2023 AT 7:58 PM
Tiny
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  • MEMBER
  • 17 POSTS
17, I don't have a thermostat
21, o2 sensor
23, air temp sensor
27, injector circuit
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Monday, May 22nd, 2023 AT 6:08 AM
Tiny
THEROG
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  • 17 POSTS
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.
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Monday, May 22nd, 2023 AT 6:10 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
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.
Images (Click to make bigger)
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Monday, May 22nd, 2023 AT 7:29 PM
Tiny
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  • 17 POSTS
Haynes Manual list of codes from flash of mil.
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Tuesday, May 23rd, 2023 AT 6:21 AM
Tiny
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  • 17 POSTS
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.
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Tuesday, May 23rd, 2023 AT 6:22 AM
Tiny
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Yes I can do those test.
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Tuesday, May 23rd, 2023 AT 2:29 PM
Tiny
THEROG
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  • 17 POSTS
There is power to injector and the ohms are good.
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Tuesday, May 23rd, 2023 AT 5:27 PM
Tiny
THEROG
  • MEMBER
  • 17 POSTS
Voltage is full strength.
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Tuesday, May 23rd, 2023 AT 5:28 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Is there an injector pulse if you check for ground via the opposite wire than power?

Joe
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Tuesday, May 23rd, 2023 AT 9:17 PM
Tiny
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  • MEMBER
  • 17 POSTS
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.
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Wednesday, May 24th, 2023 AT 3:50 AM
Tiny
THEROG
  • MEMBER
  • 17 POSTS
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?
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Wednesday, May 24th, 2023 AT 5:01 AM
Tiny
JACOBANDNICKOLAS
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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
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Wednesday, May 24th, 2023 AT 6:53 PM
Tiny
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No, I didn't disconnect PCM as of unfamiliarity with where to do so, I will readdress it this evening.
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Thursday, May 25th, 2023 AT 5:36 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

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

Take care,

Joe
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Thursday, May 25th, 2023 AT 7:36 PM

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