Cranks and idles fine. After about 5 minutes stumbles and backfires. starts jerking with acceleration. 1998 5.2 Magnum

[heyJeff-dodgeThis]

1998 5.2 Magnum New Plugs, wires, distributer cap and button. New PVC valve. Cranks perfectly, idles fine. But when I get down the road about 5 minutes it starts to backfire and stumble as it gets warmer it seems a little worse. It backfires and starts jerking when I am accelerating and at 40mph it jerks horribly when I give it gas!
I pulled the vacuum lines off the valve covers and checked for suction and it has a HUGE amount of suction while idling.
I know that the plenum gasket is very common to fail especially this old. I am trying to do cheaper, mainly easier things that it could be first before diving into such a big job on it. I just bought the truck last week so i am still learning about it.

What, in your experience are some easier things to rule out first by the symptoms I have given? I did put a new PCV valve on it this morning and it ran better even though it didn't stop the above listed symptoms. But strangely, (to me at least), the check engine light came on for the first time since I have had it after putting in the new PCV valve. It stays solid until I accelerate then the check engine starts flashing, until I am idling again. That is strange to me.

Would this be a good time to invest -$50 in an OBD scan tool? Would it be better to go the route of using the needed adapter and software instead of the scan tool? I appreciate your help!

 

[Tank Guy]

The check engine light will come on for a lot of minor things, like a leaky evap system, but it'll flash when something major is happening and it really wants to get your attention. You should be able to go to AutoZone or similar places and use their scan tool to see what codes have been generated, and that'll give you an idea where to start looking. Years ago my 02 Dodge Intrepid would start jerking the way you described and the problem was a faulty cam position sensor. The code reader I purchased (from AutoZone I might add) identified it as the problem and it was easy to replace. I've had the car since 2004 and that sensor has now failed on me twice, each time causing a problem similar to what you're describing. I can't say with any certainty that it's the same problem you're having, but start with a code reader and see where that leads you. It really makes no sense to own a modern car without one.

As to the issue showing itself as the engine warms up, expansion occurs in just about everything as it warms or heats up. This means a connection within one of your sensors (or anyplace else ) that's good when cold can become faulty at normal operating temps on a failing component.

 

[heyJeff-dodgeThis]

Thank you for replying. I will definitely keep that in mind on the cam sensor. I'm going to do some comparing the traditional scan tool vs a software and laptop version and get something next couple of days. Thank you again with your experiance.

 

[Tank Guy]

No problem. I was editing my comment when you replied. Just want to make sure you saw the edit.

 

[Daw14]

Absolutely find a way to check for codes .
Try to stick with OEM sensors , for any vehicle you might work on .
Using what the manufacturer has designed for will give you the most chance for a successful repair.
Aftermarket sensors may cost less , however, if you end up sitting on the roadside again , nothing saved.

 

[heyJeff-dodgeThis]

For sure. I want quality parts to settle this. Thank you.

 

[MrBonez]

Seems to me it's stuck in cold start fuel mode, meaning a sensor isn't working right and it's still throwing fuel to the engine as if it's still cold.
Basically all gas engines don't need as much fuel to run once they get some heat in them, an over-rich fuel mix can and will cause such things.

If you get a code about something affecting air/fuel mix, don't be suprised about it. Also make sure things like the air filter aren't bad and in need of replacement.
You can even pull a spark plug and see how it looks, if it's really dark/sooty that will more or less confirm the diagnosis of an over-rich fuel condition.

 

[Yeret]

Engine running fine when cold then rough when warm is a pretty telltale sign of a faulty upstream oxygen sensor. Clogged catalytic converter can do it too, although that's more noticeable as the engine will massively struggle for power, if it'll even run at all.

Back when my plenum gasket was blown, the spark plugs would constantly foul, and eventually the engine would misfire pretty badly whenever the transmission shifted into overdrive (around 40 MPH under light throttle). That was my bi-annual reminder to replace the spark plugs. Oh, and spark knock. So much spark knock... If *****'d spark plugs were a body count, then my truck was Arnie after a half hour of mowing down bad guys with a machine gun before I finally got the plenum fixed.

 

[Tominator223]

Coolant temp sensor for pcm. Can cause rich condition. Not the temp gauge sensor.

 

[MrBonez]

Coolant temp sensor for pcm. Can cause rich condition. Not the temp gauge sensor.

I agree since all the temp sensor really does is report what the other sensors are detecting/seeing and displaying it as a reading on the gauge you can see yourself.
Sensors that actually control air/fuel mix is what you'd need to be checking here along with other things as suggested in above posts.

 

[heyJeff-dodgeThis]

Seems to me it's stuck in cold start fuel mode, meaning a sensor isn't working right and it's still throwing fuel to the engine as if it's still cold.
Basically all gas engines don't need as much fuel to run once they get some heat in them, an over-rich fuel mix can and will cause such things.

If you get a code about something affecting air/fuel mix, don't be suprised about it. Also make sure things like the air filter aren't bad and in need of replacement.
You can even pull a spark plug and see how it looks, if it's really dark/sooty that will more or less confirm the diagnosis of an over-rich fuel condition.

I'm not getting any fuel codes. I am only getting "random miss fire", "miss fire on 8", and miss fire on 6th" cylinders. I did get a scan tool, but I am not really happy with it. I may change that out for a different one. Somehow this got past my notifications that's why I haven't replied in a timely manor.
Replaced the coil, air filter and things that are common. Replaced the 6 and 8 plugs. I really just was putting off the inevitable. I am getting things ready to replace the plenum plate and gasket and I think I will put new valve cover gaskets on it. The motor was rebuilt at 80k miles. I am getting vacuum when I pulled off the the PCV vale and plugged off the other side as well. I am getting a lot of vacuum from the vale covers. I took off the breather, (that I hate) and looked in the throttle body and there pooling in the bottom of the plenum. All signs of needing replacing from everything I have been able to research. Any other ideas would be great and thank you for the help already. I too am in central Alabama, small world. It would be funny if we are right down the road haha. I am going to make another post in a minute with new questions or at least more questions.