Hey all. 98 1500, 5.2L auto.

So this started Saturday afternoon as I was driving to work. Once I hit the freeway I started to feel a random shuddering. It wasn't constant. It just felt like it was stumbling occasionally. It's of note that I was maintaining a steady speed of about 70 mph, in overdrive, and the RPMs were running right about 2k as normal. My first thought was something in the driveline, but if that were the case, say a ujoint going south, it would be more constant and more noticeable while changing speeds, not while running steady. I watched the gauges and nothing changes when the shudder occurs. RPMs stay steady, oil pressure, temp, etc. it sort of feels like it's coming from the front of the vehicle but it's hard to tell for certain. It also does not seem to be doing this at low speeds.

Where this gets stranger is it only seems to be doing this in the mornings on my way into work. At night when I am driving home it either does not do it at all or it's barely noticeable. During the day it's a strong shudder. The only difference I can think of apart from time of day, light and dark, and the headlights being on is that in the morning my truck has spent the night parked at an angle in my driveway. It's not super steep but it is an incline. During the day it sits in a level parking lot at work.

It also has been idling a bit rough, but again only in the mornings and only the first few minutes it is running. At a stop the RPMs will drop to about 500 and it feels like it's going to stall.

The only things I have done on the truck recently was an oil change and dump a bottle of prestone fuel system cleaner in the tank. That was done Thursday, 3 days before this began. I used 10w-40 full synthetic as usual and a can of engine restore also as usual.

I would greatly appreciate any thoughts anyone has as to what to look into for the cause of this shuddering.

 

shudders at freeway speeds have been known to indicate a misfire in one of the cylinders. Does it only do it past 45ish mph when it drops into OD? Any check engine light popping up? Whens the last time you did a tune up to it? Plenum been fixed? If the plenum hasn't been fixed, I'd address that first and do a full tune up while you're in there.

 

Seems to only do it above 60 mph or so. Not throwing any codes at all. I did get a code a few weeks ago for the auto shut down relay but I replaced that when it happened. This shudder only started three days ago. Last tune up was maybe 8 months ago or so. The plenum has been addressed a few years back. I thought it may be a misfire as well perhaps caused by the fuel system cleaner. But then it would be throwing a misfire code and it has not.

 

That's what mine did when the plenum was bad, but if yours is fixed..
Maybe the cat is clogged?

 

Well the plot thickens.

Last night when I went to start it to head home from work it finally threw a CEL. Broke out my pocket scan and ripped the code P1296. MAP sensor voltage. I went and re read the thread regarding the plenum and that is sounding more and more like my culprit. So to confirm this I just pulled the number 8 spark plug



Yep, its fouled. So I called my buddy who is the mechanic that last worked on the truck. I remember asking him at the time about the plenum as he had to remove the top end to do some work, turns out that he did indeed replace the plenum plate, but with a Stock replacement piece, I.E. steel. So looks like I get to tear my top end apart here pretty soon. I am ordering the Hughes kit now.

My question is will it be ok to continue driving it until the parts come in and I can install them? (probably a week)

 

Yeah the steel plate is your culprit. Some guys still use the steel plate and put a layer of some sort of RTV flavor on both sides of the gasket, but just get the hughes kit in my opinion. Aluminum plate and everything you need to do it all is included. I'd say throw a new spark plug in there and see how it does for the week. We've all ran our trucks for miles upon miles with a blown plenum so you should be ok. Also Like mentioned before, your cat is probably bad now too.

 

well last time i had it smoked the tech told me the cat was probably on its last leg anyway. Only bitch about that is they are a bitch to get here in Commiefornia now. Just had to overpay to get a CARB approved replacement for my Jeep too.

 

You can still drive it. Ask him how long he did the OEM gasket work.

Here is my experience with the OEM gasket work this year.

https://dodgeforum.com/forum/3084644-post162.html

I'm not preaching it's better/worse. I've installed the Hughes kit on a friends rig w/o issue after almost 2 years. I'm not aware of any issues with that one. I have no experience with the one on Ebay.

You really won't know for sure until you've taken the intake off and inspected the gasket. Right now, it's just an assumption based on your spark plug. I'd do a compression test before buying anything.

 

i know when he did it. truck was in his shop about 4 years ago for another issue that involved removing the top end to get into and fix.

Im basing the assumption of the bad plenum on more than just the fouled plug too, that was just the last straw to confirm it. From what I have read the plenum will affect the MAP sensor (code it threw last night), foul the number 8 plug, and cause mystery oil loss, which I have also been having for a while now but never really thought much about. the running symptoms of the low idle also points to that being an issue

 

I got the Hughes kit, and it's held up fine (so far). I've only had it installed for about 2 weeks now.
I think you can drive with a bad plenum, but if the cat is clogged I wouldn't recommend driving it.