Cylinder 8 misfire

[Yeret]

Well guys, I'm having a bit of a problem here. Seems cylinder 8 has become El Grumpo lately.

I'm getting this incessant P0308 code and have gone through 3 spark plugs in the last two months.

I'm arranging with my mechanic to help with the intake plenum repair, though him and I work opposite times of day and he pretty much disappears on the weekends, but we'll get there.

Catalytic converter was replaced last year with exhaust work. Just for the hell of it, I did the basic test and gunned the engine at idle. She ran north of 3,000 RPM no problemo.

This is totally befuddling me. I've been driving the truck for 3 years now and never had such a problem even with the plenum needing to be repaired the whole time.

The last plug I pulled out had carbon deposits bridging the side of the ground electrode and the insulator. It also had a scent of burnt charcoal. I haven't checked the current plug yet.

Has my intake plenum gotten to the point where I'm just going to immediately foul every plug I throw in there until it's fixed? Or could I be looking at something else?

 

[dodge dude94]

Compression test time

 

[Merc225hp]

What he said.

 

[Yeret]

Ah man, was afraid that might pop up. Truck's been particularly sluggish on the hills lately too.

I'd Google it, but since I'm here, I'll just ask. What range should the cylinder pressures be? I believe +/- 10% between the strongest and weakest is OK?

 

[Merc225hp]

Min is 110. You have the percentage right. Start looking for a set of heads or better a long block.

 

[Yeret]

I should have the truck in my mechanic's shop next weekend for the plenum repair (since that needs to be done no matter what) and we'll test compression then.

She used to burp a little oil smoke at startup, but now she belts out a huge cloud. Unsurprisingly, my oil consumption has increased.

The other day on the way to work, the engine was running fine until I pulled off the highway. Then it started misfiring so bad that it felt like the whole truck was gonna shake apart!

Got no power on the hills. Even getting a run for them, I still have to bury the gas. When I do this, I can hear some kind of droning sound coming from the engine bay. Is this something of concern or is it just noises from my open-element intake? I never used to have to be so heavy on the gas after all, LOL.

Assuming a bad head, are the Odessa one's from eBay still a good buy? I don't recall a pair being $570 (thought they were cheaper a few months ago) but they still sound like nice castings.

Also, maybe worth noting. Last year, my catalytic converter plugged and circumstances forced me to run the engine much longer than I cared to (local "reputable" mechanic decided to stiff me on the job even after I drove a half hour to his shop). Not long afterwards and I noticed a significant exhaust leak in one of the manifolds. What's the likelihood of a bent valve?

 

[Yeret]

Plugged my scanner in today just to see if anything else may have arisen. Looks like cylinder 1 is misfiring alongside 8 now...

My mechanic seems to be having motivation issues. If he isn't game by this weekend, I'm looking elsewhere because this is all starting to get a bit out of hand...

On the bright side, it was an awesome day to wash the truck. Amazing how much better the chrome and pseudo-chrome looks after a little elbow grease.

 

[Yeret]

Well, I've been having communication issues with my mechanic, so I got me a compression tester to test myself.

Cylinder 8 read 130 psi. Cylinder 1 read 137. Cylinders 4 and 7 both read 165. I went back to Cylinder 8 and added a little oil. Reading increased to 145, though I'm sure the others would've read higher with oil as well.

I should note that although I held the throttle open during testing, I didn't remove all the spark plugs, only the one on the cylinder I was testing.

Also, according to my Chilton's, a variance in high/low cylinder pressure of 30 psi is acceptable, though this is well beyond the usual 10%...

Guess the bottom line is should I be worried with pressure readings/variance like this?

 

[David H]

Just a though but how old are your plugs/wires/cap/rotor? Rerouted wires per TSB?

 

[Yeret]

I replaced the spark plugs last year with Champion RC12Y.

Cap, rotor and wires are about 3 years old. BWD Select from O'Reilly's.

I rerouted the wires a couple months ago when I had multiple cylinder misfires. Took care of the problem until cylinder 8 started acting up recently.

Leaving work yesterday, the engine began stuttering and shaking real bad. Oddly enough, this did not set a CEL, even after making multiple stops afterwards. (I reset the previous CEL when I replaced my downstream O2 sensor last weekend). It also felt like the engine was surging a little bit...

I should note the engine only runs like this when it's cold and it doesn't do it every time. Once it's been warmed up a few minutes, everything seems to clear up. The problem seems worst when it's cool and/or humid.

Honestly, when the engine doesn't "act up," you wouldn't even suspect anything is wrong. Throttle response is sharp and the whole body torques with the engine while revving. When I accelerate onto the highway though, the engine has a big lack of power. It feels as if I'm pulling a boat anchor.

 

[Yeret]

Check Engine Light came on a couple days ago. Never got around to scanning it, but today while leaving Wal-Mart, the light turned back off. Assuming it was a misfire again, the ECU turns the light off if the engine goes long enough without a misfire?

An oddity that I noticed ever since I owned the truck. When accelerating up to highway speed, if you accelerate lightly, the truck runs north of 70 quite easily. However, if you accelerate heavy, the truck seems to struggle to even maintain 70. It also almost feels like the truck accelerates quicker if you're light on the throttle rather than heavy.

I had a Honda Accord a few years back that had a similar problem. Someone suggested a plugged EGR rail, but my truck doesn't have an EGR system, so that cross-reference won't work, LOL.

I'm almost starting to think my lack of power and the cylinder misfiring are separate issues. Even though the engine ran much smoother today, it still felt like I was dragging a boat anchor down the highway.

I know my compression has heavy swings between cylinders, but even the weakest one is pushing 130 psi while cold. I just have a hard time believing this is causing my problems. And I still need to replace the intake plenum (which WOULD HAVE been done two weeks ago if my mechanic didn't decide to have motivation issues...), but my plenum has been a problem since I owned the truck and the engine still used to run much stronger.

The engine would still occasionally ping if you were heavy on the gas, but now doesn't at all (current issues considered, I'm not sure if this is a good thing...), the exhaust suddenly doesn't belt out oil smoke at startup anymore...

Damn, I'm just flat confused now. I think it's time to hit the whisky and take my mind off things for a bit, LOL.

 

[dodge dude94]

Hows it shift?

 

[Yeret]

No different than it always has. If I need to get up to speed a shade brisker, the shift points seem to have increased a couple hundred RPMs, but that's probably because I have to use more throttle than I used to in order to get up to the same speed in the same time.

 

[dodge dude94]

Hmm weird. I was wondering if maybe you were laying into it and it was slipping while doing so.

 

[Yeret]

LOL, well, I'm somewhat familiar with slipping transmissions and know that if you gun it and your RPMs shoot up, but the vehicle doesn't accelerate, then the transmission is slip 'n' slidin' on ya.

But no, that's DEFINITELY not happening to me. Not sure if it's worth noting or not, but the last time I had my scanner plugged in, I noticed my TPS was reading 13.7% at idle and only 66.7% at WOT. From what I've read, it's normal for the TPS to read above 0 at idle because the throttle plates are not "perpendicular" to the bores, but rather they're held slightly open. But it does seem odd that when the plates are held "parallel" to the bores, the sensor reads that they're still held 2/3 of the way to parallel rather than all the way.

I also noticed that the coolant temperature reading wants to hold around 199 degrees at idle when the OEM is 195. My coolant isn't in the greatest condition and I'm sure the thermostat is gearing up to be replaced, but, well, just another reading off my scanner, LOL.

 

[dodge dude94]

You might be able to fix some of it by replacing the TPS and see where that puts your throttle percentage. I have a friend that completely changed the way his truck drives by just replacing the TPS.

 

[Yeret]

My mom has a '96 Grand Cherokee with the 5.2 Magnum engine, so I figured I'd try some cross referencing.

KOEO, my TPS reads 13.7% at "idle", my ma's Jeep reads 12.2. WOT, mine is 66.7%, ma's is 62%. The Jeep in question runs very strong and doesn't even lose oil. You can run north of 80 in that no problemo.

Something strange though. Same testing, my MAP was reading 28.6, but the Jeep was reading 9.2. Considering the two vehicles were 20 feet apart, I doubt the atmospheric pressure would change that much, LOL.

Since I brought it up, my mom's Jeep will NEVER start on the first, sometimes second try. And when the engine does catch, sometimes it's very grumbly when it does. Last time I heard it catch, I thought it was going to stall.

 

[dodge dude94]

Interesting. Can you rob the MAP sensor off mom's Jeep for a couple hours to do some testing?

Also, sounds like Mom's Jeep has a bad fuel pump drainback valve.

 

[Yeret]

Wish I could say that I could, but my ma drives the Jeep every day and, well, it's her "baby." Hell, she thought I was going to grill the ECU just by plugging my scanner into the diagnostic port to check the stuff that I did.

Bottom line, if I need to swap MAP's around, it'd be much less trouble for me just to buy a new one, LOL.

 

[dodge dude94]

Well ****. lol Just tryna save you money for testing purposes.

If the MAP is reading way high that'll screw things up, might go ahead and buy a new one.