I have a 98 4wd, 360, auto, 3.55 gears. Has had a recent tune up (plugs, wires, cap, rotor button). Has never had plenum replaced in 147k miles. I can't make it spark knock if I try, and it passes the tsb test and burns very little oil, so I think the plenum is ok for now. It has never had the 02 sensor replaced. It runs great and has no codes. I was thinking about replacing the front 02 sensor just to see if I could possibly get a little better gas mileage. Currently, I can get about 13 mpg, mostly highway (hilly highway), running about 70 with cruise on. So what do you guys think? Try the 02 sensor and see if it helps any, or is 13 mpg about it and I should spend the money on something else? I've seen posts where people say they get 18 mpg and what not, but I'm a little skeptical of their math.

 

with my speedo corrected for larger tires, i get about 15+ in gentle highway driving with my 5.2

it won't hurt to replace your front O2, but do this - unplug it before you buy a replacement and identify the shape of your plug - round vs square. then shop online for a NTK and see what you can find. i bought a brand new one for about 30 or 40 or so on ebay. you can probably do even better if you buy a universal, and splice it into your existing pigtail. there's only 4 wires and its foolproof.

after careful study, but not actually looking at my plug, i bought the wrong one, and then ending up splicing it in anyway, so i could have saved another 10 bucks or more by just buying the universal in the first place.

unfortunately for me, my mpg did not improve.


caution - this led me to the wrong one, square vs round.
http://www.ngksparkplugs.com/product...sors/index.asp

 

I'm sorry, but if it's a 98 with 147k on it, the plenum IS leaking! Just because you have no lights YET, you will soon enough. And by that time it will be too late. As in clogged cat and quite possibly a cracked head.

 

If they are original at that milage, replace them. The 02 sensors will only throw a CEL if one malfunctions or fails. They are suppose to be replace every 60k miles.

 

What are you basing this on (it is leaking)? What someone else told you? Was saying no light for 02 sensor. Don't necessarily think the plenum leaking would always throw a code. Or would it? It very well may be leaking, but if it runs good, burns no oil, and passes the tsb test, it very well might not. I don't plan on tearing into it unless I see some oil consumption or spark knock or even have the time. The plenum could outlast the rebuilt transmission...and if that happens, I'll be trading anyway. Anyhow, thanks for the input.

 

10-4. Will give it a try. Had previously asked a mechanic at a local shop about it. He said if no lights were on, leave it alone. Just wanted a second opinion, so thanks.

 

I'm basing it on experience, not "what some one else told me".Besides the 2000 I have, that was blown with only 78K on it, I also own a 98. And the plenum won't throw a code and neither will the o2 sensors a lot of times. And just because it passed an emissions test does not mean anything. And it's not a big deal to check the intake gasket. There are 2 ways, but the best is to remove the TB and look in the intake with a flashlite and small mirror so you can look around all 4 corners. Oh, and btw. My original post had the intention of warning you BEFORE it gets bad, right now it may be only a small leak. But it will get worse,as I stated in my 1st post.

 

I understand your intentions are good, and I thank you. I have never had an emmissions test. I have done the factory tsb test, to some degree. I plugged the pcv valve and the pcv port on the intake manifold. I partially blocked off the crankcase breather hose on the opposite valve cover with my thumb. I do not have a vacuum gauge, so I sprayed degreaser between my thumb and the partially blocked breather hose. Bubbles blew out, indicating positive pressure to me, not negative, hence no vacuum leak. I do not believe looking down the intake with the tb removed is a definitive test unless you have a considerable amount of oil in there. There is a vaccum in there under normal circumstances that would draw some oil in through the pcv valve if you have a high mileage vehicle. I do not believe that 2 vehicles that you owned having plenum issues means all magnums have plenum issues at a specified date or mileage. I have a friend who has a 97 ram that has 320k miles, has had 3 transmissions, and has never had the plenum repaired to his knowledge. I highly doubt it has a hughes or aps plate on it. I work with a guy who has a 94 with 180k on it. He has had it since 60k. 2 transmissions, no plenum repair. Again, I highly doubt it was replaced with an aluminum plate. The expansion rate theory of aluminum vs steel is not entirely convincing to me. Many engines have alum heads and iron blocks but do not suffer premature head gasket failure. Or consider alum block and iron exhaust manifolds...or alum dirt bike engines and iron exhaust pipes. In my thinking, dodge went through a period of bad gasket material in general. Many magnums have timing chain cover gasket leaks, but do people replace that gasket as preventive maintenance? I believe that you are just as likely to have a gasket fail there as have the plenum gasket fail. Will the plenum gasket fail given enough time and mileage? Yes. But not necessarily before a head gasket or oil pump. I'm sure I will get eaten alive for this, but this is my conclusion after careful study and I believe far too many people jump on the plenum bandwagon for most every problem. Again, oil consumption is not a problem for me. Spark knock is non existent. The crankcase makes positive pressure at idle (which is where a vacuum leak would show up most). I do not believe, given the facts, I have a plenum leak yet. That being said, do I think its a bad idea to go ahead and put a hughes plate, etc on? No, but I do not have the time or inclination to do so. The truck probably has another 50k in it before I semi retire it at 200k. If it has made it this far, there is absolutely no reason to believe it will not make it the next 50k. I drive it 300 miles a week, so I will definitley post if I have to change the plenum before it makes it to 200k. Thanks though, I enjoy the discussion and the learning. I just don't believe the plenum failing is an absolute, although fairly common.

 

that's an LOL,

 

Whatever DF bud. Just so you know. If you search here for the posts I've made in the last few years, you will find one or two where I state the factory TSB test is a waste of time. Both of my trucks passed that useless test. And when I took them apart, sure enough the gaskets were blown. The 98 was real bad and the 2000 was a small leak which thank God I caught before bigger problems . Which is all I was trying to tell you. Have a great Labor Day weekend and don't drink too much like I am going to. LOL


Actually how stupid am I? LOL I have worked on at least 30 various Magnum motors from Rams to Jeeps to Dakotas, helping people out. Not just my two. And before I found this site, I just replaced the lower gasket. Since then those people or there rides are long gone. So I don't know the results of how long the the gasket actually lasted.