93 Dakota 3.9L/128,000mi Starting to lose power on long freeway hill climbs. This truck has run great and still does, but in the last few months when climbing a steep grade in 5th and 4th it wants to start pinging and forces me to downshift all the way to 3rd and climb at 35mph the same hill it used to do at 55mph in 4th. On easy and moderate roads/terrain it still runs perfectly. It just doesn't like to be loaded on steep freeway hills anymore, starts pinging. I changed out fuel filter, plugs, wires, cap, rotor, and coil. The fuel pressure checks out ok at the pressure regulator and the PCV valve and crank case ventilation filter seem to be operating properly. I haven't checked the injectors yet but the truck seems to be operating perfectly on everything except hills, I'm stumped. Any input would be appreciated.

 

The Magnum motors are known for running hot cylinders and start pre-detonation/pinging.

The typical reason for pinging is a blown intake plenum gasket. Look down the throttle body and look for oil at the bottle of plenum. If there is it is leaking and needs to be changed.

Even with the plenum gasket good they can ping. Going to a colder spark plug will help. Autolite 3923's are the best for performance and cheep. What kind of plugs did you use? Platinums make it worse.

It will be anytime the engine is under a heavy load, so it is not just hills.

 

Interesting, yea when I changed the plugs I noticed that the old ones had a way bigger gap than the recommended .035, they had like an .050 gap. I totally have to baby the throttle now (under load) to keep it from Pinging up steep hills at freeway speed. None of the plugs were fouled, one just looked a little lean, I was thinking a slightly clogged injector. I'll check the plenum gasket. Thanks for your input!

 

Oh I just picked up some cheap Bosch plugs, Whatever was recommend for the application, Nothing exotic.

 

Yep oil at the bottom of the plenum, I'll Change out the gasket and post the results.

 

There are kits that replace the plenum to prevent that type of problem happening again.
http://www.hughesengines.com/Index/p...p?partid=26190

 

Okay, 8hrs and a new Plenum gasket later I run my truck up the cuesta grade.... Still pinging and losing power. So I go home and look down the TB and there is oil in the plenum pan again WTF! So out of desperation I go to the parts store to pick up a new PCV valve, this is the only thing I hadn't replaced during the tune up. The guy behind the counter starts telling me about how he had oil migrating through his one time, and then the light in my head went on.... the new oil in the plenum pan was only on the side that the PCV valve was on. I really hadn't given that part much thought because I tested it and it checked out okay. So in 60 seconds I replaced the PCV and now my truck is running better than it did when I bought it with 100k on it. I got my *** kicked by a $4 part. Many thanks to crazy4x4RT for his input, I do have a new plenum and manifold gaskets that will probably last another 100k. To summarize, if you have 129k on a tune up part, just replace it, especially when it only costs $4. It may appear to be working when it actually isn't. I also may not have put it all together had I not changed the plenum gasket and seen half a dry pan or had the guy behind the counter at the parts store not mentioned his experience.

 

So you no longer have oil in the plenum? I ask because I just replaced my PCV valve and that valve lets air get sucked into the plenum so whether the pcv valve was working or not is would still suck some oil into the plenum through that line. Sounds like your problem wasnt oil in the plenum as this seems to be fairly common but the bad pcv valve was your only problem. If you could take a look see if it still has oil in the plenum. I am def curious

 

It defiantly could be a combination of both problems. In either case you know your plenum gasket is good and now with the PCV valve replaced it is working better. One more tip, to help with pinging even more switch you plugs to Autolite 3923, they are 1 step colder than stock and they are cheep. Ask for them by part number.

 

I agree with crazy4x4RT, I could have had a combination issue with both. The deal with the PCV was that it was gummed up just enough to not close as far as it should have at high rpm (which is what it should be doing under load). Also, if the plenum gasket was blown it would have exacerbated this issue by not creating enough vacuum to allow the PCV to restrict the flow of vapor at high/loaded rpm. At this point the only way to tell for sure is to take the TB off and wipe out the plenum pan, run it and check. But seeing as how I just replaced the TB gasket, I'm feeling inclined to wait a bit before I do. I am however curious myself, to have the complete picture on that whole situation. For all I know the plenum could still be leaking (but I hope not). With 129k on the motor, like crazy4x4RT, I'm thinking it was likely a combination of both issues. I will get it wiped out and post a final assessment. It should be easy, it's only a TB gasket and 4 bolts.