I completed the plenum repair least week and so far she's still dry inside. Still was down on power but no longer pinging. Replaced the MAP sensor and that made a major difference. Still has a rough idle, like not super rough but I can feel it. No CEL or codes. The thing is I don't believe I noticed a misfire previous to the repair, just low on power and the pinging at WOT. I just bought this truck a couple of weeks ago as a summer tow rig. Looks like plugs were replaced 20k ago. Probably original o2 sensors, PCV valve looks original. The truck has 229,000 kms on it. I sprayed come throttle body cleaner around the intake and vacuum hoses (engine was cold) looking for a vacuum leak but no idle changes. ideas?

 

I forgot the TB gasket tore in 2 places so I just used a little RTV and put it back together. It could be leaking causing the minor idle roughness. I'll get that replaced and see what happens

 

When you replaced the plenum gasket, did you apply rtv to the mating surface on the 4 corners of the block before you put your manifold back on? Specifically the corners on the front, and back of the engine where the cylinder heads connect with the block. This is in addition to the gaskets for the manifold.

Also when tightening the manifold and the plenum, did you tighten in a star pattern in increments, or did you just go in a circle?

 

Yes, I did apply RTV to the four corners. Motor is dry all around the intake. I followed the instructions to a T. I torqued it down in a star patten from the inside out to the correct torque specs.

 

Hmmm when you replaced your TPS when you went to line it up to tighten the screws, did you turn into it to engage the sensor or did you turn against it? If it loses power to the point where you it feels like it is going to stall out if you let off the gas, it's upsidedown and you need to flip the way you turned it

Also make sure you didn't swap the connectors of the iac valve, and the TPS when you did all the top end work.

If those don't work, my only educated guess can be the gasket to your throttle that you said you were gonna replace

 

I meant MAP sensor, I didn't remove the TPS that was a typo. Probably should clean that though. I also have noticed that the temp gauge never reads more than just past the quarter mark. Temp sensor issue? I replaced the thermostat while I was in there. It showed the same temp previously even though there was a coolant leak and low on coolant

 

When I first got my 98 2500, I was used to my 96, and where the temp gauge generally hung out on that one. On the 98, it seemed way too low, so, instead of checking to see what the temp actually was, I changed the thermostat, only to have.... nothing change. THEN I plugged in, and discovered that just past a quarter of the way up the gauge, was indeed around 190 degrees........

 

ok cool. I changed the thermostat because there was a leak at the flange that I noticed when I did the plenum gasket. So I am thinking it's probably a leak at the throttle body gasket causing the rough idle, although could be plugs etc as this thing has had a plenum leak for quite a while I am thinking

 

Get used to those plenum leaks. That's one of the most common repairs on this engine. Only way to get rid of it for good is a solid intake manifold that doesn't have the plenum. I recommend a hughes FI air gap if you got the cash to spring for it 👍

 

A good gasket, and shorter bolts, and the plenum stops being a problem.

The Hughes intake is an alternative, but, not exactly inexpensive.... and also requires modification of the fuel rail to make it work.