Hey all! Newbie here.... I have a 99 ram 1500 with 145k and I have been experiencing an intermittent rough idle with a loud hissing sound. Rev it a little and it goes away. I have replaced int. man. Gasket and Iac sensor. No help. Any ideas would be welcome..... Thanks......

 

Replace your oxygen sensors. It's the pre-cat causing that, but the post-cat is due for replacement, too, so just knock 'em both out. Use NTK or Denso brands only.

 

Copy that. I have already changed them once, but they may be bad again from the last blown intake man. Gasket. Is there any way to test them?

 

Not really...... Even the PCM isn't entirely sure when they aren't working properly. Did you do anything about the plenum gasket? (steel pan on bottom of intake.)

 

It's possible but not worth the bother and expense for most DIY'ers. You'd need a digital storage oscilloscope and a bottle of propane, at the least. I'm sure the procedure is online somewhere. If you have the DSO already and are pretty well experienced in electronic troubleshooting with a scope, it's not all that difficult. Otherwise it's cheaper to just shotgun the part because the labor charge is greater than the part cost.

If you replaced the rear sensor in the last few tens of thousands of miles, you can just leave it. The oil that's fouled the pre-cat won't have made it through the catalytic converter to foul the downstream sensor.

You said "last blown... gasket". If your rig has made it habit to eat those gaskets, you'll probably want to swap the factory steel belly pan for an aftermarket aluminum one.

 

Yep, I'm kickin myself for not putting an alluminum one in the last time I changed it. Watched a video on YouTube,it was exactly what mine is doing. His fix was the idle air sol. Might try that one again before the o2 sensor. I'll keep you informed. Thanks for the info.

 

Make sure the IAC is clean. I have a DIY here called "clean your throttle body". But yes, bad O2 can cause this as well. Happened to me both ways.

Get NTK from Rockauto.com.

 

You also need to make sure the cat is not clogged, since the plenum was blown.