1999 Ram 2500 5.9 V8 172,000

While sitting in park from start up it idles fine. After accelerating and driving around a little it will stay on a high idle 1500 rpm. If I smack the gas pedal it will sometimes come down to normal. Then if I put it in drive and just kind of coast it will stall.

Also, During a hard acceleration after a cold start it will bog down but then come back and won't do it again.

Also, sometimes while driving at freeway speeds the RPM's will surge 100-500 RPM's at a time. I have also seen this while sitting at a stoplight but usually at that point it's 100RPM surges only.

Only other issue is a long crank to start which I already know is the fuel pump but it's been like that for years.

Here is what has been done/replaced (all US made Standard parts)
-Cleaned throttle body
-Replaced IAC
-Replaced MAP
-Replaced TPS
-Replaced Spark Plugs
-Replaced Wires/Cap/Rotor
-Checked all hoses for vacuum leaks

Think this is a candidate for a Hughes Kit? Engine has never been opened up.

Thanks for your help guys.

 

Its certainly time for the plenum to be done, but that doesn't appear to be the root of your problem in my opinion. The front O2 sensor is the likely culprit here...try unplugging it from the harness on top of the transmission and see if that erases your symptoms. This happened to me 3 months ago and I had almost the exact same issues that you're describing. And no, the front O2 sensor being bad will not always throw a check engine light. Replace it with a denso or ntk brand sensor, no bosch.

Also, a blown plenum will kill O2 sensors over time and will also ruin your cat if not addressed soon enough...time to put some serious time into the ol truck it looks like...

 

I should have also stated there is no CEL and no codes in the system.

I'll check out the O2 sensor.

Thanks for your help.

 

Not a problem...truck will throw a code once O2 sensor is unplugged. It should be immediately apparent whether or not this sensor is the problem once you take it out for a drive with it unplugged. Mine never threw a code when my front O2 was bad and it was REALLY bad by the time I replaced it...

 

Is there one or two upstream 02 sensors?

 

Federal emissions has only one, Cali emissions will have two. One on each bank.

 

Disconnected front o2 sensor. No change. What's next?

 

#6 and #8 plugs are both nice and white. There is a light oil coating inside intake but not bad. Not black like a lot of other pics I've seen.

 

Intake pic:

 

Ok. So now I pulled the horn of the throttle body so I can see what's going on. Like I said earlier. If I smack the throttle it goes down to normal (700). So I watched this. The idle speed is relative to the IAC completely. When it's idling high around 1500 the IAC is open. When it appears to be closed it's idling normal (700) So what tells the PCM the IAC what to do? TPS? I connected my scanner to watch live data. When the engine is running at 700rpm the TPS is reading 14%. Is that normal? You would think at closed throttle it would be 0. So then I disconnected the TPS from the TB and it then reads a minimum of 6%.

I also pulled all vacuum line from intake one at a time, plugged hole, and watched idle speed. No change. So no vacuum leaks from lines.