95 Ram SLT 1500 4x4 5.9L Automatic stock except for K&N FIPK and a small power amp.

For starters I have a suspected blown plenum pan gasket. Not real bad yet but getting worse. I have read up on the procedure and plan to replace it with one of the billet pans.

Problem I am trying to nail down is a drop in RPM's that I experience when I am coasting (as soon as I touch the throttle it dissapears). It is most prominent in 2nd or 3rd gear (about a 500 RPM drop off and pickup back to idle) In OD it is only around 200-300 RPM dropoff and pickup back to idle. It lopes(drops off) in a steady cadence about every 4 seconds. At idle in park it stays around 1000 RPM's with only the slightest hint of movement in the tach needle. No error codes registered.

Now, I feel my rpm problem could possibly be unrelated to the gasket failure but I can't find anything (yet) to confirm or deny that. I have not seen this as a sympton for the plenum gasket in any of my readings. Any insight to this would be MUCHO appreciated because I have failed to turn up anything specific to this and quite frankly it is driving me nuts. I do all my own work and would prefer to not send it to the shop. TPS, cap, rotor, wires, plugs, water pump, timing chain gasket all recently replaced. Cleaned out IAC but did not change it. Mabye I should but it was fairly clean to start and seemed to function properly.

Any Idea's before I continue to take wild stabs at things?
EGR, Coil, VSS, MAP, Fuel System, Cruise Control?

Thanks in advance!

How long have you had your truck?

My 2005 has an automatic dissconnect from the torque converter to increase fuel economy. Does exactly what you described. Could be the same for your truck.

How long have you had your truck?

My 2005 has an automatic dissconnect from the torque converter to increase fuel economy. Does exactly what you described. Could be the same for your truck.


I have owned it for 6 months.

Thats interesting. Could you describe your situation in detail for me?

Thanks

In the owner's manual of my truck there is a section that states the torque converter will disengage when coasting. I have to find my manual for the exact wording. In the meantime here is what mine does.

If I am coming up to a stop light and let off the gas at around 40-50 mph the rpm will drop to idle, about 7-800. If I stop, it stays the same, however if I accelerate it will pick right back up and continue driving as normal. The manual says something about this saving fuel. It makes sense. Kind of like putting your car in neutral and coasting to the stop sign.

I don't think it matters which gear I am in. Your best bet might be to get a hold of a owner's manual for your truck. I'll get my manual and post the exact wording later on. Hope this eases your fear of serious problems.

the lower intake gasket does suck air that the pcm doesn't know about so it does affect idle also did you clean the throttle body bore and is the distributor indexed properly if rpm in neutural is 1000 or more this is too high and the pcm will drive the ais motor closed too reduce the rpm and if it cannot reach the goal rpm it will then use electronic timing retard to achieve the goal rpm and intake air temp senson may be higher than normal with leaking intake pan gasket and this also affects timing wich could cause rpm issue

the lower intake gasket does suck air that the pcm doesn't know about so it does affect idle also did you clean the throttle body bore and is the distributor indexed properly if rpm in neutural is 1000 or more this is too high and the pcm will drive the ais motor closed too reduce the rpm and if it cannot reach the goal rpm it will then use electronic timing retard to achieve the goal rpm and intake air temp senson may be higher than normal with leaking intake pan gasket and this also affects timing wich could cause rpm issue



Thanks for the input guys.

Got a billet plenum pan on order. Guess the first thing to do is swap it out and go from there. Will update when I finish.

Finally got my new heughes plenum plate installed and my truck now runs awesome. Plenum gasket was in alot worse shape than I ever could have imagined.(Only 65k miles!) The sides of the old gasket were warped out about an inch and a half, when they should be perfectly straight. I had probably a 1/4 quart of oil inside the plenum when I removed it.

All the symptoms I reported earlier are gone. No more ping. No more rpm lope ( Leaking so bad it was making my IAC flip out, I suspect) and the idle in park is now 750 rpm instead of 1000. I just ran some seafoam through my PCV vacume line. Going to start it up in another hour or so and watch the carbon burn off. =)

Thanks again for the input. Hope this can help someone else and feel free to ask me any questions you might have on this procedure.