Hesitation? Misfire? Vibration under light load
 

I have a 1997 Ram 1500 w/ 5.9l V8. Things have improved after replacing my plenum pan gasket but there is a slight shudder/vibration/hesitation/misfire(?) under load and light throttle while going up slight to moderate inclines, if I give it more gas and it shifts everything seems to be smooth. Also, I have had an issue a few times where the engine will die right after starting it up. It has also died once or twice while driving at a slow speed (2 mph). Also... There is a slight shudder while coming to a complete stop while in gear, I tried stopping while in neutral yesterday and didn't notice the same vibration. I adjusted the throttle valve cable bracket the other day thinking that it might improve if it was a torque converter slippage issue but the symptoms have not improved. My next step was going to be checking and cleaning the TPS and also doing a compression check. I am definitely doing this this weekend and would appreciate some advice on whether or not you think I am heading in the right direction. I guess I should check my 02 sensors while I'm at it and am not sure how but I'll search around and see if I can find a how-to. When i replaced the plenum gasket i also replaced the intake manifold gaskets, cap, rotor, spark plug wires and rerouted them, spark plugs, cleaned the throttle body and plenum and installed a 180 deg thermostat which is getting replaced with a 195 before next winter. Thanks for the help, I'll post in the newbie section this weekend.

What plugs did you use?

The IAC can be cleaned, the TPS, not really. The TPS is pretty inexpensive at an auto parts store, O'Reillys has one with a Lifetime Warranty, IIRC.

Could also be a pre-cat O2 sensor. Do you have a CEL at all?

One sure way to tell if it's a miss, is take the truck out someplace with very little traffic, that you can go 50 or so mph, without attracting any undue attention..... Once you are there, accelerate gently from a dead stop, at about 45mph or so, your truck should hit fourth gear, and TCC should engage, if, at that point, you feel the vibration, you can bet it's a miss.

When you cleaned the TB, did you pull the IAC motor off the back, and clean those passages as well?

also check that the pin for the coil on the cap isnt loose..i just had this issue with my ram..the center pin was really loose on a brand new cap...i ran ok while accelerating but when you would let off the gas it would jerk back and forth..give it gas and t would go away...

could be that plenum leaking too,would cause everything you stated if there's a vaccum leak.

Another way to check for a miss is an RPM drop test. Old mechanics trick, with the truck running "in park if it'll stay running long enough" with some spark plug pliers; pull one spark plug wire at a time and listen. If the running condition gets worse its not that cylinder, if you come across one where nothing seems to change and its still running rough, likely that cylinder is the problem.

P.S. to clarify, pull 1 plug wire at a time, then plug it back in before moving on to the next.

Also, as jasonw mentioned, the IAC might have carbon build up and isn't closing properly. Or "smaller possibility" your EGR valve may not be closing fully.

97 shouldn't have EGR.

jasonw:
I used NGk's gapped at .035 (I checked them all).
I meant that I was going to check the TPS and clean the IAC in my original post (sorry).
Is there a way to check the pre-cat O2 sensor? I haven't researched doing that yet. I had to replace my cat a month and a half before I replaced my plenum gasket.

HeyYou:
Are you saying that it is a misfire if I get over 45 mph and I feel the hesitation? On my way to work everyday there is a stop light, flat and then gradual hill. When I accelerate from the stop light and am on the flat, everything seems normal (maybe a little hesistation? not sure, i'll check again tomorrow morning) until I hit the gradual hill at 50mph and then a little way up it I feel the hesitation or vibration, my speed starts dropping, and I have to step on the gas harder until it downshifts. It seems to me like it's dropping from 4th to 2nd when this happens and missing 3rd all together so that may be a symptom or it may just be a result of how much gas i'm giving it when
I DID NOT remove the IAC off of the TB when I replaced my plenum gasket as I just learned about that procedure after I had my engine back together after the gasket change but plan on pulling it off this weekend and cleaning it per a DIY I found.

madore33:
I'll double check the cap. I think I'll test the coil too, I was going to replace it when I did the plenum but forgot to buy it.

neilsrpcustoms:
I would HATE for it to be the plenum but i'll check for any oil under the throttle body when I take it off to clean the IAC. No oil, no oil, no oil...

97_DodgeRam:
The problem with the stalling at slow speed or right after starting her up happens VERY minimally so I'm sure I could perform this test and will do so this weekend after the IAC cleaning.

Thanks for all your help!!!

Check your TPS too......

My test was figgering more steady state acceleration, just press down on the pedal a bit, and hold it there, and see what happens. (not floored, just enough to get you accelerating up to around 50.)

Do a night time check on the wires and coil,just make sure it's dark enough that you can't see your hand in front of your face,if the spark isn't makeing it to the plug,you'll see it. It doesn't need to be under load to miss,it's just alot more noticeable then.