Hi, new here. Sorry for the long post... I have a baffling intermittent driveability problem with a 1999 Ram 1500 (Sport, 5.2, 4WD, manual transmission). I've seen mention of similar issues here, but not quite the same as mine.

The symptoms are that the engine cuts out, almost as if the ignition or fuel were switched off, at light throttle and moderate speeds, like just enough throttle to maintain speed or slightly accelerate between 20-45mph on a level road. If I press the accelerator farther it pops and bangs, a little more it cuts back in smoothly and it runs fine with normal power... too much power for a snow covered road!. Likewise, if I take my foot off the gas, the engine idles normally as I coast and slow down. There are no codes, CEL is off. The problem seems to be around 15-20% on the TPS according to the scan tool live data, but the TPS reading appears stable so it seems unlikely that that s the problem. The other sensor data from the scan tool looks normal as well. If the engine is stone cold, it doesn't do it (because it's still in open loop mode, perhaps?), and if the engine is fully warmed up (say 20 minutes of driving), it doesn't do it. Coasting to a stop or down a hill with my foot off the gas I get pop-poppop-pop-pop-pop (even when it is warmed up).

I do have what appears to be the intake plenum leak... there is a whistling sound when there's plenty of manifold vacuum, but I don t think it would cause this kind of intermittent problem... but I could be wrong? At one point I did have a code, target idle not reached (it was idling high, no doubt because of the leak), but it went away and I haven't seen it for awhile.

As the symptoms seemed like an ignition problem, I replaced the plugs, distributor cap and rotor (they looked nasty), and wires, and last night the ignition coil, without any improvement. I also cleaned the idle air control. The battery and terminals are also new and clean, so it's not a voltage problem.

I can think of any one of a number things that could cause the problem to happen all the time, but the narrow range of conditions where it happens rules a lot of them out. I've seen suggestions that a bad upstream O2 sensor could cause this behavior, (and not cause it when running open loop when cold, which fits). Furthermore oil from the leaky plenum could foul the O2 sensor but why would it go away when fully warmed up? That's what seems to be different from what others have reported.

At this point I'm pretty frustrated. I can't afford to keep throwing parts at the truck, or pay for major repairs, and it's not really worth it anyway (over 200K miles) even though the truck is otherwise solid. I can live with the leaky manifold (if that s not the cause of the problem)... it's too cold now to do it myself in the driveway before spring, and I don't have the cash right now to pay a shop to do it, the truck needs a clutch as it is. I really don't need this big a truck any more since I got rid of the trailer I was towing with it; a smaller truck would make more sense (anybody wanna trade?), but I can't sell it the way it is.

 

Unplug the pre-cat O2 sensor, and drive your truck for a few days. if the symptoms disappear, replace the sensor, if they don't, plug it back in, and start lookin' elsewhere. Given that your problem is somewhere between cold, and fully warmed up, the O2 is a good bet.

Also, I would be tempted to test the TPS with an analog meter. (with one with the needle, not a digital display) and verify that it is indeed doing what it is supposed to. Digital meters won't give you a good picture of what is going on, the scanner doesn't update often enough. The analog meter will show you exactly what is going on. If the needle does not give you a nice, smooth sweep as you open the throttle, replace it.

 

Thanks. Talking to a mechanic today, he suggested that it may indeed be the leak causing the problem... his thought is that the leak is enough to fool the computer and mess up the fuel/air ratio... until things warm up enough to cause some expansion and close off the leak enough to let the engine run more or less OK again. Disconnecting the sensor (which I haven't tried yet) would fix it either way. Of course the leak really should be fixed anyway, would cost around $4-500 on top of the the $1K to do the clutch...