RPMs dropping then recovering

Maintenance/Repairs
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1

My truck started running a little rough so I replaced plugs, wires (looks like a rat may have nibbled them in places), cap and rotor. The truck idles smoother now but there had been kind of a lurching, almost as though it was bogging down then recovering prior to replacing the parts I have done. Now the issue seems worse, if anything. It seems to be either progressive. If I try to drive the truck like this, it’s OK once it’s going, though it still sort of hesitates periodically. Getting it going, however, is an exercise in frustration. It’s got virtually no power at all and isn’t until it surges forward (recovering?) that it goes at all. I realize this may not be the best description but it’s about all I can describe it as. The other oddity is it seems intermittent. Sometimes it’ll run flawlessly after warming up, others it doesn’t. Today, it ran perfectly for 30 to 40 seconds after first starting it to move between parking spots then it began while driving it. I’m concerned about driving as I don’t want to cause damage to the engine and also, it’s annoying a heck!

The truck’s a 1989 Dodge D100 (not D150 as below; why no D100 option?) with the 318, AT and RWD only with about 225,000 miles on it. Historically, it runs without any issues whatsoever, starts instantly every time I ask it to (even now) and overall is great mechanically. (Ugly as heck but its’ a truck!) The only issue I’ve had with it is recently, the coolant overflow hose came off right by the cap and, apparently, I lost enough coolant to overheat. I noticed the temp gauge registering hot and as I pulled over I got quite a cloud of steam. It must have heated enough to hit the flash point, I guess. This happened on the freeway but I stopped IMMEDIATELY and it was fine once I put the hose back on and got coolant in it. The engine didn’t stall or act up, make noises, etc. That was a month or so ago now. Aside from that and needing the brakes done recently (can’t imagine that could be related but who knows) the truck hasn’t showed any issues at all.

Someone mentioned this could be the Idle Air Control motor but my reading suggests that’d only affect me at idle. Am I mistaken or is there something else I should be looking at?

2

By the description it does sound like an “air” problem - as in a fairly substantial vacuum leak, or a wonky IAC could give symptoms like that. The IAC is only supposed to do stuff at idle - but if its being wonky, who knows. Let it sit at idle, pop the hood & give the IAC a good whack the handle of a hefty screwdriver. If that has any effect then you could be looking at an IAC problem.

Other than that just check for vacuum leaks, and don’t forget the intake manifold.

3

Thanks, cigroller, I shall do so. I just didn’t want to ignore something that may be more major or make it worse while trying to troubleshoot the wrong thing.

Several sites mention cleaning the IAC but none say with what. Is there a moden equivalent to carb cleaner? I’m handy enough but don’t have this sort of experience.

4

I’ve always just used throttle body cleaner - which is basically the equivalent of carb cleaner. Cleaning the IAC certainly can’t hurt. You’ll want a new gasket handy, or something to cut one out of.

I’d be looking for a vacuum leak before that though. After a visual inspection I like using an unlit propane torch.

5

You also could look at the throttle position sensor. I had an Olds Ciera that would start and idle fine but when you would try to drive it it was hell and no one could tell me what was causing the problem. If you took off slow from a stop it was fine, although other drivers around me were mad because I was barely moving. If I floored it then it was fine also. Anything in between would cause the engine to almost die out and it had no power at all. I replaced the TPS and the problem went away instantly. Just my two cents, good luck!