So here we go with another problem with my '92 RS. And yes, I tried a search but can't find anything like this. The car is a 305 with a TCI 700r4 and has been converted to TPI (I used the computer, wiring, and hardware from a wrecked '92 Z, also a 305). I did the swap years ago, so I really don't think this is related to the TPI swap.
About 6 months ago, it broke down on me driving home from work and wouldn't start back up. That issue wound up being the coil shorting out and frying the ignition module. Thanks to Autozone selling me parts that were bad right out of the box, the problem was a lot harder to diagnose than it needed to be but I finally got it running and I've been driving it for a couple months.
Ever since I've gotten it running though, it hasn't been running right. IT does fine as long as the car is moving and throttle is above idle. Otherwise, after a minute sitting at a light the RPM will start to fluctuate. RPM will drop until you expect the engine to stall, then it will surge to life. Eventually it will stall. It always will start right back up. IT behaves this way in neutral or in gear, but being in gear make the problem much worse. It seemed at first like a vacuum leak but I can't seem to find it. The car pulls 19" at idle. LAtely I've thought it was a miss, because its starting to run poorly at other RPM ranges, and sounds distinctly like a miss.
Here's what all I've done: replaced EGR, IAC, plugs, wires, cap/rotor, coil, fuel filter. I've cleaned the throttle body and checked the timing. The TPS checks out in spec. I've checked all the vacuum lines, fittings, connections. Everything looks good. And again, it seems to run fine when your cruising (until lately; the problem is getting slowly worse), doesn't overheat, and it isn't leaking or otherwise losing any fluids. The problem seems worse if the engine is warm.
I'm leaning toward a faulty ignition module since it still seems like a miss, and my replacement from the no-start issue several months ago actually came from a junkyard. I know they like to act up when they get hot.
I'm also suspecting a leaking intake manifold gasket. There's a suspicious puddle of what appears to be motor oil on the passsenger side, next to the EGR. Its not the EGR leaking anything though, its as if oil is seeping up from the lifter valley.
No codes. Any other suggestions?

Thanks

Dave

 

Both of my '91 Camaros recently did basically the same thing. Turned out to be the pickup coil in the distributor.

Look the specs up in a Haynes or Chilton's or something. Take the cap off. You will see a connector that plugs into the back side of the ignition module. Disconnect that. There are two wires there. Test the resistance between each wire and ground. This should be a completely open circuit. Also check the resistance between the two wires. This is where my memory get foggy. I think it should be in the range of 1000 ohms to 1500 ohms. Don't quote me on that though.

If it tests bad then replace it (Or get an MSD distributor like I did for the 305 Replacing the pickup coil is kind of a pain. You have to drive out the pin that hold the drive gear and take the shaft out etc etc. I like just replacing the whole distributor. Usually there is a little bit of extra play in the distributor shaft by the time the pickup coil goes out.

I would also verify that you have good fuel pressure. My 3.1L did this also right before the fuel pump went out.

 

Ah, the distributor was replaced when I was trying to track down my no-start issue, so its new as well as the pickup coil... but the ignition module on the new dizzy was bad outta the box. I replaced it with one from the junkyard. Of course, if the module was bad outta the box, I suppose the pickup could be as well.
The thing is though, it runs absolutely fine off idle. If you're giving it any throttle at all, no worries.
D

 

Mine did the same thing at first before it started running bad and missing at all RPM.

One way you can check before you take stuff apart is with a timing light. Hook it up to each plug wire when the car is idling rough. Make sure that you have consistent spark on all wires. You can also check at the coil wire. That is how I found the miss initially.

Your issue could also be a rich or lean idle issue. I would pull the plugs out, clean them up, reinstall them, let the car idle for a couple min, then pull the plugs out again. If you have black sooty deposits you are running rich. If you have blisters on the insulator or electrodes you are running lean. If the insulator is a nice goldish brown color then it is not a mixture issue.

 

I just replaced the plugs last weekend, the old ones had been in there for a few thousand miles and came out gorgeous. Textbook. I don't think this is a fuel issue. My pressure is good, my injectors (Ford 19#) are less than a year old and test out good with my multimeter.
Checking for consistent spark on all wires is a good idea. Its crossed my mind to do so. I have a timing light, but I haven't gotten around to this test yet.

Yeah, at first I was really thinking vacuum leak. But this was the only issue I had after the no-start problem and the ignition was all I really messed with. Now, yeah, its starting to miss a little in more of the RPM range. Anything below 1500rpm and it might miss a little... doesn't always though and its more likely to miss when the engine is warm.
If I get a chance tonight I'll check for consistent spark and check the reading on my pickup coil.

Dave