Rough running 964
 

On the way to work this morning, my '93 C2 started running rough... hestitating at constant revs, sometimes the idle dropping to as low as 200 and sometimes fluctuating and sometimes running just a tad high (1,200 rpm). Not always, but often, it took extra revs to get it moving from a standstill (and I hit EVERY dern stoplight). Sometimes -- but only briefly -- it seemed to run okay. I got it back home with some anxiety. At a standstill, it will rev easily and with no sign of stumbling. First thing i thought of was a broken distributor belt but haven't had time to check that (it was replaced last fall, only about 4,000 miles ago) but will check tonight. Last tune-up was 18 months (8,000 miles) ago. Oil is at correct level. I've recently cleaned the ISV. Just last weekend I drove it on an 850 mile trip with no problems; fuel mileage was a little over 29 mpg. Any thoughts?

perculiar running issues often seem to involve looking at the DME relay - do you have a spare to swap out?

graham: thanks for quick response. When I saw it, I slapped my forehead. I carry a spare, new DME. I'll swap it out and see if that cures the problem.

Richard

Check out the post on recent similar problems. Could be coils, plug wires, DME relay. Some are easy to check, others very tricky. I've gone down this road because of erractic performance and have replaced coils, relays, plug wires, plugs. At various times all were, in fact, faulty. Trying to figure out just what the problem was a trick. I used Adrians book quite a bit as well as advice on the forum.

Richard, will you're in there have a look at the earth cable. Some people, including me, have had some good results after replacing this cable. Here's a thread about it. Good luck.

Today, I checked that the distributor belt was intact (it was), rotor and distributor cap (at least on the secondary distributor) were good and cleaned the Idle Speed Valve (also known in other German cars as an Idle Air Control valve); it was clean (I had cleaned it last year). Also checked all vacuum lines. Replaced the DME fuel pump relay (it was original to the car) with the now-recommended 993 unit. No change. I did notice in road testing the car that the problem of hestitancy, idle problems, etc., only occurs when the engine starts to get warmed up. Seems to get worse the warmer the engine. Will check the ground cable tomorrow.

when's the last time the fuel filter was replaced? I've got a spare the needs to go into my car, as I'm sure mine is original.

Hawk911: good idea about the fuel filter (I have a new one ready to go in once I work up some gumption) but the problem was solved by cleaning both ends of the negative battery cable. I went out into the garage last night and cleaned the battery terminals and removed the negative cable and cleaned both ends of it. As has been pointed out, both ends of mine looked clean, even upon very close inspection. However, I wire brushed both until they were shiny, which only took a few seconds. Reinstalled everything. This morning, took the car for a somewhat spirited drive and did not have any recurrence of my initial problem. Amazing! I'll be putting that negative cable on my semi-annual list of things to inspect/clean/wire brush. I wish now I had checked the battery voltage BEFORE cleaning the cable (should be around 13.5-14 volts at 2,000 rpm).

That was exactly my call when I saw the result of replacing the earth cable. Well done.