ruough start up, then ok once warm

redcoupe86 · 03-26-2006, 02:11 PM

I've done a search but I'm still unclear as to where to start.

Now that its warmer down hear in the south (or maybe because the car sat for month) when I start it up its a little rough cranking to life. Once going, say the first few seconds, its revs up to 1200 rpm or so then settles down to 900 rpm once I pull out of the drive way. However, its not a very smooth idle. Its not rythmic if you know what i mean. The roughness is all over the place. And I don't know if this is related, but if a break hard (the clutch depressed) the rpm drop alot, then bounce back up. Almost to the point of the car dying. However, once slightly warmed up - say about 5 minutes driving, the idle smooths out lpretty much, and once fully warmed up it idles like a champ.

Any suggestions?

WydRyd · 03-26-2006, 02:18 PM

Since it sounds like it's temperature related, might be the Cylinder Head Temp Sensor?

Check to see if you have the old single wire, or newer dual wire sensor. Measure resistance to ground and you should see approx 2k-3k ohm when cold (depending on ambient temp).

Replace if it's the single wire version.

Could also be a vacuum leak. Check all your vacuum connections.

wolf · 03-26-2006, 02:19 PM

pretty normal behaviour in cold weather after sitting for a while, in my opinion.

redcoupe86 · 03-26-2006, 02:49 PM

Quote:

Originally posted by WydRyd
Since it sounds like it's temperature related, might be the Cylinder Head Temp Sensor?

Check to see if you have the old single wire, or newer dual wire sensor. Measure resistance to ground and you should see approx 2k-3k ohm when cold (depending on ambient temp).

Replace if it's the single wire version.

Could also be a vacuum leak. Check all your vacuum connections.

I've checked all the usual suspects for vacuum leaks. When I take off the oil filler cap (once fully warmed up) the engine runs rough.

I forgot to mention, a couple of months ago i was trying to run down why the engine was backfiring. Turned out to be the #4 plugs wasnt pushed down all the way. Anyway, in process of diagnosis, i messed with the mixture screw - could this have ill effects on startup?

WydRyd · 03-26-2006, 02:56 PM

I'm no expert, but I would think messing with the mixture screw would affect overall drivability, not just upon startup or idle?

Maybe some of the Motronic experts will chime in here and shed more light for you.

javadog · 03-27-2006, 03:14 AM

The car should not run like this. It has a problem (or two) and the reason it runs better when warm is simple. When the car is first started, the O2 sensor hasn't reached a temperature that will allow it to operate. If your mixture is off, or the fuel pressure is wrong, etc. the car will run poorly. Once it warms up and starts providing a signal to the ECU, it can correct for slight problems with the mixture. As long as the problem is within the range of correction possible with the O2 sensor, it'll appear to run fine. There are other problems that can cause poor running during the warm-up phase but I'll stop here.

If you fiddled with the mixture, without any real means of measuring the CO, you might have caused the problem. Take it to a shop where they can check the mixture setting, fuel pressure and the rest of the usual suspects.

I hate to sound like a broken record, as it seems I have been giving this same advice lately on a daily basis, but there isn't much to be gained by trying to tune one of these systems without using a gas anaylzer.

Good luck,

JR