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HELP! engine runs rough after warm up
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TOPIC: HELP! engine runs rough after warm up

Re: HELP! engine runs rough after warm up 11 years, 8 months ago #10815

I think it is interesting that it runs fine and then starts stumbling.

Does this only happen when the car is cold? For example if the car has been sitting overnight and you start it, it runs fine for a bit and then starts running rough, right? If you then turn it off and immediately start it again does it still run rough?

If you answer yes to both of these problems, it sounds like the problem is temperature dependent. When the car starts cold, it ignores the input from the O2 sensor and runs off a predetermined fuel map with additional fuel to richen the mixture when cold. Once the motor reaches operating temp, measured from the DME temperature sensor, the DME leans out the mixture and tries to use the O2 sensor to manage the A/F ratio.

That only leaves a couple of items to cause problems:

1. The DME
2. The O2 Sensor

The DME temp switch shouldn't be the problem because that would cause it to run rough when cold, not warm.

To test the DME, you'll need a spare to swap in. To test the O2 sensor, simply unplug the connection at the firewall.

The only other suspect I can think of is the reference sensors. They can sometimes be sensitive to heat.

-bj

Re: HELP! engine runs rough after warm up 11 years, 8 months ago #10816

The idle stablizer valve is also heat related. If it stuck in certain position the may run fine cold, but not so well hot. However this should mostly be an issue at idle and not with throttle.

In my case me AFM probelms were temp reated because the AFM work differently at different temps. Once I fixed it it was fine.
Joe Paluch
944 Spec #94 Gina Marie Paper Designs
Arizona Regional 944 Spec Director, National Rules Coordinator
2006 Az Champion - 944 Spec Racer Since 2002

Re: HELP! engine runs rough after warm up 11 years, 8 months ago #10818

  • dmdirks
  • OFFLINE
  • Moderator
  • Posts: 172
Check the fuel pressure regulator and damper.

While the engine is running, pull the vacuum lines from them and see if fuel is coming out the vacuum port(s).

Recently I came across a similar problem as you describe. The diaphram inside the damper had a hole in it. Basically the engine was sucking fuel straight through the vacuum line into the intake manifold so it ran like crap at idle. But it took about 30 seconds of idling for the engine to do this, and the car ran fine at WOT.
David Dirks
www.944MotorWerks.com
2010 944-Spec National Champion
Rocky Mountain Region

Re: HELP! engine runs rough after warm up 11 years, 8 months ago #10836

  • jeffb
  • OFFLINE
  • Drivers Ed
  • Posts: 9
FIXED! Thanks for all your input. Turns out the air fuel mix was way too lean. Got that adjusted but the problem still reared it's ugly head, only not as bad. New O2 sensor took care of it. Spent the day at Miller yesterday and ran strong

Re: HELP! engine runs rough after warm up 8 years, 4 months ago #18272

  • mick
  • OFFLINE
  • Drivers Ed
  • Posts: 1
OK, so you have only replaced a few of the components. Lets go buy a coil, an ignition module, a distributor, a crank and cam sensor, new plugs and wires, a new ecm, maybe a new wiring harness, a new coolant temperature sensor, replace the intake manifold gasket, do a valve job, might as well throw a set of rings and bearings in it and a timing belt, an alternator just in case its sending out some AC current to the system, a fuse box that has ASD, ECM and other relays in it that control the system. I suggest that you also replace the fuel pump assy. and a fuel filter.
I'm not trying to be a smart ass here, but you don't throw parts at a vehicle to find the problem. You find the exact problem through proper diagnostics and then you replace the failed part(s). This is not a complicated process if you have the proper knowledge. Don't ask a chicken to fly an airplane with your family in it!!!
As a seasoned and extremely well educated automotive tech I get rattled by stupid suggestions that will most likely cost you money that you didn't need to spend.
GET A GOOD TECH TO FIX IT AND BE DONE WITH IT.
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