Dave, Here is my answer. The demand for fuel is higher when the engine is cold. The warm up regulator when cold has about a 1.5 bar of pressure (22PSI ) on top of the control plunger in the fuel distributor. This allows the plunger to rise higher allowing more fuel to flow to the injectors in the cold stage of the engine. In other words it's running richer and needs more gas. Then as the car warms up and the resistor in the WUR heats up it allows the bi-metal arm to start the change from a WUR to a CPR or control pressure regulator ( CPR ) ( 3.8 bar 55 PSI ) it puts more pressure on the FD control plunger by not allowing it to rise as high and feed as much fuel to the injectors. About the same time all this is going on the 02 sensor is also leaning out the fuel by allowing it to return to the tank. Hence: Less fuel is needed in just a fuel minuets. With-in a few minuets after all this is done and the car is popping back thru the air flow meter like a small back fire, then your running to lean and normally the WUR is running to high on fuel pressure via the control plunger if everything else is in order. If your running ok after heat up then the demand for fuel isn't there as of yet. But, If the filter isn't changed soon, then it will start leaning out even after it gets warmed and start popping back thru the air flow meter. When I'm dealing with the 911 Porsche people about this rich and lean situation, a new fuel filter is the first thing I sell them or have them put on. The reason is out of sight, out of mined on the filter and they forget about it. Then we can analyze after that. The reason they have the problem is because they redline the RPM's most of the time before they shift or it's a 930 turbo. John Hervey -----Original Message----- From: dmcnews@xxxxxxxxxxxxxxx [mailto:dmcnews@xxxxxxxxxxxxxxx] On Behalf Of doctorDHD@xxxxxxx Sent: Saturday, October 22, 2005 1:39 PM To: dmcnews@xxxxxxxxxxxxxxx Subject: [DML] RE: Strange acceleration problem Can someone explain to me how a bad fuel filter can cause hesitation and back firing only when the engine is cold? Thanks, D² & 6530 To address comments privately to the moderating team, please address: moderators@xxxxxxxxxxx For more info on the list, tech articles, cars for sale see www.dmcnews.com To search the archives or view files, log in at http://groups.yahoo.com/group/dmcnews Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/dmcnews/ <*> To unsubscribe from this group, send an email to: dmcnews-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/