My FWD, 91' Fleetwood Cadillac, w/ 4.9L V8, started to run rough about
five minutes after starting from cold and ran rough for the next 15
minutes. I would punch the accelerator petal to alleviate the symptom
for a short period of time while continuing to drive. This started six
weeks after I bought it in May 2003 and the first time I had a tank
full of 10% ethanol. The car has been doing that ever since (2 ½
years). However, in Dec 2005 it started to run very, very rough (lost
power dramatically and backfired). A few times the car's speed went
from 60 down to 25 on the freeway and as soon as we pulled on the
shoulder for a minute or two we could accelerate again for the rest of
the one hour trip. It acts like the vacuum advance stops working
temporarily. Typically happens during warm up period (about 5 minutes
after starting till 20 minutes after starting).
In early Feb I let my mechanic spend four hours on it. They road
tested it and experienced the problem. They checked the ignition and
fuel system. Replace the worn two year old rotor in the distributor.
Tested the fuel pressure and it was good. Cold cranking amps tested
just a smidgen below spec (6 month old, large Wal-Mart battery).
Trouble codes indicated Camshaft Position Sensor. They cleaned and
secured the connection for the Camshaft Position Sensor and retested.
They thought they might have had it licked.
But, when I drove it there was really no change in the symptoms. I did
discover something new on my next 130 mile trip. When it starts to run
rough I can turn off the headlights and it smoothes out ASAP. If I
turn on the headlights it runs rough instantly (no delay whatsoever)
The heater motor blower and the brake lights have a similar impact, but
not as pronounced. And, this happens now even after car has been
running for hours.