Posted by Charlie (IP: 131.204.68.117) on July 18, 2000 at 20:28:17:
In Reply to: O.K. You ASE mechanics........... posted by Craig Selvey on July 17, 2000 at 21:26:34:
Definitely change the thermostat. The ECM monitors the Engine Coolant Temp (ECT) and that has a good deal of input into fuel mixture, etc. Usually this would richen the mixture and cause black rich exhaust. However, you could be running into some major problems if the check engine light has been on for awhile. A 160 thermostat could cause the system to never go into closed loop. Closed loop operation is at normal operating tempurature. In closed loop the computer monitors the o2 sensors and decides the air/fuel ratio. If your truck doesn't reach operating temp as defined by the ECM, you might stay in open loop. In open loop the air/fuel ratio is a set standard independent of o2 sensors. What I think has happened, is that the extended open loop operation has fouled the o2 sensors and when it does go into closed loop, the o2 sensors are reading rich, causing the computer to lean the mixture and making hot exhaust.
The cause is almost certainly a lean condition, I'd check fuel pressure and if that's okay then change the o2 sensor(s). ANd change that thermostat back. As DaD said, the only real way to tell is to look at computer data with a scan tool, but from my experience this is a likely cause. If you do replace the o2 sensors disconnect the battery for awhile and step on the brake pedal to clear ECM memory. Then if the light doesn't come back you're set.
The exhaust could be plugged, especially if it is mostly original. The converters are more likely to restrict the systen than a muffler, the only real way to tell without a gauge is to remove the exhaust ahead of the cat and see if it runs better. Usually a restricted exhaust has other more identifiable symptoms, like low power, etc. I don't personally think exhaust is the main cause. -CP
: Here is my problem. I have a 1988 Ford F-250 with a 351W in it. The exhaust, in my opinion, is getting too darn hot. It literarly "cooked" and "baked" my brand new spark plug wires until they were so brittle they shorted out.
: I have replaced one "reed" valve and the hose that hooks up to it about a year ago (on the smog pump stuff). That hose is now hard as a rock.
: My thoughts....I replaced the thermostat with a 160 degree thermostat about a year ago. Maybe the computer thinks the engine is running too cold, so it increases the fuel mixture...making the exhaust get too hot.
: The truck has never overheated....just anything near the exhaust manifolds gets cooked.
: It runs pretty good "cold". Runs rough when warmed up. The check engine lite comes on and go off again.
: Any thoughts? (Please!!!)