1995 350 ecm (?) problem

I have a 1995 chevy 350 engine that WAS giving me about 17mpg. Suddenly, it drops to 10 mpg. I do the usual (plugs, wires, dist & cap, injector cleaner,
o2 sensor, etc), with NO change. There is NO code stored in the ecm, no check engine lite. I gave up and took it to a GM dealer, who charged me $150 to hook it to the computer. I was told that the engine was running too cool, so it never goes into closed loop. They said change the thermostat, the coolant sensor, and possibly the ecm module (they wanted $900). I changed the thermostat (195 degree) and the coolant temp sensor, with no change at all. I even checked the wiring to the ecm module, thinking maybe I had a broken wire, but all checked well. I changed the ecm module. No change. I changed the Knock sensor, no change. How can I tell if the ecm is going into closed loop? I monitored the O2 sensor, which after a minute or so (coming up to temperature) shows about .85 to .9 volts at idle (which is quite rich). At driving speed, it jumps so much I cant tell. There must be some voltage measurement to tell me if the ecm goes into closed loop! Anyone have any ideas whats up with this monster? I am thinking about putting in a higher temp thermostat to see if it does anything, but am not sure that this is even in the ballpark of the problem. There is no smoke, doesnt burn oil, no coolant leakage, but the exhaust smells rich. Thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
There are two coolant sensors, on for ECM and one for gauge, might sure you change the right one. The one you want is on side of engine mounted in head. You might check MAP sensor too. ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
A scan will tell what is happening if the coolant temp or sensor is reading or running to cold, no other way here but parts guess work.
wrote:

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

If you read his earlier post the dealer said his engine was running cold suggesting a bad ECM temp sensor for engine temp. It can cause engine to go over rich and use more fuel as can a bad MAP snesor can soewhat too but the temp sensor would be my first choice. It is not likely the TP sensor (throttle postition) given his problem. ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Didnt know there were 2 sensors...I just changed the one by the thermostat housing.Is the one for the ecm still called a coolant temperature sensor? Need to know what to ask for at the parts store. Learning more everyday!!! Thanks!
SnoMan wrote:

--
Message posted via CarKB.com
http://www.carkb.com/Uwe/Forums.aspx/chevrolet-trucks/200604/1
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 29 Apr 2006 16:12:17 GMT, "Bornloser via CarKB.com"

Yes there are two. One for gauge and one for ECM. They one you want mounts in head and is called a engine coolant temp sensor or ECT. That is the one you need to change. "Should" be on drivers side of engine between #1 & 3 cylinder. ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Is there a voltage or resistance test/standard for it? TIA. TomC

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

There is one. at 40 degrees it should read about 7500 ohms, at 70 degrees F, it should read 3400 ohms, at 160 degrees, 450 ohm and at 210 degrees 185 ohms. The MAT sensor has the same values. On the TPS. it should read between .45 and 1.25 voltage at idle on the blue wire coming out of TPS and icrease smoothly with throttle. On MAP sensor, it should read between 3.6 and 5.2 volts with key on but engine not running on the light green wire coming from MAP sensor. I hope this helps ----------------- The SnoMan www.thesnoman.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Agreed snoman ,but nothing was accomplished as a result. I suggested a rescan to actually see what the temp sensor was saying, not just replacing parts.

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

a scan tool will tell you if you are in closed or open loop. On my 92 the coolant temp sensor that feed the ECM is on the intake near the water neck (which i think you asked about in another post) the sensor that drives the gauge is on the cylinder head.
----------- Elbert snipped-for-privacy@me.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hopefully the rain will stop tomorrow and I can do a little checking with the info I have gathered here. But, as I do NOT have a scan tool, is there any way to check if the ecm is in closed loop with a volt meter? Thanks again
Elbert wrote:

--
Message posted via CarKB.com
http://www.carkb.com/Uwe/Forums.aspx/chevrolet-trucks/200604/1
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 30 Apr 2006 04:51:47 GMT, "Bornloser via CarKB.com"

I don't know how you can tell without the scan tool. Basically when the truck reaches operating temperature the ecm goes into closed loop. ----------- Elbert snipped-for-privacy@me.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
============ "Bornloser via CarKB.com" wrote:

==============> >>Elbert

You don't need any tools to check for closed loop, usually.
If the trans works properly.......... The lock-up convertor engages only during closed loop.
fact, not fiction.
Test :: Drive around for at least 5 minutes. Insure the engines warm and the tranny is warmed up.
At a steady 50mph LIGHTLY tap the brake pedal.
you should feel an rpm change in the engine as the convertor unlocks and locks back up.
~:~ Marsh ~no tools diagnostics~
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.