99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

[MT-2500]

You need to get that engine tempt up to 155 degrees for it to run right and get good gas mileage.
Try another thermostat to start with.
Good luck

[dr_lee29]

Changed the plug wires and the coil. I can't believe they used rivets to hold the coil on! What kind of stupid engineering is that? So after grinding down the rivets and knocking them through with a punch, I put on the new coil with the proper fasteners (they're called bolts and nuts, GM!). Regardless, the truck runs exactly the same, rough idle. I'm about ready to slide a stick of TNT in the gas tank.

[dr_lee29]

By the way, I know that it needs to get to the proper temp in order to get good gas mileage, but it idles rough cold and hot.
In case anyone is keeping track
New LIM gasket, temp sensor, thermostat, IAC, plug wires, rotor, coil, cleaned PCV, EGR, checked O2 sensors. Are there anymore Made in China parts I can replace on this heap!

[dr_lee29]

Its official, I checked the gas mileage today and I'm getting about 10 mpg.

[MT-2500]

Originally Posted by dr_lee29

Its official, I checked the gas mileage today and I'm getting about 10 mpg.

Cold running will do that.
Has it still got miss fire?
Get that engine tempt problem fixed.
No amount of throwing parts at it is going to fix it untill you
Get and do some proper testing.
Have it put on a good engine capable scanner and checked out also make sure fuel pressure is up to par.
Read out the engine running data in scanner.
AND GO FROM THERE.
GOOD LUCK

[dr_lee29]

Well, I changed the thermostat again and the spark plugs (probably should have been done a while back). The truck is still running too cold (max of 180, but most of the time 150-160). Its like the thermostat is open all the time, I can't hardly believe that I had 3 bad thermostats in a row. There has to be something else causing the truck to run cold.
The spark plugs looked pretty good overall, there was a little carbon, and they were a little wet (gas not oil). Which makes sense to me, the truck is running cold and its sucking down the gas to get it to temp, thus the bad gas mileage, rough idle. I just don't know how to get it to warm up, other than dynamite in the tank which very well may be my next move.

[MT-2500]

How are you checking you running tempt?
Check your actual running temp and check the pcm coolant sensor reading.
Do they match?

[dr_lee29]

Almost forgot, I cleared the codes (random cylinder misfire, misfire #4). I want to see if it will throw the same codes again.
Thanks again for the ideas, keep sending them my way.

[dr_lee29]

Yeah, its really cold, I stuck a thermometer in the radiator and sure enough its at 165.

[MT-2500]

Originally Posted by dr_lee29

Yeah, its really cold, I stuck a thermometer in the radiator and sure enough its at 165.

Rad stay cold in cold weather.
You need to check the engine block tempt.
What is the PCM tempt sensor reading?

[dr_lee29]

I had cleared the codes and the SES light came on again today so I read the codes again and now I only have a misfire on cylinder 4. I really wonder if the injector is bad in that cylinder? Is there a way of testing the injector?

[MT-2500]

Get it on a good engine capable scanner that has the built in injector balance test in it.

[dr_lee29]

Today after church I pulled the upper air plenum to look at the spider. Both the spider and the manifold are a little wet. I snapped some pics, let me know what you think.

[dr_lee29]

sorry I messed that up, here we go again:
[IMG][/IMG]
[IMG][/IMG]
[IMG][/IMG]

[dr_lee29]

Some of the wetness seemed more like oil than gas but its hard to say. The bottom of the spider is wet with gas but I don't know if thats normal?