99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

[dr_lee29]

Hi I have a 99 S10 4.3 4X4 with some problems, wondered if anyone out there could help.
Its idle is rough, but it doesn't seem to have lost power on the highway. I noticed really poor gas mileage (don't have exact numbers but I just burned through over a quarter of a tank in 20 miles). The temp is reading about 150 even after several miles on the highway. The oil pressure gauge is pegged at 80! The vacuum seems good, it reads 19 off the manifold at idle. The SES light just came on BUT, I've never been able to read SES lights on this truck, there's something wrong with the service port and the scanners just say error or no power (the truck has been this way for the last 6 years).
OK I started with replacing the thermostat, temp sensor, distributer rotor, IAC. I cleaned out the EGR valve and PCV. I bought one O2 sensor to check the function of the truck's O2 sensors (I swapped them out one by one checking idle after every swap). The spark plugs look clean, no carbon, no oil.
Anyone have any ideas? I wonder if the main computer is dead? The oil pressure and low temp is just really strange.

[MT-2500]

You need to check for codes and make sure engine is coming up to operating tempt.
If no scanner read out check fuses.
X or W code engine?
Cigar lighter fuse to pin 16 on diagnostic connector.
Post back code no.

[dr_lee29]

Thanks for the help!
Its an X engine.
I've checked the fuses in the fuse box located on the drivers side door these all look fine, is there another set of fuses that would run the diagnostic port? Are you saying the cigar lighter fuse runs the diagnostic connector? Or I can run power to the port by jumping a line from the cigar lighter to pin 16? If so, where can I get a diagram of the port to find pin 16? Again, I really appreciate the help!

[MT-2500]

Originally Posted by dr_lee29

Thanks for the help!
Its an X engine.
I've checked the fuses in the fuse box located on the drivers side door these all look fine, is there another set of fuses that would run the diagnostic port? Are you saying the cigar lighter fuse runs the diagnostic connector? Or I can run power to the port by jumping a line from the cigar lighter to pin 16? If so, where can I get a diagram of the port to find pin 16? Again, I really appreciate the help!

http://www.troublecodes.net/OBD2/

Fuse 2 cigar lighter on end of dash.
Put a test light on it and check for 12 volts both sides.

Yes you can run a fused jumper wire to pin 16 if all else fails.
Get power on it and check codes and then clear codes and see what comes back.
Post back code no.
Good Luck
Good Luck.

[dr_lee29]

I jumped a wire from the cig lighter to pin 16, and I was finally able to check the codes. I have a random cylinder misfire on cylinder 4. Any advice?

[MT-2500]

How ramdom?
Can you feel it miss?

[dr_lee29]

No, I can't say I "feel" it miss in the sense that there is a rhythm to the miss. If anything it seems like the truck is just idling low and rough.

So let me step back just a bit as well. When this whole thing started (rough idle, SES light, low temp), I noticed a little bit of coolant on the front of the block and thought the LIM was bad so I replaced it. After I got it all buttoned back up, there was the same problems as before but now the low temp is much more pronounced (150 instead of 180).

I just can't figure out the low temp thing, unless there is a problem with the computer (oil pressure gauge is pegged at over 80 constantly).

[MT-2500]

Make sure engine is getting up to 195 running tempt.
Check for vacuum leaks and egr and pcv valve sticking.
Check fuel presure for up to par.
And go over all computer readings for up to par.
Double check plug and wire and cap and rotor on no 4 cly.
Run a compression test.

[dr_lee29]

I'm positive the engine isn't reaching 195 degrees. I don't know how to make it get warm either, I thought the new thermostat would fix that but it didn't. I'll double check the wiring. Where is the best place to get a vacuum reading?

[MT-2500]

Originally Posted by dr_lee29

I'm positive the engine isn't reaching 195 degrees. I don't know how to make it get warm either, I thought the new thermostat would fix that but it didn't. I'll double check the wiring. Where is the best place to get a vacuum reading?

Some times new stat's are bad.
Take a temt reading of engine after warm up.

Take Vacuum reading with a vacuum gauge hooked to mainengine intake vacuum line.
Check vacuum at idle and at 2K rpm and post back reading.

[dr_lee29]

Checked the vac tonight. Off of the manifold, I get 19 at idle, 21 at 2000 rpm. The vac seems to react well also. When I snap rev the truck the vac drops to 0 then raises quickly to 20-21, decel it raises to 26 then drops to 19. I can't complain about those numbers.
I don't have a cylinder compression gauge so I haven't tested compression, but that would come out in the vac numbers wouldn't it? Besides the truck seems to have power, it just sucks down the gas and idles rough.
I checked the plug wires, they all seem good. I turned off the lights and watched for sparks and didn't see anything. I'm leaning towards buying a coil, anyone have any other suggestions?

[MT-2500]

Originally Posted by dr_lee29

Checked the vac tonight. Off of the manifold, I get 19 at idle, 21 at 2000 rpm. The vac seems to react well also. When I snap rev the truck the vac drops to 0 then raises quickly to 20-21, decel it raises to 26 then drops to 19. I can't complain about those numbers.
I don't have a cylinder compression gauge so I haven't tested compression, but that would come out in the vac numbers wouldn't it? Besides the truck seems to have power, it just sucks down the gas and idles rough.
I checked the plug wires, they all seem good. I turned off the lights and watched for sparks and didn't see anything. I'm leaning towards buying a coil, anyone have any other suggestions?

Vacuum does look good.
Make sure the engine is getting up to 195 degrees running temt.
How old are the plugs and wires?
Does it have good hot blue spark to all spark plugs?

What is the gas milage MPG in town and on the road?

[dr_lee29]

The plug wires are probably pretty old, I'll just change those out tonight, its worth doing anyway. New coil while I'm at it.
I don't know the exact gas mileage because I haven't filled it back up to check how much gas I've used but I'd guess I'm hovering around the 10 MPG mark, just about half what I normally get. That's mostly in town driving, just a little bit of highway.
So while I'm under the hood tonight, I thought I would take a look at the oil pressure sensor, where is it located?

[MT-2500]

You really need to check MPG.
You have never answered the question of.
We need information and answers to be able to help.
How is the engine tempt?

Good time to replace plugs with ac delco plugs when repacing wires.
Good Luck

[dr_lee29]

The engine is still running cool even after a new thermostat and temp sensor. I'm at 150 - 160.
I'll fill the tank when I pick up the plug wires and such.