1995 GMC Sierra Runs Rough and No Power

Maintenance/Repairs
1

I have a 1995 GMC that I have owned since new. It has 212,000 miles on it, a 305 TBI, and a manual five speed. It started running bad all at once. It idles rough, at different speeds and never settles down, (between 900 and 1300 rpm. It smells like raw gas, smokes, and has no power to get started from a stop. I checked the vacuum, it is at 19" at all rpms. The MAP sensor reads 0.3VDC when off, 4.12VDC when idling, and 1.5VDC at WOT. It appears to have 12PSI fuel pressure (after a messy test set up!) on the fuel pump side of the fuel filter (filter replaced 10,000 miles ago). The EGR valve appears to cycle OK when I suck on the vacuum line so I assume it works OK. I found a bad TBI flange gasket and replaced it. It runs slightly better but still smells like raw gas and has no power. When I try to accelerate, it immediately has no power, like a big vacuum leak. The idle is close to normal (900rpm = normal). Any ideas?? Do I just start replacing parts?

2

The problem might be with the fuel pressure regulator.

http://www.rockauto.com/catalog/moreinfo.php?pk=951154&cc=1158117

The fuel pressure regulator is replaced by removing the top of the throttle body.

Tester

3

I read some articles on the MAP sensor and it appears the voltages should be int he opposite direction, with a lower voltage at higher vacuum. Can anyone confirm how it is supposed to read?

4

Since your vehicle is an OBDI, just unplug the vacuum hose to the MAP and plug it. This will make the computer go to a default mode so the engine should run.

If the engine still runs like it’s flooding out, check the fuel pressure regulator.

Tester

5

Thanks Tester. I’ll give it a shot!

6

The problem could be a plugged vac port for the map sensor at the back of the throttle body. As “part throttle” has stated your voltages should be in the opposite direction. You should have around 1-1.5 volts at idle and around 4-4.5 at WOT. Key on engine off should be real close to 5 volts. Have ran into this problem on these old TBI trucks plenty. And this scenario will make it run RICH.

7

I ran into a problem like this once and it turned out to be the coolant switch. I believe this vintage still has the two coolant “sensors”; one an analog sender that drives the gauge and a second that is basically a switch to inform the ECM that the engine has reached operating temperature. When the switch fails, the ECM never goes to closed loop and dumps a lot of fuel into the engine thinking it is cold.

Happened to me once and the truck had almost no power, was running super rich, stumbling and almost started the carpet on fire when the cat got super hot trying to limp it home. The solution was a $5 coolant switch…

8

12PSI sounds way low for throttle body injection. If not the pressure regulator, then fuel pump?

9

9-13 PSI is specs for a GM TBI

10

I checked out the temperature sensor only because it was cheaper to check it than to buy the fuel regulator parts. I read up on the sensor and a manual mentioned something to the effect of “don’t mess up the sensor, it will have serious effects on the fuel system”. Turns out they were right! It read an open circuit at the ecm and the connector at the sensor was rusty, as was the sensor. Once you cleaned off the little posts and beared down on the sensor pins with an ohm meter, it read OK. I replaced the connector and the temp sensor and it runs great!!!

Thanks for all the ideas and hints! Especially thanks to TwinTurbo!