6.2l (1991) Rough Start / Running - Expert Opinions?

Disco Dave · December 6, 2010

I have a 1991 Silverado 1/2 ton with the 6.2l diesel. When it was in use I had bad fuel leaks around the fuel filter area at the back of the motor. This lead to (I assume) it sucking in air making the engine run rough from time to time and the lovely white smoke, but it wasn't that bad to live with at the time. Once I replaced the truck it sat for a couple of months before I decided to get back to it. My first attempt was to fix the leaks, but I believe that the block the filter mounts to may have either been damaged or I just didn't replace enough stuff (seal around water sensor etc) to get the job done.

A little bit later I inherited an older 6.2l (1985-6 ish) already removed from a truck for another project. I decided to steal the filter set-up off of that motor, as it looked dry and non leaky (and it had both proper purge plugs in it). I went to install it but the lines on it point in the opposite direction, so I was unable to mount it to the motor upright, because the line to the IP would not be long enough. It is instead lying flat on its back in roughly the same place it would be standing upright. I am unable to plug the truck in to get the engine warmed up, but my glow plugs do work. I am finding that it takes quite a while to get the truck even started (like I crank it in 15 second intervals enough to run the batteries down and have to bring in a boost). I get partial fires during that time and a lot of smoke but it just doesn't want to catch for more than two seconds at a time. When it finally is running, I am not shocked to see that it is quite smokey and running a little rough.

Back when I was driving it, if it ran rough at the start, I would just have to wait until it was a little warm and maybe rev it out a bit, then I would have a smooth clean running truck. However now, as the truck gets warmer, it starts to run rougher and rougher, with an ungodly amount of smoke. It is doing the opposite of what I would expect. There are no longer any fuel leaks around the filter (or anywhere else that I could see) but as I said, the entire fuel filter housing is now lying horizontal (filter side down) instead of vertical. So the questions:

1) Would the orientation of the filter block have that much effect on the flow of fuel? I can attempt to add a small metal tube with a 180 curve in it to meet the fuel outlet with the filter oriented the way it sould be if you think that would really help.

2) The smoke is solid white and bitter (not sweet smelling like when you are burning antifreeze on a gas engine). Is this a fuel mixture / temperature issue?

3) The engine runs rougher and rougher as it goes on. I would equate it to running waaaaaaaaay too rich with gasoline in regards to sound and behavior when revving. Would there be any reason that the IP would be providing too much fuel just from having sat unstarted for a couple of months?

4) Unrelated somewhat, but the filter block off the older truck has a third set of wires coming off it. One would be the water sensor, the second possibly a pressure sensor that sets off the 'change filter' light at the appropriate time, so what is the third set (2 wires I believe)? Is this a heater?

Thanks for your assistance.

gmtech4 · January 20, 2011

i cant read it all ,holy shit ....

GMTRUCKS · January 21, 2011

I have a 1991 Silverado 1/2 ton with the 6.2l diesel. When it was in use I had bad fuel leaks around the fuel filter area at the back of the motor. This lead to (I assume) it sucking in air making the engine run rough from time to time and the lovely white smoke, but it wasn't that bad to live with at the time. Once I replaced the truck it sat for a couple of months before I decided to get back to it. My first attempt was to fix the leaks, but I believe that the block the filter mounts to may have either been damaged or I just didn't replace enough stuff (seal around water sensor etc) to get the job done.

A little bit later I inherited an older 6.2l (1985-6 ish) already removed from a truck for another project. I decided to steal the filter set-up off of that motor, as it looked dry and non leaky (and it had both proper purge plugs in it). I went to install it but the lines on it point in the opposite direction, so I was unable to mount it to the motor upright, because the line to the IP would not be long enough. It is instead lying flat on its back in roughly the same place it would be standing upright. I am unable to plug the truck in to get the engine warmed up, but my glow plugs do work. I am finding that it takes quite a while to get the truck even started (like I crank it in 15 second intervals enough to run the batteries down and have to bring in a boost). I get partial fires during that time and a lot of smoke but it just doesn't want to catch for more than two seconds at a time. When it finally is running, I am not shocked to see that it is quite smokey and running a little rough.

Back when I was driving it, if it ran rough at the start, I would just have to wait until it was a little warm and maybe rev it out a bit, then I would have a smooth clean running truck. However now, as the truck gets warmer, it starts to run rougher and rougher, with an ungodly amount of smoke. It is doing the opposite of what I would expect. There are no longer any fuel leaks around the filter (or anywhere else that I could see) but as I said, the entire fuel filter housing is now lying horizontal (filter side down) instead of vertical. So the questions:

1) Would the orientation of the filter block have that much effect on the flow of fuel? I can attempt to add a small metal tube with a 180 curve in it to meet the fuel outlet with the filter oriented the way it sould be if you think that would really help.

2) The smoke is solid white and bitter (not sweet smelling like when you are burning antifreeze on a gas engine). Is this a fuel mixture / temperature issue?

3) The engine runs rougher and rougher as it goes on. I would equate it to running waaaaaaaaay too rich with gasoline in regards to sound and behavior when revving. Would there be any reason that the IP would be providing too much fuel just from having sat unstarted for a couple of months?

4) Unrelated somewhat, but the filter block off the older truck has a third set of wires coming off it. One would be the water sensor, the second possibly a pressure sensor that sets off the 'change filter' light at the appropriate time, so what is the third set (2 wires I believe)? Is this a heater?

Thanks for your assistance.

Sounds like to me that your injector pump is bad, in fact i can almost guarantee that is your problem, these engines were well known for bad injector pumps, and GM did absolutley nothing to fix them. Symptoms of a bad inj-pump are: Very difficult to start, engine has almost no power under accelleration, large amounts of smoke, hard to keep running at idle. Don't worry you're not the only one, my grandfather has had one 6.2 and two 6.5's in his life and they all needed injector pumps by 30,000. But atleast these engines aren't as bad as those POS oldsmobile 350 and 262 diesels, that practically blew up.