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1994 GMC Sierra 1500 6.5 diesel 4x4
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Discussion Starter #1
Hey everyone first time posting!
read many things on this site and usually always got my questions answered. I have read many things about my problem that are close but can’t find anything the exact same. I’m stumped on this one.
this is gona be a lengthy post so I apologize in advance! But I always read about not enough information so I’m gonna get everything out right off the top.

1994 GMC Sierra 1500 6.5 4x4 328000 km

mechanically the truck is stock for the most part. The P.M.D has been relocated to the fire wall on the driver side. Exhaust upgraded to 4 inch, crossover pipe 3 inch.
as far as normal ware and tare goes I have not had any major issues with it in the last 6 years that I have owned it, it has been a great truck.
(my aunt and uncle bought it new and it was his farm truck. He passed away and she had no use for it so I ended up with it)

here are the known issues.
I just replaced the water pump.
the waste gate solenoid is not working hasn’t since the day I got the truck it was wired all the way back. However the turbo still works I can hear it and it usually goes up to about 6-7 psi on the boost gauge.
if you keep the truck around or below roughly 2500 rpm when accelerating it will hold boost on the gauge. Anything over that it drops to zero faster than you can blink. going down the hwy once at desired speed it drops to zero.
some days it works just fine other days it won’t leave zero.. one thing I could never figure out is (living in Ontario). Summer it works like I just described above. Winter time.. say -10 and colder it will pull about 10 pounds of boost and never drop to zero even on the hwy at 115km it holds steady at about 4 pounds. Maybe because it’s getting constant cold air?? But it always works flawlessly in cold weather.

The glow plug relay quit working and for the price of a new one I ran two wires through the fire wall to a Togo switch never had a problem with that. Other than I can’t use the automatic start!

there is a hole in the exhaust pipe coming down just from the manifold on the passenger side where the o2 sensor goes so it’s a hole the size of the o2 sensor.

other then that problem wise that I know of that’s all.
until the last couple weeks...
The truck has been very hard to start ( it’s been anywhere from 25-30 degrees every day) even with letting the glow plugs run for 10 -15 seconds it takes about 15-20 seconds roughly to get it and It’s spitting and sputtering then quits even with the throttle applied then repeat and usually fight through the spitting and sputtering and ideals at 600 rpm. Sounds great. Until you drive it. (I should mention that the no power thing has happened to me a few times in the recent past but if I shut off the truck and started it again it would work fine)
The turbo will not kick in not on the gauge nor by sound of hearing it. It’s not got the power to pull itself up a hill. If you floor it it then it sounds like it is bouncing off the rev limiter once you hit roughly 3400 rpm just before the red line. And it will not shift into the next gear??? If you try to slowly get it up to speed it will shift..
there is no black smoke or any smoke for that matter. This all just happened pretty much instantly like I said I have never had any issues with it until now besides the water pump a few weeks ago.
I have changed the fuel filter
I run esso or petro Canada fuel through it and run fuel and injection cleaner through it every few tanks. Ido the oil changes as required. It’s generally very well taken care of.
It had the P.D.M. Installed and relocated in 2013 and the resistor replaced. All engine grounds cleaned and tightened. Lift pump and oil pressure switch replaced.
brand new full exhaust system in 2018 except for the pipes coming from the manifolds (hence the hole in it) 2 new batteries in the fall of 2019 I believe 1000 C.C.A. but could be 800 have to double check.

I'm nota mechanic by any means but I can handle most anything to fix and work on with my truck just don’t ask me to rebuild a motor lol.

I understand that the stuff done in 2013 was 7 years ago and could be bad.
I also will throw this out there.
I have never changed the injectors and I have no idea when or if ever they have been replaced.
I’m just creeping up on 100 thousand km since I got the truck 6-7 years ago. I’m sure they could stand to be replaced.
But I would like some input before I start the guessing game and waisting money and time. I really don’t know much about Diesel engines other than what I have researched and learned since I have owned the truck and until the last couple years I was hard pressed to find much information on the 6.5.
thanks guys.

16,020 Posts
Start by going through all of your Turbo vacuum system.
If you have the stock, plastic vac lines I would recommend that you replace them with rubber vac line.
The OEM plastic line becomes brittle junk and is not worth repairing.
With a vacuum gauge attached close to the vac pump, make sure you have a minimum of 22hg. of vac from the pump.
If it is less, replace the vac pump.
Replace the wastegate Solenoid with an AC/Delco brand part.
Next, your exhaust system needs attention.
If you have exhaust leaks your losing boost from the Turbo.
everything Pre Turbo needs to be Solid ( ie, manifolds tight to the block, no leaks, crossover pipe solid and tight, etc)
The Diesel has no 02 sensors. It would only be equipped with a soot trap or Catalytic Converter in some models.

Boost is not a constant. The PCM commands boost as needed with load.
You will see, maybe 1-2psi while cruising down the highway then nothing with no load.
In hilly terrain you might see 4-5 psi then drop off.
If you are hauling a heavy load and driving in hills you might see a constant 5-10psi.
That is a normal condition for the Turbo
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