I just bought a 1988 GMC Sierra with standard transmission. Strangely this truck has a turbo, which makes me think the engine is 6.5, but I may be mistaken. The stickers in the engine bay are removed. The motor has a big problem with the following symptoms:

1. Idle is very rough - as if one cylinder is not working.
2. It smokes really bad with gray smoke, not black. It smokes mostly when idling, when it is driving the smoke is not so niticeable.
3. It seems to accelerate OK, but once I shift into 4 gear and the RPM iare relatively low, I feel the motor is stumbling, as if misfiring and does not have lots of power.
4. There is severe banging from the bottom at idle. It is dull sound like banging on a door, not like hammering on an anvil. One might think that there is a camshaft/piston problem, but that hammering disapears with the slightlest increase of RPM.

After reading on this forum I decided that this might be an injector or a IP problem and was about to purchase injectors on eBay, when I discovered that the IP on the motor looks like the 6.2 pumps, not like a 6.5 pump.

So far my theory is that some of the previous owners has replaced the original 6.2 with 6.5 motor, but has kept the 6.2 pump. Is this possible?

1. If it is possible, could that be the problem?
2. Should I replace the injectors? My understanding is that it is a good idea to replace them anyways.
3. Should I buy a 6.5 IP, or that pump requires other parts to work (like a PCM)

What else can I do to identify the motor and diagnose the problem?

1. If you are saying that it 'looks like a 6.2 pump" that just means that it has a mechanical setup, no computer to mess with. :) The pumps will interchange between engines without problems.

2. Injectors are a maintenance item and most people say to replace them every 100,000 miles. It's up to you, if it turns out that it needs injectors to help fix your problem great, if it doesn't need them it would be a good idea to anyways.

3. There isn't really such thing as a "6.5" or a "6.2" pump, they are listed as DB2 pump (mechanically controlled) and as a DS4 (electronically controlled). If you have a mechanically controlled DB2, it would be best to stick with that.

As for that severe banging, by how you described it going away, I would check out your harmonic balancer, and since yours is a manual, it MAY have a dual mass flywheel, and I have heard of problems with them.

What does your turbocharger look like? Does it says 'Banks" on it anywhere?

I2. Should I replace the injectors? My understanding is that it is a good idea to replace them anyways.


I sent you a PM about the injectors.

chevyinlinesix, these are great tips. I noticed that the belt tentioner was bouncing too much at idle, but it didn't occur to me that it could be the harmonic ballancer. I will check tonight and hopefully I will have one problem less.

It is also great info that the two IPs are interchangeable - this tells me that there is nothing wrong with my setup.

I don't see any marking on the turbo. Are you saying it is possible to have turbo on the 6.2 engine? My turbo looks like a stock one.

Now the main question that remains is whether the mechanical IP that I have is causing the rough idle, the excessive white smoke and the lack of power?

My plan is to replace the injectors and see if the problem persists, and if it does, to replace the IP. Does that sound like a good plan? Is there any other way to diagnose? A mechanic was advising me to disconnect the exhaust pipes from both sides and see if the white smoke is coming from both sides. His theory is that if it does come from both sides - it must be the IP, if it comes from just one side - it must be an injector.

Yes, the 6.2's were available turbocharged, they came with banks setups designed to be installed at the dealerships.

If your turbo looks like a regular, GM-x turbo, it should have a tag directly above the turbocharger's inlet. It would say SPEC: GM-3, or GM-4, ect. Some people do in fact take all the turbocharger equipment off of a 6.5 and swap it onto a 6.2. Only way to MAYBE differentiate the 6.5 and 6.2 is by looking at the block casting number, but even then it is not 100% accurate.

I like the mechanics idea of disconnecting the exhaust, seems like a good way to help pin point the problem.

The rough idle sounds like a misfire and the white smoke is coming from unburnt / raw fuel leaving the engine. The easiest / most common reason for this happening is a bad injector, the injector kinda dribbles or squirts the fuel in the cylinder rather than spray it in a nice fine high pressure mist like it should be to burn properly.

The way you described it, saying that the noise goes away with a little RPM, it sure sounds to me like a bad injector. Bad injectors will cause a miss, make a "knocking" kinda sound, and cause it to smoke too.

The easiest way is to take a pic or two and post it. Or you can go to the garage picture of some of us or the picture thread on the FAQ, and see if it is similar to yours.

If the tensioner is bouncing, it can also be the tensioner pulley itself. The bearing may be on their way out. As a matter of fact, check all the pulleys before they go bad.

OK, I took a picture of my engine. Can you identify it - is it 6.2 or 6.5?

I removed the belt and started it without it. Without the fan I could hear the noises much better:

The loud banging is coming from underneath from the flywheel area. It sounds like the engine will explode. When I raise the RPM it disappers. What could that be and how can it be fixed.

There is another grinding sound from the top, which sounds to me like the time chain is grinding. Is that possible and how hard is that to fix?

Injector pumps don't create white smoke. Raw fuel in the cylinders equals white smoke. Comes from malfunctioning,leaking/drizzling injectors.

OK, I took a picture of my engine. Can you identify it - is it 6.2 or 6.5?

looks like factory setup for 6.5 on a 6.2. only prob is you didn't take one of the turbo. look at the rear/drivers side of engine, just where the engine has a flange that your bell housing bolts to and look for numbers. the last 3 will be 599 but if not that will say if it's a newer 6.5.

The loud banging is coming from underneath from the flywheel area. It sounds like the engine will explode. When I raise the RPM it disappears. What could that be and how can it be fixed.

if it's coming from the rear you could have some screws loose such as on the flywheel, clutch.

There is another grinding sound from the top, which sounds to me like the time chain is grinding. Is that possible and how hard is that to fix?

could be an idler pulley, alternator, IP but most likely not the chain. chain requires all front items to be removed, water pump removal, remove screws on timing cover, pull IP, remove chain and reverse what you did before. it's a job that i would give a weekend due to parts runs and removal/replacements. change out lower pulleys and water pump w/ new ones for future insurance.
and it looks like there is a sticker on the left side valve cover that has engine specs on it so i'm wondering if all of this is real or not now that i think about it.........................hmmmmmmmmmmmmmmm....... ....

There is another grinding sound from the top, which sounds to me like the time chain is grinding. Is that possible and how hard is that to fix?

could be an idler pulley, alternator, IP but most likely not the chain. chain requires all front items to be removed, water pump removal, remove screws on timing cover, pull IP, remove chain and reverse what you did before. it's a job that i would give a weekend due to parts runs and removal/replacements. change out lower pulleys and water pump w/ new ones for future insurance.

I had removed the belt, so idler pulley, alternator, etc were out of the picture. So it remains to be the IP or the chain. Does the chain have a tentioner? What in the IP can make grinding sound?

JulianK, do you hear this grinding sound just by opening the hood, or do you have to open the oil fill tube to hear the noise.

Try opening the oil filler tube by removing the cap with the engine running. If the same noise gets louder, your timing chain could be really stretched out. Remember, a good timing chain on these engines, sounds like a length of chain being dragged over a metal bar.

The timing chain does not have any kind of tensioner on it.

If the noise is coming from the injection pump, make sure all your pump mounting bolts are tight. If they are loose and letting it flex, the gears that drive the pump could grind a bit and make noise, along with extreme wear.

The grinding noise is like a broken bearing or like grinding gears. It is uneven with occasional spikes. I imagine the chain inside hitting the wall, not sure if that is possible. I have heard similar sounds from worn out alternators, but remember - I hear it when the belt is off. I hear it when I raise the hood, more clearly when I removed the belt. I did not notice before the noise to get louder when I remove the oil cap, but I will try again tonight. I also put my finger on the IP to feel for any vibrations that match the sound pattern, but could no find any.Anyways, this noise is coming from the top and inside the motor and not from the bottom or the sides.

I just bought a 1988 GMC Sierra with standard transmission. Strangely this truck has a turbo, which makes me think the engine is 6.5, but I may be mistaken. The stickers in the engine bay are removed. The motor has a big problem with the following symptoms:

1. Idle is very rough - as if one cylinder is not working.
2. It smokes really bad with gray smoke, not black. It smokes mostly when idling, when it is driving the smoke is not so niticeable.
3. It seems to accelerate OK, but once I shift into 4 gear and the RPM iare relatively low, I feel the motor is stumbling, as if misfiring and does not have lots of power.
4. There is severe banging from the bottom at idle. It is dull sound like banging on a door, not like hammering on an anvil. One might think that there is a camshaft/piston problem, but that hammering disapears with the slightlest increase of RPM.

After reading on this forum I decided that this might be an injector or a IP problem and was about to purchase injectors on eBay, when I discovered that the IP on the motor looks like the 6.2 pumps, not like a 6.5 pump.

So far my theory is that some of the previous owners has replaced the original 6.2 with 6.5 motor, but has kept the 6.2 pump. Is this possible?

1. If it is possible, could that be the problem?
2. Should I replace the injectors? My understanding is that it is a good idea to replace them anyways.
3. Should I buy a 6.5 IP, or that pump requires other parts to work (like a PCM)

What else can I do to identify the motor and diagnose the problem?


I think you need to spend a little more time reading due to the fact that you think that the original owner swapped out the 6.2 for a 6.5 they stopped making 6.2's in 92ish. but it does look like there is a 6.2 or 93 6.5 non-comuterized pump on it. What does sticker say on the valve cover? displacement Please. How does you oil look if its grinding that bad it should have some sort of silver(metal) in it. If its knocking he can loosen injector lines to see which injector is causing it if it is an injector.

I pulld out the tranny and discovered a broken DMF. I posted pictures, description of the symptoms and the reasons for them, as well as some questions in a new thread:
http://www.dieselplace.com/forum/showthread.php?t=316801