Howdy Folks,
I have a 1983 GMC diesel dually that I bought about 2 years ago. The truck is geared real low, I really can't do quicker than 55 without my fuel mileage going way down. When I go up hills, the truck will start slowing down and sometimes shift into a lower gear sometimes down to 30-40 mph or so. It blows black smoke out the tailpipe which I am sure is normal for a diesel. Does it slow down like this because of how it is geared or is there something I need to check with engine such as IP timing or something else? THanks.

If it's geared so low, you're probably extremely low on power. It also shouldn't smoke. It's possible that the timing is retarded too much, or that the IP is going out. You might also have a problem with the fuel supply or a plugged return line.

If its geared way low (4:10) you should have all kinds of power and hills should not be a concern when empty. It shouldnt downshift at all.

These motors should not smoke much, unless someone turned up the pump, but if they did that cause they lost power it aint doing much good.

Is the smoke black?

Possible worn injectors, IP, timing, or low compression in the motor.

Caution, a dripping injector can melt a piston.

Howdy Folks,
I have a 1983 GMC diesel dually that I bought about 2 years ago. The truck is geared real low, I really can't do quicker than 55 without my fuel mileage going way down. When I go up hills, the truck will start slowing down and sometimes shift into a lower gear sometimes down to 30-40 mph or so. It blows black smoke out the tailpipe which I am sure is normal for a diesel. Does it slow down like this because of how it is geared or is there something I need to check with engine such as IP timing or something else? THanks.

That's pretty much normal for a 1 ton truck with a T400 trans. The J code engine will smoke quite a bit when climbing hills and losing power. It will also heat up and crack the heads if you're not careful. The 6.2 simply does not have enough power and torque for heavy loads and hill climbing, even with the 4.10 ratio.
I drove an 86 dually brand new with 4.10 gears, T400, etc. and it was exactly the same. If climbing a long steep hill on a hot summer day, I'd have to pull over half-way up and let the heads cool a bit. Overwise, they'd crack. If you get used to it, you can tell by the smell of the paints burning on the heads. Keep in mind your 6.2 has about the same horsepower and torque as a 305 gasser which is not very much.
One note - the same rig with a manual trans will pull somewhat better. The T400 robs/wastes quite a bit of power.

Many of the crewcabs came with 4.56 gears and the low gearing runs out of steam pretty quick. I had several 83's and 84's with 4.10's and 700r4s and they would out run same year 350 gassers. If the advance piston is worn in the injection pump, the engine will be very low on power. When I was looking for vehicles, I would run them to max speed on the interstate and if they would go over 75 on flat ground, I would tell the dealer to put an injection pump in to make the deal fly. Truck would always do close to 100 with a good pump and a 700r4. With a manual trans, you would hit the governor at about 85.

I pretty much figured that it might be because of the low gearing. I had a problem with the fuel system a year ago; one of the lines on top of the driver's side fuel tank was leaking diesel down the side of the tank and the truck got air locked. I have been waiting for summer to come so I can drop that tank and fix the leak. It has been running on the passenger's side tank through the winter.
I thought it sounded like there was ticking coming from injectors on the passenger's side so I cracked the injector lines one at a time to see if would make a difference. The engine started missing until I retightened the line, what I thought was ticking didn't go away. I don't think it is "ticking" or problems with my injectors. It runs slightly rough but not bad. No black smoke at idle maybe a very slight bit of white smoke but that just may be condensation from cold pipes.
As far as my timing on my IP, I looked at the timing marks and the mark on the pump (which looks like somebody scratched the mark on there with an awl or something) is about two dime widths over to the drivers side from the mark on the engine block. Does anybody have a picture of what the mark on the IP is supposed to look like? Would advancing it farther help that much or not really worth it?
Fuel filter have all been changed so they are not restricted. I am going to run some Power service in it again to clean injectors and such.
I am kind of frustrated with myself right now for not being a diesel mechanic and instead deciding to be a refrigeration tech instead. I've been working at the coal mines here in Wyoming the past few weeks on the haul truck A/C and the diesel mechanics seem to have a lot better job than I do. But I am almost 30 and have a family so I can't switch trades at this point :o:
Well I hope this info helps ya'll with any suggestions. Thanks again for the help.

I thought it sounded like there was ticking coming from injectors on the passenger's side so I cracked the injector lines one at a time to see if would make a difference. The engine started missing until I retightened the line, what I thought was ticking didn't go away.


Well, mine was ticking sometimes when it was real hot shortly before the IP went out. It was also extremely low on power then.

I find it hard to tell anything by taking injector lines loose, but if it should tell you anything, I think you'd have to go through all of them (both sides) one after another to see if the reaction is different when taking loose different lines.


It runs slightly rough but not bad.


I a had rough idle with new IP and new injectors until I replaced the lift pump. The LP was still pumping fuel, but apparently just not enough, or not so much to keep the pressure up.

Some time later I replaced the balancer, and that made it run a lot smoother. Now, it will run rough when air got into the lines from sitting too long (takes 3 weeks with the nose up on the driveway), but smooths out after some driving when the air is out.


As far as my timing on my IP, I looked at the timing marks and the mark on the pump (which looks like somebody scratched the mark on there with an awl or something) is about two dime widths over to the drivers side from the mark on the engine block.


That can mean that the timing is pretty far advanced. You could try to retard it about one width of a dime. Even a millimeter more advanced or retarded does make a difference. It can take some trying back and forth to find a good setting.


Does anybody have a picture of what the mark on the IP is supposed to look like?


It should be a straight mark, machined, not like carved/scratched in with a hand tool. However, the mark is only an indication to turn the IP into a position where the engine will run so that from there, you can set the timing with a timing tool.


Would advancing it farther help that much or not really worth it?


Setting the timing right makes all the difference. But if you advance it too far, you will damge the engine, like burning holes into the pistons. If you retard it too far, it'll start to sound more like a gasser; if you advance it, it'll sound "harder".

If you can find someone with a timing tool, by all means have it set correctly.


I've been working at the coal mines here in Wyoming the past few weeks on the haul truck A/C and the diesel mechanics seem to have a lot better job than I do.

You could ask them to help you with the timing. Maybe someone knows, or someone knows about someone with a timing tool ...

To turn the IP, it's only three bolts to take loose with a 15mm wrench --- get a 15mm with a ratchet on one end and an open 15mm on the other end. Use a large adjustable pliers (or the special tool GM has for that ...) to turn the pump. Never try to turn the pump with the engine running, and don't run it when the bolts are loose. The engine must be at operating temperature.

Why doesn't anyone know about the "ROCKING LEVER" TEST??? Get a long screwdriver and press "in" on the rocking lever, on the side of the IP {opposite the throttle side} towards the bottom, AT IDLE...The engine should have a VERY noticable STUMBLE if the advance bore is NOT worn-out.. Do it several times inorder to verify your findings.
Someone PLEASE put THAT in your "check list"...
It's the easiest check you'll ever make and save you a helluva lotta work...

I took my 84 6.2 to a local shop and guy informed me his best mechanic said you can't time the old 6.2s there is no timing. I said thanks and left! Know any good diesel mechanics in Northern Ky?

Did you take it to a diesel shop? or a pump shop w/ drive-in service? ANY diesel service shop w/ drive-in capability worth their salt should have a TIME_TRAC timing meter!! Basic equipment for ANY diesel shop that works on any diesel motor.!! I'll check my book in the A.M. for a reputable shop in your area..
Sorry guys> I've been doing pump work for over 25 yrs and it still gets to me when I hear things like that..

Do you know of a place in Texas or New Mexico that can set the timing correctly with a tool?

I took my 84 6.2 to a local shop and guy informed me his best mechanic said you can't time the old 6.2s there is no timing. I said thanks and left! Know any good diesel mechanics in Northern Ky?

What he told you is partially true. The rotary pump that is used on the 6.2 has no extrernal adjustment for the timing advance. On some other applications, it does. When it does have that adjustment, all you have to do is turn a screw while the engine is running at a certain RPM to put the timing advance where you want it. Like I said though, there is no adjustment on the 6.2 application.

When you talk about timing, keep in mind there is the static timing which determines when the pump fires at cranking speed and low RPM. Also, there is the timing advance that advances the timing as engine speed and/or load increases. That is totally different and works by hydraulic pressure.

When the DB2 pumps get worn, the advance tends to get weak. When the engines get mechanically worn (timing chain, gears , etc.) the static timing gets late. When you loosen the pump mounts and physically move the pump, you are changing the static timing AND the final timing advance.

When you want to check timing, you have to know what the specs are and what they refer to. Gas engine is different since spark travels at the speed of light and timing is the same regardless of where you check it. Diesel ignition timing is measured in many ways, and they all differ. Can be measured when fuel leaves the pump, or when it's injected into the engine, or when ignition actually occurs inside the engine. All happen at different times. You can check with a gas engine timing light with a pulse adapter, a lubricosity probe in a glow-plug hole, etc.

When you get right down to it, if you think you're lacking power at driving speed due to retarded timing - you can just rotate your pump to make up for it. Only problem can be - your timing at engine cranking speed might wind up being too advanced.

Lubricosity probe? are you refering to a Photoelectric probe that detects combustion through light?...

Lubricosity probe? are you refering to a Photoelectric probe that detects combustion through light?...

Sorry about that. That's what happens when I type fast. I meant - a luminosity probe. Thread it into a glow plug hole and it read time of combustion. That's what GM and Ford usually goes by for the 6.2s. early 6.5s, 6.9s, and IDI 7.3s. Snap On also makes a luminosity probe tester.
Timing specs when using a probe are totally different that specs used with a timing light that reads injection pulses off a line.

Ok thats what I thought you meant, just wanted to clarify. Thought maybe a lubriscosity probe checked for the lubrication effects of diesel in the chamber....but then nahhhh that makes no sense :p:

Did you take it to a diesel shop? or a pump shop w/ drive-in service? ANY diesel service shop w/ drive-in capability worth their salt should have a TIME_TRAC timing meter!! Basic equipment for ANY diesel shop that works on any diesel motor.!! mp work for over 25 yrs and it still gets to me when I hear things like that..

Stanadyne Time Trac meter uses the same type of magnetic pulse pickup that reads injection pulses from a line, as do many others e.g. Mack, Snap On, Ferret, etc. They all work fine as long as the person knows how to use it. There's no reason that I can think of why a good shop MUST have the Stanadyne tester only. I'd be more concerned about the person doing the testing, not the brand name on the test equipment. Someone good will also know that GM factory timing specs are general, and not specific. Ford specs are more specific and dependent on fuel cetane, altitude where driven, etc. My point being that someone good will know how to custom tune a 6.2 diesel going by how and where it is used and NOT by some set of generalized factory recommendations.

The problem is to find someone who can do this. I tried a shop specialized on Diesels, but they said they don't have a tool. I tried a dealer, and the guy in the shop said he could take apart Duramaxes and put them back together all day long, but that he doesn't know 6.2s (he seemed to be too young for that anyway) --- and they couldn't even find any information about timing for them. Maybe they just don't want to do it ...

If you were to buy a timing tool and the luminosity probe to do it yourself, about how much would that cost?

I didn't mean to insinuate the Stanadyne meter was the IT tool...Just that, it was the BASIC TOOL for diesel timing at the time. {80's} It's still a handy tool to have around tho.. You wouldn't believe the resurgance of "old" pumps that are coming out of the woodwork now..I had a 5.7L this week...If I see a 4.3L, I'm stuffing my matteress w/ money, buying some ammo and growing my own food !!! Things are lookin bad..

The problem is to find someone who can do this. I tried a shop specialized on Diesels, but they said they don't have a tool. I tried a dealer, and the guy in the shop said he could take apart Duramaxes and put them back together all day long, but that he doesn't know 6.2s (he seemed to be too young for that anyway) --- and they couldn't even find any information about timing for them. Maybe they just don't want to do it ...

If you were to buy a timing tool and the luminosity probe to do it yourself, about how much would that cost?

I can't speak for the rest of the country - but in my area of central New York State, there were never any shops around that knew how to do this work. Chevy dealers did not have a clue about working on the pumps; it was just remove and replace. A friend of mine owns a small Chevy dealership near me, and he still has the unopened Kent-Moore timing tools that he was required to buy, but never tried to use.
I've been working on these pumps back to the 1960s, when they first came into use. That was mostly farm and industrial until the Oldsmobile diesel came out. At that time, GM dealers were sending their problem engines to our Deere dealership where we had our own injection pump shop. Since then, not much has changed except all the newer trucks have electronic controls and are also dianosed electronically.
You've also got to keep in mind that HD diesel shops rarely see a little Stanadyne rotary injection pump. They are not used on heavy duty equipment. In fact, rotary distributor pumps of any kind are rare, and when they are used, they are more apt to be American Bosch (AMBAC), not Stanadyne/Roosamaster. For this reason, a good tractor mechanic is apt to know more then some truck or car shop mechanics.
In regard to buying tools, I wouldn't spend money on a luminosity probe. Too expensive and too fragile. You can buy a new Ferret pulse adapter that hooks to a regular gas-engine timing light for somewhere aorund $150 new.

Thanks for the great info JD. That is the best description of diesel timing that anyone has ever given me.

I tried a dealer, and the guy in the shop said he could take apart Duramaxes and put them back together all day long, but that he doesn't know 6.2s (he seemed to be too young for that anyway) --- and they couldn't even find any information about timing for them. Maybe they just don't want to do it ...


the guy was a moron. I walked into Auto Value one day and the first thing i saw was a haynes with the 5.7, 6.2, and 6.5 from chev, and the 6.9 and 7.2 from ford. it was on sale too.

The haynes says the same thing tho. Says you can't use a conventional timing light and be accurate. the tools it recommends is a Kent-Moore (J-26925 or J-33300) or a Snap-On (MT-480 or 1480) which employs the use of a microprocessor that can read the signals from the luminosity probe and will give you a reading.

It also says that as a good rule of thumb, retard your timing by 1 degree from high altitudes.

I can't speak for the rest of the country - but in my area of central New York State, there were never any shops around that knew how to do this work.


Could be the same here ...


A friend of mine owns a small Chevy dealership near me, and he still has the unopened Kent-Moore timing tools that he was required to buy, but never tried to use.


Do they still work? Would he eventually sell them? I don't have money atm, but at some time, that could be interesting.


At that time, GM dealers were sending their problem engines to our Deere dealership where we had our own injection pump shop. Since then, not much has changed except all the newer trucks have electronic controls and are also dianosed electronically.


Do you think it might be worthwhile to check out places that repair farm equipment?


You've also got to keep in mind that HD diesel shops rarely see a little Stanadyne rotary injection pump.


The shop I was asking didn't seem to work on HD stuff --- but I can't tell for sure. It could even be what they make most of their money with since there aren't many Diesels around here. Anyway, I just expect that a shop specialized on Diesels (the call themselves something like "fuel injection shop for Diesels") has someone who's able to set the injection timing. How can they do without? They never replace pumps? Never set the timing?


You can buy a new Ferret pulse adapter that hooks to a regular gas-engine timing light for somewhere aorund $150 new.

Well, I don't have any timing light at all, but that could be an option. When I can't find someone, I'll somehow have to do it myself ...

i haven't seen that adapter on sale.... but the last test light pulse adapter i saw was $300.... but it was a snap on so some of that price was paying for the name aswell...

. . . I just expect that a shop specialized on Diesels (the call themselves something like "fuel injection shop for Diesels") has someone who's able to set the injection timing. How can they do without? They never replace pumps? Never set the timing?

Well, I don't have any timing light at all, but that could be an option. When I can't find someone, I'll somehow have to do it myself ...

Timing lights are cheap. I often see them at yard sales in the $5 ranges - I guess because they aren't used much anymore - at least not on newer equipment.

Many and probably most shops simply remove and replace complete injection pumps and rely on the timing-marks on the pump and the engine to reinstall - and just assume the advance is working as it should. On farm tractors and industrial equipment, no special equipment is needed other than a $5 plastic timing window. But, it won't work on a pump that buried in the center of a 6.2 diesel engine. Even if you could reach in to install the window, you'd have trouble seeing it.

Ferret was the cheapest around for diesel timing adapters. I've bought several for around $125 new. Model # 765. But, they are getting to be "old technology" and subsequently scarce as the price keeps going up. I just checked around a little and see several on-line stores selling them for $211 now.

In regard to accuracy, the pulse adapter is just as accurate as a glow-plug luminosity probe. The issue is NOT accuracy, it's figuring out what specs to use. When reading with a pulse adatper, you are reading the time when the injection pulse makes a sound at the part of the injection line you're hooked to. Depending on the injector opening pressure and length of that line, the degrees of difference (injection lag) between that reading - and a reading done with a probe will vary. Generally speaking, what a pulse adapter "hears" on a line occurs 5-8 degrees earlier than the time of actual combustion that a probe will read. So, zero degrees TDC read with a pulse adatper can be equal to 6 degrees ATDC read with a probe.

Just for the record, all 6.2 and 6.5 diesels need to run with the timing advanced around 12 - 14 engine degrees when run fast and hard. Not much different than gas engines. Since an injection pump on a 6.2 only runs at half-engine speed, it's advance is also half. So, 6 degrees advance at the pump equals 12 degrees of advance at the engine flywheel or front damper. With minor variations, a 6.2 pump starts to advance at 1400 engine RPM - up to 2 engine degrees. At 1500 engine RPM it can advance up to 3.5 engine degrees, and by 2800 - 3200 engine RPM, engine advance is at 12 degrees.

Getting away from specs and tech equipment - here's a general rule-of-thumb. If you start your 6.2 up in mild weather (50-80 degrees F),and then, right away, try to rev the engine - and it breaks up a bit under no-load, but smooths out when driving - or smooths out altogether (load or no load) once hot when you rev it - then you can assume the timing is not advancing enough.

My truck acted up like that once. At about 280000 kms. Lack of power going up hills and smoking. Trouble passing people. It was the advance piston in the IP. After getting it rebuilt it was like a new truck. Now my 6.2 only smokes when I do a cold start.

If you start your 6.2 up in mild weather (50-80 degrees F),and then, right away, try to rev the engine - and it breaks up a bit under no-load, but smooths out when driving - or smooths out altogether (load or no load) once hot when you rev it - then you can assume the timing is not advancing enough.

"Breaks up"? What does that mean?

Isn't it supposed to run smooth once it's warmed up?

"Breaks up"? What does that mean?

Isn't it supposed to run smooth once it's warmed up?

Yeah . . . what part of what I wrote makes you think otherwise? I said . . . if it breaks up at high revs when cold (no load) - but smooths out once warm - the timing is probably causing it. Now, if it breaks up cold and also still breaks up hot - there might be (and often are) other issues involved. When a engine breaks up, it's misfiring, and with a diesel, a misfire means that the compression-ignition cycle is not working as it should - and that can be caused by many things if it happens all the time.

And, there are extremes. I've run diesels that had absolutely no top piston rings left and it could take half a can of ether to get them started -even in warm weather. Same engines, if you really worked them hard and got them hot - ran pretty good. I ran a Deere 350 crawler all summer, and built a pond with no top rings left on any of the pistons.

Yeah . . . what part of what I wrote makes you think otherwise?


Exactly the part of what you said that I quoted made me wonder. You seemed to say that either if it misfires when revved up while cold, or when it smoothes out once warmed up or when it doesn't misfire at all, that then the timing isn't advanced enough.

But it doesn't matter, I think now I see what you mean: If it misfires when revved up cold without load, it's probably the timing. If it still misfires when hot, it could be something else.

Wouldn't the timing have to be quite a bit off if it was misfiring when cold and revved up without a load? So much that you don't get much power anyway?

if you are in an area with tractor repair shops, look for a shop that deals with allis chalmers. the old allis' used a roosamaster rotary pump. also thanks jd for the specs.

Some John Deeres also used stanadyne pumps.

if you are in an area with tractor repair shops, look for a shop that deals with allis chalmers. the old allis' used a roosamaster rotary pump. also thanks jd for the specs.

In reference to farm tractors and industrial equipment that used Roosmaster/Stanadyne rotary pumps - John Deere, Ford, International Harvester, Oliver, Case, Allis Chalmers all used them - and there are others.

Wouldn't the timing have to be quite a bit off if it was misfiring when cold and revved up without a load? So much that you don't get much power anyway?

Depends on what you call a lot. When working on industrial equipment, it's much easier to see what's going on. I've noticed that a pump that's lacking 3-4 degrees advance is enough to make the engine break up at high revs - and 3-4 degrees at the pump equals 6 - 8 degrees at the engine since the pump runs at 1/2 engine speed. On a tractor, I can use a timing window with degrees marked right on it - no high tech equipment needed, with the same Stanadyne pumps.

I've had the same happen on my GM 6.2s, Ford 6.9s and 7.3s, and also on my Dodge Cummins that uses a Bosch rotary - not a Stanadyne. Although I've got the technical equipment to check and adjust accurately, I usually don't bother. In most cases, loosening the pump mount and moving it in the advance direction 1/16" of an inch, or sometimes a bit more, is all it takes. 1/16" movement of the pump equals 3 degrees at the pump, or 6 degrees of engine advance. Seems that most Stanadyne pumps in auto and truck use get sluggish advances once they get over the 120K mile point (generally speaking). On my 92 Dodge Cummins, it ran flawlessly unit it got to 220,000 miles. Then, it started running ratty at first start, and sometimes took 10-15 minutes of driving before it smoothed out. It has the Bosch rotary pump. I loosened the pump, moved it a little over 1/16", and it's been fine since. Starts as well as when new, fuel mileage is the same, power is the same, etc. Good enough for me.