A smokin' Cummins - Help!!!!

[CJLennon]

Hey Folks,
As always, thanks to all that read this. I have a 1974 Autocar with a 370 Cummins motor (don't ask, the boss can't let go of the past) that is smoking bad from the moment you start it up. The smoke is from fuel, not oil. This has beed verified. We replaced the injectors, set their travel using the dial indicator method and adjusted the valves but that did not change things at all. We next went to the pump and checked to see if there were any pressure shims/washers that we could remove and try that, but there were none. We then pulled the heads off just to see if we could find anything that might give us a hint about why this fuel is not being burnt properly. We found 2 cylinders, not on the same head, that were soaked with fuel. The valves all look good - no burnt edges.
So, at this point and because of a lack of books and info on this motor, I'm at a lose. Does any one have experience setting up the fuel systems on these old Cummins motors? I am skilled in diesel engine repair and calibrations so I should be able to understand you. Thanks again to all for your time!

Chris

 

[Oldiron]

Do you have some more model information on that engine??? NTC ?? NTA
If it is an NTA in stock trim they were terribly smokey on start up. Low compression ratio coupled with late static timing . Most PT cummins especially the higher HP small cams blow white smoke until they are warm . also check for sticky rockers on the wet cylinders. you will need a book to put it back together,:drinkup injector setting is kind of fun .

 

[CJLennon]

Hey Oldiron,
Thanks for the info. From what I understand, it is an NTA engine. We are looking into checking the pump timing. This engine smokes from start to finish. We set the injector travel using the dial indicator but it didn't make much difference. Now, with the head off, it will be easy to do pump timing. By the way, any idea where to find the engine tag that gives the pump code in order to do the timing? Thanks again for answering this post. I really appreciate the help!

Chris

 

[bolt thrower]

White/blue smoke? Does it clear up while you are pulling a load? If so its running like an old cummins! Old Iron is right, these things smoke alot. The PT fuel system varies the injection timing from retarded at no load idle to full advance at full load/ rated horsepower. It is an unintended side effect of the injector design. You mentioned checking pump timing. The pump is not timed. The fuel injection timing is changed by adding shims between the cam follower housings and the block. You cannot advance the timing on these things enough to make them quit smoking at idle because the timing at full load would be advanced so much it would ruin the engine. Cummins designed two modified PT fuel systems to adress this problem before going to a computerized fuel system. The first was MVT (Mechanical Variable Timing). The second was STC (Step Timing Control). Both were very effective at eliminating light load/idle smoke. You may be able to convert your engine to MVT. It would take 10 pages to explain MVT & STC so if you are interested, I would suggest looking for some old service manuals. good luck.

 

[CJLennon]

Hey Bolt T.,
Thanks for the info. The truck smokes all the time, especially at idle. We are going to do the check required to verify pump timing and see what that comes up with. We have the head off becasue we had a slight water leak so it should be pretty easy to check timing with the head off. But, it sounds like this is just the way it is. Thanks again for your knowledge. I really appreciate the help.

Chris

 

[Will Musser]

Sounds like a hole in a liner. Probably just rusted in it. The tag should be on the timing cover up front by the large pulley on the front of the engine. Also the truck probably doesn't have a pressure type cooling system so the compression that leaks into the cooling ssysten through the hole would be vented. Also I think this because you were loosing water.

 

[td25c]

Check the injector setting

Hi CJLennon,Cummins recommends using the dial indicator method for setting non (top stop) injectors.Im guessing you have the non top stops in the 1974 370 cummins engine.Might run the rack & check the settings close as if they are a little loose the engine will smoke white .And yes as other members have said some older cummins 855's like to to smoke at idle.As far as timing go's ,thats done at the cam follower housing with shim gaskets,You will need the cummins timing tool to check that.Here are a few pages out of the cummins manual on the (dial indicator method).Good luck with the Autocar

 

[CJLennon]

Folks,
Thank you to all who responded. I would not be able to do this without you. Got an update for you on things. This is what I found:

1.) I found the pump code which is BM. This says that the injector rod travel is between .052 and .054.

2.) After checking pump timing, all cylinders are between .104 and .106.

So, now I have the following questions if you have another moment:

1.) When I have excessive travel of the injector rods, does this mean my timing is retarded or advanced? My guess is advanced. True?

2.) It sounds like I will need an offset key because I am off by .052 which is more than a gasket change will do. Correct?

3.) If my injector lift is at .104 instead of .052, would this cause all this smoke and raw fuel on top of the cylinder when I pulled the head off? (By the way, all cylinders had raw fuel when we pulled the head off for another reason.)

Thanks again for everyones time!

Chris

 

[CJLennon]

One more thing I failed to mention. This is a small cam engine. Thanks again!

 

[Oldiron]

If you don't have the engine spec plate , I would suggest using the book specs for the NTA, the Pt pump is very common to a lot of cummins engines with the pt system , pumps were changed on engines with the internal mods made but the tag number left alone. If I remember right less travel is more fuel?? at least when setting them with an inch pound torque wrench less torque made for more fuel delivery.some one may have changed that engin over to NTC specs or set the injectors to NTC specs and caused the problem in the first place.

 

[bolt thrower]

CJ I am confused: (doesn't take much!). How did you come up with .104-.106 readings? Are you using the special cummins tool with two dial indicators? It measures piston travel and injector plunger travel simultaneousley to figure timing. This tool is the only way I know to check timing. The pages TD25 posted are for adjusting the plunger travel only. If the plunger travel is more than specified it is merely an adjustment issue, and it will make her run pretty poor. The idea behind setting the plunger travel is to actaully make the injector plunger bottom out in the injector cup when the rocker is all the way down, injecting all the fuel in the cup and sealing the spray tip closed during the power stroke. This is a crucial adjustment, too loose she runs poor, and carbons injectors up; too tight and she wears out cam lobes. As far as I know there are no offset keys available for this. Let us know what you come up with.

 

[td25c]

Sorry Fellers I should have also posted pics of the timing procedure

 

[CJLennon]

First of all, I can't thank you folks enough for all the info you have copied and put on this forum. Now to answer your questions and give you an update. We had to pull the head on this engine because it was leaking water. While the head was off I did the measurements for pump timing. Without the head on, it makes it really easy to do these measurements becasue you don't need all the special holders and adapter - just two dial indicators. I feel I must have previously done the test wrong because I got these crazy readings so I went back and redid them. The numbers I got this second time where much more inline with the spec. They ranged from .050 to .060 so I feel my timing is on. I believe I need to do a better job on the injector settings which I will do when I get the head back on.
As long as I have you folks here, is there any info out there on how this injector system works? It's killing me to understand how these injectors work. They appear to not "fire" when they are seated but it seems that most of the time the cam lobe for the injector is "down" causing the rocker arm to retract from the injector. So it seems they would be firing all the time. My first thought is that these injectors somehow operate on a two-stage process where they are "loaded" and then they release. Is this true? Again, I need to understand things before I dive into them and this system is one of the first that has me stumped. Any info on how this fuel system works would be MOST appreciated. If I don't get it straight in my head, it will haunt me. (For me, it's either understanding this system or Prozac and I would much prefer the former.) Again, thank you for all the help you have given me!

Chris

 

[bolt thrower]

CJ, the PT fuel system is probably the simplest fuel system ever made. I am going off memory here, so some one more up on these may correct me or have something to add. The injector plunger will be depressed (seated rocker down) for most of the power stroke, all of the exhaust and intake strokes, and part of the compression stroke. That means the plunger, shaped just like a sharpened pencil is seated in the cup, which has the same internally tapered seat, with the 6 spray orifices drilled into the end of it. At a certain point in the compression stroke, the rocker comes up, and the plunger with it. Now pressurised fuel from the pump flows into the cup through a screened orifice. How much fuel that enters is a function of pressure ( determined at the pump, varies by throttle position) and the time the plunger is "open". Thus the name "Pressure Time" or "PT". At a few degrees before top dead center the plunger starts traveling down. This is where the timing will vary- if the engine is at full horsepower/load, the cup will be full of fuel, and injection begins as soon as the plunger starts moving. But, if idling or @ light load, you have to imagine, the cup is only, say, 1/3 full. Now the plunger has to travel 2/3 of the way down before injection starts, because of the void above the fuel. All the while the piston is still traveling up, so this translates into retarded timing. Injection ends when the plunger seats in the cup. So, one could say this fuel system has a variable start of injection, in relation to piston position, and a constant end. Better designed (in my opinion) fuel systems, like a Cat have a constant start and variable end, so static timing is the same regardless of the amount of fuel injected. If you still have your injectors out, and they aren't top stops, the plunger will pull right out and you can see the innards. Hope this all makes sense.

 

[bolt thrower]

Just to add a few lines for clarification, the rocker actuated injector plunger is what actually creates the high pressure of the fuel necessary to inject and atomize the fuel. this pressure is probably in the neighborhood of 3000 psi. The PT pump only puts out a low "rail pressure " running between about 20 - 60 psi. A cool trick you can do with an old cummins injector is pull out the plunger, and dribble a little fuel in the barrel/cup. Put the plunger back in, hold on to the injector body and strike the plunger with a hammer. You can see what a job the injector does atomizing that fuel firsthand. Just be careful not to get a finger near the spray tip when you hit it, cuz that fuel will get injected under your skin.

 

[CJLennon]

Folks,
Great info. Thanks! Last night I mapped out the movement of the injector rocker arm and found the most interesting thing I've seen in my 40+ years of auto work. As you folks said, the rocker rises as the compression stroke approaches. At TDC (compression), there is another rise, about an additional .015", and then the level drops back to where it was at TDC (about .100") and holds that postion until TDC (intake/exhaust) at which time it returns down to 0.0" lift position. It then stays at 0.0" until 90 degrees BTDC (compression) at which time the rocker starts to move into the injector nozzle and the cycle starts over again. My question is, why does the rocker push into the nozzle for that .100" from about 45 degrees ATDC (compression) until TDC intake/exhaust? Other than this, it is all starting to make sense thanks to you folks. So, it seems that we have a two-stage type nozzle that (first) loads when the lift is at 0.0" and the (second) disperses when the lift reaches its full travel around TDC. I'm assuming that when we have that .100" "lift" from just after TDC (compression) up to TDC (intake/exhaust) that this is the time when nothing is happening with the injector (i.e., not loading with fuel and not spraying) so it is in some type of neutral position. Let me know if this makes sense.

One last thing - pump timing. It seems if you change the thickness of the cover gasket on the cam follower cover, that will either push the timing "forward" or move it "back" and where the timing is has to do with the injector travel distance from .2032" BTDC out to the 90 degrees ATDC. It sounds like a distance that is less than the spec for a specific pump code means that the timing is too advanced. If the distance is greater, it is retarded. Is this right?

Finally, if I do everything right and everything lines up perfect with regard to the specs and this truck still heavily smokes after warmed up and going down the highway (and the smoke is not oil or water), any tricks to leaning out the motor?

Again, I can't thank you folks enough for all this time you have taken. If you ever get to the Carolinas, look me up for a free meal and I mean that seriously!

Chris Lennon
(mellott_info@nc.rr.com)

 

[CJLennon]

Gents,
Thank you again for everything you have done for me here. I hope one day I can repay you. Now I understand how all this works and it is very interesting to say the least! Now, let me ask you something else. Once I get this engine back together and set everything up perfectly and it still blows lots of smoke at both idle and running down the highway, are there any tricks I can do to back the fuel down? Here are things I'd like to get your opinion on:

1.) I took the plug out of the fuel pump and noticed that there are no pressure control washers in the pump. I've heard stories where you can add washers in order to increase power (and most likely burn up the motor!) and remove them in order to reduce smoke. This pump has no washers so I can't try that but could this pump be putting out too much pressure causing excessive smoke? Any words of wisdom in this area?

2.) It seems that if I extend the travel distance of the injectors from say the spec of .170" up to .180" that this would reduce the amount of fuel that would get injected into the cylinders. If this is true, couldn't I try this in order to reduce the smoke?

One thing I forgot to mention is that this engine has more power than you can imagine! It runs extremely strong so I don't think we have a lack of fuel in this baby. OK, that's it for today. Again, I owe both of you big time. If you ever get to North Carolina, look me up.

Chris Lennon
Mellott Contractors
Chapel Hill, NC

 

[CJLennon]

OK, I didn't mean to do a double post here. I did not see the first one after I typed it in and so I thought that it got lost. So, I did the second one. Sorry for all the text.

 

[bolt thrower]

Hi Chris. Just to verify, are you talking about alot of blueish smoke while the engine is running and the truck is sitting still? The kind of smoke that makes your eyes water? If so, Retarded Injection Timing is your problem. Like I tried to explain before, this is very common on early cummins engines. The PT fuel system is designed to operate the best at full rated horsepower, not idle or light load.
Leaning out a diesel. Since diesel engines have no throttle "butterfly", the pistons suck as much air into the cylinders as they can every time the intake valves open, unlike a gas engine. There is no lean mixture in a diesel. The less fuel you inject, the slower the engine runs. in fact there is no rich or lean mixture in a diesel, because the fuel burns the instant it is injected into the cylinder.
Increasing injector travel. No. The purpose for adjusting the injector travel is this: The total lift of the injector lobe of the camshaft is .170". When the lifter is on the high part of the lobe (outer base circle), the injector rocker is down, the injector plunger has to be seated in the injector cup. The only way to accomplish this is to adjust the injector travel to match the travel of the cam lobe. If the plunger doesnt seat, combustion gasses will leak back into the fuel system and make a big mess. The injectors are designed to travel the exact amount that the cam lobe lifts, no more, no less.
Spacers in the pump. I think you may be talking about the "button", AKA the idle spring plunger. Installing a smaller than stock button will give higher horsepower, and more black smoke at full power, but will do nothing for smoke @ idle or light load.

 

[CJLennon]

B/T,
Now I really get it. Thanks for the lesson. Starting out on gas engines really gets in the way of fully understanding diesels. So, bottom line, it's all about the injection timing. I will check the timing again and make sure I am right on the money. It sounds like if the actual timing number is low, then the timing is advanced and if the number is high, then it is retarded. Did I get this right?
Thanks again for this great education. I'll pass it on to the next poor slob that tries to look at a diesel like it is a gas engine.

Chris