Hello gang, may I first start off by saying how stupid I was for not finding this forum sooner! :notworthy I've got me a Cat 12G grader that I run on a daily basis and i'm getting tired of fighting with this issue. You have to fill it full of fuel every day (7-9 hours of running) or it'll start sucking air from somewhere. It only does it when it gets down below 80% fuel by the stick. The motor & injection pump was rebuilt in it probably 8 years ago so I'm hoping to rule that out. The way I see it it can't be sucking air after the transfer pump to the injection pump because that's a steel line. You would think that if its sucking air, as fine a particles as diesel fuel is (everybody knows diesel fuel will find a hole) that it'd leak through. But there are no obvious leaks anywhere. When/if you do run it past 80-75% and it starts to act up you have to fill it up, then crack every single injector line off of the pump 3 separate times and it won't give you any more trouble. I've been putting up with this problem for going on 5 years now and i'm at wits end. The primer pump valve was also replaced along with the gasket, but there is a possibility that it could have gotten hard. Anybody have any ideas on where exactly to start with this situation? The grader runs constantly and cannot be pulled down for maintenance other than maybe replacing a few small things like the line from the bottom of the tank to the primer/filer, and the other line to the transfer pump. Thanks guys, I really look forward to hearing other peoples opinion.
This may help ,but this has happend to me more times then I can remember if your hoses are kind of old , maybe 3-4 years old , the hoses may be pitted inside or have small cracks , now diesel as we all know is hard on rubber fuel hose , and like you said diesel will find the leak , but sometimes before it can leak out of a crack it can suck air and not leak fuel, now it would take alot of small cracks to make it suck air and still not leak fuel its happend to me a million times ,but why it would do it at 80% or lower I dont know , this is all just a helpful hint , maybe not in your case , I could be so far off on this problem its not even funny , hope it helps
I don't remember if the 12 has a pickup tube inside the tank or not. Could be a crack or pinhole in that. We had a wheel loader that would suck enough air to stall after about 20 minutes running, each time the primary filter would be half empty. It had just had the injection pump rebuilt and all new lines and fittings. I finally talked the owner of the injection shop to come out and help. He found a crack or pinhole in one of the new brass fittings I had installed. It did not leak a drop even though it was lower than the bottom of the fuel tank. He found it by putting a lot of shaving cream foam on everything and it was obvious where the suction was. Of course I was feeling dumb as I had thought it was something he had done.
The 12G's don't have the pickup tube so far as I know. I tried looking at it through taking the strainer out and I didn't see any, I also tried looking it up in the parts book & service manual. So far as I know they only have just a petcock at the bottom of the tank. Then its a rubber line that goes all the way to the top of the tank to the canister fuel filter & the primer pump, then another rubber line down about 1/2 way level of the tank to the transfer pump on side of the block, then its a steel line to the injection pump. The rubber lines are the same that have been on the machine since we've owned it which was about 8 years ago, the motor and injection pump was rebuilt right before we bought it, it was part of the deal. Sweet deal right? I know for a 100% fact that its air simply because you can crack the injector lines and nothing but bubbles comes out. I've took a many a side shield off to look at the tank, the tank isn't leaking, don't 100% know but I don't think there is a pickup tube, then again it wouldn't make a lot of sense IMO to have a pickup tube and a petcock at the bottom. I'll talk to the rest of the big heads (3 others) and tell them what I've found out so far and we'll go from there. But if anybody thinks of any other ideas on what it could be it would be greatly appreciated! Thanks guys!
Maybe the air vents are stopped up in the tank. Does the tank suck in air when you remove the cap to fill it?
I'd say your transfer pump may be faulty. When the tank is full, it's running on gravity feed. Check your fuel pressure.
I agree with Cmark. I'd first replace the lift pump, and the rubber fuel hoses, then take it from there. Neither is terribly expensive nor time consuming to do.
I'll agree with the rubber lines, but the transfer pump i'm going to have to disagree with. The lack of fuel pressure won't cause an open void and make air. The fuel just won't have as many psi as it would if it worked under its full potential, that arises the question of "where is the air coming from" I'm not arguing against your post, just trying to rule that out myself.
Not sure.. didn't really know the tank had an air vent. I'll have to investigate this one.
Hi Jonesbros. I don't want to give you a hard time or contradict you or anything, but if this problem's been on your back for five years, lets get it cleared up quick. To a mechanic this seems (at first) to be a simlple problem. The symptoms are predictable, ie airlocking. The circumstances are predictable, ie fuel below 80%. Also consider the fact that the fuel tank is above the engine and the highest point in the fuel lines is the prefilter which is located approximately two thirds up the tank. A mechanic would think; If rubber lines are leaking air, then why only when the tank is 80% full? And then; If the breather is blocked, then why do the symptoms only appear when the tank is 80% full? And then; "Where is the air coming from? - Must be cavitation". I repeat, check the fuel pressure. If you don't have a gauge then disconnect the return line at the FIP. Put your finger over the outlet and run the engine. You should feel some pressure. How much pressure is hard to define but pretty much anything above 5 psi will make 3306 run, and 5 psi isn't much. I think that maybe after 5 years, you can't see the wood for the trees.....
Ahhh, air can pass through places that one would not even think of. We had an older Case 855C loader that ran like a champ. That thing would fire off with it -20 degrees with no starting fluid. But you'd run it for a for about ten minutes and the engine would slowly crap out like you just ran it out of fuel. It had a piston plunger type lift pump mounted on and driven by the cam inside the injection pump. I found that the lift pump had a plastic check valve to prevent backflow and the plastic valve would stick, so it wouldn't move fuel. One would think the engine would simply...quit. But no, it seems the hyd head in the injection pump would keep pumping fuel inside the injection pump until it just "aired out" the injection lines. Every time this happened, I'd free the stuck plastic valve, then had to "bleed all the air from the pump and injection lines." Where did the air come from? :beatsme My best guess is the blockage at the lift pump prevented fuel delivery but the hyd head still had demand (vacuum), so it was sucking air around the pump shaft seals until there was no more liquid (fuel). After doing this several times, :Banghead I replaced the lift pump. No more problems. Using logic in troubleshooting is necessary, but don't let it prevent you from thinking outside the box of possibilities. My suggestion of replacing lift pump and fuel hoses is based on your original premise that the machine runs constantly and can't be pulled from service for extensive repairs. Replacing lift pump and fuel hoses are not terribly time consuming and eliminates two possible sources of your problem. Checking the venting of the tank suggested by Galute is also a good idea.
Thanks Cmark & Atco, its starting to make better sense to me. I'll give Caterpillar a call tomorrow and have them send a transfer pump and the rubber lines. Might as well replace all 3 of them (2 supply and the 1 return). I understand where your coming from Cmark.. after 5 years of this aggravation its hard to see anything!
My 12G 61M7378 has a water seperator, one more thing to check if yours has one
Nope, neither one of ours has a water separator. Just a canister filter inside the bowl and the filter on the injection pump. Thanks though DPete!
Are you talking about one of those cannister filters that has a bolt going through the top of the filter housing to draw the can up snug under the housing plate? If so, have you check the seal washer under the head of the bolt?
That's the one. We pulled all that apart last winter and checked everything out but I for some reason don't recall a seal washer under the bolt. Only the giant square o-ring type seal in the housing itself that the canister draws up to.
Whoa, right there! Just to be clear; If you have "that bolt" that goes through the top of the filter housing plate, it requires some form of seal. Most of the time it's either a copper seal washer or a fibre seal washer. There are some that use an o-ring, they generally have a bevel cut in the hole of the filter housing to accept the o-ring. Check your parts book, you should see some form of seal under the head of the bolt.
I'll do it first thing tomorrow morning. I could be highly mistaken, there could be one there there couldn't. My memory for some reason isn't what it used to be. Thanks Atco!
LOL, mine isn't either...I think...ugh, I can't remember! :drinkup
Also, don't buy a new transfer pump. Thats mucho dollars You only need a few O rings and 3 valves and you can rebuild it yourself for a fraction of the cost. Resealing the prefilter is also a good idea. However you have to think that if it is letting air in when running, it would be leaking diesel out when the machine is stopped (with a full tank of fuel).
Well I thought i'd bring this thread back to life with the results. Since we didn't have a pressure gauge we went ahead and sent it to our local machinery mechanic to have it tested accurately. We explained to him what everybody here had mentioned and he said he'd get on checking the fuel pressure. We left the tank full when we took it to him just for the heck of it to see if it was easily found. He immediately called and said that the pressure was horribly low and the transfer pump was completely shot. He went on to replace the petcock, hose from the petcock to canister filter housing, hose from canister filter to transfer pump, and the transfer pump. We also got him to replace the infamous 3306 seal on the injection pump shaft (shaft was in fact inspected and was alright), and my #6 injector was leaking. Now we have upwards of 40 psi and she's running like a champ. So i want to personally thank everybody here that has helped me especially Cmark for keeping me on the right path. As I stated this problem has been riding on my back for the past 5-6 years. Even though I wasn't running the grader the whole time when it was happening, I was still the poor bastage that had to crack the injectors and smell like diesel for the rest of the day. But even though I didn't name everybody, your help, information, and advice was and always will be greatly appreciated!! HEF Rocks!!
Thanks for the feedback. Appreciate that.