I have a 3126B in a Cat D5N that I’m stumped on. Boss just got this machine. He had a CAT tech throw ET on it about a month ago. Tech said heui pump was bad. So now it’s my problem. Engine was puffing black smoke and misfiring. Erratic rpm. Since then I have replaced the heui pump, fuel filters, fuel lines from tank to pump. injectors were removed and tested. All were only meeting the bare minimum specs for working so I replaced them. At same time I also replaced the injector sleeves. No score marks or any other issues in the cylinders as I ran a scope into them while I was waiting on the injectors. Still runs about the same. I put a sight tube in the fuel return line. Fuel looks foamy. A lot of air coming from the head in the fuel. Installed sight tube between tank and pump. No air bubbles. All orings in the steel fuel lines and adapter fittings have been replaced. There was some insulation that had came off the wiring that goes to the fuel pressure sensor. I repaired that. I have also replaced the fuel pressure regulator. Every procedure to complete all this work was followed exactly the way Cat SIS said. I borrowed a computer with Cat ET on it. No codes. I have not replaced the fuel pressure sensor or the Injection Actuation Sensor yet. But I don’t feel those would cause the excess air in the fuel. Secondary fuel filter drains were also replaced. Engine oil pressure is steady at 60psi with mechanical gauge. Anyone have any ideas or tips to help me figure this one out? Thanks in advance.
Welcome to HEF MurphHeavyMech : Since there is no codes it might be an idea to try sending return fuel from the injector through a clear piece of hose if there is return lines from these injectors. Trying to remember if they had them. If it does, it would give an idea of how much air is coming from an injector to return. Could put air to the fuel tank with a blow gun and a ball made of rags over the hole with the blowgun pipe shooting 1-2 PSI of air into the tank. It would tell you if there were any leaks up to the filter or head, which could be sucking air when running. Need 2 people to do this test. Does that engine have a banjo bolt at the fuel pump that might have a filter hiding inside. Even though they are new injectors, presuming they are OEM could they be wrong part. The fuel filters sometimes have an o-ring on center thread that doesn't get removed when filter is changed. Is the turbo ends of shaft not too loose and turning freely, or is there a wastegate that is acting up. Just a list of somethings that have happened on this site in the past. Simon C
The injectors get their fuel from a common passage in the head. I did use a clear tube tool that I purchased from Cat between the tank and transfer pump to eliminate the possibility of air leaking in the hand pump and fuel/water separator housing. Perfectly normal red diesel there. When I used the clear tube on the return side, it was after the pressure regulator and before the existing fuel return line. That’s where I have foamy fuel. Fittings from the pump to the secondary filter and from the secondary filter to the head are flat face oring. I replaced every oring in the flat face side and the boss oring sides of those fittings. Thanks
Maybe someone else will chime in. Did see another post with similiar problem, but no solution yet was posted. Your foamy fuel is the one that makes me wonder? Simon C
I searched the other posts multiple times looking for something similar. Didn’t locate anything. The foamy fuel is what baffles me also. I want to think it’s something simple but I’m unsure. Kinda makes me rethink how good of a mechanic I am. Only been working on diesels engines and heavy equipment for about 30 years now…. Between my boss and myself, we can fix almost anything but this one has both of us stumped. And with no external fuel leaks it is really difficult to see what is wrong. There’s no bubbles from compression in the radiator and the antifreeze had no traces of fuel so theoretically that should eliminate the injector sleeves as well.
Wonder if those new injectors might be faulty. Reason I say is that they are difficult to test and have seen new parts faulty before. That things got fuel racks doesn't it or was it only on the 3116 engines. If it does could the racks be out of adjustment. Simon C
Maybe Nige can help out tomorrow as he is very sharp. Simon C
No fuel racks on it. Injectors are all electronically controlled on this one. We have an IT62 Cat loader with a 3126 that is the MUI system. It has the racks on it. We had to have the Cat tech there to adjust that one and replace the injector sleeves on it cuz we didn’t have the tooling to do it. Same tech is the one that diagnosed this D5N as I was tied up in another project at the time but he was too busy to actually repair this D5. It’s possible that the new injectors could be faulty but I don’t think it would be having the same issue. Thanks for letting me bounce ideas off of you though.
Are those new injectors or OR injectors? I didn’t catch that. It wouldn’t surprise me if you got a bad injector, or maybe a rolled “o”-ring.
Not familiar with HEUI but worked with MUI, but they are not so diff, Those crap systems have life of theyr own, basically u need 2 clean things "Fuel rail pressure" and "HYD rail press" as the injectors are hydraulically actuated , the opening signal thats electricall, wich add another set of problems... In MUI was not uncommon leaks on the fuel rail, that would be seen in engine oil. Cracked heads also found one. U have fuel bubling ... and that can only come from compression, can be a seat leak or a "O" ring, or both. i remember MUI were sensitive to seat leaks. Good luck with it.
What's the actual fuel pressure going into the head? Have you looked at the fuel return check?
If I had to bet money I would say the problem is the O-Rings #7 in one or more cylinders. The illustration below may or may not be exactly the same as your actual cylinder head, but seeing as you didn't give a Serial Number I would not guarantee it.
I appreciate everyone’s advice. All o rings, injector sleeves, injectors and the heui pump were new from CAT. I won’t be back to work on it for a couple days but had a random thought about the fuel pressure sensor. With it being partially plastic, I’m wondering if it could be the culprit letting air in. I will definitely post on here when I get it figured out. Thanks again everyone
Fuel return check valve was replaced. I’ll have to rig up a gauge between secondary filter and the head
Did you use the recommended retaining compound on the sleeves and give it time to cure?
Those o-rings and the lower portion of the sleeve with retaining compound only seal coolant. It's pretty much impossible for compression to enter the fuel system from those areas. You can get compression into the fuel system by the lower seal on the injector itself.
I followed CAT guidelines to a t with sealant from Cat for the sleeves. Everything I replaced was OEM from Cat. I suppose it could be a rolled ORing on installation but since it has the same issue as before I tore it down and replaced everything, I find it hard to believe. I’ll try to get some pictures of the fuel in the sight tube tomorrow and post them.
Pic of fuel in the sight tube and of my fuel pressure right before the head. I didn’t get a video or pic of the fuel injection actuation pressure. That was bouncing in a range from 400-1600 psi. I talked to Cat Tech support and they said that it shouldn’t do that. I put a manual gauge in the oil galley right beside the injection actuation pressure sensor and it was steady pressure. I then replaced the IAP sensor and it runs absolutely perfect now. I wanted to post my results so if someone else has a similar issue, it might save them some time and money. Thank you everyone for your ideas and advice.
Thanks for telling us what final outcome was. Simon C
Hard when not there to state the solution. The solution was in your first post IAP Sensor not changed. Sometimes simple but missed stuff is the solution. You had Cat ET on it and nothing special was revealed. We all mentioned different things to check or prove. One thing is foresure, not one of us is right all the time, or else we best head to stock market to find out otherwise. Glad you posted all you did as most of us will add to our archives. Simon C