ok guys, im new here so take it easy on me lol. 3 years ago i bought a 99 fld 120 with a 98 n14 celcet plus in it over the first 2 1/2 years the truck ran fine but the ecm had 2 million miles on it and the person i bought it from did not know how many miles was on the first rebuild. i was starting to get alot of oil and water leaks on the engine so i had it rebuilt on my terms before it went on me. i pretty much done a major over haul including the cam without pulling the block. the truck was still running fine when i took to the shop. the day i picked it up from the shop, i started having back fire issues, 10 to 15 times a day, i took it back to the guy that done the rebuild and he changed the cam position sensor and that did not fix it so he sent me a bill for 200$. im not here to bash the guy but i basically told him what to do with that. well any way i started changing parts and have been unsuccessful so far but i do have an issue that i found and maybe one of you might can help. i have my own diagnostic laptop and was driving down the road and watching the parameters and noticed that my manifold boost pressue was staying on 6 inh20 but my mechanical boost gauge would go from 0 to 40 depending on the load. i replaced the manifold sensor and no changes on ecm, it just stays at 6. i checked the continuity of all 3 wires on the harness and all tested fine, i checked the voltage and 5v is what its supposed to be and was so my question is, could my ecm be bad or going bad? any help would be greatly appreciated
I have a Cummins Celect and Celect plus troubleshooting and repair manual. It does not really have a tree for backfire only misfire. As far as a boost sensor issue it has a short tree but all related in troubleshooting section for connections and harness. The ECM section is actually brief, probably on purpose. But with that said the fact that this engine was disassembled that far still setting in the rails tells me a very close inspection of engine/injector harness needs to be done. Plus every ground connection and or grounds period. There could be tons of fault codes logged, have you pulled all codes? And yes Those ECMs can and will go bad, but the fact as you say it ran ok before rebuild then issues after tells me connections and or harness, Check the harness leading to cam position sensor. In most cases on those old Cummins harnesses you will need to remove the black loom and inspect all of it. I have found many injector harnesses on Celect engines where the insulation covering the wire is full of cracks.
Thanks for the reply, if it was a ground issue, wouldn’t I have other things not acting correctly? I do not have any active codes unless I initiate it by unplugging something. I have done a cylinder test and I get the same results from each cylinder. I did video it but I’m not sure the file will upload. It like the fuel shuts off and back on instantly and cause the back fire. Is some cases it violent enough to kick cruise control off. The picture that is shown was at high idle and each cylinder responded the same way.
Intermittent ground possibly. Beyond that It's hard to figure-Much easier if it just went dead-Intermittent electrical issues with equipment especially a engine baked harness are problematic all on it's own. Truthfully you need to hook up with OEM diagnostic equipment.
What was the boost psi before overhaul? One thing to check is the fuel shutoff solenoid. Plus what brand of cam position sensor was installed.
Boost was 0 to 35 before overhaul. I had new turbo put on and now 0 to 40 and have only put Cummins parts on truck before the rebuild and after
All of the harness was replaced 2 years ago along with the injectors
Ok-more info learned a shred at a time. You have new harnesses and injectors, all cummins parts including turbo. Plus a new cam position sensor. Not much left. Either-fuel shutoff solenoid {which won't leave a code} or ECM. My shot at your dart board.
Thanks for the info. A dartboard it is, I have a buddy that has an 97 celect plus. Trying to get in touch with him and see if I can swap them out and see if I have the same problem,
We had 6 Celect II engines in our fleet for years. We lost some crank seals, a pantload of radiators (an International problem, not Cummins) and other than that, ALL other failures were ECM related. Our best mechanic moonlighted hauling fuel for an outfit that ran dozens of these engines. Their parts room had a reman ECM on the shelf. YMMV
well guys i do believe that i have found my problem i hope, i pulled the manifold pressure sensor and hooked it back to the wiring harness and blew air into the end of the sensor and watched for a change in pressure on the laptop and nothing happened. since the continuity is fine on all three wires, i do believe that the ecm is the root cause of my problem. i want to thank all for your help and my last question is are there any recommendations on where to get the ecm repaired or to buy another one?
No but just another point about potential corrosion between the fuel cooler plate behind the ecm. I've removed my ecm and cleaned this off a couple times over the years. I put some dielectric grease all over the plate last time to try to keep the moisture from getting in there but i haven't had it apart since. TRUCK SHOP, what's your solution for the corrosion or is this only more common in my area? I assume too much corrosion may cause the ecm to overheat?
Kind of a funny thread, the company I work for had 16 Celects-never lost a ECM and some of those were well over 800,000. I haven't found a decent aftermarket for ECM's personally. I have been told corrosion can cause a problem but have never seen it myself.
It seems odd to me your getting 40 PSI boost on the manual gauge. The ECM should cut the fuel way back if the boost sensor does not see enough boost pressure. Are you sure you have the right sensor?
Yes it is the correct sensor, I changed the sensor thinking the old one was bad. Everything worked correctly before the rebuild. The ecm was removed along with the cooling plate to paint the block. I know things can just go bad and maybe it did. The truck has over 2 million miles on it and I believe it is the original ecm so it might be warning me lol. I will take it out around Christmas and have it tested and fixed if needed. I believe the fuel is being miss calculated ant that’s why im getting the back fire and miss at times. You are correct that the boost sensor should read the same as the gauge but it is stuck on 6. When I unplug the sensor I do get a code thrown for low voltage to the sensor and when I plug it back in it goes to stored memory but I clear it any way
My concern about the boost pressure is that it seems way high. 40 PSI converts to around 80 inches of mercury which is about twenty more than I've ever seen in any engine. The hottest engines I've worked on were the twin turbo K Cummins running at about 60 inches or or 30 PSI at full load. Those were in 50 ton Haulpak rock trucks and pulling around 750 horsepower. I can see 40 inches in a highway truck fixed up hot to pull the mountains out west here. No way could I understand 80 inches on any 855 Cummins pulling freight.
Can you backprobe the wires on the boost sensor to see if the sensor readings actually change? I would confirm that before condemning the ecm. The spark ignition side of my brain says either a intake valve isn't sealing or the spark plug wires are on wrong. Is it possible the wiring harness was put on the injectors wrong?
When I had the engine rebuilt, I had them put the largest turbo made for the n14 on and the boost before the rebuild was 35. Not sure why you haven’t seen a higher boost, I’ve seen 50 on a highway truck so 40 is not really that much but back to the sensor. This problem started the day I took it from the shop, before a load was ever put on it. The man that rebuilt it said bring it back when a check engine light comes on. Well that hasn’t happened yet
I done a continuity test on all three wires and they tested fine. I get power to the sensor but apparently the signal doesn’t return or it does return but the sensor signal doesn’t transmit, either way something is not clicking to make this ecm compute the correct boost pressure, I put air to the sensor but no changes were made to the ecm
Could the sensor itself could be the problem and not the ECM?
I changed the sensor thinking it was bad but even after the new sensor was put in, nothing changed
Process of elimination says ECM then.
That’s what I’m thinking. At least it has been running so I think it will make it til Christmas and I will take it out of the truck and get it tested. Thanks for the reply
The NTC 475 Cummins with twin turbos and variable timing could pull 48 on boost. Detroit DDEC V would pull 50 psi on boost. But 40 is high depending on compression ratio and who knows what CPL the engine or turbo is. The trouble with these electronic thread problems is getting all the correct info in exact order. It's the usual dart shoot.
as long as we are throwing darts..... I would look at the throttle position and switches for loose connections or intermitting connections (as well as fuel solenoid), I'm not sure that if the ecm is seeing the wrong boost pressure that it would cause back firing?
The fuel pedal and harness was replaced 2 years ago along with the fuel pump and I changed those because at certain times when I would make a turn I would loose throttle response but that did not fix the issue then. Like I mentioned in a previous post, just after Christmas I will pull the ecm and have it tested
Just out of left field, from a non mechanic, what's the chance your new manifold pressure sensor is no good? Can you confirm its correct operation in any way?
I’m not sure if I can test it other than putting another one in. The one I took out was only 1 1/2 years old I think. This problem started after the rebuild and I’m just getting ideas as to what to check. I will be calling Cummins back tomorrow about this. They are open 24/7 for tech support but the guy I talked to today said he just knows general things and I should call back tomorrow.
How many wires does the sensor have? Frequently, you will have one wire grounded. One wire fed +5 volts or something. And the third wire returns the value to the computer. Apply shop air to the sensor at various pressures 0-50 psi and see what the third wire says. Should vary between ground and the +5 volts. However some sensors are pulse modulated instead. These are harder to troubleshoot. You need to find the troubleshooting procedure for your particular unit. Make sure the ground and +5v is present at the sensor. Then make sure the variable signal is making its way all the way back to the computer. After that you can condemn the computer.
I just done that today, I do have the 5v at the sensor but when I applied the air to the sensor, there was no change to the value from the ecm. I checked the continuity of all three wires and all tested good so that leaves me thinking that the ecm is stuck on stupid
Leave everything hooked up. Back probe the signal wire that goes into the ECM and verify that it is delivering a variable voltage to the ECM when you apply pressure to the sensor. Continuity checks are not good enough when this much money is on the line. You need to be 10)% sure the ECM is actually receiving the signal.
I'm with Birken. The only sure way to test out those 3-wire (ECM-supplied +, GND to ECM, and signal) sensors is to have them hooked up to the wiring harness and have a set of connectors for the DMM capable of back-probing the connector pins. I refer to them as spoons, maybe they have other names as well. Without knowiing what pressure sensor this particular one is then it could be a PWM type that would require a DMM capable of measuring duty cycle in order to measure the sensor output signal.
Just a thought. There is not a bent pin in the connector at the ECM?
I checked that and all pins seem straight and I also cleaned the harness and pins
Guys i want to thank all of you for your help.. It turns out that my diagnostic laptop was the faulty part on the boost pressure. I have boost pressure to from my sensor. but however i did finally found the whole problem with my performance issues. Turns out i lost #3 injector intermittent and was able to see that when i dropped the cylinders. i have done that test many times without it showing up bad. i pulled the injector today and will put the new one in tomorrow. i had new pass troughs put in during the rebuild but like i said, i used the old injectors which was only 2 years old and found that oil got pass the harness into the pass through which in turn shorted the injector out. when i install the new injector, i will let you know if my problem went away
So, couple silly questions. First, have you verified that the wires for the boost sensor are in the correct cavities? You said you checked continuity, did you check for correct orientation and cross-bleed? Second question, do you have Jakes or C-brakes sitting on top? When were they last overhauled?
Yes actually I bought a scangaugrkr and the boost pressure shows just fine on it. That’s when I done the cylinder test again and found #3 dead. However while driving the cylinder comes and goes whenever it feels like it. I will be getting the new one tomorrow and putting it in.
The jakes was overhauled with the engine 3 months ago
If the problem persists, i would plug the oil supply to the Jakes, and drive it around a few weeks. See if that leads to anything.
The solenoid dictates injection volume, i don't think it could possibly cause a backfire. The injection is still mechanically actuated via cam and lever. Have you inspected the rocker box for a crack in the center section? Have you checked overhead adjustment for that cylinder? Finally, with the rockers loose, if you blow air in through the injector opening, can you hear air escape through intake or exhaust? If you never generated an error, i find it unlikely that the pass thru oil contamination is the root cause. The celect were very simple and logical. No fault, no problem ( at least electrically) that's why I feel you wanna look for mechanical faults or external influence. Just my opinion, good luck and keep us posted, please.
Ok thanks for the heads up
Last sentence in post #4.
If none of my suggestions bare fruit, This has to be better than a dartboard
A lot of shops around here us those! Roll the Diagnostic Dice and replace parts. Not fixed roll the Diagnostic dice again and replace some more parts. Eventually the right parts get replaced or the owner runs out of money.
What kind of fruit you got? I like peaches.
Little fuzzy wrinkly ones
I have a dealer level diagnostics laptop that determined the dead cylinder. I’m not sure why it would not read boost pressure and that is something that I need to investigate but however I had a regular scan gauge 7 years ago that came from camping world that caught a bad injector before it even threw a check engine light and I replaced the injector and all was fine. It’s like a second opinion from another doctor and might catch it before the first doctor. This 99 never threw a code and had me very frustrated at the whole situation. It’s been going on since I paid 32000 For a rebuild so I was hoping for a miracle The scan gauge did not catch this injector but the laptop did
Sounds a little old and over ripe.