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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8761
Experience:  I am ASE Master Truck Certified.
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2002 GMC 7500 with 3126, 3126 ser# CKM64044, We have tried

Customer Question

2002 GMC 7500 with 3126
JA: What is the complete model and serial number of your machine?
Customer: 3126 ser# *****
JA: Are you fixing this yourself? What have you tried so far?
Customer: We have tried everything, has new injectors, pump, cam sensors, sensor harness and injector harness. Truck came in, customer complaint was sometimes truck would run great and other times run terrible or not start at all. We have 200 rpm cranking and 1200 to 1800 pump psi
JA: Anything else you want the equipment mechanic to know before I connect you?
Customer: Does smoke and smells of fuel
Submitted: 8 months ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 8 months ago.
Thanks for asking your question. I will try and help you out.
Expert:  Cat Man replied 8 months ago.
Has the fuel pressure been checked while cranking?
Are there any codes in the ecm?
Customer: replied 8 months ago.
No to fuel PSI while cranking. No codes in system, we did get running but smokes like it is running rich
Expert:  Cat Man replied 8 months ago.
Thanks for the information.
Sounds like it may be sucking air.
Have you tried running it out if a bucket and bypassing the truck tanks and fuel inlet line?
Customer: replied 8 months ago.
Ran out of bucket still no different
Expert:  Cat Man replied 8 months ago.
Thanks for the information.
Does the engine run? Or is it currently a crank no start?
Customer: replied 8 months ago.
Does not want to start, we did spray and to get it running
Expert:  Cat Man replied 8 months ago.
Thanks for the information.
Since you have replaced the injectors, heui, and harness. There are four other things I would suspect.
First is low fuel pressure. Low pressure can be caused by a weak fuel pump that's on the back of the heui, faulty fuel pressure regulator on the back of the head. Or a plugged hand primer if the engine has one.
Second is bad fuel. If you haven't already. I would run the engine out of a bucket of known good fuel.
Third is the engine has been over sped and all the valves in the head are slightly bent and not sealing. It's rare but I have seen it a few times.
Lastly is a faulty injection actuation pressure sensor. They can read wrong and cause no start with smoke and no codes.Please reply back if you need more help or have more questions. I am happy to assist.
Expert:  Cat Man replied 8 months ago.
Hey. I just wanted to see how things were going. Please let me know if you need more help. I am happy to assist. If my information was helpful, please leave a good or higher rating so that I receive credit. Thanks.
Customer: replied 8 months ago.
We have low fuel PSI, we will let you know what we find, just have not had time to work on it.
Expert:  Cat Man replied 8 months ago.
Ok sounds good. Please reply back if you need more help or have more questions. I am happy to assist.
Expert:  Cat Man replied 8 months ago.
Hey. I just wanted to see how things were going. Please let me know if you need more help. I am happy to assist. If my information was helpful, please leave a good or higher rating so that I receive credit. Thanks.
Customer: replied 8 months ago.
OK we have spent some time on this, When running out of bucket and returning to bucket truck runs great, has 70 PSI when running. Shut truck off and try to start right away it will not start and fuel PSI only go's up to 25 PSI, have to spray to get started again.
Expert:  Cat Man replied 8 months ago.
Thanks for the information.
25 psi should be enough pressure to start.
I would suspect a bad primary engine speed sensor. Your engine has two. The lower one is the primary start sensor. If it fails, the engine will be hard to start but when starting fluid is used, the engine spins fast enough for the top high speed sensor to work.
Customer: replied 8 months ago.
We did replace these sensors, the lower one did not OHM correctly. When cranking over now with scanner on it shows 152 RPM then once in awhile it will jump to 290 for a split second, still no start.
Expert:  Cat Man replied 8 months ago.
Thanks for the information.
Can you tell me how much actuation pressure you have?
Have you tried checking AC to the injectors while cranking?
Customer: replied 8 months ago.
800 PSI out of Heui pump, it will start with a squirt of either
Expert:  Cat Man replied 8 months ago.
800 is not enough. It takes at least 870 psi to start. If you only have 800. I would suspect a faulty heui pump or leak at an injector.
Customer: replied 8 months ago.
I am told that it fluctuates 800 to 1200PSI
Customer: replied 8 months ago.
What is the min engine speed to start
Expert:  Cat Man replied 8 months ago.
There should not be any fluctuation. That indicates an actuation pressure problem. Most likely a faulty heui pump, iap sensor, or leaking injector.
150 Rpms are enough to start.
Customer: replied 8 months ago.
Ok, So miss understanding, We have a variation because of battery voltage, charger on we have higher numbers.
Right now cranking it reads 940 steady and 150 RPM.
Expert:  Cat Man replied 8 months ago.
That should be enough.
Next I would check AC to an injector while cranking. You should see 20-40 AC while cranking.
If you do not. You have either low voltage while cranking, faulty ecm, low actuation pressure. Or no TDC signal to the ecm.
Customer: replied 8 months ago.
We have 22 AC at injectors with injector unplugged, we have 154 cranking RPM and 970 HEUI pressure.
Expert:  Cat Man replied 8 months ago.
Thanks for the information.
If you have AC to the injectors and the engine does not run. You have either bad injectors, bad fuel, or no compression, or the cam to crank timing is off.
When AC is at the injectors. This indicates the ecm has power, the ecm is getting an rpm/TDC signal, and the ecm is detecting it has over 870 psi.
The only other thing I have seen is an iap sensor say it has over 870 psi but in reality it does not.Have you checked the iap with a manual gauge?
Customer: replied 8 months ago.
We have 900 PSI on mechanical gauge and scanner shows 900 PSI?We can not even get it to run with either nowThis is very frustrating!
Expert:  Cat Man replied 8 months ago.
Thanks for the information.
Sorry to hear you are having this trouble.
Unfortunately if the engine will not run on starting fluid. I must suspect a compression problem. Most likely bent or non sealing valves.
I would remove the intake and exhaust piping and crank the engine. If the valves are not sealing you will hear popping out intake or exhaust.
Customer: replied 8 months ago.
What does not make sense is we have had running and it ran good, how can it just not have compression now?
Expert:  Cat Man replied 8 months ago.
The only time I have seen ether not work is when there is no or low compression. Compression is all ether needs.
Customer: replied 8 months ago.
What is the min spec on compression?
Expert:  Cat Man replied 8 months ago.
Unfortunately Cat does not release a compression spec. Sorry. I would suspect it's around 250 psi.

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