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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8812
Experience:  I am ASE Master Truck Certified.
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I have a 2002 gmc 6500 with a 3126. The truck starts hard in

Customer Question

I have a 2002 gmc 6500 with a 3126. The truck starts hard in the cold but if you plug it in, it fires right up. The wait to start light comes on for about two seconds then goes off. I have power at the connector for the solenoid. There is power at the big wire on the solenoid also. I do not get a ground from the computer. I ohmed out the wire to the computer it is is fine. If I unplug the air temp sensor the light will stay on in the truck for about 30 seconds but the power still doesn't go to the intake heater. I also cannot connect my scan tool to the truck to see what the intake and coolant sensors are reading, I'm not sure why it won't connect. The serial number of the motor is ckm38790
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 1 year ago.

Thanks for asking your question. I will try and help you out.

What is the temperature where you are at?

When you say it starts hard, do you mean it's hard to start? Does it run rough at start up? Or does it not start?

Customer: replied 1 year ago.
the morning it wouldn't start it was About 40 degrees out. The truck would crank and puff a little White smoke but not start. I plugged it in for about 45 min and it fired right up
Expert:  Cat Man replied 1 year ago.

Thanks for the information.

If it's 40 degrees the inlet heater should stay on. The ECM will leave the heater on until the sum of the coolant temp and intake air temp exceed 120 degrees.

But to be honest. The engine should not have any trouble starting in 40 degrees. The inlet heater is not there to start the engine but to eliminate smoke at start up. It's an emissions thing.

I would suspect you have another problem.

The first is a low or no fuel pressure condition. The engine needs at least 25 psi to start. If it is low, the injectors will not get filled and it will not start. If the engine has a hand priming pump, and it gets stiff when operating it, the engine has enough pressure to start. If there is no hand priming pump, I would connect a psi gauge and measure the pressure. If the pressure is low, I would change the fuel filters first if you haven’t already. If it is still low, I would suspect a failed fuel pressure regulator on the back of the head or a weak fuel transfer pump. The pump is located on the back of the HEUI pump.

Second is a low injection actuation pressure. This is the high pressure oil system that activates the injectors. The engine needs at least 870 psi to start. If it is low, you have either a failed Injection actuation pressure control valve in the HEUI pump, a weak HEUI pump, a high pressure oil leak at an injector, or a faulty Injection actuation pressure sensor. To test the sensor, just unplug it. If the engine starts, the sensor is bad. It has 3 wires and is located on top of the engine near the front and screwed into the head.

Third is a failed engine speed timing sensor. Your engine has two. They are located on the driver’s side of the engine on the back of the engines front cover under the HEUI pump. The bottom sensor is the primary start sensor. If it has failed, the engine will be hard to start. The top one is the primary run sensor. They come as a pair.

Next is low battery voltage to the ECM while cranking. The ECM must have at least 7.5 volts DC to operate. I have seen weak batteries or a poor connection cause low voltage. You could have a poor connection at the batteries or starter.

Next is a faulty engine ECM. The ECM has an injector driver module inside. This is what powers the injectors. If it fails, the ECM will log injector codes for all the injectors.

Lastly are worn injectors. Over time the injectors can wear if they have a lot of hours on them. To verify, I like to check AC to the injectors while cranking. You should have between 20-40 AC while cranking. If you do, and you have good fuel pressure, the injectors are worn.

Most often when these things are happening there will be diagnostic codes in the ECM. If your engine has cruise control switches, you can pull the codes. If you have never done this, let me know. I can get you the procedure.

Please reply back if you need more help or have more questions. I am happy to assist.

Customer: replied 1 year ago.
The truck has a new transfer pump and regulator that are about 2 months old. So I might have another problem but there is also one with the intake heater also correct?
Expert:  Cat Man replied 1 year ago.

Yeah I would think so. It should stay on for a minute or so at 40 degrees.

Please reply back if you need more help or have more questions. I am happy to assist.

Customer: replied 1 year ago.
why when I unplug the intake temp sensor the light in the truck stays on for 30 seconds but the intake heater doesn't have power. And also do you know what to check for when my scan tool won't communicate with the truck
Expert:  Cat Man replied 1 year ago.

The ECM will want to keep the light on when there is a fault with the inlet heater. Since it does not know the inlet temp. It will keep the light and heater on.

Most often when a scan tool will not communicate. There is a problem with the wiring from the connector to the ECM.

Customer: replied 1 year ago.
Also out of all those possible causes which one could it be if plugging the block heater in makes it start right up
Customer: replied 1 year ago.
Do you have a wiring diagram of the data link connector
Expert:  Cat Man replied 1 year ago.

I would suspect an engine speed timing sensor problem. I have seen them be the problem when cold but not when hot. I will see if I can find a diagram.

Customer: replied 1 year ago.
Thank you that would be great
Expert:  Cat Man replied 1 year ago.

All I have is the wiring connections for the ECM. It shows where the data wires connect to the ECM. I hope this helps.

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