Hello all. As the title says I’m trying to track down the cause of an intermittent engine knock. I’ve got some good info to go off of so I’m hoping y’all can help as I’m out of ideas.

I’m working on a 2003 Dodge Ram 3500 with 297k miles. About a month ago I goofed and put gasoline in my truck. I drove maybe 2 miles after I put gasoline in it and parked it for the night. The next morning it wouldn’t start. I had it towed to the shop and they told me about the fuel and recommended that I put new injectors in it since I’ve never had that done. They replaced the injectors with stock remanufactured Bosch’s.

All was good when I picked it up from the shop. However, I did notice the knock every now and then. I took it back to the shop and asked them to look into it and they could not find anything wrong. They could hear it when I dropped it off but could not get the truck to recreate the problem. This happened a couple times. The mechanic mentioned the Fuel Control Actuator could have been damaged by the gasoline so I changed that and it still persists, although maybe not as pronounced. He also mentioned the gasoline could have damaged the injection pump but that is a $2,000+ part so I’m hoping that’s not the case. I haven’t put any injector cleaner through the system other than the Howes anti-gel I regularly use.

Here are some of my observations. It doesn’t do it when it’s cold, only after I’ve been driving around, shut the engine off, and then start it again. It then only does it at idle. After I have been driving around for a few minutes the problem goes away. When I start it up, it is worse when I shift into reverse or park. I just recently noticed that it comes and goes when there is an apparent strain on the electrical system. Something is pulling on the power system and when the lights dim a little bit, the knock really kicks in. I put a couple video below showing the symptoms.


Thanks in advance for any ideas.

 

First, I'm no expert, but since you've had no response in a couple days, I'll give my 2 cents.

I believe what you're seeing is the intake grid heater cycling on and off. I notice a bit dimming of the lights when the grid cycles, but no effect on engine performance. If the grid heater is pulling power from weak batteries, it may effect performance. Watch your ammeter/voltage gauge and see if voltage drops when the lights dim. How are your batteries and alternator?

I don't know when the grid heater circuit activates and I know you mentioned it doesn't occur when the engine is cold, so I may be off track, but it may be worth looking into a bit further. The grid heater pulls more amps than your alternator puts out, so the balance of power is drawn from your batteries. I believe the ECM uses the incoming air temp at the intake to determine when to close the grid heater circuit. It looks like outside temp at the time of your test was 34 degrees.