Hard start and knocking sound!!! plz share your knowlege

[Brandon]

Hard Start and Knocking Sound

Hello all,

Thank you for sharing your knowledge and helping this diesel newbie out.

I am having a hard starting issue with my 6.2 suburban and a knock sound at idle and low throttle when engine is not fully warmed up, knock sound almost goes away once engine is at normal temperature. The truck starts really hard after being turned off a while and sometimes will barley start and then stall out, and then I have to give it a lot of gas to get it to start, once running engine is strong but still has knocking sound.

Not my video but my engine sounds almost exactly the same: http://www.youtube.com/watch?v=j9LPFEb6aRY

Things about my truck:
6.2 Detroit diesel with 114K miles
New air filter
New fuel filters (2)
New lift pump
New check valve on return side of injector pump
Injectors and injector pump was supposedly replaced a year ago (previous owner had receipt)
High idle solenoid disconnected
Aftermarket spin-on secondary filter

Possibilities for issues:
Air leak: I have pressurized the fuel system a few times and never been able to find a leak. I swapped out the rubber line on the return side of the injector pump with a clear vinyl line. When engine is running there are no bubbles flowing through the vinyl line. After around an hour of truck being off noticed small bubbles floating to the top of the vinyl line and the bubbles are coming from the return side and not from the injector pump. Would air on return side of fuel line cause the hard starting and the knocking sound I described?

Injector pump timing: I believe my injector pump timing is very slightly retarded. Line on pump is barely left (toward passenger side) of the line on block. Would this cause any of the issues I’m experiencing?

Injector: Could the issue be from a failing injector or an injector that is not popping at the right psi?

anything else you guys can think of? Step by step instructions are greatly appreciated. Thank you all so much

 

[fredflinstone]

Start with the basics

If you have a multi-meter you can eliminate if it is electrical. Measure your voltage at the terminals with nothing on. Should be around 12 - 12.8 volts. Turn on just the lights with the high beam and see how fast the voltages drop. Shouldn't be dropping more than a volt every 10-20 seconds. If the voltage drops off fast it is the battery. Lastly ,check the voltage at the terminals with the truck running, should be 13.5 to 14.5 volts. If not the altenator is shot. If this is not the problem I think you are going to need a diesel doctor judging by all the stuff you are after changing. This is the hard start part only.

 

[Brandon]

If you have a multi-meter you can eliminate if it is electrical. Measure your voltage at the terminals with nothing on. Should be around 12 - 12.8 volts. Turn on just the lights with the high beam and see how fast the voltages drop. Shouldn't be dropping more than a volt every 10-20 seconds. If the voltage drops off fast it is the battery. Lastly ,check the voltage at the terminals with the truck running, should be 13.5 to 14.5 volts. If not the altenator is shot. If this is not the problem I think you are going to need a diesel doctor judging by all the stuff you are after changing. This is the hard start part only.

Thanks for the input, I had the alternator checked a few months ago and it was perfect. The truck starts great if it hasn't been sitting for long and the starter turns the engine over at a fast speed, I also hooked up jumper cable to a running vehicle and then started the suburban and it started with the same results as starting on just the battery. Thanks for the advice Fred

 

[Jims86]

Thanks for the input, I had the alternator checked a few months ago and it was perfect. The truck starts great if it hasn't been sitting for long and the starter turns the engine over at a fast speed, I also hooked up jumper cable to a running vehicle and then started the suburban and it started with the same results as starting on just the battery. Thanks for the advice Fred

I would say do no more diagnostics untill you get the timing confirmed, otherwise, you are chasing your tail.

 

[crazy4offroad]

I would say the knock is from someone using starting fluid/ether to start it, it is supposed to be very bad for a diesel's bottom end since they are so much higher compression. As for the hard start I would suspect an air leak or the injector pump, try to find out for sure if the pump is good.
*I'm no diesel expert but have heard of similar problems being blamed on the pump, and bottom end knock being caused by the use of ether to start a hard-to-start diesel engine.

 

[Brandon]

@Jims86: how do i confirm timing on the 6.2 diesel? thanks for the advice

@crazy4offroad: Would a air leak on the return side cause the issues? The hard starting and knocking sound started at exactly the same time. Should i advance the pump timing to try and alleviate the issue?

Thanks guys

 

[Jims86]

@Jims86: how do i confirm timing on the 6.2 diesel? thanks for the advice

@crazy4offroad: Would a air leak on the return side cause the issues? The hard starting and knocking sound started at exactly the same time. Should i advance the pump timing to try and alleviate the issue?

Thanks guys

Thats something I personally would take to a diesel shop, and have done. I dont work on diesels much, and cant see spending $190 on the reluctor attachment for a timing light. I would also run a can of Sea Foam in the tank, just to help clean things a bit.
I wouldnt want an air leak on either side.