Light knocking after replaceing spark plugs

[WESTTC]

I have a 2003 Silverado 1500HD with about 130,000 miles, and it's having trouble starting. Before replacing the fuel pump, I replaced the fuel filter, spark plugs and plug wires. I now have a light consistent knocking from the engine that I only hear while driving, not at idle. I hear it from within the cab. This was the first spark plug replacement on the truck and have had no issues till now. I used the GM recommended replacement plugs - AC Delco Iridium 41-110, which replace the 41-985 plugs, and according to recommendations I left the plugs gapped at .04 (though I did not check each one individually). I visually inspected each plug and they locked the same, though I realize .005 - .01 would be difficult to spot visually. Is it worth removing the plugs and gap checking each one?

[Jsdirt]

From here that sounds like a heat range problem rather than a gap problem. Maybe they got the wrong part number? It happens. Might need to go one step colder in heat range. Or the engine is just loaded with carbon.

[holty]

Did you also replace the plug wires? If not, you should.

[mhenshaw]

You should not gap or check the gap on Iridium plugs.

[txab]

Might have carbon. You may have spark knock sensors that are getting lazy. Might give it a little time for the ECM to "learn". What did the old plugs look like and were they proper plug #? At that mileage wires should be replaced as well.

 

Starting problem: Did you check your fuel pressure regulator? What is your fuel pressure, key on, engine off?

[WESTTC]

I did also replace the wires, specifically using AC Delco wires just to avoid any aftermarket spec descrepencies. Probably not necessary, but if I only do this once every 8 years, I'm ok paying a bit more.

[WESTTC]

The old plugs looked pretty good for 133k miles - no indication of fowling or soot - very clean. Probably didn't need replacing, but they were the originals (I bougt the truck new in 2003, so I know the exact history).

 

I have done no diagnostics on the fuel pressure, but that's my next step. Not trulying knowing how these fuel pumps work, my assumption is that the fuel pump seals used to actually pump the fuel may be weakening, and the pump is not supplying enough pressure to start the engine, but once started it seems to run fine. To start it cold, I can press the accellerator once during starting and it starts fine; otherwise tries to turn over but won't. Only other thing I can think of is a regulator or the ECM either sensing something to throw off the fuel deliver, the ECM malfunctioning (not likely - no codes generated as of yesterday), or an ignition problem. My gut is a fuel related issue. Will check pressure next.

[markw8]

By the PNs you have the correct plugs.

 

Mark

[txab]

The old plugs looked pretty good for 133k miles - no indication of fowling or soot - very clean. Probably didn't need replacing, but they were the originals (I bougt the truck new in 2003, so I know the exact history).

 

I have done no diagnostics on the fuel pressure, but that's my next step. Not trulying knowing how these fuel pumps work, my assumption is that the fuel pump seals used to actually pump the fuel may be weakening, and the pump is not supplying enough pressure to start the engine, but once started it seems to run fine. To start it cold, I can press the accellerator once during starting and it starts fine; otherwise tries to turn over but won't. Only other thing I can think of is a regulator or the ECM either sensing something to throw off the fuel deliver, the ECM malfunctioning (not likely - no codes generated as of yesterday), or an ignition problem. My gut is a fuel related issue. Will check pressure next.

 

 

On your truck you should have a fuel pressure regulator under the hood, drivers side fuel rail. It can cause your starting problems, or your fuel pump may be getting weak. Check the FPR and also check the fuel pressure. 55 - 62 key on engine off

[WESTTC]

txab, you may have been correct about letting the ECM "learn". The knocking has indeed stopped, so thank you for that. Still having the starting problem. I bought a fuel pressure gauge but haven't had much time to spend on it (3 other vehicles with problems, including rebuilding an AMC 258 for a 1980 CJ-5). I did put the gauge on the valve and the pressue jumps up to about 60, but drops to about 50 after about 2 seconds and then declines quickly from there. I need to time it, and see how far it drops. I have the test procedure, so I'll follow it when I get time. Thanks for the advice.

[Debo511]

Pull the vacuum line off the regulator , if it is wet with fuel, replace the regulator. Common problem.

[WESTTC]

I did actually pull the line off and it looked dry (though I didn't study it closely), but I didn't do the part of the test procedure that says to pull it and command the pump on. I'll take another look this evening. Thanks.

[WESTTC]

UPDATE: I pulled the vacuum line off the regulator and it's dry - no indication of leaks. I put a pressure gauge on the test valve, and the first two times of turning the key to ON, the pressure barely moved. On the third attempt it jumped to about 60 for about 2 seconds, then dropped to about 50 immediately, then slowly dropped to around 20 over about 2 minutes; I didn't watch beyond that. The test procedure wasn't real clear to me, but this seems to indicate a fuel pump problem. So, anyone have recommendations or cautions on dropping the fuel tank to replace the fuel pump? Any cautions or tips on how to best drop the tank without breaking or damaging fuel lines?

[txab]

Some people raise the bed.

[WESTTC]

UPDATE and final post on this topic: As previously indicated, knocking went away - apparently the computer adjusted itself.

 

Starting problem has been corrected - replaced fuel pump. Didn't have time to repair myself so took to the dealer. They apparently ran the appropriate tests and determined the check valve to be bad, which is part of the pump. This explains the rather rapid drop in pressure when signaling the pump prior to stating. Also explains why the truck ran fine once started.

 

Thanks to all who contributed.