Trouble codes

Seadragon · February 25, 2006

I have a 1993 GMC Sierra 350 TBI. I just recently had a new crate engine put in and had almost all the sensors replaced. Now the SES light is on and I am reading two codes. They are 43 and 54. I know that the Knock sensor was replaced. I also put in a new ECM. I have had an intermittent problem with it starting and running. On occasion it will not idle and will not move unless you really pump and play with the throttle. I just found the code 54. How do you check the fuel pump relay? or any of the rest of the fuel pump circuit? I think I will go ahead and replace the relay, they're pretty inexpensive. The knock sensor thing has me really frustrated, well, all of ti does. This problem has been plaguing me with the older engine and this one. HELP

GM-Tech · February 25, 2006

Sounds like it might have a bad fuel pump relay. See if you're getting power when you turn the key on (without trying to start).

Knock sensors are very sensitive to their installation torque. You can't just screw it in and tighten the snot out of it. If you do, you'll damage it. Seems to me the torque is something like 12 lb. ft.

Seadragon · February 26, 2006

Thanks, I did replace the fuel pump relay ($8.00). I then unhooked the negetave battery cable and reset the computer. Started the truck and no SES till I reved the engine a little. Checked the code and all I got was 43. So, I'm hoping I solved one of my problems. Trouble is, it has been such an intermittent problem, I'll have to wait and see if it occurs again. Now, I suppose I'll have to put a new knock sensor in, or, can I reseat this one? I was told on another forum that I should get 4 to 6 volts on the wire from the knock sensor, if I unhook it and check it with the engine running. Is that the correct way to eliminate the wiring? I really do appreciate your responce. I haven' had a lot of luck with my questions on the other forum.

PaulQ · February 26, 2006

Hey sorry to jump on your post like this, but I'm having the same fule pump relay problem myself, and could use any help I can get. Where is this thing located?

Thanks for any help you can provide and sorry again for jumoing in and not helping on your other problem.

Paul Q

GM-Tech · February 26, 2006

I don't recall if, only the older models, the PCM supplies a bias voltage. The knock sensor itself generates an AC signal voltage when it picks up vibrations. It has a piezo element. It's like the little gizmo in gas barbque grills that lights the flame when you press a button.

I'd also want to know (but don't have access to manuals at home) what the actual test that's run to determine if it's working. Some of the older ones simply advanced the timing X amount and expected to see a spark knock signal (detonation). If the base timing is not properly set (retarted), the PCM can't advance the timing enough to create detonation and it would falsely set a knock sensor code, when in fact it didn't pick up a knock only because one wasn't generated.

Seadragon · February 26, 2006

Hey sorry to jump on your post like this, but I'm having the same fule pump relay problem myself, and could use any help I can get. Where is this thing located?

Thanks for any help you can provide and sorry again for jumoing in and not helping on your other problem.

Paul Q

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No problem PaulQ. My fuel pump relay was on the passenger side firewall. There is a black cover about a foot wide and four inches deep, sorta "T" shaped. There are fuses and relays under it. The cover is held on by three plastic nuts on my truck. Good luck with your problem. Mine has driven me nuts, I have even dreamed about it!

Seadragon · February 26, 2006

Thanks GMTech, One thing the mechanic said was he had trouble setting the timing correctly. I'm going to try to get it set to specs and see if that might help.

Seadragon · March 5, 2006

OK, help, I thought the new fuel pump relay had solved some of my problem, but. I started the truck yesterday (after using it for three days in a row) and it wouldn't stay running. It would act like it ran out of gas at an idle. When I tried to drive it , I had to constantly pat and play with the throttle to get it to move. Then, after it warmed up to operating temp, it ran fine! Now, it will probably start and run fine for a week or so then do the same thing again! I'm ready to set fire to it! HELP

GM-Tech · March 5, 2006

Can't offer much help without some basic diagnostic info unless you just want me to make a wild guess?

Seadragon · March 5, 2006

Could you give me some places to start looking? I can't get the thing to happen predictably enough to take it to a mechanic. I never know when it will happen again.