I need help!!!!! GM mechanic needed please

[Terry Flynn]

Is there any GM mechanics out their that could please help me?

I have a 1997 GMC seria 4x4 with the 5.7 and it wont start!

well this is what happened. yesterday I was driving and it just died. it would not restart right away opened the hood and checked the wiring plugs to see if anything came loose. everything was fine tried to restart and it ran, drove about 5 miles parked the truck for the afternoon got pack in and it started right up drove home no problem. this morning went to leave and it wouldn't fire. checked fuel pressure I had nothing. BINGO bad fuel pump. I pulled the box off and replaced the fuel pump. well guess what it still wont run!!!!!! so I checked to see if I had power to the pump. no power. ok lets change the relay.

new relay. we got power the truck started WHOO HOO, it died no power to the pump again. so one more trip to the parts store new oil pressure sending unit. well guess what it ran idled for about 5 min. shut it off now it wont start again.

no power to the fuel pump. now here's were it gets strange sometimes it will have power and sometimes it wont.

I turn the key on it has power then it goes off. just like it should 5 sec. you crank it over and it will show a constant power.

plug in the fuel pump and it will start. runs about 10 sec and dies. check to see it there is any power to the pump and nothing. what else controls power to the fuel pump?

Thanks for any help.

Terry

 

[NITROSNIFER]

Is the security lamp on in the dash??? The PCM (power train control module) will cut power to the fuel pump if it does not recognize the key. When you changed the oil pressure sending unit did all the wiring look ok?? After initial start up the PCM looks for a pressure signal from the oil pressure switch if it does not see it it will also kill the power to the fuel through the PCM. I know this does not help a whole hell of a lot but these two systems are directly related to the fuel circuit. I am thinking the PCM is having issues and cutting fuel. The problem is you'll have to see your GM dealer to get the PCM and dash/immobilizer programed.

 

[shawdaddy]

I have seen the fuel cut off switch do the same thing when going bad. It is part of the safety mech due to a crash and kills the pump. Can't remember what it's called. Ryon

 

[Terry Flynn]

Is the security lamp on in the dash??? The PCM (power train control module) will cut power to the fuel pump if it does not recognize the key. When you changed the oil pressure sending unit did all the wiring look ok?? After initial start up the PCM looks for a pressure signal from the oil pressure switch if it does not see it it will also kill the power to the fuel through the PCM. I know this does not help a whole hell of a lot but these two systems are directly related to the fuel circuit. I am thinking the PCM is having issues and cutting fuel. The problem is you'll have to see your GM dealer to get the PCM and dash/immobilizer programed.

This truck dosn't have a security system. And yes from what I could see the wires look to be in good condition.

 

[Terry Flynn]

I have seen the fuel cut off switch do the same thing when going bad. It is part of the safety mech due to a crash and kills the pump. Can't remember what it's called. Ryon

I don't think this truck has this? it's a Z71 so I would think that some good mudding would act like a crash. and this truck has never been off road it's still spotless underneath.

 

[SayMikey]

I have seen the fuel cut off switch do the same thing when going bad. It is part of the safety mech due to a crash and kills the pump. Can't remember what it's called. Ryon

I don't think this truck has this? it's a Z71 so I would think that some good mudding would act like a crash. and this truck has never been off road it's still spotless underneath.

Get hold of Ken Stang on here or Charles Perdue you need a wiring diagram and a test lite or volt meter to track it down

 

[TERMINATOR]

I have seen the fuel cut off switch do the same thing when going bad. It is part of the safety mech due to a crash and kills the pump. Can't remember what it's called. Ryon

I don't think this truck has this? it's a Z71 so I would think that some good mudding would act like a crash. and this truck has never been off road it's still spotless underneath.

Get hold of Ken Stang on here or Charles Perdue you need a wiring diagram and a test lite or volt meter to track it down

it sounds like a ignition module to me terry .. if it goes south and it sounds just like one going bad they will shutdown fuel to the motor because the computer sees no signal so it will not supply fuel to the engine ..so it does not flood it out. try taking it out and having it checked out at the parts store "before you buy one..and maybe have the pickup coil checked out as well" .. most certianly you will find one or the other is bad. and at that point just replace the whole distributor so it is all fresh and new and they cost pennies more than the two parts anyways.. good luck

tk

 

[Terry Flynn]

I have seen the fuel cut off switch do the same thing when going bad. It is part of the safety mech due to a crash and kills the pump. Can't remember what it's called. Ryon

I don't think this truck has this? it's a Z71 so I would think that some good mudding would act like a crash. and this truck has never been off road it's still spotless underneath.

Get hold of Ken Stang on here or Charles Perdue you need a wiring diagram and a test lite or volt meter to track it down

it sounds like a ignition module to me terry .. if it goes south and it sounds just like one going bad they will shutdown fuel to the motor because the computer sees no signal so it will not supply fuel to the engine ..so it does not flood it out. try taking it out and having it checked out at the parts store "before you buy one..and maybe have the pickup coil checked out as well" .. most certianly you will find one or the other is bad. and at that point just replace the whole distributor so it is all fresh and new and they cost pennies more than the two parts anyways.. good luck

tk

Thanks Terry

But the only problm is some times I have no power to the pump, even when you turn the key to the on position prestart. you should get power for 5 sec and someting it's there and sometime not?

 

[dwilfong]

Get a code reader from the local auto parts store. They don't coast much any more . It will save you a lot of time and money in the long run and this wont be the last time you will need it.

you can spend a lot of money in parts changing trying to guest at it.

 

[shawdaddy]

Terry, If it is fuel injected it has the fuel cut off switch( called an Inertia switch). If you are close to the parts house they will run a code test for free and get the parts you need. while you are there.

 

[NITROSNIFER]

That's weird your 97 Z-71 does not have a factory mobilizer mine does. Computer is on the drivers fender well right???

 

[Bob Howard]

I am a GM tech. Will pull up some diog. and see if I can locate anything. Have had lots of problems with ignition swithes.

Bob

 

[Andy Greene]

I have a friend dealing with the same issue-

Not questioning you, but how does the ign switch not give signal to the fuel pump intermittently ??

He did find a poor wiring connection(bared wires contacting each other intermittently) at the OP Sending unit- and it hasn't acted up since- but its only been a day or 2

Nitrosniffers , explanation makes a LOT more sence to me after my buddy was trying to explain how the OP switch effected fuel pump operation. GM sure did some stupid stuff back then, you could get your lunch handed to you in a heart beat as a tech. :huh:

Andy

PS - Keep us posted, I'm curious ......

 

[NITROSNIFER]

I have seen GM ignition switches cause start up issues. If one part of the switch contacts fail under load it can interrupt power to one or more systems required to send power to relays or modules. Gm use a ground activated relay on some systems so there is a 30 power (always there) load power for the relay coil. The ground comes from a switch or the CPU and grounds the coil in a relay or grounds out a motor to activate. This can be an intermittent issue that can come and go dependent on the contact through the switch. Your vehicle did not die while driving so it makes more sense that a switch issue on initial power up (switch to start position) may be your issue.

 

[NITROSNIFER]

GM is not the only company that used the OP switch to complete the fuel pump circuit. Ford/Chrysler/ Nissan/Jeep all similar systems. On key engine off the fuel pup primes. Then after start up the fuel pump circuit waits for a signal from the op switch to complete the circuit. It was designed to not allow the engine to run if no oil pressure was present. Any op switch with a 3 wire configuration is of this design it may not disable the same system but it's for similar reasons. Oil pressure closes the contacts in the switch sending ground to the coil in the fuel pump relay keeping it on.

 

[nitrotwin]

FUEL PUMP !!!!!

Check for power at relay should be about 3 sec. check for fuel press at port gm has one sorry fuel pump no cooling when fuel gets low

 

[Terry Flynn]

I have seen GM ignition switches cause start up issues. If one part of the switch contacts fail under load it can interrupt power to one or more systems required to send power to relays or modules. Gm use a ground activated relay on some systems so there is a 30 power (always there) load power for the relay coil. The ground comes from a switch or the CPU and grounds the coil in a relay or grounds out a motor to activate. This can be an intermittent issue that can come and go dependent on the contact through the switch. Your vehicle did not die while driving so it makes more sense that a switch issue on initial power up (switch to start position) may be your issue.

it did die on me when i was driving. it took about 5 min then it restarted and ran fine.

Thanks for all the help guys please keep it comming.

Terry

 

[Bob Howard]

If it died because of lack of fuel you either lost power at ECM B fuse or lost ground or power at connector C 103 which is located under brake booster. Ground is a black and white wire and the power is a gray wire. Check the ground to see if it is loose at the center of right hand cylinder head.

Anotherthing that I just remembered is that the new fuel pumps come with new connectors because of problems at the connector.

Bob

 

[NITROSNIFER]

Hey Bob forgot you still worked at GM. Power for ECM B Fuse is from the ign switch is it not???

 

[Bob Howard]

ECM B fuse is hot at all times. On SI I can only go back to 98 on most items but engines go back a few more years so I dont have any of the wiring ecept engine.

Bob