94 suburban issues

[GT8]

checked the lift pump already. talked to 3 different diesel mechanics that i know and they all are pretty sure its the IP.

[sathackr]

Originally Posted by GT8

lately it has been acting strange. its not everyday but more often lately the truck will sputter. doesnt matter if going down the highway or just idleing. almost like it hiccups. i just had a heath pmd isolator installed in march. and once in a while the gauge cluster will show like the key is turned forward but but not actually running when it actually is. battery light is on and rpm's aren't even registering. from what i am reading it could be anything from a bad ground to the ip to the optic sensor in the ip. the local shop i take it to said most likely its the ip but could be the optic sensor and should put some sea foam in to possibly clean out the sensor. does this make any sense to you guys? and if it is a bad ground then where would that most likely be at? getting frustrated. thanks in advance for any feedback

Maybe a bad ignition switch?

[FloridaHorseman]

Originally Posted by GT8

... i just had a heath pmd isolator installed in march. and once in a while the gauge cluster will show like the key is turned forward but but not actually running when it actually is. battery light is on and rpm's aren't even registering...

I can't quite figure out what you mean by "the gauge cluster will show like the the key is turned forward but not actually running when it actually is". Key ON and the gauges go to a test position first and then go to actual engine signals after start. Are you saying the gauges go full travel while the engine is running and then return to normal engine readings?

The erratic gauge and battery light behavior shouldn't be caused by a faulty IP... but inversely, a bad ignition/key cylinder could cause the IP to cut in and out. Have you seen any correlation between the odd gauge activity/battery light and the sputtering condition or are they independent of each other?

If the IP problem started after you installed the PMD relocation kit you should probably check the extension harness. And make sure the ground wire is still attached to the torx screw on top of the IP cover plate and not attached to the new heat sink. It wouldn't hurt to Tee a fuel pressure gauge into the input line at the IP with an extension long enough to reach your windshield cowl and note the fuel pressure when the sputtering occurs. That should isolate fuel supply (LP or clogged fuel tank sock) problems from IP/PMD electrical problems.

Without more information and test results I don't know how your "mechanics" all agreed it's a bad IP. It sounds to me like either the ignition circuit is shorting somewhere (bad key cylinder?) causing the IP to cut out or you have two unrelated problems, electrical and fuel supply, each skewing the diagnosis. A fuel pressure gauge should narrow it down some. ~FH

[jrsavoie]

I have had to replace the ignition switches in all but one of our vehicles.

I had various undiagnosed problems that were cured with a new GM/AC Delco switch. Do not use an aftermarket.

How many miles are on the Burb?

It sounds as if you are having an electrical problem as apposed to a fuel problem.

If you are having an intermintent fuel problem it is good to check the tank sock. Stuff cvan plug the sock and then slosh around away from it only to be sucked back against it again later.

Did you hot wire the lift pump and run about 3/4 gallon out the drain while noting thhe flow?

I use a glas gallon pickle jar, let it sit 20 minutes and look for crud and water bubbles on the bottom.

When you change the fuel filter, do you suck out/clean out the bowl? I use a 12 volt in line fuel pump. A lift pump will work. If I suck out the excess fuel and there is still crud at the bottom of the bowl I use Sea Foam to clean it up and suck it out with the fuel pump again

[sathackr]

Originally Posted by jrsavoie

I have had to replace the ignition switches in all but one of our vehicles.

I had various undiagnosed problems that were cured with a new GM/AC Delco switch. Do not use an aftermarket.

How many miles are on the Burb?

It sounds as if you are having an electrical problem as apposed to a fuel problem.

If you are having an intermintent fuel problem it is good to check the tank sock. Stuff cvan plug the sock and then slosh around away from it only to be sucked back against it again later.

Did you hot wire the lift pump and run about 3/4 gallon out the drain while noting thhe flow?

I use a glas gallon pickle jar, let it sit 20 minutes and look for crud and water bubbles on the bottom.

When you change the fuel filter, do you suck out/clean out the bowl? I use a 12 volt in line fuel pump. A lift pump will work. If I suck out the excess fuel and there is still crud at the bottom of the bowl I use Sea Foam to clean it up and suck it out with the fuel pump again

Just as a maintenance item, I dropped the tank on mine a few weeks ago to check the sock. Not sure if I'm the exception, but my sock had a bypass valve in the end of it that opens if the sock clogs so that fuel will still flow.

[57diesel]

That sock bypass was something I knew GM did in the early passenger diesels. I seem to recall its described in the service manuals. It said it was for cold weather operation. Apparently they felt that the fuel could be gelled enough to not go through the sock so they made that bypass. I was sort of thinking that if it was that gelled that just the soc would not be your only problem.

[GT8]

Originally Posted by FloridaHorseman

I can't quite figure out what you mean by "the gauge cluster will show like the the key is turned forward but not actually running when it actually is". Key ON and the gauges go to a test position first and then go to actual engine signals after start. Are you saying the gauges go full travel while the engine is running and then return to normal engine readings?

The erratic gauge and battery light behavior shouldn't be caused by a faulty IP... but inversely, a bad ignition/key cylinder could cause the IP to cut in and out. Have you seen any correlation between the odd gauge activity/battery light and the sputtering condition or are they independent of each other?

If the IP problem started after you installed the PMD relocation kit you should probably check the extension harness. And make sure the ground wire is still attached to the torx screw on top of the IP cover plate and not attached to the new heat sink. It wouldn't hurt to Tee a fuel pressure gauge into the input line at the IP with an extension long enough to reach your windshield cowl and note the fuel pressure when the sputtering occurs. That should isolate fuel supply (LP or clogged fuel tank sock) problems from IP/PMD electrical problems.

Without more information and test results I don't know how your "mechanics" all agreed it's a bad IP. It sounds to me like either the ignition circuit is shorting somewhere (bad key cylinder?) causing the IP to cut out or you have two unrelated problems, electrical and fuel supply, each skewing the diagnosis. A fuel pressure gauge should narrow it down some. ~FH

its weird, you'd have to see it. when the truck is running sometimes it seems its in the test position. but only the battery light is on and the rpm's don't register until you start moving, but once you stop the needle falls back down again like its not running. its not all the time and there is no correlation between that and when it sputters. i changed the alternator because from what i understand is that the rpm's are controlled by the alt. haven't seen any problems since saturday when i changed the alt. i guess we'll see. checked the extension harness and did not see any problems with it. it is grounded to the torque screw on the top of the IP. as far as the ignition we have not checked that yet.

[GT8]

Originally Posted by jrsavoie

I have had to replace the ignition switches in all but one of our vehicles.

I had various undiagnosed problems that were cured with a new GM/AC Delco switch. Do not use an aftermarket.

How many miles are on the Burb?

It sounds as if you are having an electrical problem as apposed to a fuel problem.

If you are having an intermintent fuel problem it is good to check the tank sock. Stuff cvan plug the sock and then slosh around away from it only to be sucked back against it again later.

Did you hot wire the lift pump and run about 3/4 gallon out the drain while noting thhe flow?

I use a glas gallon pickle jar, let it sit 20 minutes and look for crud and water bubbles on the bottom.

When you change the fuel filter, do you suck out/clean out the bowl? I use a 12 volt in line fuel pump. A lift pump will work. If I suck out the excess fuel and there is still crud at the bottom of the bowl I use Sea Foam to clean it up and suck it out with the fuel pump again

it has 127k on it. i will look into the ignition switch, i'm sure its a lot cheaper than the IP. haven't done anything with it yet. would like to rule out the small things first.

[stefuel]

There are other grounds on the back pass side of the engine you should look at.

[GT8]

it is giving two codes. one is for the glow plug relay and the other one is 36. one of the mechanics said it was the ip. would a bad pmd make it give any codes?