The Cursed 96 GMC Is Back

[Marcos Beltran]

Well i put this truck on tbe back burner for about 2 months now and figured let me give it another try and get this truck running. The truck would crank but wouldnt start. Ended up this time being the the gas hose between the fuel sending unit and pump ruptured. Easy fix fit or so i thought. The thing is the hose was less than a couple weeks old at the time. Well replaced it and truck ran for the first time in 2 months. Ran for 2 days and hose ruptured again. Replaced it again and now i realized that im getting an erractic/ high idle and this could be the reason for all the times the hose got damaged. It idles anywhere from 1000 to 1300 rpm when cold starting, sometimes and doesnt come down. Then sometimes it will be from 900 to 1100 rpm. If lm lucky when it warms up if will idle nicely at 650 rpm but if i put it in gear it sill rise to 800 to 850 rpm with my foot on the brake. If i shut it off and turn it back on it will idle at 1000 rpm and wont come down. Its all over the damn place and no i dont have a vacuum leak. I have a homemade smoke tester and no leaks. Replace tps and idle air control valve. Helped a little bit but still has the problem. Could the IAT sensor be the culprit? What else controls the idle? Map sensor was cleaned a while back and replaced with a known working one from a buddies truck and had no effect so put the old one back on. O2 sensors? What about the computer? Speed sensor?

 

[Frank Enstein]

Please tell us year, make, model, engine, and transmission so we can better guide you.

If the fuel line in the tank isn't submersible it won't last.

Is it for high enough pressure? The Vortec engine runs 58 psi and regular fuel hose will fail in short order.

Don't overtighten the hose clamps. The little bulge in the tubing where you push the rubber hose on is where it seals.
The clamp is there to prevent the hose from expanding enough to let it get over the bulge and pop off.

If you see rubber coming through the clamp it's already too tight!

Get a code scanner on it. You should be able to zero in on what the issue is pretty quickly if you can get live data on it.

Look at the IAC commanded value vs. the actual IAC position and check the TPS as well. Check the passage for the IAC for gunk.
Clean with throttle body cleaner NOT carb cleaner. Carb cleaner can damage the electronics.

Inspect all the vacuum hoses (smoke may not come out but air might sneak in) and the PCV valve as well as the EGR passages/EGR operation.

Any and all of the above can effect idle speed.

Once you get some values from the scan tool if you aren't familiar with what they mean ask us here. Shurkey will know.

I just bought a Snap On MT2500 for like $80 off Facebook marketplace so I'm still learning!

 

[stutaeng]

Yep, what Frank said.

I think I remember your truck now...
So the ruptured fuel hose was the reason why you were losing pressure? What the heck?!

...indeed that bi&$t@#ch is cursed!

 

[Marcos Beltran]

Please tell us year, make, model, engine, and transmission so we can better guide you.

If the fuel line in the tank isn't submersible it won't last.

Is it for high enough pressure? The Vortec engine runs 58 psi and regular fuel hose will fail in short order.

Don't overtighten the hose clamps. The little bulge in the tubing where you push the rubber hose on is where it seals.
The clamp is there to prevent the hose from expanding enough to let it get over the bulge and pop off.

If you see rubber coming through the clamp it's already too tight!

Get a code scanner on it. You should be able to zero in on what the issue is pretty quickly if you can get live data on it.

Look at the IAC commanded value vs. the actual IAC position and check the TPS as well. Check the passage for the IAC for gunk.
Clean with throttle body cleaner NOT carb cleaner. Carb cleaner can damage the electronics.

Inspect all the vacuum hoses (smoke may not come out but air might sneak in) and the PCV valve as well as the EGR passages/EGR operation.

Any and all of the above can effect idle speed.

Once you get some values from the scan tool if you aren't familiar with what they mean ask us here. Shurkey will know.

I just bought a Snap On MT2500 for like $80 off Facebook marketplace so I'm still learning!

96 gmc 4.3 v6. Yes gas hose is submersible, its the hose that comes with the pump and not too tight. . Fuel pressure is within range. PVC valve, hose, and elbow are new. Tps readings are at 0 when at idle. I got a scanner but it doesnt read the values for the iac, i know not much help,lol. I cleaned the throttle body when i first bought the truck back in April of 21. Took completely apart and replacd gaskets. But i will check the passage way tomorrow. Will also check other hoses again.

 

[Marcos Beltran]

Yep, what Frank said.

I think I remember your truck now...
So the ruptured fuel hose was the reason why you were losing pressure? What the heck?!

...indeed that bi&$t@#ch is cursed!

No, never figured that out. Truck ran fine after i basically replaced the fuel system, fuel injectors, regulator, and fuel pump.

 

[Marcos Beltran]

No other suggestions?

 

[1998_K1500_Sub]

What's your fuel pressure under no-vacuum conditions (e.g., WOT or engine off, you pick)?

Measure fuel pressure with the engine off by energizing the pump via the little wire over near the ABS pump. If it's exceedingly high (at or above 68psi, thereabouts), then there's a problem somewhere... and possibly causing that hose failure.

Or, starting from key-off / engine-off, measure the fuel pressure during the first few seconds of pump operation after a key-on event.

I once had an obstruction in the return line and the fuel pressure was running around 80psi (from memory) constantly.

 

[Schurkey]

the gas hose between the fuel sending unit and pump ruptured. Easy fix fit or so i thought. The thing is the hose was less than a couple weeks old at the time. Well replaced it and truck ran for the first time in 2 months. Ran for 2 days and hose ruptured again. Replaced it again and now i realized that im getting an erractic/ high idle and this could be the reason for all the times the hose got damaged.

Hose rupturing and idle speed have no relationship. Idle speed does not pop fuel hoses. Excess pressure pops fuel hoses. FIND OUT YOUR FUEL PRESSURE UNDER ALL CONDITIONS--hot, cold, idle, WFO, cruise, priming, etc.

Could the IAT sensor be the culprit? What else controls the idle? Map sensor was cleaned a while back and replaced with a known working one from a buddies truck and had no effect so put the old one back on. O2 sensors? What about the computer? Speed sensor?

Connect a scan tool, look at the data stream. Verify all sensor inputs, and computer outputs. Especially, verify IAC operation. Use the scan tool to increase/decrease the idle speed as proof that the computer and IAC system has control of the idle.

 

[Schurkey]

I just bought a Snap On MT2500 for like $80 off Facebook marketplace so I'm still learning!

You'll like it. It's very obsolete; and you'll have no capability for graphing sensor output. Seeing the sensor output as a graph can be very helpful...but for eighty bucks, you've got a great tool with some bi-directional control; ABS, Air bag, instrument cluster, and transmission/body computer interfacing (vehicle dependent) as well. The newest-possible software is for '09; mostly what I see is 2003 or 2006. Euro software is fairly rare.

Snap-On did a good job with allowing the various OBD-1 connectors to be used with newer scan tools, so the adapters that you get for the '2500 can transfer to the newer scan tools in the Snap-On arsenal.

 

[1998_K1500_Sub]

Hose rupturing and idle speed have no relationship. Idle speed does not pop fuel hoses. Excess pressure pops fuel hoses. FIND OUT YOUR FUEL PRESSURE UNDER ALL CONDITIONS--hot, cold, idle, WFO, cruise, priming, etc.

Building on Schurkey’s comments:

Keep an eye on your system voltage under various “engine-on” operating conditions. IF there’s a condition causing high system voltage it would enable high fuel pressure due to other causes (e.g., return line blockage).