**FIXED!!**DEFINITELY***Super HIGH idle after multiple parts replacement - TBI 1990 K1500 350engine

[BoondocK5]

I picked up a can of carb cleaner about 30 minutes ago. I'm going to give that a shot tomorrow.

When you have the engine running at an idle(running a bit faster than you think it should), spray the carb cleaner around the base of the throttle body. If that turns out to be the problem, it will actually slow the idle speed down as it temporarily cuts off the supply of extra air. This, causing a slower RPM.
This shouldn’t really be that hard to find, as it’s raising the idle speed so high.
I’ve also seen the the rod that is connected to the butterfly plates in the bore of the throttle body, the horizontal bore where the rod sits will actually develop a vacuum leak.
The repair for that was to get some pieces of teflon from an old Holley double-pumper rebuild kit's. Take the rod and butterfly plates apart, force a bit of that teflon bore repair into the bore for the rod, then, reinstall the butterfly plates and rod. The rod will force the teflon to “seal” the bore. Back in the day, this find and repair was considered black magic. No one knew about, let alone how to repair it.
Find that leak the same way as the base leak. Spray carb cleaner, listen for the RPM drop.

And, Bob’s your uncle

 

[dyeager535]

What EGR did you get?

GM used two largely different types of EGR, aftermarket (and probably even GM at this stage) has no idea what "right" is for those things.

The hard starting. Does the fuel pump prime when the key is first turned to run? Need about 15 seconds or so between key turns to make it prime again (internal ECM timer), but that'll tell you if the fuel pump relay (or it's circuit) is bad.

 

[ZombieK5]

What EGR did you get?

GM used two largely different types of EGR, aftermarket (and probably even GM at this stage) has no idea what "right" is for those things.

The hard starting. Does the fuel pump prime when the key is first turned to run? Need about 15 seconds or so between key turns to make it prime again (internal ECM timer), but that'll tell you if the fuel pump relay (or it's circuit) is bad.

The EGR I purchased was OEM and it was the exact part that came off the truck. The one that came off looked like it was the original from 1990.

The fuel pump appears to be functioning as designed. It was integrated into the sending unit so I did not need to install it separately. It is a Delphi pump and sending unit.

 

[dyeager535]

The EGR I purchased was OEM and it was the exact part that came off the truck. The one that came off looked like it was the original from 1990.

The fuel pump appears to be functioning as designed. It was integrated into the sending unit so I did not need to install it separately. It is a Delphi pump and sending unit.

If it's OEM number (and not one of those "fits 38 different applications with these 38 different GM part numbers) then you are probably good to go.

Does the fuel pump prime when you turn the key to run? Thats important towards figuring out the hard start issue. By functioning as intended I would assume yes, but necessary to know that specifically as that is the approximate time GM says it takes for the oil pressure switch to close and power the pump if the relay fails.

 

[ZombieK5]

If it's OEM number (and not one of those "fits 38 different applications with these 38 different GM part numbers) then you are probably good to go.

Does the fuel pump prime when you turn the key to run? Thats important towards figuring out the hard start issue. By functioning as intended I would assume yes, but necessary to know that specifically as that is the approximate time GM says it takes for the oil pressure switch to close and power the pump if the relay fails.

I'll stick my head under the truck and have somebody turn the key on this afternoon.

The EGR came in a GM labeled box. Let me go find the receipt and pull up the part number.

 

[ZombieK5]

If it's OEM number (and not one of those "fits 38 different applications with these 38 different GM part numbers) then you are probably good to go.

Does the fuel pump prime when you turn the key to run? Thats important towards figuring out the hard start issue. By functioning as intended I would assume yes, but necessary to know that specifically as that is the approximate time GM says it takes for the oil pressure switch to close and power the pump if the relay fails.

Confirmed... GM Genuine EGR

 

[BoondocK5]

Let’s hope this turns out to be a hose that got left off by mistake, or a plug that didn’t get put back on. It happens…
Maybe an intake gasket leak? Vacuum leaks are the single most common cause of poor running conditions.

 

[ZombieK5]

Let’s hope this turns out to be a hose that got left off by mistake, or a plug that didn’t get put back on. It happens…
Maybe an intake gasket leak? Vacuum leaks are the single most common cause of poor running conditions.

I plan on painting the top of the engine with carb cleaner this afternoon. I should know more by 4:00 p.m. today.

 

[gmbellew]

there never was a pic in the first post of the sensor missing the vac line. please post that up.

 

[ZombieK5]

there never was a pic in the first post of the sensor missing the vac line. please post that up.

I actually figured out what that was. It was the Breather line for the EGR valve.

 

[BoondocK5]

At least you found it.

 

[ZombieK5]

At least you found it.

Yeah, but I was hopeful that it was the source of any leaks I am having. Unfortunately that is the way it was designed. I should know more about vacuum leaks today. I'm going to go ape s*** crazy with the carb cleaner.

 

[Wes Harden]

That is a vent to allow the vacuum on egr to escape. It should have a foam filter on it, and possibly a cap over filter. That egr control solenoid could still leak vacuum there if it is not closing completely

 

[gmbellew]

That is a vent to allow the vacuum on egr to escape. It should have a foam filter on it, and possibly a cap over filter. That egr control solenoid could still leak vacuum there if it is not closing completely

the EGR solenoid has a foam covered vent. the EGR valve does not.

 

[ZombieK5]

*******UPDATE*******

I got the carb cleaner and there was a slight change in RPM when I sprayed it near the back of the TBI. I tightened all of the Bolts and they all seemed a little loose. It now seems to run perfectly fine for the most part. Son of a b**** if that wasn't the problem.

The thermostat just opened up and the idle is holding between 800 and 825 RPM. I thought it was supposed to be about 750 rpm. Seems like I'm almost there. I do not want to remove the cap blocking access to the idle screw however if the idle does not drop to where it needs to be I might just file down the end of the idle screw just a hair.

Any thoughts on adjusting the idle at this point? I really don't want to dick with it anymore now that it's close.

*****UPDATE 2******

I turned the engine off and restarted it. The idle dropped to about 700 to 725 RPM. About 30 seconds after I started it it's surged up and down for about 45 seconds then settled into the 725 RPM range.


*****UPDATE 3*******

The service engine soon light is back on. I have not checked with the code is yet.

 

[gmbellew]

700-725 is about right in park/neutral. it should drop to about 525 when you put it in drive.

 

[ZombieK5]

******UPDATE 3******

The engine is fully warm and when I stop it and start it again it will always immediately go into surge mode. The video attached is what it is doing but after about 4 minutes it settles into a normal idle. Right now it is idling at 700 RPM

And now for something new, when I throw it into drive the RPMs dropped to that about 550 rpms. When I let off the brakes and put the brakes on again the RPMs go up and down correspondingly.

View attachment 20250503_174943.mp4

 

[Wes Harden]

booster is leaking, when brakes are applied. Do not adjust the idle rpm screw. If you think you need to adjust that run the minimum air procedure.
pull the pcv and the crank case vent tube, plug both valve cover grommets and see what the rpm does.

the EGR solenoid has a foam covered vent. the EGR valve does not.

yep I was talking egr solenoid.
I can see where my statement might have confused. "Vent is to release vacuum operating egr valve".

 

[ZombieK5]

booster is leaking, when brakes are applied. Do not adjust the idle rpm screw. If you think you need to adjust that run the minimum air procedure.
pull the pcv and the crank case vent tube, plug both valve cover grommets and see what the rpm does.


yep I was talking egr solenoid.
I can see where my statement might have confused. "Vent is to release vacuum operating egr valve".

The steel tube going into the intake manifold was loose. I'm not sure if I loosened it or if it was like that already. I tightened it down and did not seem to make a difference other than to fix the issue of the RPM fluctuating when I pressed on the brake pedal . The rubber tube from the brake booster to the intake manifold is brand new. I sprayed down both ends of the rubber hose with carb cleaner and it did not change the rpm. I have everything tightened down now including the air cleaner and it's still surges. It will surge over and over again for several minutes before calming down to a steady idle. If I shut the truck down for 30 seconds or a minute and start it again, it starts to Surge again immediately. When it's steady idling at about 700 RPMs it's smooth as glass. I'm really really confused now and what could be causing this. I sprayed carb cleaner all around the edges of the intake manifold and nothing changed with the rpm.

 

[ZombieK5]

booster is leaking, when brakes are applied. Do not adjust the idle rpm screw. If you think you need to adjust that run the minimum air procedure.
pull the pcv and the crank case vent tube, plug both valve cover grommets and see what the rpm does.


yep I was talking egr solenoid.
I can see where my statement might have confused. "Vent is to release vacuum operating egr valve".

Could the brake booster itself be bad internally? Would that create a vacuum leak that would not necessarily ingest the carb cleaner? If I pinched off the rubber hose coming out of the brake booster would that be a valid test of that theory?