Loosing all boost off idle

[danieljleach]

94 K2500
6.5 TD
Vin F
Remote mounted PDM, new top end, new T-chain, GM4 turbo, 99 water pump, duramax fan.


I’ve been semi restoring this truck and I’ve rebuilt the top end of the engine with new heads, and an newer model water pump, rebuilt the trans, and rear end.

I’m not trying to make a super truck, I pulled a camper from Delaware to Montana and back last summer and I’d like to do it a couple more times. I take it easy on her and have a good set of gauges for trans temp, egt, and boost.

My problem is with my boost control. It worked fine after the rebuild. I switched back and forth between a mechanical boost controller and the stock system a few times just experimenting a bit. Then I ran the manual controller for the last year... I decided to switch back to the stock system again to take a quick trip to VA and get a good read on towing fuel mileage as I never did leave it stock long enough to compare. My problem is it’ll boost when I first accelerate but immediately drop off to 0psi. If I let it back down to idle it will build vacuum back up to the wastegate. The vacuum pump is holding 25in the whole time this happens. I replaced the wastegate solenoid, intake air temp sensor, map sensor on the intake manifold (not the one the firewall).

I don’t mind just switching back to the mechanical spring and boost fooler but I am wondering if this is exposing another problem. I’ve always thought I was slightly under powered and if the computer is deboosting, it must think that it needs to and maybe it defueling as well. I have a hard time deciding if it is underpowered as I know these things aren't power houses and I don't have any experience with any other similar vehicles.

I’ve wondered if the computer needs to relearn after having the boost fooler on it. So I have taken it on several short trips the last week but nothing has changed

If nobody has a better suggestion, I’m going to take a real careful look at wiring and ohm check them from the sensors to the computer. I might be able to take it to my buddies shop and put a scanner on it to see data.

Otherwise there are no codes and it runs pretty well with the manual controller on it.

[Robyn]

If the computer is complaining the SES lamp will come on and you will know whats up.

Check and make sure the waste gate vacuum pod is not leaking off.

Be sure all the hoses are in good shape and connected correctly.

[danieljleach]

If the computer is complaining the SES lamp will come on and you will know whats up.

Check and make sure the waste gate vacuum pod is not leaking off.

Be sure all the hoses are in good shape and connected correctly.

Thanks, The vacuum hoses are brand new and the waste-gate controller holds a vacuum infinitely. For whatever reason, it seems to me that the computer is intentionally lowering the boost. That is why I targeted the MAP sensor. I am under the impression that it uses pulse width modulation to raise and lower the boost pressure based on intake temperature and the boost pressure reading from the MAP sensor.

Appreciate any extra insight you have for me.

[Robyn]

Are the vacuum lines factory units that will only connect one way ????

If they are simple tubes/hoses try swapping the lines at the solenoid on the Valve cover...

[danieljleach]

Are the vacuum lines factory units that will only connect one way ????

If they are simple tubes/hoses try swapping the lines at the solenoid on the Valve cover...

Yes they are, thank you though. I've done this before but the end result is you get full, or almost full boost at all times. That was a good suggestion though.

[danieljleach]

If the computer is complaining the SES lamp will come on and you will know whats up.

Check and make sure the waste gate vacuum pod is not leaking off.

Be sure all the hoses are in good shape and connected correctly.

OK, I apologize if I wasted anyones time. When I pulled the waste gate actuator (or pod as you called it) off the shelf, I tested it with a hand pump and it held vacuum. I must have damaged it putting it on. Your comment inspired me to go back and check, which I should have anyway. I always tell my kids and employees when you are having a really hard problem that makes no sense, it is probably you. The waste gate actuator won't even engage with the volume that I can produce with a hand pump. I appreciate the help guys. I have a new one on the way and I assume that it will fix it. If it doesn't, I will definitely be back to pester you all.

[Robyn]

Glad you found the gremlin....:):)

It was likely not damaged putting it on....the diaphragm probably just gave up.

[danieljleach]

OK, I apologize if I wasted anyones time. When I pulled the waste gate actuator (or pod as you called it) off the shelf, I tested it with a hand pump and it held vacuum. I must have damaged it putting it on. Your comment inspired me to go back and check, which I should have anyway. I always tell my kids and employees when you are having a really hard problem that makes no sense, it is probably you. The waste gate actuator won't even engage with the volume that I can produce with a hand pump. I appreciate the help guys. I have a new one on the way and I assume that it will fix it. If it doesn't, I will definitely be back to pester you all.

The new diaphragm helped, however, the new borg warner activator solenoid isn't working right. I fiddled with it for a while, and put the old one back on and everything worked perfectly. I switched back and forth a bunch of times and it clicks and does work at times but even when it is clicking away, it will loose vacuum altogether. Thanks again for the help.

[Robyn]

Many new parts are DOA