Waste gate control and idle question

[JM1995]

Hello
I recently bought the truck and trying to sort through the changes made by a previous owner. The truck runs excellent and I haven’t had any problems with it. The person who I bought it from was not person that did the changes so his knowledge of work done was limited. It’s a 1995 3500 it has had the db4 to db2 swap done. It looks like they tapped into the intake on the engine side of the turbo and ran a hose to the waste gate and from there to under the dash where it is plugged. I want to make sure this is effective way of regulating boost. (Pictures added). Also the truck idles at 1k if I pull back on lever at IP it will come down to 700. Once you throttle again it will come back down to 1k. It’s not the high idle holding it it seems sticky inside of pump. I attached a picture of the linkage set up. Is there a fuel treatment that will help this or is there some other issue? This is my first 6.5 so I’m still learning thanks for any help.

[DmaxMaverick]

That hose from the intake is NOT boost control. It is simply a line into the intake that goes nowhere. Probably intended for a boost gage.

You have 2 ways to control boost. One is the wastegate. The OEM vacuum controlled WG may or may not be working, but it won't allow any boost if the DS4 IP is missing. If it made any boost, you'll probably find the vacuum actuator has been replaced with a spring-type regulator at the wastegate. It should have an adjustable spring to regulate relief (boost) pressure. Maybe show us a pic of the turbo/wastegate? The other is your right foot. More fuel = more boost. Less is less. That is, of course, as long as the wastegate is able to close and stay closed while it makes boost.

If your throttle is sticking, you're just going to have to figure out what's sticking. Usually it's something mechanical and external, like the throttle pedal, cable or a coupler. If it's internal, you may try some fuel additive or ATF (added to the fuel, just 1/2 fill the filter when you change it). If fuel isn't leaking from the throttle shaft, then the shaft probably isn't what's sticking. If it still sticks internally, probably best to have a tech get a look inside. If it's sticking a a grand, what's to say later it won't stick at 3 grand?

What's controlling the transmission? The original PCM won't do it, with the electronic IP missing.

[JM1995]

Thanks for the reply
It may not be the correct way or even functional but that’s exactly what they were trying to accomplish. The hose goes from the intake to the actuator that controls the wastegate “T”s from there to under dash where it is plugged. Is it possible that it’s set up to actuate wastegate from boost pressure?


And I’m not sure how the trans is being controlled, I assume it has a stand-alone TCM. It shifts fine no issues. I’m slowly trying to dissect what was done.

[JohnC]

Well, that's pretty funny.... The original system used vacuum to keep the wastegate shut, and when boost increased, it modulated the vacuum to allow the exhaust pressure to push the wastegate open. By applying manifold pressure to the actuator all you do is force it to stay open all the time. You'll be lucky to get 3 psi of boost that way. I suppose if you put a big spring on there to pull it shut and then used manifold pressure to override the spring, you might be able to get something to work, but a lot simpler is just use a spring to pull it shut and allow exhaust pressure to push it open. I did just that one day when my vacuum pump failed, so I could get home pulling my horse trailer.

[DmaxMaverick]

Yeah, what John said. Boost pressure could be used to regulate the wastegate (like the LB7 Duramax does), but it certainly wouldn't be practical with the original actuator, and operating the actuator (diaphragm) opposite probably wouldn't last long. It simply isn't designed for that.

Either spring (pardon the pun) for a Turbo-Master controller, or make your own, like John said. The 92-93 model MFI wastegates use a similar setup.

[JM1995]

Yea I thought it was a strange set up. I’m going to go with the manual spring style. I have a boost gauge ordered and I want to hook it up the way it is just to see if it’s making any boost before I change it out just for s&g. Thanks again

[JM1995]

I got the gauges hooked up. I have 0 boost at idle, 5psi running 60 egt at 500 and up to 11psi with my foot in it going uphill egt 800. Does this sound correct?

Also on the idle issue I put a heavier spring on it it’s now idling at 850 if I quickly flip the throttle with my foot or if I pull it back at IP with my finger with little force it comes down to 650. It kind of feels like it’s dragging on something. I have eliminated the throttle cable and the throttle position sensor being the problem. Is there something I can check internal on the IP that would be causing this?