I have a 1997 turbo diesel 2500 4X4 chevy pick-up [i think it has a
6.5L or 6.3L motor]. Just had a jasper transmission and motor installed [rebuilt, 90,000]. Problem is, I got it back about two weeks now from the shop, and it drives perfect. However, when i take it up to 55-70 mph and punch the pedal, it starts bucking like crazy. Now I can cruise pass that speed all the way up the speedometer, but I get that problem only when I punch the pedal within that range. I did an ODBII scan on it and got a reading of P0238 [Turbocharger Boost Sensor A Circuit High]. I have no clue what that means, and any help will be appreciated. Thank YOU!!!
While I've not seen your specific problem, it appears that your boost pressure is out of whack because (most likely) the turbo dump valve for the waste gate is either sticky or leaking, allowing it to open and close the turbo wastegate causing a surge or you have a leaky vacuum hose, bad vacuum solenoid, or faulty vacuum pump. Fortunately this should be an easy item to diagnose and fix.
First, check the waste gate valve on the turbo itself. It is located on the turbo and operated by a vacuum line that runs from a vacuum solenoid on the opposite side of the motor on top (one of three). Disconnect the vacuum hose to the valve and with a pair of pliers move the arm of the valve back and forth. If it doesn't want to move then the vacuum actuator is jammed, either because the valve itself is bad or the wastegate in the turbo is sticky; if it moves too freely then the diaphragn inside the actuator may be leaking or torn; but if it moves slowly and you can hear the air rush in and out of the unplugged port then it should be ok.
Next check your vacuum lines to and from the three vacuum solenoids on the drivers side of the truck. A cracked or leaking line could cause issues and is very easy to replace. You could also have a bad vacuum solenoid. The decal under the hood will show the vacuum hose routing and which solenoid operates the dump valve on the turbo.
Lastly, check the vacuum pump itself. Remove the serpentine belt and find the vacuum pump located on the passenger side of the motor. Turn the pulley by hand. If it spins freely it's bad and needs to be replaced.
All you have to do to check the vacuum is to pull the hose off of the dump valve at an idle. It should have full vacuum at an idle. If it don't then go back to the solenoid which is on the back of the drivers side valve cover and see if you have vacuum to that. If not most likely it would be a broken plastic vacuum line. Trace that back to the pump looking for a break. I would look at the boost sensor on top of the manifold. Look at the wires for chaffing. If it looks ok I'd try replacing the boost sensor. I doubt that the vacuum can be a problem at all, because that code is a overboost and no vacuum gives you low boost because they use vacuum to close the dump valve so without it you would have very little boost.
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