[srudder]

My 2007 VR4 has been having a boost issue my mechanic and I have been troubleshooting for some time. The car bucked like crazy while building boost. The car also had trouble building boost. It would build if it was built up slowly, but when WOT the car would buck like crazy. Recently something changed, the car had no trouble building boost, but MAXED out at 14 psi and would then buck violently. We found the issue and I'm posting here so that others don't have to wrack their brains trying to find a solution...

The problem appears to stem from a faulty MDP (Manifold Differential Pressure) sensor. The MDP was likely going bad when the car would not build boost - sending signals to the ECU to open and close the waste gates in the turbos in an oscillation while accelerating. If low RPM and light load, boost would build since the load charts must have been below the threshold of the sendor value being sent by the MDP, but when higher engine load was initiated the MDP triggered the ECU to open the waste gates and the car would buck as it transitioned in and out of boost. Later the MDP failed entirely - so it wouldn't send any signal to the ECU triggering the waste gates to open when boost got too high and I think that the fuel pump couldn't keep up so the car bucked/backfired. It took a while to track this all down. We tested the waste gate solenoid by monitoring the solenoid's voltage and found that we saw change between engine idle and acceleration, but no change at all when boosting. This lead us to the MDP sensor and voila a quick 60 second change out from the old to a new MDP and the car is working like a charm!

I hope that this thread helps others that experience strange issues with their VR-4 boost fluttering or failing to build pressure.

 

[nissanzack]

Very nice job with the diagnostics! This is very interesting, especially as it would be specific to '96-up.

 

[bboyalan]

2007? Factory MDP is mainly for emissions operation, such as EGR... just saying heh

Likely open = leak = over work turbos = higher load

-sent from my Galaxy Note 9

 

[TurboSinceBirth]

I agree it was probably just a big boost leak. If you haven’t ever pressure tested the intake and intercooler system definitely do it.

The MDP sensor is a 1 bar sensor and measures vacuum up to zero psi. 14.5 psi absolute = 0 psi gauge. It only affects the purge and EGR solenoids not the boost solenoid. You can find the full disassembly of the MDP functions within Chrome over on 3SGTO.

 

[SSN Sailor]

Nice job and thanks for sharing! Reminds me if when I was getting fuel cut under boost at WOT. Got a real boost gauge and hooked it up, 23 psi on stock everything then fuel cut. The vacuum lines to the waste gate actuators were bad. (Posting here to hopefully help others).

 

[92vancouver]

Nice job and thanks for sharing! Reminds me if when I was getting fuel cut under boost at WOT. Got a real boost gauge and hooked it up, 23 psi on stock everything then fuel cut. The vacuum lines to the waste gate actuators were bad. (Posting here to hopefully help others).

I am wondering how I could test the hoses going from my boost controller to each or the turbo actuators. I do have a hand held vacuum pump or should I be using my air compressor to pressurize it and listen for hissing/leaking air sound. I am experiencing bad bucking right now, trying to solve it... Thank you

 

[SSN Sailor]

To be honest I am pretty sure I was replacing plugs and wires and decided to replace all the vacuum lines at the same time. The vacuum line connections under the upper plenum, where they connect with a hard pipe that goes to the front of the car , were really hard and brittle. After replacing them the overboosting and fuel cut stopped.

 

[Paul H.]

I am wondering how I could test the hoses going from my boost controller to each or the turbo actuators. I do have a hand held vacuum pump or should I be using my air compressor to pressurize it and listen for hissing/leaking air sound. I am experiencing bad bucking right now, trying to solve it... Thank you

Instead of trying to test pressure on small hoses like that use a vacuum pump. That way you can actually see if you turbo actuators are working as well.

 

[3KGTANG]

just to confirm, the MDP fix only applies to 2nd/3rd gens right? I'm still sorting through my hesitation issues as well. I have new vacuum lines not installed yet (boost/leak tested fine after a few fixes so wasn't in a rush to swap the lines). Maybe worth it though if it made a difference for you...