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Zcardude

Need help diagnosing boost control issue

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Posted (edited)

I'm working on a 73 240Z with an L28ET with a larger turbocharger running on a MegaSquirt II running msns2e. Honestly not sure the type of car is all that important as I think this is more of a general turbocharger issue that I've never crossed paths with before. The car runs pretty well and is fun to drive but is doing something sort of funny with regards to the turbocharger and boost control. Here's a graph of a 3rd gear pull from megalogviewer:

 

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Essentially what's happening is, when I go to full throttle, the turbo produces some boost (about as much as it was making previously at part throttle) and just... continues to slowly build more and more boost. Ideally It would hit some sort of boost level and hold it. Or even hit some boost level and creep up from there would be fine, I would be totally stoked to tune for that. Additionally the boost is getting a little hot - maybe this is expected but it feels like the turbo is working hard to achieve the given level of boost. I was seeing AITs in the 130 F at 70 F ambient range on this pull, with a small same-sided intercooler, hot air intake, and a windshield wiper sprayer hitting the IC with watermeth. This might be ok given the crappy intercooler setup, crappy hot air intake. Additionally coolant temps rose to approx 200F from my thermostat's 180F and quickly fall back to 180F over 15-20s  - I'm thinking this might be a symptom of me using re-routed heater lines to the water cooled section of the turbocharger, essentially dumping hot water from the turbo directly on the coolant temp sensor but maybe a symptom of something else. I have a large arizona Z car aluminum radiator and shrouded electric fan and new water pump so the cooling system is sufficient unless something else is wrong.

 

Since taking this datalog, I have fixed 5 very, very tiny pinhole boost leaks in some welds in my intercooler piping using a boost leak tester, but I am thinking it would take significantly more than that to produce this behavior so I am looking for more ideas. I will be re-testing with these very small boost leaks fixed shortly but don't expect that it will resolve the issue due to how tiny they were. When using the boost leak tester, releasing the line from my air compressor let a whoosh of air out the boost leak tester's fitting, telling me that the system held pressure pretty well and that the leaks were minor.

 

I keep thinking maybe there is a wastegate problem. The wastegate is an integral dual port wastegate with a 7psi spring. I've run the vacuum line directly into a vacuum port before the throttle body to the top vacuum port on the wastegate, and also with a boost controller on that same vacuum line. In this run it was hooked up to a boost controller, but the same behavior happens without a boost controller and the boost controller did not make an appreciable difference in controlling boost. The wastegate actuator can be moved by hand and has good tension and seems to shut fully (I can hear it kind of "clack" shut if I release the spring by hand. Maybe there could be a problem with the diaphragm in the wastegate however? The arm on the wastegate actuator is threaded, so I could also try tightening it up. The arm also goes to the wastegate from the actuator at a slight angle and is not 100% straight.

 

Happy to answer any other questions that I can about the setup if it helps. The car otherwise drives great and is fun to take on spirited drives, doesn't smoke or anything weird, but I'd like to get some proper boost control in place here and start making some decent power, right now it's sort of underwhelming when you stomp on it. Also worth noting that the turbocharger and exhaust manifold seem tight, all the bolts that I can get to easily are snug, no large amounts of sootiness around anything.

Edited by Zcardude

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Okay slight update: I just put my intercooler piping back together and it seems to be holding boost. I just retested my system with my compressor set at approx 25psi and am not getting bubbles anywhere. One thing of note is that I was watching the wastegate actuator as I was hooking up the compressor to the boost leak tester and it did not move at all with 25psi in the system. Would that point to a bad wastegate actuator? 

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Okay another update: I yanked the actuator and hooked it directly to my compressor and it does not move at all. Instead all of the air seems to push around the outside of the diaphragm housing where it is crimped together. So seems like this is the issue and the boost just seems to do whatever it wants as the exhaust pushes the wastegate puck open

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Posted (edited)

I hope people are enjoying my troubleshooting thread! I believe I fixed my wastegate actuator off the car. I then bent the actuator arm so it does not get cocked at a funny angle to reach the turbo. When retesting the car with the boost leak tester again, the wastegate now seems to go in and out as desired, and I also noticed... one more huge boost leak! The BOV piston is leaking under boost letting a ton of air by. I've disassembled the BOV and permatexed the crap out of it - hoping that it seals or I will need to get a new one. Will retest and post results shortly once the permatex dries. If it's not one thing, it's another!

Edited by Zcardude

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I ended up not driving the car tonight but I did retest the system and the BOV no longer leaks air by. Instead of driving I spent the evening building a cold air intake out of extra intercooler piping while I had things apart, so hoping to see reduced IATs as well as good boost on a drive tomorrow. 🤞

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Posted (edited)

Update!

 

Boost-wise, things are looking much better. It looks like it makes positive pressure at about 1800rpm, full boost at 3k, creeps by a few psi by 4k and tapers back to wastegate by 6k where it is pretty much out of steam. So this was likely caused by a combination of some boost leaks and a non-working wastegate actuator but the boost curve now looks totally normal so I think my original questions were resolved! I hope others find my troubleshooting useful.

 

Outside of that, the car sounds and feels a bit better, seems like it is not working as hard to do what it's doing, but it is extremely rich under boost (saw high 9 afr's at some point when glancing at the gauge?) and a little stuttery and not making a ton of power because of that. I'm going to manually remove some fuel from the fuel table before attempting to let autotune do its thing and will go from there.

 

Also wanted to note: Realized my last datalog the other day where I was complaining about the temperature jumping to 200 under load is because the electric fan wasn’t on. I definitely need to get this under megasquirt control. Today I had the fan on and the temperature held steady. I have a skillard front splitter and I think that the radiator and engine bay gets very little cooling without the electric fan pushing in air so this is really really important.

Edited by Zcardude

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Pin hole leaks are just a nuisance and likely don't affect your issue, but good to get them sealed.  If you have boost creep then it would indicated you need to port the wastegate hole on the turbine housing to increase flow.  Post up your actual log file and I will take a look using LogViewer and post the graphs. 

 

Regarding IAT, strive to be within 20F of ambient so if you are seeing 50F above ambient then likely your intercooler sucks.  Garrett 24x12x3.5" core works exceptionally well and I see less than 20F over ambient at 34psi in the summer. 

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55 minutes ago, 240Z Turbo said:

Pin hole leaks are just a nuisance and likely don't affect your issue, but good to get them sealed.  If you have boost creep then it would indicated you need to port the wastegate hole on the turbine housing to increase flow.  Post up your actual log file and I will take a look using LogViewer and post the graphs. 

 

Regarding IAT, strive to be within 20F of ambient so if you are seeing 50F above ambient then likely your intercooler sucks.  Garrett 24x12x3.5" core works exceptionally well and I see less than 20F over ambient at 34psi in the summer. 

 

34psi? Are you running an L28ET or something else? 

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Yeah definitely this thing could use more intercooler but I am happy keeping things around 120F at the moment with my new CAI. I want to redo all of my piping and do a larger core intercooler that enters and exits on opposite sides of the engine bay and a proper CAI and I'm sure I too can strive for 20F over ambient.

 

I don't mind the boost creep at all - I think it should be totally manageable with tuning and I am probably not going to be doing anything about it. I'm fighting what I think is a bit of an ignition issue at the moment but it seems to hold ~17psi pretty well after a small spike when using a boost controller which is a good target for this current setup. I'll shoot for more when I get a bigger intercooler

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