Yet another low-boost problem post...

Jazzer · Post by **Jazzer** » 20 Dec 2016, 17:24

Hi guys,
Hear me out if you will; 96 platinum 850 wagon with193k. Just rebuilt the head a few months back, engine has been running great; no codes, but turbo barely boosts into the white, perhaps an 1/8" when I'm pushing it on the turnpike. I replaced all of the old dry rotted vacuum lines, and any line that was showing age. Checked all hoses and intercooler lines. Thought it might be a bad TCV and swapped that out with a flea-bay TCV (perhaps it's bad as well?). No difference. I pulled the hose off of the wastegate actuator and tested that the turbo was in fact boosting (it did). I checked the diaphragm on the turbo when I had the engine apart and it was solid, no tears. So, there you have it. Any ideas?

Post by **rspi** » 20 Dec 2016, 19:48

You have a small hole somewhere, or a pending code in the system. When my car dropped a pending P0420 code, (no cel), it would not boost. One open vacuum port hole can cost you 6 psi. Also, get a real boost gauge to test your boost.

Post by **abscate** » 21 Dec 2016, 04:42

TCV probably should be bought new, as it's a part which ages to fail, as opposed to digitally good/bad like a purge valve.

Of course, if you have a friend with a turbo you can test it out, go fir the PnP

Post by **pkc303** » 21 Dec 2016, 04:52

I have a similar issue, and have the CEL on. Will pull codes this afternoon. Will start with vacuum lines. Good luck wih your low boost. Mine only goes up about 3/4 of max on the internal gauge.

Post by **rspi** » 21 Dec 2016, 05:12

That is where it should be, 3/4 into white.

Jazzer · Post by **Jazzer** » 21 Dec 2016, 05:37

Mine used to go about 1/2 way into the white, and even that was a nice boost compared to now

Is there a way to test the TCV with a meter perhaps? It seems like that would be the quick fix, anyway.

Jazzer · Post by **Jazzer** » 21 Dec 2016, 05:41

I'm also thinking it may be the line that I can't see, the one that goes back to the gas tank and runs along the fuel lines. I'll see if I can locate a link on here that will help, I seem to remember seeing it...

Post by **abscate** » 21 Dec 2016, 06:52

Jazzer wrote:Mine used to go about 1/2 way into the white, and even that was a nice boost compared to now
Is there a way to test the TCV with a meter perhaps? It seems like that would be the quick fix, anyway.

The DC resistance spec is 29.7 Ohms with no tolerance given. Ive measure this +- 3 Ohms on a small number of units that were working as well as a brand new unit.

The valve is not operated in DC mode, though - it is driven by an AC signal of something like 70 Hz IIRC. You could get a rough idea on the duty cycle if you can hold load steady by watching the voltage on the pins as a fraction of the TCV, but this isnt going to be very accurate.

Like most things ECU, the TCV gets a constant 12 VDC and is fired by ECU grounding the other side of the solenoid, so be careful what voltages you are applying where when testing.

Post by **abscate** » 21 Dec 2016, 06:54

Ahhh...from jimmy57

The TCV is a winding of wire that when energized produces a magnetic force to pull the pin with sealed tip off its seat on one port and closes the other end on another port. When the relay closes the TCV has battery voltage or alternator charge output voltage on that pin and when not grounded on the other pin that pin also has the same voltage. When ECM grounds that pin it will for a few thousandths of a second have nearly 0 volts and then a for a few thousandths of a second it will be batt voltage again.
Duty cycle is how pulsed fixed frequency signals are measured. The TCV is cycled between non-energized to energized postions 70 times per second if I remember the figure. On each of those operating cycles the grounding percentage can change. Higher percentage of the time grounded leaks more manifold pressure back to the hose going to MAF-to-turbo hose and reduces the pressure going to wastegate actuator. This gives higher boost. If the boost is desired to be reduced by ECM then it reduces the percentage of the time the TCV is grounded (reduced duty cycle low) and the valve stays in the position letting manifold pressure (turbo outlet pressure on the 5 cylinder engines technically) be sealed in to raise wastegate actuator pressure therefore opening wastegate and reducing boost.
A voltmeter is slow to read and the average voltage will be shown on Vmeter when change in voltage is fast.. If the batt voltage is 12.0 and the duty cycle is 50% then the meter reads 6 volts. If the duty low is 75% then the meter would show 3 volts. This assumes the ECM transistor is able to give a no loss, 0 V signal. Realistically it is likely .7V when grounded so maybe 6.35 V @ 50% and 3.17V @ 75%.
Duty low is ground or low volt % of time and duty high would be the remainder of the time when signal is at high level. Duty low and duty high always equal 100% when added.

Jazzer · Post by **Jazzer** » 21 Dec 2016, 15:33

Wow, that's a lot of good info. I think I'll just buy a new one lol.