<DIV class=smallfont>Boost numbers up and down </DIV>
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<DIV>Okay guys, just got my pyro and boost gauges installed. I went for a test ride when I pulled out and stepped on it the boost jumped up to 15# but then dropped off to 5# (foot still in it). Bounces back up 15 and then back to 5. Took it to a pretty good hill for around here and motored up it at a constant 75. Boost goes to 10 and then drops off to 3. This continued all the way to the top. Up and down, up and down, up and down. Now that I have the gauges I can understand the performance I've been getting.

Is this normal??

There are no codes showing. Obviously the wastegate is opening and closing but this really seems to be a long cycle. Shouldn't it close down faster to keep the boost up without overboosting the engine? I have searched the forums and didn't find anything that seemed to match this one. Most of them point to problems with the wastegate solenoid, the actuator, or the vacuum lines. Would a mechanical wastegate controller fix this?

I'm a little confused where to go next.</DIV>

Sounds like a leak in the Boost gage line, but -


Not normal - Boost may spike to 15 or so, but level off to ~7psi under load.


When cruising 65-70, may be between 3 to 5psi.


No wild swings unless commanded by the driver.


Put a vacuum gage in the line between wastegate and solenoid.


Use a 'tee' and a long enough hose to reach around into the cab, then observe vacuum vs Boost pressure under same conditions you just related.


Should be 25" vacuum at pump, 15" vacuum at 'gate actuator at idle to start with.


Post your V\B readings here for further, compared with EGT readings.


What you're seeing should be reflected in EGT swings.Edited by: gmctd

Gking


With boost fooling I can get the guage to stay toward 13-14 psi all the way up the hill. Pyro will climb to 900-1000 F by the top.

Got the gauge on the vacuum circuit. 20 inHg at idle. 25 inHg while pulling. Pulled the electrical connection to the wastegate solenoid. 0 vacuum at idle. Book says this is normal. Wastegate actuator holds vacuum, no leak down.


There was a comment in the Diagnostic Aids section of Mitchells that states, "The turbocharger wastegate actuator uses a pulse width modulated vacuum solenoid that causes vacuum to fluctuate under normal conditions." What does this mean?

It means the PCM allows the WG to modulate or very the amout the waste gate dumps through the waste gate duty cycle. The stock WGDC is 65% at idle and drops as the voltage from the MAP increases. With a pot. on the MAP you can turn it up to 99.7% at idle by reducing the voltage reading the PCM gets.

Why didn't they just say so.


Any other thoughts on fixing this thing? Don't want to just start throwing parts at it.


Forgot to mention that this boost cycle is about 5 to 10 seconds from the high boost to low and back up. No corresponding change in EGT. When I was testing the other day with constant throttle, load and speed EGTs were about 800-850Edited by: GKing

Is the gauge defective?

Another idea is a leaking/failing wastegate actuator. If the WG actuator doesn't pull the wastegate lever strong enough, the wastegate could open due to the pressure of the exhaust gas on it. Boost pressure would drop, EG pressure too, and the WG actuator would pull the WG lever up again, until the EG pressure opens it again.

With that, you would see constant vaccum on the wastegate tubing, but boost pressure going up and down. The WG lever moves eventually slowly, and adding some delay for the turbine spinning up and down, you could get to those cycles of 5--10 secs.
Edited by: 0lee

Olee, what you say makes a lot of sense but I used a vacuum pump attached to the WG actuator and it held a constant vacuum. No leak down. Tried it at 5 in. Could move the WG by hand. Tried it at 10 in. Was tighter. Tried it at 15, etc up to 25 in. Where it was real tight and wouldn't open. No leak down at any of these levels.

The resistance on the MAP will sustain 15 psi boost unless your exhaust is leaking.

Don't really know what you mean by "resistance on the MAP" Don't know what a MAP is.


Exhaust is new from turbo back and not leaking.

Hm, I'm just guessing from what I've been reading here ... I've a boost gauge installed yesterday, and it doesn't jump.

Maybe the PCM sees some reason to lower the output of the engine, but afaik it would do so by either dropping the boost or by cutting down fuel delivery. If you've got constant vacuum from the solenoid to the wastegate while driving and if the WG actuator is ok, I'd say that the PCM doesn't try to lower boost.

Instead, cutting the fuel delivery could reduce the amount of exhaust gasses that drive the turbo charger. But I've no idea if that can result in a difference of 10 PSI on the boost gauge.

I'd expect that you would notice the loss of boost when driving, as power would decrease and increase ...

In any case, I'd first check the boost gauge --- if you get a false reading from it, you'll have a hard time in solving a nonexistant problem :/

In any case, I'd first check the boost gauge --- if you get a false reading from it, you'll have a hard time in solving a nonexistant problem :/



http://www.dieselplace.com/forum/smileys/Clap.gif http://www.dieselplace.com/forum/smileys/HiHi.gif


LOL. Truer words were never spoken.

Your vacuum switch is faulty, I've seen these exact symptoms before. PCM commands specific WGDC but the switch cannot accurately comply and the wastegate either closes too far or not enough causing the PCM to overcompensate resulting in the extreme readings your getting.

An example example of a MAP resistor.


http://www.dieselplace.com/forum/uploads/DE8_Boostswitch6.jpg


and a true potential switch


http://www.dieselplace.com/forum/uploads/E45_Boostswitch3.jpg


Good for 1-5 psi increase. That is, boost won't drop much from 15 or more on your GM-8 turbo.

Texas Diesel Guy really came through. http://www.dieselplace.com/forum/smileys/Clap.gif His diagnosis was right on. Replaced the WG solenoid and now the numbers are as they should be.


Thanks to all who responded.