code 78 Waste Gate Selinoid

[kevin77]

I have replaced the vacuum waste gate controller with a mechanical waste gate, I have left the waste gate selinoid plugged in and I still get a 78 code - usually happens under full load - when I producing boost.

I looked at alldata.com at the diagnostic chart for code 78 and it does not mention boost as a problem - am I correct thinking I have an electrical gremlin? Or will boost on the turbo throw a 78 code?

Kevin

[gmctd]

Put the required gages on your truck and you'll see what's happening - only a few more cool months before you break your engine.

PCM wants 7psi Boost max because you have no charge-air cooler - you've defeated PCM's ability to control Boost, so when you exceed 7psi for a few minutes, PCM sets the DTC.

If it were summertime, you'd also get sudden low power as PCM desperately tries to protect the engine from high Intake Air Temps and high EGT's.

Gages first, then cooling and exhaust, then power-ups

Word up, dude.............

[kevin77]

So the boost sets a code 78 waste gate controller error and not 61 and 62 boost high/low circuit fault?

Kevin

[gmctd]

Those are sensor codes - sets when the connector is unplugged, or the sensor is failing\failed.

Over-Boost is 78, to prevent excessive Intake Air Temps which cause excessive Engine Coolant Temps and excessive EGT's - and excessive cash outlay.

[kevin77]

Ok, I checked the boost pressure this morning on my way to work - and if I am reading the tech 1 correctly, at about 3000 rpm with the pedal to the floor I am at 158 Kpa absolute pressure - I assume you take the this value subtract the barometric pressure (100) to get 58 which translates to about 8 psi - so I need to loosen the spring just a wee bit, right?

Kevin

[gmctd]

Good - a Tech1 - 1st thing you can do is, at the beginning, select English instead of metric so you can read in psia, degF, mph, gallons, and such.

Makes it easier, but , yeah, that is correct formula for Boost - try backing off the spring a little for 7psi.

Then, do a search here for boost fooler and such - an adjustable offset for the MAP sensor so you can run more than stock Boost

Remember, tho - without EGT\Boost gages, that can be very dangerous in summer heat - keep any eye on your IAT now for a reference, to see why.

[DickWells]

Kevin: You got hooked up with the right guy when you got GMCTD. He's helped me out too many times to mention.
I don't see that you have an intercooler. That's probably the reason that I was able to block off the vacuum to my WG solenoid, and run for years without any problems on my 6.5. I had the large SpearCo IC. I also had the boost pressure fooler on the sensor, too. But, I never had a mechanical boost controller, either. I typically saw 3-5 psi while at low throttle cruising, while towing, and up to about 14 psi under WOT on hills.
I can't tell you why I didn't see code 78 after I blocked off the vacuum. Probably GMCTD can. Actually, all I did was jumper the vacuum lines around the solenoid. I still had vacuum going to the waste gate off course. Innitially, I had code 78, so I put in a new solenoid. It lasted about a month. Got out and jumped the vacuum around the solenoid with a "T" and a 4 inch piece of hose, and never looked back for the next 85 K miles.
Just for what it's worth.
Dick:)

[gmctd]

When did you add the 'boost fooler', Dick?

DTC sets when over-Boost condition - greater than PCM is demanding - occurs for more than ~20 seconds

[DickWells]

GMCTD: I put the boost fooler (from Kenedy) in back in 99 or 00. I talked about it with you regularly on here for some time. Never did get it through my head why I ran so clean with it, in spite of your best efforts to talk me through it. I just learned to live with it and enjoy it.
From my seat here in my trailer in Brownsville (for the moment), I can't for the life of me remember what I did for boost control after I put in the mechanical pump. Seems I just left it hooked up where it was. Must be it's wired in separate from the IP. Anyway, the old Burb still runs great, and I don't question the reasons.
Thanks again for all the help you've given me over the years.
Dick:)