Code 78

[Kenneth]

I was driving home from work today at 65 mph. All of a sudden I noticed my SES light on. Then it went off. I checked the guages and everything appeared to be fine. I kept driving. The light came on one or two more times, then went out. I had a similar exerience last summer where code 78 was set. The situation seemed similar. My question is, last summer when it happened Heath Diesel convinced me my vacuum setup was going bad and to replace it with the Turbomaster. I plugged off the turbo wastage solenoid with the little plastic ball provided in the kit. Is it possible to get a code 78 (wastegate solenoid fault) when I am using the turbomaster?

My truck is essentially stock. I think the one thing to add is that back in May or so, I soldered in a potentiometer inline with the turbo boost signal going to the PCM. I then turned up the boost to ~10-12 psi (I don't have a guage so I am not totally sure).

If anyone could chime in and give me an idea. Do I really need to replace the wastegate solenoid? Or do I need to check the soldering on the potentiometer (I have it right next to the PCM behind the glovebox). Any help would be much appreciated. Thanks

[More Power]

Is it possible to get a code 78 (wastegate solenoid fault) when I am using the turbomaster?


Yes.

The better 6.5 programmers can work around that problem. Some here feel the vacuum boost control is best. Personally, I don't have an opinion one way or the other, except I would want what works...

Replacing the wastegate solenoid wouldn't do anything while you have the mechanical turbo wastegate actuator installed.

Jim

[chickenhunterbob]

Code 78 is caused by overboost, as Jim indicated, your wastegate solenoid has nothing to do with it, it's not controlling the wastegate.

Try turning your turbomaster nut whichever way reduces boost a tad.

[Kenneth]

That's funny. The reason why - I soldered a 10K potentiometer into the boost signal going back to the PCM. Also, I did a lot of towing this last month. I towed an 18 ft car trailer loaded one time with construction materials, the other time with a 68 mailbu. I also towed an 18 ft enclosed car trailer loaded with my brother's family's things (about 6-8,000 lbs) from Chico to Gridley - about 45 min with a trailer each way - (Then my buddy hooked up his 08 Duramax with only 3,500 miles on it and towed it the rest of the way).

Anyway, what I am asking and I guess pointing out, is that why would I get an "overboost" code when cruising down the freeway at 65 mph empty, and not when I am doing somewhat heavy towing? My truck struggled a little to pull that enclosed trailer. It was downshifting a lot and I figuired if it would set an overboost code that would have been as good as time as any.

Don't get me wrong, I appreciate all of the reply's and believe them, I am just trying to understand. I have been thinking of turning the boost back down to about 8 psi anyway, so now is a good time. Another thought just came to me, I thought when you get an overboost code, the engine defuels and goes into limp mode.

BTW, I haven't had the code set again, and the truck seems to run great. Thanks again for the replys.

[kaylabryn]

It can get this code for an underboost condition if I remember correctly. It usually means the ECM is seeing a condition outside the norm at the MAP sensor, and it can not correct for it.

Mike

PS: The 94-95's are very sensitive to this, more so at lower altitude.

[More Power]

The PCM looks at a variety of input data to make a decision whether or not to set a boost code. One of those input data is engine RPM. A code 78 can set if the engine RPM exceeds 2500-rpm and if the boost pressure isn't what the PCM wants to see. If you're not aware of it, this RPM relationship can make it a bit confusing when trying to pin down a pattern for setting the code.

For example, it might be that while you're towing you don't exceed 2500-rpm, but you do occasionally while running unloaded.

Jim

[Kenneth]

Okay. That makes sense. I didn't know it was the same code for an overboost as well as for an underboost. I have been meaning to change out my rubber/silicone peice between the turbo and upper intake. I did change both upper and lower manifold gaskets when I had to pull the intake to install the fsd resistor. I also noticed recently, that I have a leak between the turbo and the RH manifold - you can see the soot. This is due to the fact that the two studs facing the engine are missing the nuts. So only the outside two bolts are holding the turbo onto the manifold. Horrible I know. So, to make a long story short, there probably could be an underboost condition.

As far as the RPMs, both times it set the code I was only going 65 mph; ~2050? I know I have a 62mm alt. pulley and 265 tires, so it could be off a little. Anyway, I understand what you are saying More Power. The PCM sets a code if the boost doesn't fit within range at a given RPM.

Sorry for such a long post. I appreciate all the help. From what you guys have told me, and from what I am seeing, I should be fine. I'll turn down my turbo to about 8 psi, replace that silicone piece, and tigthen the turbo down with ALL four nuts. I just need it to last about another year so I can trade it in or sell it. I am looking to buy the new 4.5L 1/2 ton duramax or an 2009 3/4 ton 6.6 duramax. Thanks again for the help.

[JohnC]

I struggled with this problem for about a year, 12 or so years back. I don't have the documentation any more, but I'm pretty sure the DTC can set anytime the RPM is over 1800 and the fuel rate is over something like 20 mm3. I'm also pretty certian that it will not set for under boost, only over boost.

Put a scanner on the truck and go for a ride. Watch the waste gate solenoid duty cycle and manifold pressure. You should see the duty cycle slowly go lower and the boost stay the same or increase. After 30 seconds the duty cycle will drop to 0 and the dtc will set. The cause is usually a sluggish solenoid, plugged solenoid vent, or possibly a sticky wastegate. If the problem occurs more in hot weather or when the under hood temps are high, like after a fuel stop on the highway on a hot day, it's most likely the solenoid.

Oh, yeah, you have a turbo master. Well, the conditions are the same, but I guess the solenoid can't be the cause. You'll still see the solenoid duty cycle drop off as the PCM tries to bring the manifold pressure down, and drop to 0 when it gives up. A resistor in series in the sensor line is not really a good way to control it. The PCM senses voltage and you're regulating current. A voltage divider like JK's "boost fooler" works better, or just change the programming.

[More Power]

DTC P0236 will set if:

1- The engine speed is greater than 2400-rpm.
2- The fuel rate is greater than 20 mm3.
3- The boost pressure is greater than, less than, or equal to 20 kPa from desired (Internal to PCM).
4- The conditions were met for 10 seconds.

Or, DTC P0236 will set if:

1- The engine speed is greater than 1800-rpm but less than 2400-rpm.
2- The fuel rate was greater than 20 mm3.
3- The conditions were met for 12 seconds.

This code can be a little confusing to troubleshoot because of the variety of conditions that have to be met. Based on experience, the most common cause of a DTC P0236 or DTC-78 is due to a lack of vacuum acting on the turbo wastegate actuator. This could be because of a faulty vacuum pump, break in a vacuum line, defective wastegate solenoid or a ruptured diaphragm in the turbo wastegate actuator. The wastegate solenoid usually winds up being the most common cause among the various possibilities.

Jim

[JohnC]

OK, I lied. I still have an All Data CD, it's just a PITA to use.

It says:
DTC 78 will set when
^ RPM greater than 1800
^ Fuel rate greater than 20 mm.
^ Boost pressure greater than or less than desired.

I guess the logic was simpler in the ODB I PCMs.

The reason I say it doesn't set for low boost is I once towed for 250 miles with a failed vacuum pump. Maximum boost was about 3 psi. I never set a code.

[More Power]

It's all good!

The conditions for setting OBD I's DTC-78 are probably less stringent than it is for OBD II's P0236 (which I have manuals for). OBD II was a step toward preparing the way for more emissions control monitoring and tougher EPA emissions regs.

Having a boost pressure gauge sure helps to identify the problem. ;)

Jim

[Kenneth]

Thanks for all the input guys. I don't think it's really that big of deal. Code 78 has only set on my truck twice. Each time it went on and then off a couple times. Last summer I was on the freeway doing 70mph and had been driving for about 30 min or so and then again this summer when I was driving home from work going 65mph for about 20 min. The code has never set any other time. I was just curious what info and input you guys had on the topic/problem. I ordered a new silicone coupler from Heath today and I am replacing the two missing nuts on the turbo. I figure my engine and turbo is wearing out and could be producing either high or low boost levels. Either way, I don't want to put too much money into the truck, but I do want to properly maintain the vehicle. That is why I love this site. There are a lot of people with a lot of experience. Thanks again.