Power Loss under load - Code 78

[rdurh]

I did a search on this and really didn't find an answer.

Today I had an issue with the 1994 6.5TD Blazer today that is new to me. I just want to be sure it isn't something I need to worry about. Took off today from Colorado Springs (6,025 ft. elev.) to head up to a friend's place to get some parts and attend a meeting. I was pulling an empty trailer made from a short bed Ford pickup. Problem happened on I25 going over Monument Hill (7,500 ft. elev.). I no longer have a tach in the truck so I have no idea what the RPM was but speed was 60-65 MPH as I was dropping speed to keep the EGTs down. Transmission had dropped out of overdrive into drive. I was watching the EGTs and working to keep them under 1000 degrees. EGTs were about 900 degrees or a little more. Boost showed about 11-12 psi. Almost to the top I get a quick surge and then no boost. The SES light comes on but there is no smoke at all. Got it turned around and on one short hill I got 3 psi boost, just enough to move the boost needle. Maximum speed was 45 MPH. Distance traveled from the house was 15 miles one way. On the way home it never smoked even under acceleration. By the time I got home I had zero boost and the wastegate arm would move freely with the engine running.

I didn't have time to check the truck out then but six hours later when I got home I checked the wastegate and vacuum pump. Wastegate worked fine and held vacuum with a handheld vacuum pump. The vacuum pump worked putting out a steady 21 inches. Took the truck around the block and everything worked fine with boost going up to 11 psi when accelerating. Pulled the codes from the computer and it had only code 78.

Wastegate solenoid and boost sensor were replaced in January along with rebuilt injectors. Also in January I went through all the electrical connectors and cleaned them up and put in some dielectic grease. Also add in more ground straps from engine, batteries, firewall and frame using 4 gauge battery cable. I put in a different PMD with a #5 resistor last month, The PMD was used but had been checked out to be good. The seller was an eBay business with a good track record. I have no idea as to what the resistor was in the original PMD as it is still on the IP. The original PMD was replaced because it was starting to cut out when hot. This is the first that this truck has really pulled anything since I bought it last fall.

[Patrick m.]

looks like loss of wastegate control.
you will need to check for vac at w/g actuator at idle, should have strong vacume.
also trace and inspect the plastic vac tube from one end to the other, these tube get old brittle and sometimes crack.
also need to check w/g solenoid, but intermittants are hard to pin point (if it is the solenoid).
my bet is you will find a cracked vac tube.

[rdurh]

Should have added:

All vacuum lines were replaced in January. When I got the truck back to the house I checked all the lines and they were still intact and attached to their respective vacuum nipples.

I also gutted the catalyst and added a new downpipe and crossover pipe from Summit Racing (Flowmaster FLO-17220).

When I got the truck home I had no vacuum at the wastegate, as it would move freely with the engine running. However, six hours later I had vacuum at the wategate as it took considerable effort to open the wategate with the engine running, but once I pulled the vacuum line I could move the wastegate arm freely. Didn't check the actual psi just went by how the wastegate was operating.

Code 78 seems to be a catchall code, that is too much boost/not enough fuel or too much fuel/not enough boost so it's really not telling me much. I have to wonder if I have an intermitent electrical problem, since six hours later with the engine cold it was back to normal. Alternative, if the resistor in the PMD is wrong so that I'm not getting enough or too much fuel. Can an incorrect resistor cause the problem that I had? I had absolutly no smoke after this happened, black, white or grey, so I have to wonder if it just wasn't getting enough fuel for the grade it was pulling. However, would this cause the no boost to continue once I had the engine cooled off heading home? Would the computer/PCM need to reset/cool off after setting a Code 78?

Monument Hill, north of Colorado Springs, isn't a real steep grade on I25 but it can slow you down. I hit the bottom of the hill at 80 MPH but had to drop the speed as I went up to keep the EGTs under 1000 degrees. I still had plenty of boost from the engine to keep my speed up, but the EGTs climbed pretty rapidly over 1000 degrees if I tried to keep the truck at 75 MPH.

[diesel65]

Since you replaced the boost solenoid recently, I would not suspect it being bad already and you say the vacuum lines have been replaced.
You need to connect a vacuum guage in place of the waste gate actuator, with the engine idling you should have approx 15inches of vacuum; With the engine still idling try wiggling the wires going to the boost solenoid and watch the vacuum guage to see if it drops to zero, do not be afraid to move the wires around. If the guage jumps up and down as you move the wires, you found your problem. I think you might have a broken wire by the solenoid connector.

[ronniejoe]

This has to be either a sticking waste gate solenoid (I know it is very low time) or an electrical connection problem that disabled the solenoid.

[JohnC]

I would not call DTC 78 a "Catch All " code. It means that the PCM could not control the boost while the engine was under load. On ODB I trucks I'm pretty sure it only comes on if the boost goes too high; low boost goes undetected.

I have seen several boost solenoids that failed when they got hot and then worked fine later, after they cooled down. Or, it could be an electrical problem.

I happen to have a brand new solenoid left over from my '95 you can have for half price plus postage...

[bluuzman]

I recently got code 78 quite frequently (also SES) when accelerating above 55mph. One day I checked the waste gate rod with engine idling and I could move it easily - no vacuum. Well, I bypassed the solenoid valve from the waste gate vacuum line to get the port closed and gave it a try. Everything worked just fine and no codes. Smooth and good performance too ;)

After this quick trial I then reconnected the wastegate solenoid back in business and have not seen the code since. Based on this quick trial it seems that you can get code 78 also if you have too low boost, which must be the case here.

-ari

[JohnC]

Based on this quick trial it seems that you can get code 78 also if you have too low boost, which must be the case here.

My conclusion otherwise was based on 2 weeks of driving without a vacuum pump. Never set a code.

[bluuzman]

..well, in that case I think the only one who really knows is the one who wrote the PCM software.

Anyway, I'm pretty sure I would have seen the code if just pushing the truck hard enough with the solenoid bypassed (at least should have). Normal daily driving for a couple of days did not activate it, however. As I don't have a boost gauge I don't have an idea about the actual boost pressure.

No big deal but an engineer would always like to know ;)

-ari

[Warren96]

My '94 Blazer did the same thing,even with new rubber lines.It would intermitantly set the code then work fine for a while.I rode around with a vacum gauge under the windshield wipers for a week before I found the problem to be a leaking hose adapter to the boost solenoid.

[moondoggie]

Good Day!

"...it seems that you can get code 78 also if you have too low boost, which must be the case here." Personally, I doubt it, but have no hard data.

Don't discount an intermittent Wastegate Control Solenoid. I had that happen on our Sub. It made troubleshooting a little harder, but not that much. My el cheapo boost gauge helped me eventually figure out for sure what was going on.

Blessings!

[bluuzman]

...a quote from a manual:

"DIAGNOSTIC TROUBLE CODE (DTC) 78 Will Set When
-RPM greater than 1800.
-Fuel rate greater than 20 mm.
-Boost pressure greater than or less than desired (internal to PCM).

---> Boost will shut down. Reduce maximum fuel."


I can not really say if this is true but this matches to what I have experienced. It should be easy to duplicate it by disconnecting the vacuum. Might try it out some day. Actually I got the code usually when accelerating but sometimes just by increasing the speed up to about 60mph on a "highway".

To me as an electrical engineer it makes sense to detect both cases, either too low or high boost. The engine sure likes none of them. The PCM tries to control the boost and if it fails to do that, it will set the code.

-ari

[JohnC]

Bluuzman:

what year manual was that from? I don't have mine any more, but I don't recall my '95 manuals saying that. Could be wrong, though... (BTW, just received the2006 manuals, all 25 lbs of them...)

I had a Kennedy exhaust on and the truck would make 3 or 4 lbs of boost with the wastegate flapping in the breeze, so maybe that's why it never set a code.

[rdurh]

I have the 6.5 Diesel Electronic Fuel Injection manual from GM (pt#16015.15-2, May 1994). That's the information it has at page 5-44. However, it doesn't say anything about " Boost will shut down. Reduce maximum fuel." Though it does say "If an overboost condition exists, the PCM will reduce fuel delivery to prevent engine damage."

I've been wanting to take the truck out and see if I can recreate the problem but everything has been getting in the way. I'm going to try this afternoon after work and take a run up I25 to Monument Hill and see if the boost cuts out again. If it goes out I'll have my vacuum gauge/pump with me and my plan is to check the vacuum at the pump and then at the wastegate. Then wiggle wires to see if I have a hot short. May also take an ice pack and if wiggling wires doesn't work see if cooling down the wastegate solenoid will get my vacuum back. Also take the multimeter and see if the electrical connections are working. It looks like I should have 12v power at the Lt Blu/Blk (539) wire at the solenoid, and a PCM controlled ground at the Yellow (972) wire.

[rdurh]

I tried to recreate my loss of vacuum/boost a couple of times earlier this week by running the truck up Monument Hill on I25. However, I had cool days and couldn't get the EGTs over 800/850 degrees and didn't lose any boost. Truck ran normal up and down the hill at 75+ mph Today I had some free time and a warm afternoon and took it back up the interstate. Hit Monument Hill and kept the EGTs just over 1000 degrees. Two thirds of the way up I lost all boost. Took about 20 seconds give or take with EGTs over 1000 and speed at 75 mph. I also had the SES light come on, and checking later when I stopped I had only a code 78.

I got off the interstate at the top of the hill and into a pull off area. No vacuum at the wastegate, but had 20 inches on the vacuum pump side of the wastegate solenoid. Had 12v power on Lt Blu/Blk (539) wire at the solenoid and was getting a variable ohm reading at the Yellow (972) wire. Ohms ran from around 60 ohms to 50 ohms.

I was going to cool down the wastegate solenoid with some water, so I turned off the truck so I could get the water out of the back of the Blazer. I had hooked up the vacuum gauge at the wastegate before I turned the truck off and had zero vacuum. When I turned the engine back on I had 15 inches of vacuum without cooling down the wastegate solenoid or doing anything else. I then checked the codes and found only a code 78.

I am thinking the wastegate solenoid is intermittently bad. That when it gets over 1000 degrees (as per the EGTs) the wastegate solenoid sticks open once it gets that command from the PCM, but of course won't shut afterwards giving me the code 78. Once it cooled off, with the hood open, turning the truck off somehow reset the solenoid so it came unstuck. I assume that I could also have a hot short somewhere in the wires, but giving the wires a good wiggle/tug didn't give me back any vacuum, or cause a short when I had vacuum to the wastegate.

[rdurh]

Should have added that boost going up the hill was at 11/12 psi just before I lost all boost.

[moondoggie]

Good Day!

"I am thinking the wastegate solenoid is intermittently bad. That when it gets over 1000 degrees (as per the EGTs) the wastegate solenoid sticks open..." Your PCM has no idea what your EGT is.

"...turning the truck off somehow reset the solenoid so it came unstuck." I'm reasonably sure the PCM restores normal boost control caused by a code 78 every time you turn the truck off; the code will stay in memory for 50 (I think) key cycles. So, shutting off your truck is what actually restored (temporarily) your boost. To prove it, you could cycle the ignition key off then on while rolling down the highway. I do NOT recommend doing this. I've done it myself many times, but cannot recommend doing this due to obvious safety concerns. If you try this anyway, & the boost comes back as you feed in footfeed, then you can be reasonably sure the removal of boost was due to the PCM, not an intermittent wastegate actuator solenoid.

I don't have hard numbers, but I think a factory EPROM doesn't like 11 - 12 PSI boost, at least at sea level. In high country, the PCM should allow higher boost to compensate for altitude, but I have no idea how much at a specific altitude. I wonder if it's possible your baro sensor is lying to the PCM, telling you you're at low altitude, so the PCM shuts you down due to overboost. Hmmm - hopefully someone more knowledgeable will chime in.

Blessings!

(signature in previous post)

[bluuzman]

Hi,

The manual I referred to is a database.

Moondoggie's conclusion about PCM being the one shutting down the boost sounds logical. However, if the wastegate is stuck closed due to a faulty solenoid, the only way to reduce boost is to limit fuel injection. If the boost is completely down but you have not lost all hp, I'd think PCM still controls the waste gate somehow...

Carrying out debugging the wastegate solenoid circuit with an ohm meter is probably very difficult. Solenoid has VCC that you can measure but the other wire is connected to the PCM, which controls the solenoid duty cycle.

Again, I do not have the PCM schematic drawing, so just guessing but I would think there is a solid state switch that grounds the solenoid control line. This is done probably at quite high frequency being the reason the voltage or ohm meter won't be telling much. An oscilloscope would be needed to see the pulses. A scan tool might also help.

-ari

[moondoggie]

Good Day!

"...shutting down the boost..." In normal operation, it doesn't shut it down, it modulates it to a level the algorithm likes. When my 95's are idling (per JK), the Wastegate Actuator Solenoid is provided a ~ 75% duty cycle, so I see ~ 75% of the vacuum level the vacuum pump produces at the Wastegate Actuator. At high rpm/load, it modulates it down from where it'd go with 75% vacuum, so boost seems to stay ~ 5-8 PSI.

"...if the wastegate is stuck closed due to a faulty solenoid..." Or, the wastegate and/or actuator are stuck fully closed.

"...the only way to reduce boost is to limit fuel injection." 10-4. That's what happened when I ran my truck without PCM boost control for awhile - I substituted the Actuator for a piece of wire. If the PCM de-fuels do to overboost, however, power drops dramatically - you couldn't not notice it when it happens. (Wish I had time to write this event up - it's pretty funny. The PCM saved my motor from my ignorance.)

"If the boost is completely down but you have not lost all hp, I'd think PCM still controls the waste gate somehow..." Yep, it vents the Actuator to atmosphere - no boost until restart.

Blessings!

(signature in previous post)

[bluuzman]

"Yep, it vents the Actuator to atmosphere - no boost until restart."


...vents the actuator.. by controlling the wg solenoid or by some other means?

Well, this seems getting more academic tech-talk rather than solving the practical issue ;) Anyway I don't mind..

[rdurh]

Well after all of the above I'm not sure just what to do to cure the problem. JohnC is selling me a spare wastegate solenoid that was little used. My plan right now is to install the new solenoid and see if the problem reoccurs.

Should have posted that the wastegate acuator moves freely and holds vacuum without a problem. So binding of the acuator shouldn't be a problem. Also this is a recent problem as the truck has been up in the mountains before with the boost and EGTs running at comparable levels.

The only other thing I may do before the new solenoid gets here is to run back up Monument Hill and keep the boost at 10psi or a little less but get the EGTs up to 1000+ degrees. I am aware that the PCM doesn't measure EGTs, just that I am wondering if there is a hot short in the solenoid or the mechnicals within the solenoid are binding when the heat gets up there inside the engine bay.

Thanks for all the help and all comments & suggestions are appreciated.

[Warren96]

You are on the right track by replacing the solenoid.I still have one on the shelf to swap in mine if she sets another code 78.Keep us posted !

[JohnC]

I spent a lot of time wrestling with this issue a few years back. That's how I ended up with an extra solenoid. Here's how it all works, as best as I could tell. Preface: this applies to a '95 and may or may not be correct for a '94.

First off, there is a default duty cycle for the wastegate solenoid, roughly 75%. At all speed and power settings below (IIRC) 1800 RPM and 20mm3 fuel rate, the wastegate runs at 75% and the PCM does not make any decisions based on manifold pressure.

Whenever the RPM exceeds 1800 or the fuel rate exceeds 20mm3, the PCM monitors manifold pressure and adjusts the solenoid duty cycle up or down to maintain the desired boost for the throttle and load.

The solenoid alternately applies vacuum to the wastegate control and vents both the vacuum and the actuator to the atmosphere. It does this something like 20 times a second, so the solenoid is closing and opening 20 times a second. When the solenoid is switched off no vacuum is applied to the actuator and the actuator is vented to the atmosphere. When the solenoid is switched on vacuum is applied to the actuator through a calibrated orifice. By varying the relative time spent on and off, the closing pressure of the wastegate is controlled and hence the manifold pressure.

Apparently the common failure mode for the solenoid is that it cannot switch between the on and off positions fast enough. Since the duty cycle favors on over off, the sticky solenoid tends toward on and boost generally starts to climb. If you have a scanner you can watch the duty cycle fall off as the PCM tries to reduce manifold pressure, and the boost remain the same or climb until the PCM sets the code, at which point the duty cycle drops to zero. Soon after the solenoid sluggishly slips into the "off" position and boost drops to zero or a few psi.

As soon as the RPM drops below 1800 and the fuel rate drops below 20mm3 the code clears and the PCM goes back to 75% duty cycle. Boost is restored and the cycle starts over. No need to cycle the ignition.

High under-hood temps seem to aggravate the problem. After having problems all fall, my truck ran flawelessly through the winter and didn't set the code again till the first really warm day of spring. Heat soak also aggravates the problem. Run 350 miles without a problem, shutdown for fuel and it acts up as soon as you hit 65 again.

That's my story and I'm sticking to it!

[rdurh]

New low hours wastegate solenoid from JohnC came in today's mail. (Thanks again John.) Got it installed and made two runs up Monument Hill on I25. Started each run at the bottom of the steep section at 60 mph. Interstate speed limit of 75 mph. Boost max'ed out on each run with the boost gauge showing about 11 psi of boost. First run EGTs just over 1000 degrees and the second run they were up around 1050 degrees. No loss of boost on either run. The second run I actually had to back off the throttle as I pegged the speedometer. Made another run up another steep section of road I know about and still no loss of boost with the EGTs just over 1000 degrees.

Thanks all for the help.

[JohnC]

New low hours wastegate solenoid from JohnC came in today's mail. (Thanks again John.) .

IIRC, that solenoid has only about 10 minutes of run time on it. Glad it solved your problem! One thing I learned is they are awfully difficult to pin down when they are acting up.

[dealwithit]

hmmmmm, my truck (96 gmc 1500 4x4 6.5) had blown lots of black smoke since i bought it in febuary. i replaced the vac pump a couple months ago, but it didn't fix anything and the ses light has been on ever since. it has been dropping to limp mode all the time recently and its really starting to piss me off..... i recently just hooked the vac line directly to waist gate to see if this would clear my smoke issue and it did! it also got alot more power too. this worked fine for abit around town, then i hooked up my trailer to it today and started pulling. it went a few blocks fine then i went up abit of a hill with the foot down and it started missing then started slowing down all together. i shut it off for a sec while rolling and it came back to life as normal when i restarted it. i have noticed 3 different power levels today while driving, 1 full power full rpm, 2 full power only up to about 2200 rpm anything over that it just makes noise but no more power, and 3 limp mode, zero power and will only rev to 2200 in neutral. i've got some tests to do tomorrow but im wondering what keeps setting this dam limp mode, it only does it when its warm, and the PCM has already been moved to behind the bumper with its own heatsink.

[JohnC]

Fix the wastegate. When the PCM senses too much boost it cuts the fuel rate back in an attempt to limit boost and temperature.