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6.5TD turbo issues

I

icenutt

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North Dakota
I have a 1994 k1500 6.5TD. My turbo has been working off and on for about 6 months. I have replaced vacuum lines, vacuum pump, wastegate solenoid. My turbo will work fine until I come to a stop sign. I take off and no turbo until I come to the next stop sign and then the turbo starts working again. I have just figured out that if I manually shift from OD into 3rd gear, the turbo starts working and works perfectly. As soon as I shift back into OD the turbo stops. Once in awhile the turbo will continue to work in OD until I come to a stop. When I take off the turbo stops working but I manually shift into 3rd gear and the turbo spins right up and works. Anyone have any ideas what is causing this? Thanks.
 
Do you have a boost gauge that you are referencing, or how are you determining the turbo is not working?

In OD the RPMs will be low and not be able to build as much boost. The PCM controls boost too, dropping it to near nothing at some points. It would perform better with an aftermarket PCM PROM, that doesnt dump boost as much.

It could be that you have missed a leak, have plugged the solenoind in backwards, or there is something wrong with your turbo actuator, or wastegate lever in the turbo (could be sticking open).

You can try plugging the vacuum pump directly into the wastegate actuator and see if you get lots of boost, which would show nothing wrong with all that equipment.
 
Yes I have a turbo gauge that shows boost levels but I can also hear the whine of the turbo when it's working. When the turbo is working in OD, I still have been 2 to 5 lbs of pressure while driving around. The gauge will drop to zero when it idles, like it should, but when the turbo is working properly I always have some reading on the gauge. I have also checked the wastegate actuator and everything checks out fine. Like I mentioned earlier, when I am in 3rd gear, the turbo never stops working. It's only when I am in OD that I have the on and then off again issue. So that also tells me that the actuator and vacuum is working fine. I believe it is all tied to the wastegate solenoid and the computer shutting it off and on. That is what I am trying to figure out is why the computer tells it to shutdown the solenoid at times and then startup again. Something deeper is giving the computer a wrong reading or something and I think it is tied to the OD in some way shape or form.
 
When you say "in overdrive" do you mean when actually driving in 4th gear, or just having the selector set to OD, the turbo also works less in 1st, 2nd and 3rd while accelerating?

How much boost do you get while accelerating?
 
When I say in OD i mean the gear select is on the "OD". I manually pull it out of "OD" and put the selector on "D" and the turbo works perfectly and never stops working. The on and off issues disappear until I put the gear selector back into "OD".

When I am on a normal grade and just driving around I have 1 to 2 lbs of pressure all the time on the guage and if I accelerate I have 3 to 4 lbs of pressure on the gauge. If I am going up a hill it will go up to 5 to 6 lbs of pressure. Out on the highway at 65 mph I have 2 to 3 lbs of pressure. If I step on it to pass or something it will jump to 4 to 5 lbs of pressure. Yes, at idle there is zero pressure.
 
So are there on and off problems while selected "OD" and accelerating from first to third gear, or just once it shifts into 4th? Just trying to figure out if this is normal PCM shenanigans that drives a lot of people to use a mechanical wastegate actuator or you have something else wrong. What gears and tire size do you run, does it give you low cruise RPMs or high cruise RPMs? Do you get a bunch of black smoke when you hit the pedal?

Your boost is a little low, seems like you might actually have an issue with the wastegate. Do you ever get an SES light?

I will arm you with some info. Your factory boost curve looks very similar to this one, you can see in the right RPM and fuel rate range it goes to zero. Yours is actually worse, because where this one rises a bit at higher RPM at low fuel rates, yours actually just stays near zero. When you get into OD the RPMs and fuel rate are likely in this zero range or not enough fuel rate to be on the curve. Although it also seems like you have a loose wastegate, or one that cannot close all the way, like stuck open because you should be able to hit 8-10psi of boost when accelerating.

There are two pics of the back of my turbo, that wastegate can get dirty and sticky and not close all the way, happened to me, took it apart, freed it up. Then there is a homemade mechanical setup I had on mine, so the PCM cannot cut boost and its completely proportional to the amount of fuel rate and RPMs you are at. A loose spring will drop all your boost while cruising too. You dont want much boost if cruising empty, 2-3psi in OD is plenty, but your problem is you have none. I prefer the vacuum setup after programming it to hold boost better. Other things that can result in low boost is a failing boost sensor on the intake. With a scanner you should be able to see the feedback it provides, and if its too high then PCM will cut it back. You could also have an "optic bump" on the OS that fools the fuel rate lower to the PCM which will cut boost. With the scanner you could see if your idle fuel rate is near 8mm3 where it should be, but if its 0-3mm3 it won't function properly. Even the free GMTDScan trial will help you a lot, look for it at Engh Motors, and you'll need the GM OBD1 ALDL cable to USB to plug it in.

Stock Boost Curve.jpgGM4 turbo_Turbine housing labeled.jpgGM4 turbo_turbine housing wastegate open.jpgGM4 turbo_exhaust elbow removed-labeled.jpg
 
When I have gear selector in OD and the turbo is working, I have turbo pressure all the time. First gear, second gear, third gear, fourth gear, etc. When I am under normal gas acceleration I have between 4 to 5 lbs of pressure. I can make the turbo pressure jump to over 8 lbs if I floor it or climb a good size hill but under normal vehicle acceleration from a stop sign I get about 4 lbs on average. When the turbo is working in OD or D for that fact, the turbo works the exactly same. 2 to 3 lbs under town driving between stop signs. 4 or 6 lbs if I step on it. 8lbs or more if I floor it. On the highway when I am trying to get up to 65 mph it will run around 5 to 6 lbs of pressure until I get to my speed I want and then when i level off the gas pedal it will drop to about 2 to 3 lbs and just cruise there. If i need to pass a vehicle and stomp on it a bit the turbo will go to 5 to 6 lbs. If I floor it the pressure goes between 8 to 10 lbs.

Also, when I floor the truck I have never had alot of black smoke at all. I have floored the truck a time or two and it will smoke just a little bit of black smoke but that is it. I have seen some diesels that just pour the black smoke out when they step on it but not this truck. Good or bad, I don't know.

In regards to the SES light, it never comes on when the turbo is working properly. The only time it will come on is when I am on the highway and the turbo quites working. It will come on but as soon as I let my foot of the gas pedal for a split second it disappears. It will continue to come on as long as the turbo is not working because the MAP sensor is not sensing any turbo pressure in the system. No error codes though. The time delay between the ses light coming on is about 10 minutes when the turbo is not working. The ses light does not come when I am just driving around town when the turbo is not working though, only out on the highway.
 
Getting closer to the problem. All relevant information.

Specifically though, when the turbo is not working, its not working in any gear? You get no boost on acceleration when its in OD and not working. Then put the gear selector in D and the turbo starts working in first gear too. When my turbo wasnt working intermittently because of a cracked vacuum line it would throw out a lot of black smoke at any speed when I hit the fuel pedal. I spoke with another person that swore their turbo wasnt working and came to find out his gauge hose was loose, but he kept tightening the wastegate to get more boost. When he was done he fixed the hose and realized he has 20psi of boost. I am not saying this is your scenario, but since we cannot be in the truck with you we have to rule out a lot of things with questions.

Since you are getting the SES light we can presume its for no boost, if youre not seeing any, or maybe its a pulsewidth or EGR code? It shouldn't code in town, when its not working, because it has to be at least 2psi out of tolerance for 10 seconds straight. That 10 seconds straight causes it not to code, since you likely arent accelerating that long and not going fast enough to create the load and rpm to require boost once going in town.

Have you attempted to check the stored codes? Anytime the SES light comes on it should store a code, even if the light shuts off. Although strange things do happen and maybe it doesnt if its only lit for a few seconds.

Also since it is coding, it must not be the PCM controlling it to zero. It really sounds like a cracked vacuum line that opens and kind of seals the rest of the time. Or a flaky boost solenoid. You could T in a vacuum gauge right at the wastegate actuator, run it in the cab or onto the windshield, and when its not working see if its because no vacuum at the actuator. The gear you have it in should make no difference when its in first gear accelerating off the line. It might be coincidence, if its intermittent anyway. Actually verifying the vacuum with a gauge is your best way forward. That will tell you where to look. Otherwise, if you dont want to get a gauge, use a known good piece of hose to connect the vacuum pump directly to the wasteage actuator and see how much boost you get, or if you can get it to stop working. It might code for too much boost at some point if all of that is working right.

The scanner would help you. If the Boost/MAP sensor on the intake, or the one on the firewall was being flaky it may actually start reading high boost, cut your boost as much as possible, and then code for being too high, even though you have none. Have you done anything to either MAP sensor?
 
I have not done anything to the MAP sensors. I have replaced all vacuum hoses, wastegate solenoid and vacuum pump. Now when the turbo is not working, it doesn't work at all. As soon as I manually shift out of OD to D, the turbo starts working in all gears and continues to work in all gears. When I am in D the turbo never fails. I have turbo all the time. first gear, second gear and third gear. It stops working as soon as I go back to OD. Like I said earlier, every once in a while the turbo will continue to work when I am in "OD" and when that happens it works in first gear, second gear, third gear and fourth gear. It will continue to work until I come to a stop sign or stop for some reason. Then it won't work. Usually but not always, at the next stop the turbo will whirl back up and work fine. Then stop again on the next stop. I can get out of this loop by simply putting the gear selector into "D".

I can try connecting the vacuum pump directly to the actuator but I don't know what that will prove at this point. I know the system works just fine in D. I have driven around three straigh days in "D" only and the turbo has never missed a beat. Yesterday I tried putting the gear selector into OD to see if it was still happening and sure enough, zero went the pressure and no turbo. Slide the gear back into "D" and presto.. pressure immediately and the turbo started to work. So if there was actually something wrong with the vacuum system or the turbo then why would work perfectly in "D". It has to be something computer related somewhere. Something is telling one of the solenoids to close and cut the vacuum but what is it? I have checked and rechecked the DTC codes and I get nothing.
 
icenutt said:
I have not done anything to the MAP sensors. I have replaced all vacuum hoses, wastegate solenoid and vacuum pump. Now when the turbo is not working, it doesn't work at all. As soon as I manually shift out of OD to D, the turbo starts working in all gears and continues to work in all gears. When I am in D the turbo never fails. I have turbo all the time. first gear, second gear and third gear. It stops working as soon as I go back to OD. Like I said earlier, every once in a while the turbo will continue to work when I am in "OD" and when that happens it works in first gear, second gear, third gear and fourth gear. It will continue to work until I come to a stop sign or stop for some reason. Then it won't work. Usually but not always, at the next stop the turbo will whirl back up and work fine. Then stop again on the next stop. I can get out of this loop by simply putting the gear selector into "D".

I have driven around three straigh days in "D" only and the turbo has never missed a beat. Yesterday I tried putting the gear selector into OD to see if it was still happening and sure enough, zero went the pressure and no turbo. Slide the gear back into "D" and presto.. pressure immediately and the turbo started to work. So if there was actually something wrong with the vacuum system or the turbo then why would work perfectly in "D". It has to be something computer related somewhere. Something is telling one of the solenoids to close and cut the vacuum but what is it? I have checked and rechecked the DTC codes and I get nothing.
Click to expand...

Now this should have been your first post, the best explanation, better to have too much info than not enough. You are right, this sounds like an electrical issue, but a very strange one. I would not guess it has something to do with the PCM doing it on purpose because you are in OD, because I have not seen any kind of boost limiter by gear in the code. More like when your column shifter is moved its shorting something and making the boost solenoid stop working, unbeknownst to the PCM.

The transmission is electronic, except the linkage. The problem presumably could be in the colum or I/P or wires caught up on the linkage? The feedback to the PCM on the tranny being in OD may be shorted, dropping reference voltages, which should cause other codes too. I will look through some schematics to see if there is anything that jumps out. Perhaps the position selector line(s) run near a ground or the PCM pin that controls the boost solenoid

A scanner could still be valuable and an O-scope, in seeing if the PCM actually stops commanding the boost solenoid pulse width, or if its just not getting there.

A mechanical controller would resolve the issue although you could have more down the road. When the selector is in OD, it does upshift into 4th right?
 
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Yes it does shift into 4th. I have paid close attention to the shifting sequence many times and I can hear and watch all four gears and finally OD. This tranny has always shifted very smoothly and I have never had any slippage at any time.

Let me ask you this, at what rpm should this truck run at 65 mph? I only ask because I have had several other Chevy K1500 trucks and I think this one runs at a higher rpm than the others but I have never had a 6.5TD before. Just curious to know if its just me or maybe I do have an OD issue. Any thoughts?
 
I'm curious ... does your Torque-Converter clutch lock? (if you're driving in 'D' at 55, then shift to 'OD', you should feel an upshift then another.. your rpms should drop twice. Pushing on the pedal a bit will 'unlock' the TCC for a bit more power.)

Reason I ask, is if your TCC isn't working, I don't think your turbo will, either... your PCM should read this as a soft code, and cut the boost.

I may have missed it, reading through here (sorry) ... does your Check Engine light work?
 
I am pretty sure that it is locking up as I have watched and listened as the truck shifts and when it gets to the 4th gear it does upshift one last time. That last upshift does drop another 200 to 300 rpms. My concern is that it should drop further and not run as high as it does. I see you have a 1994 diesel with the same rearend ratio so can you tell me at what rpm does your truck run at 65 mph?
 
What are your RPMs?

In my truck you dont feel the TCC lock in 4th as its already locked in 3rd.

When driving it "D" you should feel the shift to 2nd, the shift to 3rd, and then it lock up the torque converter. That might feel like 3 upshifts. Then shifting into OD (th 4th gear) should drop your RPMs again.

With 3.73 gears at 65mph you should be around 2000rpm in OD with the torque converter locked. Although most times, you have a replacement alternator with the wrong pulley and it causes the tachometer to read high. Tire size also affects RPMs, do you have stock tire size?

If TCC lock was the issue then the problem would only present itself in 3rd and 4th gear, and not 1st and 2nd.
 
At 65 mph, mine runs at about 2300 to 2400 rpms. The 2000 rpms sounds more like what I thought. Now I am wondering what else I have going on. It shifts very smoothly but it sounds like my torque convertor is not locking up correctly. How can I truly test this on my own? I don't have any error codes other than DTC 74 which is the Input speed sensor and I just replaced that.

And yes I have stock tire size on this truck. The alternator was replaced before I got it but its an AC Delco alternator so I just assumed it was the right one for this truck.
 
Easy way to tell if your TCC is locking is to drive along about 60, up a gentle incline so you're pushing on it a bit, then gently touch the brake with your left foot... as you push down on the brake, your TCC should unlock and it will feel like you 'downshifted'... and you'll gain about 300 rpm.
 
In regards to the error codes. After I put the new TIS sensor in, I cleared the codes and everything was good. I have driven it around and I checked the codes the next day and still clear. I will check them again today if I get the chance and let you know. It has been about a week now since I put the new TIS sensor in.

In regards to testing the TCC. Yes, if I touch the brake while going up an incline I can feel the TCC unlock and my rpms will jump about 300.
 
OK, that clears up the DTC from being relevant to the turbo problem, either directly or indirectly from a shorted signal or reference voltage.
 
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