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Old 07-14-2019, 09:27 AM   #1
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Turbo Waste Gate Issues

Just finished diagnosing the "Low Power", "Come and Go, issue on a 3126 for a very nice couple from PA. The gentlemen stated his power would come and go. Sometime it pulled like it did when new, other times it couldn't get out of it's own way.

I plugged in my SL to monitor the boost pressure while setting the cruise at 60 mph on the same part of the highway with inclines. Being the exits on the highway were 2 miles apart I ran the same road conditions three times to find the boost pressure varied from 24.8 one time to 14.7 the next to 21.2 the next, same incline, same attempted speed set on cruise. as the boost pressure was low, so was the power performance.

I pulled the outlet hose off the turbo to find only .0025" run out of the turbo shaft, perfect. I then applied 25" of vacuum to the actuator and measured the stroke fully closed and fully open. Did this multiple times to find the actuator shaft did not return to the closed position consistently, open position was spot on. I reached around the back of the turbo to find the actuator arm for the waste gate could "wiggle". Again applied vacuum to the actuator to find this time the arm was tight (fully closed). That said, I pulled an actuator off a spent turbo in the shop, calibrated the stroke relative to " of vacuum and now found consistent closure of the waste gate. Not closing 100% is a prime condition for "low boost"/"Low Power".

Test drove the same highway at 60 mph, on cruise, to find the turbo pressure was now at 24.5 +- 1 psi each time. Problem solved.

I cant express the importance of having some type of gauge to measure the turbo pressure accurately under same driving conditions. Without being able to measure this data would have lead a person to R&R many components and possibly not find the root cause unless replacing an expensive component, the turbo! Not sure what the waste gate actuator cost itself, being that I had one on an old turbo, but I'm sure it is far less than a whole new turbo.

Hope this helps many that experience the voodoo power come and go issue.
Have a great day.
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Old 07-14-2019, 09:53 AM   #2
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Informative post! Do you own/work at a Diesel shop? Rook
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Old 07-14-2019, 10:50 AM   #3
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Hi Rook,
I'm a 40 year plus "motor head" and have helped out folks that I have met through other items I have designed in the past for HEUI systems. Do I have a shop? Yes, but not a formal every day repair center. I do this for folks when they are told they need to replace big $ items and they are uncomfortable with what they are being told. This particular couple was told that the HUEI along with other sensors needed to be replaced to resolve the problem. That was not true. I gauged the HEUI in my testing to prove that the HEUI was not of issue nor was the IAP or any other sensors(that so often can cause issues).

This issue was a matter of diagnosing off of pure data, not guessing. I may have missed the inconsistency of the turbo pressure if I did not conduct repetitive tests to prove the root cause. As my wife would say: "You are anal" Yep, I sure am. Best way to be when you are counted on to provide accurate diagnosis.

Travel safe, George
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Old 07-14-2019, 11:17 AM   #4
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If you had a professional repair shop - and were charging for this job, the coach would have ran perfect during the entire test run. Then once you returned it to owner, it would have started acting up again.

Nice job!

How do you calibrate the waste gate?
If it's adjustable, is that a means to provide a little additional boost - through the adjustment?
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Old 07-14-2019, 04:58 PM   #5
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CountryB,
Here is the Cat method: https://www.catengine.info/workshop/...adjusting.html

I use vacuum on the diaphragm to verify full closed position. The issue is normally a waste gate slightly open, loss of or low boost, and applying vacuum to the actuator forces it to fully closed position. If I experience a vacuum leak then the diaphragm has a failure, thus causing lower or inconsistent boost when pressure is applied while running the turbo connected normally.

Can you increase the boost by miss-adjusting the rod? YES. However, setting a high boost pressure causes several other issues. I shoot for 25 psi of boost on 3126 and C7 and they seem to perform the best with no ill affects. Over adjustment, on actuators with threaded adjusters, can cause the waste gate to hit the cover and either bend the cover or waste gate itself. You also run the risk of excess heat in the system and potentially damaging the turbo.

The actuator performance is based on the delta P between the actual boost pressure, post turbine, verses the actuator spring force. As the boost pressure rises, relative to the demand, the pressure begins to crack the waste gate open proportional to the actuator spring pressure/force or in this case 25 psi. As the delta attempts to increase the waste gate actuator would open further to maintain 25 psi max.

This is not only true with our Cat engines but many car engines with turbo's. Many times I have forced the waste gate actuator open and then let it snap shut to knock off any exhaust debris to find the boost pressure is now back to normal.

IF an engine experiences inconsistent boost, comes and go's, then I always look at the waste gate as the culprit. IF an engine just flat out produces low boost,then I check the waste gate first, turbine run out second but normally leads to a failed silicone hose or leak post turbine: failed clamp, pin holed CAC....... Remembering, the turbine puts out considerable volume on high engine demand. That said, pin holes in a CAC would have to be pretty large to dump that volume. Low boost, in my experiences, comes from waste gate leakage or leaking hose's post turbo. Verify the source of the pressure first. If the turbo cannot create the pressure then removing other items are just guessing.

Have a great day
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Old 07-15-2019, 06:12 PM   #6
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One last item regarding the turbo's on these Cat engines.
The great debate about full syn oil verses conventional is valid mainly regarding the turbo lubrication. Here is why. You and the wife are traveling down the highway at 60-65 mph for the past three hours. Up pops a rest area or restaurant you may want to try. Take your coach off cruise and down the off ramp you go having the right away to slide right in the parking spot. Here is where pure syn oil has an advantage. IF you shut down the engine without enough cool down then the oil in the turbo begins to rise in temperature extremely fast. Conventional oil will scorch and burn a film on the lubrication areas of the bearings, shaft and housing. Pure syn is more tolerant to this higher level of temperature. IF the owner continues this bad habit to shut down prior to allowing enough cool down then the turbo life is drastically reduced. Exponentially reduced with conventional oil regardless of how often the oil is changed.
The big Q: How long is cool down? Depends on many variables; air temp, engine load.... I use a simple rule of thumb. Regardless of all conditions, 5 minutes minimum.
This allows the oil flow through the turbo to help cool the turbo down while idling before shutting down.
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Old 07-15-2019, 06:26 PM   #7
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All are great, valuable posts. Thank You. Rook
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