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Bucking, Hesitation and Poor Economy Fix - Cleaned EGR and Intake (throttle) Valve

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Hello Everyone. I have been a following member for quite a while and have found the information in this forum very helpful. I have a 2014 CTD that i purchased in September 2013. I drive mixed 80/20 highway/city. I maintain the car well and its been very reliable for me, but I have experienced a few issues that i have been able to relate to on this site. Bucking (mild and severe) after coasting or light throttle, and a huge lag (almost a stumble) when hot in start/stop situations. I've also experienced several random hard downshifts. These problems first occurred when the car was about 1 year old (not that bad or often) and they seemed to improve after the first emissions recall. Shortly after the latest recall, the symptoms reappeared and gradually became worse.

Background/history:
107,000km
O2 Sensor replaced for P026C code - Mid 2015
DEF Tank Heater - Aug 2016
Emission Recall 15594 - Ran great after recall, but still got occasional P026C code
Emission Recall 17089 - Above mentioned symptoms re-appeared and gradually got worse

Two posts caught my attention: 15 cruze won't start or run, and Help diagnose bucking during regens. Since reading these posts, i have cleaned my EGR valve and the intake valve (throttle plate). Thank-you IndyDiesel for the write-up on cleaning your EGR valve - it went well, just as you described it. I did not replace the gasket plate but probably will if I service the EGR valve again. There is coolant behind the gasket plate and if you have any pressure in the cooling system, it will drip while the EGR valve is out (i had a small panic when i saw this). I have attached four pictures below. 1.) Location of EGR and gasket plate 2.) Intake pipe removed looking into throttle body 3.) close-up of the very gummed up throttle valve 4.) valve body after cleaning

I did the EGR first, about 5 days before i cleaned the throttle body. I noticed a significant improvement in fuel economy and acceleration. The valve was not stuck open, but it was not moving very free and it would sometimes bind up. I used brake clean and worked the valve back and forth with an attached vise-grip. I was pleased with the improvement in performance, but the car was still bucking, mostly first thing in the morning.

When I removed the intake pipe at the throttle body i pushed the plate closed with my finger and it did not release on its own spring tension, but would pop free after a second or so. I figured this could be the cause of the bucking. Cleaning this was much more challenging. I used throttle body cleaner sprayed on rags and a small plastic scrapper to do most of the cleaning. Since so much crap was falling down onto the bottom of the intake (just on the bottom where i could still see it) , i rigged up a small plastic hose (about 3/8 inch) connected to my shop vac to suck out all the carbon debris. What a mess in there - really disappointed to see. I think i could have removed the whole throttle body assembly as well which would make this much easier and allow access to clean further into the intake. Care should be taken to not spray any cleaners directly into the intake since any residual solvents would ignite and combust very different than diesel fuel, potentially causing significant engine damage.

So not sure why my intake got so gummed up. Are there any other reports on this? I've always used the proper engine oil and my UOAs have looked very good. I drive a lot of highway miles, but i could be a bit light on the throttle. All that stuff had to come through the PCV, so I may have to look at that further (catch can....ect.). I also find it interesting these symptoms came back after the latest emissions recall. I will be keeping an eye on the intake valve for sure. The car is running great again with fuel economy back up to near the best i've ever seen, and the bucking is gone, even during the regen it did today. I sure love this little car and hope this helps anyone having similar issues.

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COTM Winner
2014 Cruze Diesel, 2007 Cobalt, 1981 Camaro Z28, 2017 Volt
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89 in-lb
 
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COTM Winner
2014 Cruze Diesel, 2007 Cobalt, 1981 Camaro Z28, 2017 Volt
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for both, egr and throttle valve?
Yup! The throttle valve is a straight torque, the EGR had another step in there, like 51 in-lb then 89 in-lb. Given how small the torque is, that's kind of surprising they even bother.
 
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COTM Winner
2014 Cruze Diesel, 2007 Cobalt, 1981 Camaro Z28, 2017 Volt
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So that you neither under torque OR over torque.
Sure, but when it comes to that low of torque, it is rare to see. Especially when you look at other parts with the same torque spec not needing it.

But it's likely to make sure it settles down on the seal evenly.
 
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