Chevrolet Malibu Forums banner
1 - 4 of 4 Posts

·
Registered
Joined
·
6 Posts
Discussion Starter · #1 ·
About a month ago, my wife started getting an intermittent CEL in her 2014 Malibu (2.5 liter engine, 176k miles). It was throwing the P0171 code (system too lean). The car was performing well and the problem was not persistent enough to successfully troubleshoot. I suspected either the heated (upstream) O2 sensor was fouled or beginning to fail, or a small, intermittent vacuum leak. I ordered a new O2 sensor to have on the shelf in case that turned out to be the problem. After a week or so, it seemed to clear up.

Last week, my wife was attempting to drive to work when the car started to lose power. It stalled a couple of times and she brought it home. She reported that the car performed okay at higher speeds and most poorly at very low speeds. At that point I was pretty certain I was dealing with a vacuum issue. There was no CEL, but the Stabilitrac light was on. I checked for codes and this is what I got:
P0106 (Manifold Air Pressure Sensor)
P0131 (HO2S Circuit Low Voltage)
C0700 - this one isn't listed in the Factory Service Manual; I think it's related to the Stabilitrac light.
The CEL came on while I was scanning.

I replaced both PCV valves (the primary was clogged), the PCV hose, the oil cap, and the upstream O2 sensor. I checked the condition of all the vacuum lines and they were fine. I then cleared the codes and test drove the car. It barely got down the road. I drove it about a mile and a half at around 35-45 mph and nursed it home.
After that test drive I scanned for codes and got the following:
P0030 (HO2S Heater Control Circuit)
P0053 (HO2S Heater Resistance Sensor)
We're up to six, if you're keeping count.

At this point I turned to YouTube and found a couple of videos where people were showing Gen 8 Malibus with 2.5 liter engines where the intake manifolds were so loose they could be physically shaken. I had been all over the manifold and the throttle body and hadn't noticed any movement, but I tore it back down to double check. I tried to move the manifold and throttle body and they wouldn't budge, but I decided to check the bolts just to be sure. All five were loose. I got at least 1-1/2 turns on each of them before even snugging them up. One of them turned 3 or 4 times. Once I got them all snug, I tightend them securely. (There wasn't nearly enough room for my torque wrench, so I just went by feel, taking into consideration that I was tightening steel bolts on a plastic intake manifold into an aluminum head.)

I put everything back together and test drove the car. It performed perfectly on an eight-mile trip at speeds ranging up to 50 mph and averaging around 35-40 mph. I scanned it when I got home and there were no codes.

As a PS to this post, I'll just note that I suspect the Stabilitrac issue was a result of insufficient brake booster vacuum.

I hope this is helpful.
 

·
Super Moderator
2016 Malibu 1LT 1.5T/6-speed 6T40
Joined
·
4,245 Posts
Thank you for sharing that experience. Checking intake manifold bolts is a frequent suggestion on this forum for a handful of common codes of the 2.5L. I'm not sure why they keep loosening but it's the best of all fixes - targeted and cheap!
 

·
Registered
Joined
·
6 Posts
Discussion Starter · #3 ·
The likely reason the manifold bolts become loose is because the intake manifold is plastic and the gasket is rubber - two really stupid moves (IMO) by GM "engineers" who either don't understand the material characteristics or just don't give a flying flip about quality and customer satisfaction (I suspect the latter).

Metal changes shape and size when it heats and cools but, unless the temperatures are extreme, it returns to its original dimensions. Fiber gaskets aren't significantly affected geometrically by temperature transients. On the other hand, many plastics and rubbers shrink over time when subjected to cycling temperatures. (That's why rubber valve cover gaskets on some vintage cars start to leak sometimes, requiring the bolts to be tightened.)

I didn't realize the intake manifold was plastic at first. I was working in the dark and the manifold is down behind the engine. Had I known it was plastic, I would have been suspicious of the bolts sooner. Oh well, live and learn, right?
 
1 - 4 of 4 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top