Ideas wanted, rough running LDK LONG - Cobalt SS Network



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Old 12-05-2017, 06:24 PM   #1
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Ideas wanted, rough running LDK LONG

The other day when I was visiting friends I noticed my car was making a somewhat Subaru type sound. It was pulling a few miss fires, but not many. 5 or less here and there. The first thing I thought was spark plug and/or coils, although those normally show up as miss fires on the interceptor. I swapped in a known good set of coils and plugs, same result.

I also pulled the valve cover and looked for broken or loose rockers, all were in place perfect and no unusual marks on cams, although I may pull it again and very closely inspect the lash adjusters and springs to triple check with a bright penlight.

Here are some things of interest. I had just filled up with gas that morning. Bad gas cause a rhythmic sound ever? Also, the car has been backfiring and popping on cold starts. Any other time its fine, just on cold starts a few seconds. It had popped a few extra times, I'm wondering if I hurt a map sensor. Is it possible for them to be bad with no CEL? There are no CELS. What makes me wonder even more, when I'm coasting down hill in 5th gear, I no longer see any fuel shutoff. Usually Lambda goes up to 1.99 and afr goes to 29 something in a coast. I did not yet do a compression test. I had just thrown my useless Harbor Freight tester as far as I possibly could. Going to get a good new one. I'm wondering if anyone has ever had any similar situations, or ideas. I have 2 new MAP sensors coming also. Thanks guys
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Old 12-06-2017, 11:45 AM   #2
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Sounds like an engine issue. But I would also check simple things like Evap Purge Valve.

It will cause the exact issues you described.
How are your fuel trims?


You need to log the car with a scanner or OB2 reader.
You should be able to see if the 02 sensor is reading correctly and see if it's actually going into DFCO.

Edit: Sorry I missed your sentence about Lamda.

Last edited by umrdyldo; 12-06-2017 at 12:05 PM.
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Old 12-06-2017, 11:51 AM   #3
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Isn't afr=29 at or around band edge? Did you record afr during DFCO before? I think it's a red herring here. I'd look at the cam actuator solenoid valves, too.
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Old 12-06-2017, 12:18 PM   #4
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Will do, checking the Evap and cam actuator solenoid . I was actually just thinking about that because of the weird cold starts. I have a spare set I can swap in. Compression turned out ok. A bit on the low side but dead on consistent. Fuel fuse pulled, throttle wide open and all plugs pulled, but I didnt have a battery charger on. 126,131,130,131. Not sure if the comp tester is accurate, but with stage 2 cams and increased ring gaps over oem due to filing, it may yield a lower cranking pressure. Being its so even I'm not worried. The 29.9 would only appear before when coasting down hill in 5th. That is when the fuel shuts off and it defaults to that 29.9
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Old 12-06-2017, 12:22 PM   #5
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Oh so you're saying it DOESN'T go to 29 now?
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Old 12-06-2017, 12:45 PM   #6
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Yes. It used to hit that going downhill, and would make a slight audible change as the fuel cutoff comes in. No I'm seeing neither. Very strange, and may have nothing to do with my issue, just an observation
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Old 12-06-2017, 12:53 PM   #7
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Quote:
Originally Posted by cw383 View Post
Yes. It used to hit that going downhill, and would make a slight audible change as the fuel cutoff comes in. No I'm seeing neither. Very strange, and may have nothing to do with my issue, just an observation
It has to be above 35mph and something like 2500 or 3000 RPMs.

If you confirm it isn't working, I would look at 02 sensor, Cam Actuators, Evap Purge, compression tests etc.

How is the AFR when cruising?
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Old 12-06-2017, 01:07 PM   #8
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what tune are you running? and stft at idle and ltft?
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Old 12-08-2017, 11:52 PM   #9
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Well I found the problem. There was indeed a rocker screwed up. Whether it was this way the last time i had the valve cover off a few days ago and I didnt notice somehow, or it was on its way out and fell apart after that I'm unsure. I pulled the valve cover to do another inspection, as I had a nagging feeling I may have missed something before. All the rockers were in place, but my light wasnt the best before, so I redid it today. Low and behold there it was, the very first rocker on the exhaust side was perfectly in place on the lash adjuster and valve, but.......no bearing. Nothing anywhere to be found! What held it right there in place I dont know.

There was some slight damage to the cam lobe. I stuffed some rags under the cam and used various grades of emery cloth to smooth it off the best i could. I'm not sure if that will work or not. My plan is to pull the VC again in a few days and see how everything is mating. As for the missing bearing, Im going to dump the oil tomorrow and try a flexible magnet in the drain hole to see what all I can get out. The #1 exhaust valve rocker has a straight shot to the space behind the front cover, and I'm sure thats where everything went. Into the pan. No signs of anything in the head. Thank you to all who gave advice.
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Old 12-09-2017, 01:57 PM   #10
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Glad you found it
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