Motor problems

[jared09lgt]

Okay guys, i need ideas.

 

quick history

09 lgt all stock with about 115k miles, no prior problems.

 

1 year ago i got a cylinder 4 misfire, which ended up being low compression due to worn exhaust valves. I removed the motor, took the heads off and took them to a shop to be returned to factory specs. When i put the heads back on and motor back in the car i got a misfire in cylinders 1 and 3. 2 and 4 were fine, no problems in either. We took it to the shop, they said low compression in those cylinders. I then took the one head back off, took it back to the machine shop, they went back through the head and they said it all looked good, maybe just not sealed to the block all the way, they suggested running the car to temp may do the trick. Put the head back on, ran it, code for 1/3 cam position sensor went off, checked it out, wire in plug came loose, fixed it, and still 1/3 misfire after that. Car runs, rough, of coarse, backfire occasionally. We noticed glowing red exhaust at downpipe, shut down car. Replaced downpipe and cat with cobb catted pipe (turbo back to mid), also replaced upstream o2 sensor because it looked fowled, replaced spark plugs on 1/3, also checked compression all about 105-110 on cold motor (all 4 cylinder) similar numbers with wet test. We have good spark, fuel, and compression. Still have mis in 1/3. Checked exhaust temp at upstream cat and was about 500 f. Checked at downpipe and was around 300 f. wondering if upstream cat is fouled and could be causing miss in 1/3.

Sorry for the long post, just wanted to share all symptoms and current solutions. Let me know if you have thoughts or ideas. Thanks

[ehsnils]

First thought here is the cam belt timing that it's correct and not messed up some way.

 

 

The ignition coil connectors shall be black for the rear coils. Also make sure they are correctly connected.

 

 

Also pull out the spark plugs and look at them. Keep track of which cylinder each goes to. They shall all be identical in color. If you haven't replaced them with new plugs it's time to do it.

 

Third step is to pull all injectors and have them checked and matched.

 

 

And since you had the engine out - check the main connector to the engine for damage. Also check that the grounding wires between engine and chassis are good.

 

 

If you can, post pics of the spark plug electrodes/insulator if you don't know what to look for.

 

 

A side note - did you remove the banjo bolt filter?

[jared09lgt]

Checked timing first time motor was pulled, it was good, havent checked since 1/3 cylinder head was reinstalled, thats another thing i was going to do. -thanks

 

The plugs in 1/3 did were black/dirty at electrodes on both plugs, I replaced both. Cylinders 1/4 plugs were both in great shape. Ill double check that the plugs wires are oriented correctly.

 

Ill pull injectors and take a look at them, but we can smell unburned fuel in exhaust leading us to believe that we are getting fuel, also because of the black on the plug electrodes.

 

Ill double check main connector. Grounding wires were replaced on both sides on reinstall.

And yes, I did remove banjo bolt filters. I saw that on another post here on the forum.

[ehsnils]

My concern is rather that the injectors run too rich on some cylinders causing a problem. Too rich on some cylinders means too lean on other, and too lean is also bad.

 

 

And since you had the engine out I hope you didn't forget a rag in the intake. This has happened to another member here causing a bad but not fatal result.

[jared09lgt]

Running too rich and lean makes sense, i definitely put rags in intake. What would cause the inequality in the air fuel ratios between cylinders?

[ehsnils]

If a rag goes out into one part of the intake like the 1+3 side then it could mess up things quite a bit, especially air/fuel ratio since those cylinders would get a lot less air. So the rags may have stuck on the TGVs.

 

But does the cylinders 1+3 fire at all?

 

If the injectors aren't working right or you have an air leak on the other bank (2+4) giving the total mix to the O2 sensor the wrong data then it can also give an issue.

[jared09lgt]

Update:

 

I took at look at the coil wires and, in fact, they were switched! I put them as they should be, started the car, everything ran great. A p0171 code came up, I looked around and found a vacuum line that had come undone from the IC to the intake manifold. Plugged that back in, cleared code, turned the motor over and it was running perfect! Love an easy fix, all thanks to ehsnils! Thanks man!