cam timing question

I just completed installing a refreshed engine (new short block, newly machined heads), which included installing the cams and setting the timing. When I start the engine, I get a rough idle and a very loud backfire - one every 3-5 seconds or so. I only ran it a few times for a few seconds before shutting it down. I followed the factory manual for setting the cams and timing: Crank at 60 deg BTDC, install the bank 4-6 cams with the lobes oriented towards the inside, locking them with the locking tool (smaller circular cutouts facing outward), rotating the crank to TDC, then locking the chain sprockets. Advance the crank to 60 deg BTDC and repeat for bank 1-3 but with the cam lobes installed facing outward. When done, at TDC, both banks have the smaller cut-outs facing outwards, like they're supposed to. My question is: With the crank at TDC, and the small circular cutouts facing outwards, is it possible that the timing could still be off (presumably by 180 deg.)? Is it possible I over-rotated the crank between setting bank 4-6 and bank 1-3? I would think if that happened, then bank 4-6 would have the smaller cam notch facing inward. ??? If so, could this be the cause of the backfiring? Other than new plugs and new coil packs, everything else is the same from when the engine ran fine. -JS

Forgot to say... engine is an M96 3.6L (3-chain) -js

After further thought, I think I answered my own question. At TDC (regardless of compression or overlap cycle), the circular cutouts on Bank 1 vs Bank 2 should be opposite. If Bank 1 has them toward the outside, Bank 2 should have them toward the inside, as viewed from their respective ends. Can someone confirm this? Thanks-

You should consider purchasing Jake Raby's engine assembly series on the M9X. He covers step-by-step how to time the 3-chain engines. Most of the information he shares is not in the factory shop manuals. https://www.theknowledgegruppe.com

Also, may want to check for a vacuum leak somewhere. When I reinstalled my engine after recovering from a cracked head, my first few starts exhibited rough running and backfiring. I also thought that I had somehow mis-timed the engine. Turns out I had a massive air leak at the brake booster to intake plenum connection. When I fixed the leak, engine purred like a kitten. Best of luck.

Thanks- I will definitely check for vacuum leaks. But I'd like confirmation on the cam half-moon positions because if I got them wrong I don't want to start the engine again. Jon

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Yes, he covers 3 chain and 5 chain in detail. It's easy to do it the wrong way, especially if your done it on air-cooled engines. He also explains how to be certain you did it correctly. I would watch that before starting your engine again to make certain you did it correctly.

Hi Jon. See https://rennlist.com/forums/996-foru...-question.html and specifically post #4.

Doug's right. You missed a step. You need to rotate the engine one turn between setting bank one and two.

OK, I'm definitely off 180 deg on Bank 1. But not sure why, since I wasn't just checking/tweaking the timing, I was installing the cams and setting the timing. Here's the first step quoted from the factory manual after completing Bank-2 cams installation and timing, with the crank at TDC: "Turn the crankshaft by means of the pulley or vibration balancer clockwise (running direction of engine further still until the marking U6 on pulley or vibration balancer coincides with fixing bore in crankcase (60� before TDC). Fix with locating pin P 9595/1." Isn't this the 360 deg. rotation you're talking about? Its actually 300 deg, but it'll be turned another 60 deg before the Bank 1 sprockets are locked. If so, short of turning the engine backwards 60 deg, you HAVE to rotate the crank in order to reposition it to 60 deg BTDC to install the Bank-1 cams. Or are you saying you have to rotate 1 full turn (back to TDC), then another 300 deg to U6? That would put the Bank-1 cam back to where it was. Maybe that's what I mistakenly did -- 1 full turn, then another 300 deg to U6. Don't recall doing that, but that would explain things. Regardless, I need to redo Bank 1. Looks like I can do it with the engine in the car (Cayman). Hopefully.

It sounds like you followed the manual, and therefore have created a problem that I see over and over again. Lots was lost in the German- English translation. Throw that book away. This manual causes problems, and always has. This is the exact reason why I shot my video series, I lay this process out in a very simple, easy to follow manner. You can bail yourself out from where you are now very easily. Don;lt feel bad, I've charged dealerships to consult them on this issue many times. Still today I see dealer techs botch this procedure. Put the engine at TDC and lock the crank.Lock the B1 cams using the proper tool, Remove the bank 1 scavenge pump, and then remove the B1 chain tensioner. Then remove the 4 bolts on the exhaust cam sprocket, and the single bolt on the intake cam/ vane cell adjuster. You will need the proper tool to hold the vane cell adjuster while breaking torque on it. Do not place load on the cam timing tool on the opposite end of the camshaft! This is critical. Unlock the crank, and then spin the engine exactly 360+ back to TDC, (the timing chain will spin the sprockets on B1 freely) then install the chain tensioner, and then install the 4 bolts back into the exhaust cam sprocket, and the single bolt in the vane cell adjuster. Torque to value using the proper tools. replace the exhaust sprocket bolts. You have just reset the timing. Rotate the engine at least 7 times, and check cam timing on both banks. When you complete this, go burn that damn manual, and never use it again.

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I once rebuilt a GM manual transmission, assembled everything, but I could tell right away something was wrong when it wouldn�t smoothly operate. Retraced all my steps, couldn�t figure it out, and then I had to take it to a transmission expert. He said the manual left out a part - a shim ring near the input shaft. He showed me a parts diagram that had the part that was clearly missing. He said �happens more than you�d think.�

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I haven't opened damn factory manual since 2007. It gets people in trouble continually. Even the Turbo manual screws up the cam timing, it shows pictures that conflict the descriptive text, and on top of that my factory tools were marked backwards.. The only way you figure that one out is by creating your own resource material when the engine comes apart. I help people with this very situation dozens of times per year. If a pro makes this mistake it costs him 350 bucks for me to perform a bail out. they get to pay their way through their mistakes, and it makes my classes look cheap.

The key factor that things are wrong is that the cam lock tool currently fits both banks at TDC. It should only fit 1 bank, and require a 360 rotation to fit the other bank. I don't know if I'm ready to burn the entire shop manual, but I certainly won't trust the "Engine" section anymore! Thanks Jake. -js

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Yes, the cam timing tool should NEVER fit both sides of the engine simultaneously while at TDC. One bank should be phased completely opposite. We have now made a special cam timing tool that allows both banks to be set simultaneously, since the tool is made opposite for the opposing bank. The book leaves out 3 critical steps, and has very confusing text that I address directly in my M9X video series; especially so this mistake won't be made.. You didn't make a mistake, except for the fact that you followed the manual, which leaves a lot of detail to be desired. Don't feel bad, I made this exact same mistake with he first M9X I built. The difference was there were no forums to help, and no one I could call for help, since these engines were not being opened up at all at that time. I figured it out, and never made that mistake again. It's all because of the manual, most people do make this mistake that follow the manual.

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So with the B1 cams locked at TDC the B1 valves are all closed (enough) to avoid contact when the engine is spun over? Not doubting, just surprised. I'm sure this is not the first time you've given that instruction.

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Yes, nothing is on lobe. The engine design is such that valve timing events depend on engine oil pressure. I learned to do this when doing test work on the dyno, and I waned to change cams, or cam timing without full disassembly of the valve train.

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Ah! Makes sense. Very cool.

All fixed, per Jake's instructions. Actually quite easy to do in the Cayman since the sprocket ends of the cams face the rear on Bank 1 - nothing in the way other than exhaust pipe. Related to the previous post -- With the B1 cams 180 deg out of timing, could there have been piston/valve contact when I started it?