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Discussion Starter #1 (Edited)
After a water pump job, my engine is idling quite roughly around 750 RPM. It sits on Lower poly mounts, but even before it was a very dead smooth idle. It looks like they took the cover off instead of using a holder for the water pump pulley. Could one of the balance shafts have slipped a tooth if the chain went slack enough? The engine seems to run fine everywhere else, except below 1000 RPM. Throttle body cleaned, mass air flow sensor replaced

On a related note, it seems there's oil leaking from behind the water pump? Right where the pump butts up against the back of the front engine cover. I've already replaced the valve cover gasket, didn't see any leaks on the old gasket, but that old gasket was hard and needed replacing. But I should be asking is should there be a gasket between the water pump in the back of the engine cover? I'll get a picture when I get back under the car again to address a loose heat shield on the cat.

To add, the car dips to almost 500 RPM coming to a stop sometimes. Sounds like it almost stalls out but catches itself

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If the header wasn't tightened in the correct way, there could be an inbalance (although not sure how this would be via RPMs). Valve covers require 2 passes to be in complete balance. I wish I had the proper docs for that on hand.

Otherwise, are you getting any misfires? CELs?
 

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Discussion Starter #3 (Edited)
No CELs, no misfires. Valve cover tightened with an even pattern. I'm guessing the rear balance shaft may have skipped a tooth during the water pump job. The access hole is untouched, and the timing cover has a new gasket. They didn't use the 'cheat' tool. I may have some funds coming my way. If so, I'll have a stealership replace all chains & guides and be done with it. I noticed slight slack in the timing chain between the cam pulleys when replacing the cover gasket. The engine was SOOOOO smooth before, even with the bottom poly mounts. I swear, if the fuel economy and fun/performance wasn't there, I'd be shopping for a new car. Too much like an old friend, though. Given the age, I can let certain things slide. I'm also going to ask the stealership to check the alignment of the balance shaft before they remove the chain.

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I would think that the balancer chain being off would affect the engine at all RPMs. With the waterpump being part of the belt system, isn't it possible that one of the pulleys is wobbling?
 

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Discussion Starter #5
The water pump is driven off a chain from the crank, same one as the balancers, but not the same chain that drives cams. All engine RPMs are affected, but it's simply more pronounced at dead idle.

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