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GM3100 Crazy Engine Issues


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Ok so I have a wild issue I cannot figure out. Me (decent mechanic) a mechanic friend and two shops have not been able to solve the issue.

1996 Buick century wagon

bought this car with a busted intake gasket, replaced it did NOT mix pushrods up, flushed it, everything seems fine except this issue below.

the engine idles poorly, up and down and I hear a clacking sometimes right under the left side of the intake, I have no idea what it is, could be unrelated. The idle is bad enough the tensioner shakes badly.

things I’ve done

1 new intake gasket (no cross contamination anymore and everything was cleaned up)

2 temp sensor, cam sensor, crank sensor, tps, map and mass air (yes this one has both) complete re-hosed vaccuum and water, water pump, power steering pump. Plugs wires coil packs ignition module, belt tensioner and tons of other unrelated things or ones I’m forgetting.

1st shop - asked to fix idle and steering issues. They aligned everything and put new tires on it since I rebuilt the suspension prior and needed professional machine to do it. Fixed motor mounts and replaced a ton of seals to fix an oil leak, replaced fuel injectors and I’m sure they did lots of testing etc. everything fixed exempt shitty idle.

2nd shop, wanted window regulator fixed , ac fixed, small water leak fixed, and the idle fixed. Wound up getting another water pump timing gasket and 2nd oil pan gasket for some unknown reason, oil change, ac topped off, and “the window regulator is not longer made (I found out it’s easily ordered online) and a this is just the way old cars run.

the idle is crap, I mean under load of turning at low speed out of a parking slot the power steering doesn’t have enough power. It’s blown two belt tensioners. As much as I can tell under full load the engine makes no noise I can hear and the engine runs smooth. It doesn’t get a lot of miles but has run this way for 20k and not fell apart. Aside from that the car runs like a brand new vehicle as it should considering the complete overhaul I did. I have spared nothing this isn’t a beater is a car left in a garage and is in beautiful condition. It’s like a member of the family and I have expressed that money is not an issue I just want the motor to run correctly. I know these can, I put 300k on a 3400 with proper maintenance and it ran like new and when sold the new owner drove it 5 years.

im sure the shops have all done the dummy tests to correct a bad idle just as I have. WTF is wrong with this? Lol

any ideas would be awesome, I’m just at an age now I don’t want to work on it, I want a shop to freaking fix it. I have no issues riding around town in it but don’t feel safe on a 10 hour trip to the beach. We want to drive this like the griswolds did in Christmas vacation (not the same car but very similar)

it’s not something easy it’s something inside the motor. Anyone else run into something like this?


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Check O2 sensor, make sure all other sensor wiring is clean, and clips properly clean and seated. Check for frayed wires leading to sensors, check where the wires touch their clips. Clean up your grounds. Go over whatever electrical your mechanic had to undo for the work he did, tighten/clean anything and everything that is dirty, loose or tampered-with by anyone else.

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Oh, and also, an old man once told me that you should replace the power steering fluid regularly, once you start feeling stiffness at the wheel. He used a turkey baster to suck everything out of the reservoir, then poured in fresh fluid. Once or twice a year for this, was his routine.

Edit: It's not always easy to find proper torque specs for each specific year, engine, and optioned variants of FWD GMs. Even very good mechanics might not have the documentation specific to your exact vehicle... And valve-train work tends to be unforgiving in its tolerance for this... I bet that if you hear a clackedy-clack after some of the more invasive work done, it might just be a nut/bolt that didn't get tightened properly during reassembly.

But I would treat a stumbling idle as an electrical/sensor/vacuum issue, probably separate from audible, isolated clackedy-clackedness.

It wouldn't be too complicated to remove a valve cover and check for play under there, yourself.

It wouldn't hurt to put an ear to the timing chain area and listen for irregular sounds between cold and warm.

Also, I once had a simple serpentine belt tensioner throw me off because it was rattling like valvetrain tick.

Edited by Robbie
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Posted (edited)

All new O2 sensors, new power steering pump and flush. I think the clacking noise is a result of the 3rd tensioner failing due to the engine making it jump around idling. Last time the tensioner solved the noise, motor still idles piss poor, changing the tensioner today, engine will probably quiet up again for the time being but still idle like crap.

question, why does this silly motor have 2 crank sensors, what does each one do, the one on the side of the block I have changed. The one by the balancer is original.

odd thing is this car cranks faster than both of our Hondas, runs smooth as can be (aside from the typical piston noise or whatever these all have for the first 10 seconds), then idles down after a few minutes or more and when it’s idling at the right RPM it’s having trouble maintaining it. Usually idles better when cooler, but still not a correct idle.

I’m going to see if the mechanic putting the 3rd water pump on it solved the slow water loss, maybe I got a bad one, I couldn’t find evidence it was leaking but if the coolant stays right at least some head gasket gonna blow thoughts will be gone and I’ll drive it more and just let it vibrate the steering wheel. I’m at a loss. I’ve looked everywhere for any vac leaks can’t find any.

I can even tell it’s “idling” poorly running down the road with no foot on the gas as when I give it a little gas it jumps in the game a little more than normal, it feels sort of like the old clink clank of a bad u joint but just the clink and less subtle and only felt not heard. I know it’s the motor popping out of crappy idle into running right. TPS and it’s wiring are new IAC is good, all the stuff, and the OBDII system thinks everything is absolutely fine, but I can see the rpm’s bounce around on the scan tool, we’re talking like 600-900 I think, probably supposed to be 750 if I hazard a guess, been awhile since I was the passenger with the scan tool monitoring as someone else drove. Just don’t feel like repeating tests over even if done over an 8 year span when it’s always been the exact same problem the whole time. I’ve taken 3 hour interstate trips with it, it got where it needed to be. I also don’t have much experience with it before overhauling it (2 days before I realized I was sold a car with a bad LIM gasket) I don’t know how the previous owner treated it other than the engine was fairly clean and orderly and the paint was good. It had been run hot in the past as the cam sensor wiring was melted near the manifold, being an old car, I basically started changing everything I had to remove when I did the LIM job with quality parts, age had put a lot of stuff near EOL etc. I could tell as things like the alternator had browned wiring field coils, power steering fluid was crap as someone else mentioned so egh why not just redo the whole thing when I could practically sit under the hood it was so torn down. Then chased my tail forever on any sensor or related part after. Especially since I tried to get it professionally diagnosed with no success.

I also know the computer is at least functioning well enough to throw codes, it guided me to a bad tps when it went and also had emission related issue codes clearing the shit out of the LIM path to it solved that problem, and of course the EGR has been inspected and looks like new.

I just refuse to pull the LIM when it acts so well after the good gasket was put on, plus unless it’s sucking in air from the engine compartment I have no idea where it would be getting unmetered air, the fluids are prestine and have been since replacement, everywhere it’s mated up to the heads or block looks clean as can be, I don’t think I messed the LIM job up, or if I did it would have other symptoms besides mine like running like crap no mater the rpm etc.

I know I’m rambling on but I have a station wagon fetish lol

Edited by AshTray900
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1.  What are the misfire counts for each individual cylinder?

2.  Check for bent pulleys, and/or failing crankshaft torsional damper.  The belt tensioner should not be jumping around.  For the record, tensioners have a damper built into the mechanism to prevent them from jiggling.   Old tensioners can have failed dampers, but a new tensioner shouldn't.  That's why I'm thinking you've got a bent pulley, or something not running true.

3.  In my driveway, it'd get a cranking compression test of all six cylinders, perhaps a leakdown test of all six.

4.  Some engine computers have a method of disabling the fuel injectors for each cylinder in sequence using a scan tool to perform a cylinder-balance test.  I don't know if that vehicle has that capability.  If it does, you should use it.

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I replaced the tensioner, it’s not as bad now but still does it (the bouncing) the noise is gone, I think I may still be leaking water, possibly from the water pump area, just ever so slightly but it could just be leftover mess from the new pump being put on by the shop, I’ll have to test drive more and monitor levels, it’s not the LIM leaking water and all surfaces on the motor look clean and I unstained, you may be correct on the pulley, the ps pulley might need cleaned and I think the belt should be replaced, it looks great however it has a hump in it from not being driven and sitting around the alternator. Not sure how much it would effect it, but it kinda formed a curve matching the little alt pulley.

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