Hello,


So, I have a 99 SC2 automatic with 178k on the ticker. It's been sputtering and surging while in gear and the engine has a load on it. It's way worse with the A/C on, but if it's in park or I pop it in neutral, it smooths right out its worst at idle and acceleration. It seems to mis when at speed and I am at a level RPM. The idle is where it should be (ballpark of 700RPM). I just replaced the valve cover gasket (full kit) as it appeared to be original, all new AC Delco copper plugs (.040) and wires. The coils aren't that old so I just cleaned them up and replaced the fuel filter and pcv. Cleaned up the throttle body, AGAIN... My next step is to AGAIN replace the coils as they are cheap enough and put new injectors in. Any other suggestions??? Oh, replaced the transmission filter and fluids about a year ago as well if this a difference. Thanks to all that reply.

 

don't quite understand.

DOHC A/T.

Park/Neutral (no load) "smooths right out"

But right after that you said "worst at idle".

Then you say it misses at speed + constant RPM -- which implies it is not surging etc.


So you see how someone might get confused as to how to interpret your description?

Are you trying to say with no load, it runs like crap if you let it idle in park or neutral

AND

it runs like crap in the same manner if you are driving and try to accelerate? But popping into neutral smooths the engine right out when you are at speed and obviously not at idle?

Please restate

Conditions were it runs like crap and specifically how
Conditions were it runs seemingly fine and the specific circumstances under which it can be made to run seemingly fine.
Any SES activity? If so, codes?

thanks

 

Yeah, sorry. So basically with no load ( in park or neutral) The engine runs smooth. With a load whether accelerating or in gear with my foot on the brake at idle, it runs rough, hesitates and feels like a misfire. With the air conditioning on its way worse. It is an automatic 1999 dohc and no codes, no idiot lights.

 

If you're not getting a flashing SES, you're not experiencing a true misfire..

Is the engine truly misbehaving under load, or is the car just shaking and vibrating like crazy?

I'm wondering if your upper torque axis engine mount is shot.

This would be consistent with increased vibrations in gear as with a load on the engine; everything wants to rotate (physics). Hence the torque axis mounts (upper and lower to keep it from rotating under the hood.)

When the upper mount goes bad, it often transmits very significant vibrations to the cabin. There is usually a bad resonance in the DOHCs to start with near idle, and I suspect that when you put it in drive, you are dropping the RPM enough to hit that resonance and shake the crap out of your car. Whenever you try to accelerate, you are trying to add more twist under the hood, and the mount simply can't handle it.

With the e brake ON

Have a buddy stand on the passenger side of the engine and note the amount of movement of the engine with the car warmed up and idling in park. Then have him go back to the exhaust pipe and listen to the exhaust note. Should be smooth and even.

Shift to drive and observe under the hood for amount of movement for a good 10 sec. If upper mount is bad, engine will be a dancin', comparatively. Simulate acceleration ( slightly, as you are line-locking it and straining the tranny). The accel should elicit the same response.... Vibration and engine movement. Have your buddy check the exhaust note. It should still be smooth and even, even if the car is vibrating like a cheap motel room bed.

The upper torque axis mount is located to the left of the cam cover as viewed from the front of the vehicle. It is a upside down u shape bolted to the frame at each end, with stiff rubber across the U gap. With e find off, check for torn or sagging rubber. The other highly scientific test which I have not used but seems dependable is is to try to fit a #2 pencil between the top of the mount and the rubber. If it fits, the mount is supposedly shot.

Also, make sure the nuts were torqued down snug on the three studs it DO NOT OVERTORQUE these as the studs will twist off.

If this is indeed the issue, go OEM with the mount. Aftermarket TAM s suck.
Also, get a new set of mounting studs and nuts. Ask them for the torque spec, then look it up to see if they were right. It will feel like you have another 1/4 turn left, and then it is in your hand.

If this is not the issue, consider yourself reasonably educated on upper TAMs.

Search the site; I know I've explained several times how to do this properly without denting your oil pan. You must also loosen the lower torque axis mounts once the engine is supported, then install the upper TAM, then tighten the bottom back up. This is to keep from locking in a static stress betw the two mounts.

 

So. It's definitely not just a vibration. If I have the a/c on and go to take off from a dead stop. The car sputters and almost stalls before getting any power.

 

Correct answer...Bad coil. Won't be buying from China again. Lol!!!

 

glad it's fixed.
I've never had an OEM coil go bad on an S car.
Yes it does happen....

But these were not OEM that failed. Why did the last set of coils get swapped out?