170k mile 99 LS400 here, just recently did a starter replacement. The car starts up and runs, but I have a very weird problem. I accidentally fuel-fouled my spark plugs while cranking the starter to try to get my car to start, resulting in fuel being dumped with no spark. So I let the fuel evaporate for a day and replaced the spark plugs. The car now runs and drives much better, however it sounds awful. It will idle alright to the point where it sounds unproblematic, but the second I try to drive it anywhere, the entire car begins to shake like crazy and it feels like my car lacks power. Then when I let off the gas and come to a stop, the car will dip its idle so low (100-200) that it struggles to stay alive. Sometimes it actually stalls completely. Before I did the starter replacement, I had 0 issues with anything. Does anybody have any clue as to what could possibly be causing this rough running?

 

You didnt power spray clean it did you?

 

I'm assuming you mean with a power washer or something. In that case, no I did not.

 

Is the check engine light on?

 

Yes it is, as well as the VSC light. I'm currently looking for somebody who has an OBD scanner I can borrow as I don't think the car will make it to the nearest Autozone and I don't have the money to buy one myself.

 

Sorry to hear that. Suffice it to say, getting the codes read is the next step to figure out what happened. Otherwise all I can say is to spray around the intake hoses and intake manifold with brake cleaner to see if RPM increases, and check every electrical connector within reach to make sure nothing was left loose.

 

I'll try the brake cleaner trick, if the RPMs do increase, what does that tell me?

 

If the rpms increase, you know that you have an air leak. then do your best to narrow down exactly where it’s coming from and fix it, whether it means tightening clamps further, cleaning fittings, replacing a cracked hose, checking bolt torque values or replacing a gasket or o-ring somewhere.

On the other hand I am thinking of things that you might have removed and reinstalled that could have resulted in the VSC light coming on and making the motor run very poorly. Can you double check your throttle body electrical connections?

 

All electrical connections are very secure, and I can't find any hoses/clamps that could be disconnected. In fact I replaced a majority of the vacuum hoses in search of a fix. However, I just noticed from a wet spot under my car that when the car is running, it pisses coolant from the rear of the engine. I'm fairly certain it must be the gasket from the coolant bridge on top of the starter, as I did remove it during the process. I replaced one gasket as it was all bent and ripped, but thought it would be fine to reuse another. Looks like I thought wrong! In any case, is there any way this coolant leak could be interfering with how the motor runs? I can't think of anything besides coolant getting seeping into the intake chamber and to the valves, and somehow interfering with the air/fuel combustion. However this seems extremely unlikely in my eyes. Any thoughts?

 

Thats a good finding but I also doubt that it’s the cause of your rough running. I’d beg or borrow a scan tool at this point. You can get one for really cheap too; Walmart sells one for $20 and you can go even cheaper with a Bluetooth scanner that communicates with your phone.