My car began performing very poorly after re-filling the coolant. Can someone help?

[Deparneaux]

I recently increased the amount of coolant (50-50% water with ethylene glycol) on my two cars (1989 Buick Park Avenue & 2001 Chevrolet Cavalier, with 71K and 112K miles respectively). I increased by about roughly 6-8 ounces, until the amount of coolant reached the appropriate fluid level line (this left plenty of open space in the reservoir for overflow of hot coolant). The coolant was the appropriate percentage of each constituent (water and A/F), did not have any foreign contaminants, and still had plenty of shelf life (in the case of the ethylene glycol).

No matter, when I started the car after filling the reserve tank on each vehicle, the ignition wouldn't turn over at all, despite running perfectly before and having fully functional starter motors and powerful batteries. It sounded like the pistons couldn't even fire, and I immediately suspected the possibility of hydrostatic shock. However, I was unable to detect the presence of an discoloration in the motor oil due to contamination by coolant fluid. Moreover, once running, each engine idled perfectly fine, with little hesitation or knocking, although there was a moderate loss of power (5-10%) and absolutely massive loss of fuel economy (the Buick instantly dropped from 24mpg/City to around 2-3mpg/City, which was mind-baffling), particularly in the higher RPM range. There was also a far slower throttle response, and it created more internal engine stress to achieve RPM increases.

I would have initially suspected a potentially cracked head gasket allowing the seepage of engine coolant into the oil, but tests performed on the head gaskets revealed full functionality and health. Also, the remaining components of the cooling system (radiator, water pump, hoses, etc.) all work fine (and always have) too. Can anyone offer any insight into my problem?

[Blue Bowtie]

6-8 ounces of coolant added to the reservoir should not have caused any problems unless coolant was spilled or leaked onto or into something sensitive. For this to occur with two vehicles is highly suspicious.

Another concern is that the description implies that the same coolant mixture was used for both vehicles. The '89 Buick was supplied with an older formula (conventional) coolant which included silicates and higher pH corrosion inhibitors, whereas the '01 Chevy was supplied with a silicate-free mildly acidic coolant formula. Mixing the two types can cause problems, but not the kind of problems described above.

The comment that both vehicles idle perfectly fine would also tend to indicate that nothing is mechanically damaged. The symptoms including slower throttle response and poor fuel economy can be indicative of air intake, fuel delivery, and/or ignition problems, and serious problems should be detected by the engine management systems and result in a "Check Engine" warning being displayed on the instrument panel. Is this the case?