87 Astro mushy throttle with backfire

[xv920r]

1987 Astro quit! Ignition problem. I replaced cap, rotor, pickup coil and plug wires. Engine runs great in open loop but closed loop throttle response is very bad and it will backfire at slow engine speed upon modest throttle opening. It sets no codes. Once the engine "gets used" to a throttle position it will steady out and run well. The speed sensor has never worked. The fuel pump is 2 yrs. old and the filter is fresh. Ideas? Thanks.

[Blue Bowtie]

Is this the 262 V-6 of the 151 4-holer? I'm presuming it's the 6, since the 4 was very rare in '87.

Sine it supposedly runs well in open loop, the fuel pressure, TPS, and ignition timing should be reasonable. I'd suspect a problem with the O² sensor, or possibly the CTS. There is also a slim possibility that the EGR is opening when it shouldn't, causing some of the symptoms.

It would be good to verify the O² is producing good signal, but since you need to remove it to perform an appropriate bench test it might be just as well to replace it if it has more than 50K miles on it. This is one of the older single lead units and they are relatively inexpensive, but tend to lose voltage at about 50K, even sooner with contamination from coolant or fuel additives.

The CTS can be tested with an ohmmeter and its resistance compared to a standard at a known temperature.

[xv920r]

To Blue Bowtie,

Thanks for your reply...I will check it out tomorrow.