c4cruiser

My 87 has been in the shop for a few weeks trying to fix an idle surging problem At idle, I would see as much as a 300 RPM swing. We wound up replacing the OEM injectors with some Bosch III's from FIC. Also replaced the original fuel filter.

While the engine ran much better than it had before there was still a slight surge. So we picked up a reman AC Delco ECM and re-installed the Hypertech Thermomaster chip that was in the car. That completely solved the idle issue and we checked everything with a scan tool and found all parameters were right where they should be.

Now the car has a bog with any sort of quick throttle application. Only a fraction of a second and it recovers quickly. Happens with a part-throttle or full-throttle action. Don't see it at very light throttle either off idle or something like passing a car on the freeway as long as you ease into the throttle. TPS voltage is where it should be and the IAC is working fine. Just this bog which is just like a bad accelerator pump in a carb'd car.

So right now it runs and drives fine as long as you don't apply too much throttle too fast. Runs smooth and starts very quickly even when cold. We're going to try another ECM as we can do it under waranty from the supplier to see if that helps.

Any ideas? And our shop tune-up techs have lots of experience in FI cars, but this one problem s stumping them.

AGENT 86

Maybe a drop in fuel pressure is happening, when demand is high. Have you checked your pressure, under load and when gas pedal is punched ?

c4cruiser

Pressure is running around 41-42 psi at idle and about 38-40 at cruise speed. We checked fuel pressure early on and was within spec, even the pressures after sitting for an hour or so.

The big concern is that I had not noticed this bog before. But with the idle surge it was something I had not been paying attention to. Driving in city traffic is fine as is freeway cruising. It's just when you jump on it with anywhere from medium to full throttle. Seems to happen at any given RPM even off idle in neutral.

383vett

Maybe time for a new 02 sensor. If it is crudded up, the motor might be fooled into thinking it is running rich and lean down the mixture. This might be more apparent when you stomp on the throttle and the motor becomes too lean and hesitates.

AGENT 86

But what happens to fuel pressure on initial start of acceleration ? Does the pressure drop for a second, or does it increase, as it should ?

c4cruiser

O2 sensor is new. I drove the car for about 30 miles this afternoon; mixture of freeway and city driving. On the freeway, I stayed in 4th and no OD. On the city streets, I stayed in 2nd for speeds up to 35.

c4cruiser

We can check that tomorrow. When we looked at fuel pressure early on it was within spec including while driving.

AGENT 86

Also a thought, any knock counts during tip in ?

c4cruiser

I'll check that tomorrow too.

reidry

Any update? I have the same issue.